From 92ebf8abb57aba6d9f7fe5f5d5f421ff1c07f4a0 Mon Sep 17 00:00:00 2001
From: Kartios <kartitripathi@gmail.com>
Date: Tue, 11 Aug 2026 20:22:12 -0700
Subject: [PATCH] feat: add authenticated realtime presence

---
 ARCHITECTURE.md                    |  10 +
 CONTRACT.md                        |   5 +
 index.html                         |  39 ++
 server/README.md                   |  27 ++
 server/src/app.ts                  |   4 +-
 server/src/realtime/index.ts       |  13 +
 server/src/realtime/service.ts     | 664 +++++++++++++++++++++++++++++
 server/src/routes/realtime.ts      | 186 ++++++++
 server/src/services.ts             |   3 +
 server/src/test/realtime.test.ts   | 663 ++++++++++++++++++++++++++++
 src/engine/scene.ts                |  44 ++
 src/interiors/officeScene.ts       |  39 ++
 src/main.ts                        | 281 ++++++++++++
 src/realtime/client.ts             | 567 ++++++++++++++++++++++++
 src/realtime/index.ts              |  23 +
 src/realtime/presenceIndicator.ts  | 121 ++++++
 src/realtime/scenePeers.ts         | 359 ++++++++++++++++
 src/test/presenceIndicator.test.ts | 114 +++++
 src/test/realtimeClient.test.ts    | 412 ++++++++++++++++++
 src/test/scenePeers.test.ts        | 173 ++++++++
 20 files changed, 3746 insertions(+), 1 deletion(-)
 create mode 100644 server/src/realtime/index.ts
 create mode 100644 server/src/realtime/service.ts
 create mode 100644 server/src/routes/realtime.ts
 create mode 100644 server/src/test/realtime.test.ts
 create mode 100644 src/realtime/client.ts
 create mode 100644 src/realtime/presenceIndicator.ts
 create mode 100644 src/realtime/scenePeers.ts
 create mode 100644 src/test/presenceIndicator.test.ts
 create mode 100644 src/test/realtimeClient.test.ts
 create mode 100644 src/test/scenePeers.test.ts

diff --git a/ARCHITECTURE.md b/ARCHITECTURE.md
index 73ed4fb..b2805ae 100644
--- a/ARCHITECTURE.md
+++ b/ARCHITECTURE.md
@@ -118,6 +118,16 @@ adapters whose absence cannot stop the city, office, traffic, or sky from
 running. This is the replacement path for the retired Phaser World 1; it is not
 a migration of that renderer.
 
+The first hosted adapter is now concrete: the Fastify API owns short-lived,
+in-memory authoritative presence sessions and streams validated deltas over
+authenticated POST/SSE. `src/realtime` remains renderer- and transport-neutral;
+the browser transport, interpolation buffer, procedural peer renderer and small
+status UI are separate adapters. A city or full-depth office opts into remote
+peers explicitly, while an anonymous/public scene does not even allocate their
+assets. Interest changes clear the old coordinate frame before a new one is
+shown. Game-state deltas never carry webcam or screen media; those remain
+separate consent and authorization lifecycles.
+
 ---
 
 ## 3. Three rules that keep Apache 2.0 honest
diff --git a/CONTRACT.md b/CONTRACT.md
index 7641935..d3eca8a 100644
--- a/CONTRACT.md
+++ b/CONTRACT.md
@@ -193,6 +193,11 @@ Three backends were designed for one box — three ports, three frameworks, thre
   public box.
 - `Cache-Control` is fail-closed: a global hook stamps `private, no-store`, and
   a route opts in to public caching explicitly.
+- Authenticated realtime is a route module in this same process, not another
+  daemon. Its state is bounded and memory-only. Event credentials travel in a
+  POST body, never a URL; the server owns interest filtering, motion validation,
+  token rotation and revocation. The auth subject is not a peer-visible entity
+  id, and no media is carried on the game-state stream.
 
 ### 5.1 Zero-config boot must actually boot
 
diff --git a/index.html b/index.html
index 017b72a..78d30b4 100644
--- a/index.html
+++ b/index.html
@@ -446,6 +446,42 @@
       }
       .tier.god .who { color: var(--amber-ink); }
       .tier a { color: var(--amber-ink); text-transform: none; letter-spacing: 0.04em; }
+      #presence-host {
+        position: fixed;
+        top: calc(var(--s4) + env(safe-area-inset-top) + 2.5rem);
+        right: var(--s4);
+        z-index: 3;
+        max-width: min(15rem, 65vw);
+      }
+      #presence-host:empty { display: none; }
+      .tera-presence {
+        display: flex;
+        align-items: center;
+        gap: var(--s2);
+        padding: 5px var(--s2);
+        color: var(--ink-3);
+        background: rgba(8, 12, 17, 0.76);
+        border: 1px solid var(--hairline);
+        border-radius: var(--r-sm);
+        box-shadow: var(--shadow);
+        backdrop-filter: var(--blur);
+        -webkit-backdrop-filter: var(--blur);
+        font-size: 9px;
+        letter-spacing: 0.04em;
+      }
+      .tera-presence[data-presence="live"] { color: var(--ink-2); }
+      .tera-presence__retry {
+        padding: 2px 6px;
+        color: var(--amber-ink);
+        background: transparent;
+        border: 1px solid rgba(242, 177, 52, 0.3);
+        border-radius: 3px;
+        font: inherit;
+        cursor: pointer;
+      }
+      body.presence-on .corner {
+        top: calc(var(--s4) + env(safe-area-inset-top) + 4.55rem);
+      }
       .profile-trigger {
         min-height: 28px;
         padding: 4px 8px;
@@ -781,6 +817,8 @@
           right: var(--s3);
         }
         .tier { top: calc(var(--s3) + env(safe-area-inset-top)); right: var(--s3); }
+        #presence-host { top: calc(var(--s3) + env(safe-area-inset-top) + 2.5rem); right: var(--s3); }
+        body.presence-on .corner { top: calc(var(--s3) + env(safe-area-inset-top) + 4.55rem); }
         .minimap-frame { height: 10rem; }
         .rail { right: var(--s3); bottom: calc(var(--s3) + env(safe-area-inset-bottom)); }
         .source { left: var(--s3); bottom: calc(var(--s3) + env(safe-area-inset-bottom)); }
@@ -949,6 +987,7 @@
     </div>
 
     <p class="card tier" id="tier" hidden></p>
+    <div id="presence-host" hidden></div>
 
     <aside id="corner" class="corner" aria-label="Plan view">
       <div class="card" id="minimap">
diff --git a/server/README.md b/server/README.md
index d833e8b..23ed2aa 100644
--- a/server/README.md
+++ b/server/README.md
@@ -31,6 +31,10 @@ without breaking the typecheck, which is the wrong order to find out.
 | `GET /api/v1/markers` | — | `MarkersBody` | **private; 401 unless signed in** — public empty body when `TERA_MARKERS_SOURCE=none` |
 | `GET /api/v1/offices/:id` | — | `OfficeDoc` | public offices only |
 | `GET /api/v1/offices/:id/presence` | — | `PresenceBody` | **private; 401 unless signed in** — never publicly cached |
+| `POST /api/v1/realtime/join` | — | strict `JoinRequest` or `ResumeRequest` | **private; never cached** |
+| `POST /api/v1/realtime/events` | — | fetch-streamed SSE, opened with a strict `ResumeRequest` | **private; never cached** |
+| `POST /api/v1/realtime/pose` | — | one owned `EntityPoseSnapshot` | **private; never cached** |
+| `POST /api/v1/realtime/leave` | — | session id + opaque resume token | **private; never cached** |
 
 Every body is declared once, in `src/server/wire.ts` in the **root** package —
 type-only, so it compiles to nothing and both the browser build and this service
@@ -48,6 +52,29 @@ reply before any route runs, and a route opts in explicitly. A request that
 arrived with an `Authorization` header or a cookie never gets a public policy,
 whatever the route asked for.
 
+### Realtime sessions
+
+Hosted presence is an optional, in-memory adapter. It stores no pose history on
+disk and disappears cleanly on restart; the static city and office continue to
+work when it is absent. Joining requires the deployment's existing auth, and a
+session owns exactly one opaque actor plus a bounded list of interest cells.
+The server validates entity ownership, monotonic sequence/time, cell membership,
+speed, climb and turn rate before broadcasting a pose only to the same cell.
+
+The event stream is deliberately a **POST consumed with `fetch()`**, not an
+`EventSource` URL. Session/resume credentials stay in the request body and
+therefore out of browser history, referrers, Caddy access logs and copied URLs.
+Tokens are random, stored only as SHA-256 hashes, rotated on resume, and bounded
+by absolute TTL and disconnect grace. Reconnect, room capacity and pose rate
+are capped. Authentication subjects remain server-only; peers see page-scoped
+opaque entity ids and no profile name, face texture, email, or media locator.
+
+California, city, office, floor and room cells are distinct coordinate and
+authorization boundaries. Disk-backed office packs are checked before a floor
+or room may be joined. The three public demo offices bundled into this repo may
+use their office envelope without a duplicate server pack; that exception does
+not invent floor/room access and does not apply to arbitrary tenant ids.
+
 ## Regions
 
 **A caller's coordinate is never forwarded upstream. It only selects among the
diff --git a/server/src/app.ts b/server/src/app.ts
index d1b1d47..9b54ae6 100644
--- a/server/src/app.ts
+++ b/server/src/app.ts
@@ -20,6 +20,7 @@ import { registerHealth } from "./routes/health.ts";
 import { registerMarkers } from "./routes/markers.ts";
 import { registerOffices } from "./routes/offices.ts";
 import { registerPresence } from "./routes/presence.ts";
+import { registerRealtime } from "./routes/realtime.ts";
 import { registerSatellites } from "./routes/satellites.ts";
 import { registerSession } from "./routes/session.ts";
 import { registerWeather } from "./routes/weather.ts";
@@ -51,6 +52,7 @@ export function buildApp(config: Config = loadConfig()): FastifyInstance {
   registerMarkers(app, services);
   registerOffices(app, services);
   registerPresence(app, services);
+  registerRealtime(app, services);
   registerSession(app, services);
 
   app.setNotFoundHandler(async (_req, reply) => {
@@ -86,7 +88,7 @@ function registerCors(app: FastifyInstance, allowed: string[]): void {
 
     reply.header("access-control-allow-origin", origin);
     reply.header("access-control-allow-headers", "authorization, content-type");
-    reply.header("access-control-allow-methods", "GET, OPTIONS");
+    reply.header("access-control-allow-methods", "GET, POST, OPTIONS");
     // Whatever the response ends up being, it depended on the Origin header.
     reply.header("vary", "Origin");
 
diff --git a/server/src/realtime/index.ts b/server/src/realtime/index.ts
new file mode 100644
index 0000000..176c6c4
--- /dev/null
+++ b/server/src/realtime/index.ts
@@ -0,0 +1,13 @@
+export {
+  createRealtimeService,
+  type RealtimeConnection,
+  type RealtimeFailureCode,
+  type RealtimeJoinInput,
+  type RealtimeJoinResult,
+  type RealtimeListener,
+  type RealtimeResult,
+  type RealtimeResumeConnection,
+  type RealtimeService,
+  type RealtimeServiceOptions,
+  type RealtimeSessionView,
+} from "./service.ts";
diff --git a/server/src/realtime/service.ts b/server/src/realtime/service.ts
new file mode 100644
index 0000000..9fc149a
--- /dev/null
+++ b/server/src/realtime/service.ts
@@ -0,0 +1,664 @@
+/**
+ * Dependency-free, in-memory authoritative realtime sessions.
+ *
+ * The service owns credentials, interest filtering, sequence allocation and
+ * movement validation. HTTP/SSE routes are adapters only. No poses survive a
+ * process restart and usable bearer tokens are never retained in memory.
+ */
+
+import { createHash, randomBytes, timingSafeEqual } from "node:crypto";
+import {
+  isEntityPoseSnapshot,
+  isInterestCell,
+  validateMotion,
+} from "../../../src/realtime/protocol.ts";
+import type {
+  EntityPoseSnapshot,
+  InterestCell,
+  JoinGrant,
+  MotionLimits,
+  PoseDelta,
+  RealtimeMemberRole,
+  ResumeGrant,
+  ResumeRequest,
+  ServerRealtimeMessage,
+} from "../../../src/realtime/types.ts";
+
+const UINT32_MAX = 4_294_967_295;
+
+export interface RealtimeServiceOptions {
+  now?: () => number;
+  sessionTtlMs?: number;
+  disconnectGraceMs?: number;
+  maximumSessions?: number;
+  maximumSessionsPerCell?: number;
+  maximumInterestsPerSession?: number;
+  maximumPoseUpdatesPerWindow?: number;
+  rateWindowMs?: number;
+  actorMotionLimits?: MotionLimits;
+  vehicleMotionLimits?: MotionLimits;
+}
+
+export interface RealtimeJoinInput {
+  requestId: string;
+  actorId: string;
+  subject: string;
+  role: RealtimeMemberRole;
+  interests: readonly InterestCell[];
+}
+
+export interface RealtimeJoinResult {
+  sessionId: string;
+  token: string;
+  grant: JoinGrant;
+}
+
+export type RealtimeFailureCode =
+  | "invalid"
+  | "unauthorized"
+  | "expired"
+  | "capacity"
+  | "conflict"
+  | "rate-limited"
+  | "sequence"
+  | "interest"
+  | "ownership"
+  | "motion";
+
+export type RealtimeResult<T> =
+  | { ok: true; value: T }
+  | { ok: false; code: RealtimeFailureCode; message: string };
+
+export type RealtimeListener = (message: ServerRealtimeMessage) => void;
+
+interface Session {
+  id: string;
+  tokenHash: Buffer;
+  previousTokenHash: Buffer | null;
+  previousTokenExpiresAt: number;
+  subject: string;
+  actorId: string;
+  role: RealtimeMemberRole;
+  interests: readonly InterestCell[];
+  interestKeys: ReadonlySet<string>;
+  createdAt: number;
+  expiresAt: number;
+  disconnectedAt: number | null;
+  lastClientSequence: number;
+  rateWindowStartedAt: number;
+  rateCount: number;
+  proposedPoses: Map<string, EntityPoseSnapshot>;
+  poses: Map<string, EntityPoseSnapshot>;
+  listeners: Set<RealtimeListener>;
+}
+
+interface ResolvedOptions {
+  now: () => number;
+  sessionTtlMs: number;
+  disconnectGraceMs: number;
+  maximumSessions: number;
+  maximumSessionsPerCell: number;
+  maximumInterestsPerSession: number;
+  maximumPoseUpdatesPerWindow: number;
+  rateWindowMs: number;
+  actorMotionLimits: MotionLimits;
+  vehicleMotionLimits: MotionLimits;
+}
+
+export interface RealtimeService {
+  join(input: RealtimeJoinInput): RealtimeResult<RealtimeJoinResult>;
+  authenticate(sessionId: string, token: string): RealtimeResult<RealtimeSessionView>;
+  connect(sessionId: string, token: string, listener: RealtimeListener): RealtimeResult<RealtimeConnection>;
+  resume(request: ResumeRequest, subject: string): RealtimeResult<ResumeGrant>;
+  connectResume(
+    request: ResumeRequest,
+    subject: string,
+    listener: RealtimeListener,
+  ): RealtimeResult<RealtimeResumeConnection>;
+  submitPose(
+    sessionId: string,
+    token: string,
+    snapshot: unknown,
+  ): RealtimeResult<PoseDelta>;
+  leave(sessionId: string, token: string): RealtimeResult<null>;
+  revokeSubject(subject: string): number;
+  cleanup(): number;
+  sessionCount(): number;
+}
+
+export interface RealtimeSessionView {
+  sessionId: string;
+  actorId: string;
+  role: RealtimeMemberRole;
+  interests: readonly InterestCell[];
+  expiresAt: number;
+  snapshot: readonly EntityPoseSnapshot[];
+}
+
+export interface RealtimeConnection {
+  view: RealtimeSessionView;
+  disconnect(): void;
+}
+
+export interface RealtimeResumeConnection extends RealtimeConnection {
+  grant: ResumeGrant;
+}
+
+function finitePositive(value: number | undefined, fallback: number): number {
+  return typeof value === "number" && Number.isFinite(value) && value > 0 ? value : fallback;
+}
+
+function finiteCount(value: number | undefined, fallback: number): number {
+  return Math.max(1, Math.floor(finitePositive(value, fallback)));
+}
+
+function resolveOptions(value: RealtimeServiceOptions): ResolvedOptions {
+  return {
+    now: value.now ?? Date.now,
+    sessionTtlMs: finitePositive(value.sessionTtlMs, 5 * 60_000),
+    disconnectGraceMs: finitePositive(value.disconnectGraceMs, 20_000),
+    maximumSessions: finiteCount(value.maximumSessions, 1_024),
+    maximumSessionsPerCell: finiteCount(value.maximumSessionsPerCell, 96),
+    maximumInterestsPerSession: finiteCount(value.maximumInterestsPerSession, 16),
+    maximumPoseUpdatesPerWindow: finiteCount(value.maximumPoseUpdatesPerWindow, 120),
+    rateWindowMs: finitePositive(value.rateWindowMs, 10_000),
+    actorMotionLimits: value.actorMotionLimits ?? {
+      maximumHorizontalSpeedMps: 24,
+      maximumVerticalSpeedMps: 18,
+      maximumTurnRateDegPerSec: 540,
+      positionSlackM: 1.5,
+      maximumDeltaSeconds: 2,
+    },
+    vehicleMotionLimits: value.vehicleMotionLimits ?? {
+      maximumHorizontalSpeedMps: 105,
+      maximumVerticalSpeedMps: 45,
+      maximumTurnRateDegPerSec: 240,
+      positionSlackM: 4,
+      maximumDeltaSeconds: 2,
+    },
+  };
+}
+
+function hashToken(token: string): Buffer {
+  return createHash("sha256").update(token).digest();
+}
+
+function tokenMatches(expected: Buffer, token: string): boolean {
+  if (typeof token !== "string" || token.length < 16 || token.length > 512) return false;
+  const found = hashToken(token);
+  return found.length === expected.length && timingSafeEqual(found, expected);
+}
+
+function interestKey(cell: InterestCell): string {
+  switch (cell.kind) {
+    case "california-tile": return `tile:${cell.level}:${cell.x}:${cell.y}`;
+    case "city": return `city:${cell.cityId}`;
+    case "office": return `office:${cell.officeId}`;
+    case "floor": return `floor:${cell.officeId}:${cell.floorId}`;
+    case "room": return `room:${cell.officeId}:${cell.floorId}:${cell.roomId}`;
+  }
+}
+
+function cloneCell(cell: InterestCell): InterestCell {
+  return { ...cell };
+}
+
+function clonePose(pose: EntityPoseSnapshot): EntityPoseSnapshot {
+  return JSON.parse(JSON.stringify(pose)) as EntityPoseSnapshot;
+}
+
+function entityKey(pose: EntityPoseSnapshot): string {
+  return pose.entity === "actor" ? `actor:${pose.actorId}` : `vehicle:${pose.vehicleId}`;
+}
+
+function poseInterestKey(pose: EntityPoseSnapshot): string | null {
+  if (pose.pose.space === "local") return interestKey(pose.pose.cell);
+  // Geographic coordinates are interpreted through the session's authored
+  // outdoor interest: either one exact California tile or one exact city.
+  return null;
+}
+
+function validIdentity(value: string): boolean {
+  return typeof value === "string" && value.length > 0 && value.length <= 256;
+}
+
+export function createRealtimeService(options: RealtimeServiceOptions = {}): RealtimeService {
+  const settings = resolveOptions(options);
+  const sessions = new Map<string, Session>();
+  const epoch = randomBytes(16).toString("base64url");
+  let serverSequence = 0;
+
+  function nextSequence(): number {
+    serverSequence = serverSequence >= UINT32_MAX ? 0 : serverSequence + 1;
+    return serverSequence;
+  }
+
+  function geographicInterestKeys(session: Session): string[] {
+    return [...session.interestKeys].filter(
+      (key) => key.startsWith("tile:") || key.startsWith("city:"),
+    );
+  }
+
+  function poseBroadcastKeys(session: Session, pose: EntityPoseSnapshot): string[] {
+    const local = poseInterestKey(pose);
+    return local === null ? geographicInterestKeys(session) : [local];
+  }
+
+  function broadcast(delta: PoseDelta, keys: readonly string[], excluding?: Session): void {
+    if (keys.length === 0) return;
+    for (const peer of sessions.values()) {
+      if (peer === excluding || !keys.some((key) => peer.interestKeys.has(key))) continue;
+      for (const listener of peer.listeners) listener(delta);
+    }
+  }
+
+  function removalDelta(session: Session): PoseDelta | null {
+    const removedEntityIds = [...session.poses.keys()];
+    if (removedEntityIds.length === 0) return null;
+    return {
+      type: "pose-delta",
+      protocolVersion: 1,
+      serverEpoch: epoch,
+      sequence: nextSequence(),
+      timestampMs: settings.now(),
+      updates: [],
+      removedEntityIds,
+    };
+  }
+
+  function broadcastRemoval(session: Session, keys = [...session.interestKeys]): void {
+    const delta = removalDelta(session);
+    if (delta) broadcast(delta, keys, session);
+  }
+
+  function remove(session: Session, announce = true): void {
+    if (announce) broadcastRemoval(session);
+    sessions.delete(session.id);
+    session.listeners.clear();
+    session.proposedPoses.clear();
+    session.poses.clear();
+    session.tokenHash.fill(0);
+    session.previousTokenHash?.fill(0);
+  }
+
+  function cleanup(): number {
+    const now = settings.now();
+    let removed = 0;
+    for (const session of sessions.values()) {
+      const graceExpired =
+        session.disconnectedAt !== null && now - session.disconnectedAt > settings.disconnectGraceMs;
+      if (now >= session.expiresAt || graceExpired) {
+        remove(session);
+        removed += 1;
+      }
+    }
+    return removed;
+  }
+
+  function authenticate(
+    sessionId: string,
+    token: string,
+    allowPrevious = false,
+  ): RealtimeResult<Session> {
+    cleanup();
+    if (!validIdentity(sessionId)) {
+      return { ok: false, code: "unauthorized", message: "Realtime session is invalid." };
+    }
+    const session = sessions.get(sessionId);
+    const matchesCurrent = session ? tokenMatches(session.tokenHash, token) : false;
+    const matchesPrevious = allowPrevious && session?.previousTokenHash !== null &&
+      settings.now() <= (session?.previousTokenExpiresAt ?? 0) &&
+      tokenMatches(session?.previousTokenHash ?? Buffer.alloc(0), token);
+    if (!session || (!matchesCurrent && !matchesPrevious)) {
+      return { ok: false, code: "unauthorized", message: "Realtime session is invalid." };
+    }
+    if (settings.now() >= session.expiresAt) {
+      remove(session);
+      return { ok: false, code: "expired", message: "Realtime session expired." };
+    }
+    return { ok: true, value: session };
+  }
+
+  function view(session: Session): RealtimeSessionView {
+    const snapshot: EntityPoseSnapshot[] = [];
+    for (const peer of sessions.values()) {
+      for (const pose of peer.poses.values()) {
+        const visible = poseBroadcastKeys(peer, pose).some((key) => session.interestKeys.has(key));
+        if (visible) snapshot.push(clonePose(pose));
+      }
+    }
+    return {
+      sessionId: session.id,
+      actorId: session.actorId,
+      role: session.role,
+      interests: session.interests.map(cloneCell),
+      expiresAt: session.expiresAt,
+      snapshot,
+    };
+  }
+
+  function join(input: RealtimeJoinInput): RealtimeResult<RealtimeJoinResult> {
+    cleanup();
+    if (
+      !validIdentity(input.requestId) || !validIdentity(input.actorId) ||
+      !validIdentity(input.subject) ||
+      (input.role !== "member" && input.role !== "admin") ||
+      !Array.isArray(input.interests) || input.interests.length === 0 ||
+      input.interests.length > settings.maximumInterestsPerSession ||
+      !input.interests.every(isInterestCell)
+    ) return { ok: false, code: "invalid", message: "Realtime join request is invalid." };
+    if (sessions.size >= settings.maximumSessions) {
+      return { ok: false, code: "capacity", message: "Realtime service is at capacity." };
+    }
+    if ([...sessions.values()].some((session) => session.actorId === input.actorId)) {
+      return { ok: false, code: "conflict", message: "Realtime actor id is already active." };
+    }
+    const unique = new Map(input.interests.map((cell) => [interestKey(cell), cloneCell(cell)]));
+    for (const key of unique.keys()) {
+      let count = 0;
+      for (const session of sessions.values()) if (session.interestKeys.has(key)) count += 1;
+      if (count >= settings.maximumSessionsPerCell) {
+        return { ok: false, code: "capacity", message: "Realtime interest cell is at capacity." };
+      }
+    }
+    const now = settings.now();
+    const id = randomBytes(18).toString("base64url");
+    const token = randomBytes(32).toString("base64url");
+    const interests = [...unique.values()];
+    const session: Session = {
+      id,
+      tokenHash: hashToken(token),
+      previousTokenHash: null,
+      previousTokenExpiresAt: 0,
+      subject: input.subject,
+      actorId: input.actorId,
+      role: input.role,
+      interests,
+      interestKeys: new Set(unique.keys()),
+      createdAt: now,
+      expiresAt: now + settings.sessionTtlMs,
+      disconnectedAt: null,
+      lastClientSequence: -1,
+      rateWindowStartedAt: now,
+      rateCount: 0,
+      proposedPoses: new Map(),
+      poses: new Map(),
+      listeners: new Set(),
+    };
+    sessions.set(id, session);
+    const grant: JoinGrant = {
+      type: "join-grant",
+      protocolVersion: 1,
+      requestId: input.requestId,
+      sessionId: id,
+      actorId: input.actorId,
+      role: input.role,
+      serverEpoch: epoch,
+      serverTimeMs: now,
+      nextSequence: serverSequence >= UINT32_MAX ? 0 : serverSequence + 1,
+      resumeToken: token,
+      interests: interests.map(cloneCell),
+      initial: view(session).snapshot,
+    };
+    return { ok: true, value: { sessionId: id, token, grant } };
+  }
+
+  function connect(
+    sessionId: string,
+    token: string,
+    listener: RealtimeListener,
+  ): RealtimeResult<RealtimeConnection> {
+    if (typeof listener !== "function") {
+      return { ok: false, code: "invalid", message: "Realtime listener is invalid." };
+    }
+    const found = authenticate(sessionId, token);
+    if (!found.ok) return found;
+    const session = found.value;
+    session.disconnectedAt = null;
+    session.listeners.add(listener);
+    let connected = true;
+    return {
+      ok: true,
+      value: {
+        view: view(session),
+        disconnect() {
+          if (!connected) return;
+          connected = false;
+          session.listeners.delete(listener);
+          if (session.listeners.size === 0) session.disconnectedAt = settings.now();
+        },
+      },
+    };
+  }
+
+  function updateInterests(
+    session: Session,
+    requested: readonly InterestCell[],
+  ): RealtimeResult<readonly InterestCell[]> {
+    if (
+      !Array.isArray(requested) || requested.length === 0 ||
+      requested.length > settings.maximumInterestsPerSession || !requested.every(isInterestCell)
+    ) return { ok: false, code: "invalid", message: "Realtime interests are invalid." };
+    const unique = new Map(requested.map((cell) => [interestKey(cell), cloneCell(cell)]));
+    for (const key of unique.keys()) {
+      let count = 0;
+      for (const peer of sessions.values()) {
+        if (peer.id !== session.id && peer.interestKeys.has(key)) count += 1;
+      }
+      if (count >= settings.maximumSessionsPerCell) {
+        return { ok: false, code: "capacity", message: "Realtime interest cell is at capacity." };
+      }
+    }
+    const changed = unique.size !== session.interestKeys.size ||
+      [...unique.keys()].some((key) => !session.interestKeys.has(key));
+    const previousKeys = [...session.interestKeys];
+    if (changed) broadcastRemoval(session, previousKeys);
+    session.interests = [...unique.values()];
+    session.interestKeys = new Set(unique.keys());
+    if (changed) {
+      // A pose from the old cell cannot become the first authoritative point in
+      // a new cell's speed check or leak through its next snapshot.
+      session.poses.clear();
+      session.proposedPoses.clear();
+    }
+    return { ok: true, value: session.interests };
+  }
+
+  function rotateToken(session: Session): string {
+    const token = randomBytes(32).toString("base64url");
+    session.previousTokenHash?.fill(0);
+    session.previousTokenHash = session.tokenHash;
+    session.previousTokenExpiresAt = settings.now() + 10_000;
+    session.tokenHash = hashToken(token);
+    return token;
+  }
+
+  function resume(
+    request: ResumeRequest,
+    subject: string,
+  ): RealtimeResult<ResumeGrant> {
+    // The immediately previous token is accepted only for resume. This small
+    // overlap lets a client retry when the rotated grant was lost in transit,
+    // without letting an old token continue publishing or leaving a session.
+    const found = authenticate(request.sessionId, request.resumeToken, true);
+    if (!found.ok) return found;
+    const session = found.value;
+    if (session.subject !== subject) {
+      return { ok: false, code: "unauthorized", message: "Realtime session is invalid." };
+    }
+    if (request.serverEpoch !== epoch || request.lastReceivedSequence > serverSequence) {
+      return { ok: false, code: "sequence", message: "Realtime resume point is invalid." };
+    }
+    const updated = updateInterests(session, request.interests);
+    if (!updated.ok) return updated;
+    const continuous = request.lastReceivedSequence === serverSequence;
+    const token = rotateToken(session);
+    return {
+      ok: true,
+      value: {
+        type: "resume-grant",
+        protocolVersion: 1,
+        requestId: request.requestId,
+        sessionId: session.id,
+        serverEpoch: epoch,
+        serverTimeMs: settings.now(),
+        nextSequence: serverSequence >= UINT32_MAX ? 0 : serverSequence + 1,
+        resumeToken: token,
+        continuous,
+        snapshot: view(session).snapshot,
+      },
+    };
+  }
+
+  function connectResume(
+    request: ResumeRequest,
+    subject: string,
+    listener: RealtimeListener,
+  ): RealtimeResult<RealtimeResumeConnection> {
+    if (typeof listener !== "function") {
+      return { ok: false, code: "invalid", message: "Realtime listener is invalid." };
+    }
+    const resumed = resume(request, subject);
+    if (!resumed.ok) return resumed;
+    const session = sessions.get(request.sessionId);
+    if (!session) return { ok: false, code: "expired", message: "Realtime session expired." };
+    session.disconnectedAt = null;
+    session.listeners.add(listener);
+    let connected = true;
+    return {
+      ok: true,
+      value: {
+        grant: resumed.value,
+        view: view(session),
+        disconnect() {
+          if (!connected) return;
+          connected = false;
+          session.listeners.delete(listener);
+          if (session.listeners.size === 0) session.disconnectedAt = settings.now();
+        },
+      },
+    };
+  }
+
+  function submitPose(
+    sessionId: string,
+    token: string,
+    input: unknown,
+  ): RealtimeResult<PoseDelta> {
+    const found = authenticate(sessionId, token);
+    if (!found.ok) return found;
+    const session = found.value;
+    const now = settings.now();
+    if (now - session.rateWindowStartedAt >= settings.rateWindowMs) {
+      session.rateWindowStartedAt = now;
+      session.rateCount = 0;
+    }
+    session.rateCount += 1;
+    if (session.rateCount > settings.maximumPoseUpdatesPerWindow) {
+      return { ok: false, code: "rate-limited", message: "Realtime pose rate exceeded." };
+    }
+    if (!isEntityPoseSnapshot(input)) {
+      return { ok: false, code: "invalid", message: "Realtime pose is invalid." };
+    }
+    const snapshot = input;
+    if (snapshot.sequence <= session.lastClientSequence) {
+      return { ok: false, code: "sequence", message: "Realtime pose sequence is stale." };
+    }
+    if (snapshot.timestampMs > now + 5_000 || snapshot.timestampMs < now - 10_000) {
+      return { ok: false, code: "sequence", message: "Realtime pose timestamp is stale." };
+    }
+    if (
+      (snapshot.entity === "actor" && snapshot.actorId !== session.actorId) ||
+      (snapshot.entity === "vehicle" && snapshot.driverActorId !== session.actorId)
+    ) return { ok: false, code: "ownership", message: "Realtime entity is not owned by this session." };
+    const localKey = poseInterestKey(snapshot);
+    if (localKey !== null && !session.interestKeys.has(localKey)) {
+      return { ok: false, code: "interest", message: "Realtime pose is outside the joined interest." };
+    }
+    if (localKey === null && geographicInterestKeys(session).length !== 1) {
+      return {
+        ok: false,
+        code: "interest",
+        message: "Geographic pose requires exactly one outdoor interest.",
+      };
+    }
+    const key = entityKey(snapshot);
+    const previous = session.proposedPoses.get(key);
+    if (previous) {
+      const motion = validateMotion(
+        previous,
+        snapshot,
+        snapshot.entity === "vehicle" ? settings.vehicleMotionLimits : settings.actorMotionLimits,
+      );
+      if (!motion.ok) {
+        return { ok: false, code: "motion", message: `Realtime motion rejected: ${motion.reason}.` };
+      }
+    }
+    session.lastClientSequence = snapshot.sequence;
+    const authoritative = clonePose({ ...snapshot, sequence: nextSequence(), timestampMs: now });
+    const removedEntityIds = [...session.poses.keys()].filter((existing) => existing !== key);
+    for (const removed of removedEntityIds) {
+      session.poses.delete(removed);
+      session.proposedPoses.delete(removed);
+    }
+    session.proposedPoses.set(key, clonePose(snapshot));
+    session.poses.set(key, authoritative);
+    const delta: PoseDelta = {
+      type: "pose-delta",
+      protocolVersion: 1,
+      serverEpoch: epoch,
+      sequence: authoritative.sequence,
+      timestampMs: now,
+      updates: [clonePose(authoritative)],
+      removedEntityIds,
+    };
+    broadcast(delta, poseBroadcastKeys(session, snapshot));
+    return { ok: true, value: delta };
+  }
+
+  function leave(sessionId: string, token: string): RealtimeResult<null> {
+    const found = authenticate(sessionId, token);
+    if (!found.ok) return found;
+    remove(found.value);
+    return { ok: true, value: null };
+  }
+
+  function revokeSubject(subject: string): number {
+    let count = 0;
+    for (const session of sessions.values()) {
+      if (session.subject !== subject) continue;
+      const message: ServerRealtimeMessage = {
+        type: "membership-revoked",
+        protocolVersion: 1,
+        serverEpoch: epoch,
+        sequence: nextSequence(),
+        timestampMs: settings.now(),
+        sessionId: session.id,
+        actorId: session.actorId,
+        reason: "membership-revoked",
+        reconnectAllowed: false,
+      };
+      for (const listener of session.listeners) listener(message);
+      broadcastRemoval(session);
+      remove(session, false);
+      count += 1;
+    }
+    return count;
+  }
+
+  return {
+    join,
+    authenticate(sessionId, token) {
+      const found = authenticate(sessionId, token);
+      return found.ok ? { ok: true, value: view(found.value) } : found;
+    },
+    connect,
+    resume,
+    connectResume,
+    submitPose,
+    leave,
+    revokeSubject,
+    cleanup,
+    sessionCount: () => sessions.size,
+  };
+}
diff --git a/server/src/routes/realtime.ts b/server/src/routes/realtime.ts
new file mode 100644
index 0000000..f7095a1
--- /dev/null
+++ b/server/src/routes/realtime.ts
@@ -0,0 +1,186 @@
+/** Authenticated HTTP + SSE adapter for the in-memory realtime service. */
+
+import type { FastifyInstance, FastifyReply } from "fastify";
+import type { InterestCell } from "../../../src/realtime/types.ts";
+import type { ResumeRequest } from "../../../src/realtime/types.ts";
+import { validateClientRealtimeMessage } from "../../../src/realtime/protocol.ts";
+import type { RealtimeFailureCode } from "../realtime/index.ts";
+import type { Services } from "../services.ts";
+
+const BODY_LIMIT = 32 * 1024;
+const UNAUTHORIZED = { error: "unauthorized", message: "Realtime requires a signed-in member." };
+
+/** Public, statically bundled demos that exist even when TERA_OFFICES_DIR is empty. */
+export const BUNDLED_REALTIME_OFFICE_IDS: ReadonlySet<string> = new Set([
+  "lumbridge-hq",
+  "frontier-valley",
+  "mateo-court",
+]);
+
+interface CredentialBody {
+  sessionId?: unknown;
+  token?: unknown;
+}
+
+interface PoseBody extends CredentialBody {
+  snapshot?: unknown;
+}
+
+function text(value: unknown): value is string {
+  return typeof value === "string" && value.length > 0 && value.length <= 512;
+}
+
+function failureStatus(code: RealtimeFailureCode): number {
+  switch (code) {
+    case "unauthorized":
+    case "expired": return 401;
+    case "capacity":
+    case "rate-limited": return 429;
+    case "conflict":
+    case "sequence":
+    case "interest":
+    case "ownership":
+    case "motion": return 409;
+    case "invalid": return 400;
+  }
+}
+
+function sendFailure(reply: FastifyReply, result: { code: RealtimeFailureCode; message: string }) {
+  const status = failureStatus(result.code);
+  if (status === 401) reply.header("www-authenticate", "Bearer");
+  return reply.code(status).send({ error: result.code, message: result.message });
+}
+
+async function interestsExist(interests: readonly InterestCell[], services: Services): Promise<boolean> {
+  for (const cell of interests) {
+    if (cell.kind === "california-tile" || cell.kind === "city") continue;
+    const doc = await services.offices.get(cell.officeId);
+    if (doc === null) {
+      // Only the office envelope is public in a client bundle. Without a
+      // server pack we have no authoritative floor or room catalogue, so those
+      // finer cells remain unavailable rather than guessed.
+      if (cell.kind === "office" && BUNDLED_REALTIME_OFFICE_IDS.has(cell.officeId)) continue;
+      return false;
+    }
+    if (cell.kind === "office") continue;
+    const level = doc.floor.levels.find((candidate) => candidate.id === cell.floorId);
+    if (!level) return false;
+    if (
+      cell.kind === "room" &&
+      !level.floorplan.rooms.some((room) => room.id === cell.roomId)
+    ) return false;
+  }
+  return true;
+}
+
+export function registerRealtime(app: FastifyInstance, services: Services): void {
+  app.post<{ Body: unknown }>(
+    "/api/v1/realtime/join",
+    { bodyLimit: BODY_LIMIT },
+    async (req, reply) => {
+      // Authentication precedes interest parsing so anonymous callers cannot
+      // use office cell errors to learn private identifiers.
+      const viewer = await services.auth.resolve(req);
+      if (!viewer.authenticated || viewer.subject === null) {
+        return reply.code(401).header("www-authenticate", "Bearer").send(UNAUTHORIZED);
+      }
+      const parsed = validateClientRealtimeMessage(req.body);
+      if (!parsed.ok) {
+        return reply.code(400).send({ error: "invalid", message: "Realtime join request is invalid." });
+      }
+      const request = parsed.value;
+      const interests = request.interests;
+      if (!(await interestsExist(interests, services))) {
+        return reply.code(400).send({ error: "invalid", message: "Realtime interest is unavailable." });
+      }
+      if (request.type === "resume-request") {
+        const resumed = services.realtime.resume(request, viewer.subject);
+        if (!resumed.ok) return sendFailure(reply, resumed);
+        return reply.send(resumed.value);
+      }
+      const joined = services.realtime.join({
+        requestId: request.requestId,
+        actorId: request.actorId,
+        subject: viewer.subject,
+        role: viewer.admin ? "admin" : "member",
+        interests,
+      });
+      if (!joined.ok) return sendFailure(reply, joined);
+      return reply.code(201).send(joined.value.grant);
+    },
+  );
+
+  // POST is deliberate: usable session tokens must never enter URLs, reverse-
+  // proxy logs, referrers, or browser history. Clients consume this SSE body
+  // through fetch streaming rather than the URL-only EventSource constructor.
+  app.post<{ Body: unknown }>(
+    "/api/v1/realtime/events",
+    { bodyLimit: 4 * 1024 },
+    async (req, reply) => {
+      const viewer = await services.auth.resolve(req);
+      if (!viewer.authenticated || viewer.subject === null) {
+        return reply.code(401).header("www-authenticate", "Bearer").send(UNAUTHORIZED);
+      }
+      const parsed = validateClientRealtimeMessage(req.body);
+      if (!parsed.ok || parsed.value.type !== "resume-request") {
+        return reply.code(400).send({ error: "invalid", message: "Realtime resume request is invalid." });
+      }
+      const request: ResumeRequest = parsed.value;
+      if (!(await interestsExist(request.interests, services))) {
+        return reply.code(400).send({ error: "invalid", message: "Realtime interest is unavailable." });
+      }
+      const connection = services.realtime.connectResume(request, viewer.subject, (message) => {
+        if (!reply.raw.destroyed) reply.raw.write(`event: ${message.type}\ndata: ${JSON.stringify(message)}\n\n`);
+      });
+      if (!connection.ok) return sendFailure(reply, connection);
+
+      reply.hijack();
+      reply.raw.statusCode = 200;
+      reply.raw.setHeader("content-type", "text/event-stream; charset=utf-8");
+      reply.raw.setHeader("cache-control", "private, no-store");
+      reply.raw.setHeader("connection", "keep-alive");
+      reply.raw.setHeader("x-accel-buffering", "no");
+      reply.raw.flushHeaders();
+      reply.raw.write(`event: resume-grant\ndata: ${JSON.stringify(connection.value.grant)}\n\n`);
+      const keepalive = setInterval(() => {
+        if (!reply.raw.destroyed) reply.raw.write(`: keepalive ${Date.now()}\n\n`);
+      }, 15_000);
+      keepalive.unref();
+      // The POST request body finishes immediately; listening on IncomingMessage
+      // `close` would therefore tear the stream down as soon as credentials
+      // were parsed. ServerResponse stays open for the lifetime of the SSE feed.
+      reply.raw.once("close", () => {
+        clearInterval(keepalive);
+        connection.value.disconnect();
+      });
+    },
+  );
+
+  app.post<{ Body: PoseBody }>(
+    "/api/v1/realtime/pose",
+    { bodyLimit: BODY_LIMIT },
+    async (req, reply) => {
+      const body = req.body;
+      if (!body || !text(body.sessionId) || !text(body.token)) {
+        return reply.code(400).send({ error: "invalid", message: "Realtime credentials are required." });
+      }
+      const submitted = services.realtime.submitPose(body.sessionId, body.token, body.snapshot);
+      if (!submitted.ok) return sendFailure(reply, submitted);
+      return reply.code(202).send(submitted.value);
+    },
+  );
+
+  app.post<{ Body: CredentialBody }>(
+    "/api/v1/realtime/leave",
+    { bodyLimit: 4 * 1024 },
+    async (req, reply) => {
+      const body = req.body;
+      if (!body || !text(body.sessionId) || !text(body.token)) {
+        return reply.code(400).send({ error: "invalid", message: "Realtime credentials are required." });
+      }
+      const left = services.realtime.leave(body.sessionId, body.token);
+      if (!left.ok) return sendFailure(reply, left);
+      return reply.code(204).send();
+    },
+  );
+}
diff --git a/server/src/services.ts b/server/src/services.ts
index 9866c37..e7ec385 100644
--- a/server/src/services.ts
+++ b/server/src/services.ts
@@ -12,6 +12,7 @@ import { createFlightsService, type FlightsService } from "./flights/index.ts";
 import { createMarkerStore, type MarkerStore } from "./markers/store.ts";
 import { createOfficeStore, type OfficeStore } from "./offices/store.ts";
 import { createPresenceStore, type PresenceStore } from "./presence/store.ts";
+import { createRealtimeService, type RealtimeService } from "./realtime/index.ts";
 import { createSatellitesService, type SatellitesService } from "./satellites/index.ts";
 import { createWeatherService, type WeatherService } from "./weather/index.ts";
 import type { Config } from "./config.ts";
@@ -24,6 +25,7 @@ export interface Services {
   markers: MarkerStore;
   offices: OfficeStore;
   presence: PresenceStore;
+  realtime: RealtimeService;
   auth: AuthService;
   /** Epoch milliseconds, for `uptimeSeconds` on the health body. */
   startedAt: number;
@@ -43,6 +45,7 @@ export function createServices(config: Config, log: ServiceLog): Services {
     markers: createMarkerStore(config, log),
     offices: createOfficeStore(config.offices.dir),
     presence: createPresenceStore(config.presence.dir),
+    realtime: createRealtimeService(),
     auth: createAuth(config.auth),
     startedAt: Date.now(),
   };
diff --git a/server/src/test/realtime.test.ts b/server/src/test/realtime.test.ts
new file mode 100644
index 0000000..fca3da1
--- /dev/null
+++ b/server/src/test/realtime.test.ts
@@ -0,0 +1,663 @@
+import assert from "node:assert/strict";
+import { createHmac } from "node:crypto";
+import { after, describe, it } from "node:test";
+import type {
+  ActorPoseSnapshot,
+  FloorInterest,
+  InterestCell,
+  PoseDelta,
+  VehiclePoseSnapshot,
+} from "../../../src/realtime/types.ts";
+import { buildApp } from "../app.ts";
+import { loadConfig } from "../config.ts";
+import { createRealtimeService } from "../realtime/index.ts";
+import { BUNDLED_REALTIME_OFFICE_IDS } from "../routes/realtime.ts";
+
+const FLOOR: FloorInterest = {
+  kind: "floor",
+  officeId: "lumbridge-hq",
+  floorId: "level-1",
+};
+const OTHER_FLOOR: FloorInterest = {
+  kind: "floor",
+  officeId: "mateo-court",
+  floorId: "level-1",
+};
+
+function actorPose(
+  actorId: string,
+  sequence: number,
+  timestampMs: number,
+  xM: number,
+  cell: InterestCell = FLOOR,
+): ActorPoseSnapshot {
+  if (cell.kind === "california-tile") throw new Error("fixture needs local interest");
+  return {
+    entity: "actor",
+    actorId,
+    kind: "humanoid",
+    sequence,
+    timestampMs,
+    pose: {
+      space: "local",
+      cell,
+      xM,
+      yM: 0,
+      zM: 0,
+      headingDeg: 0,
+      pitchDeg: 0,
+    },
+    velocity: { xMps: 0, yMps: 0, zMps: 0, yawDegPerSec: 0 },
+  };
+}
+
+function geographicActor(
+  actorId: string,
+  sequence: number,
+  timestampMs: number,
+  lat = 37.7,
+): ActorPoseSnapshot {
+  return {
+    entity: "actor",
+    actorId,
+    kind: "humanoid",
+    sequence,
+    timestampMs,
+    pose: {
+      space: "geographic",
+      lat,
+      lng: -122.4,
+      altitudeM: 10,
+      headingDeg: 0,
+      pitchDeg: 0,
+    },
+    velocity: { xMps: 0, yMps: 0, zMps: 0, yawDegPerSec: 0 },
+  };
+}
+
+function vehiclePose(
+  driverActorId: string,
+  sequence: number,
+  timestampMs: number,
+): VehiclePoseSnapshot {
+  return {
+    entity: "vehicle",
+    vehicleId: `vehicle-${driverActorId}`,
+    kind: "model-x",
+    driverActorId,
+    sequence,
+    timestampMs,
+    pose: {
+      space: "local",
+      cell: FLOOR,
+      xM: 0,
+      yM: 0,
+      zM: 0,
+      headingDeg: 0,
+      pitchDeg: 0,
+    },
+    velocity: { xMps: 0, yMps: 0, zMps: 0, yawDegPerSec: 0 },
+    steering: 0,
+    wheelRadians: 0,
+  };
+}
+
+function joined(
+  service: ReturnType<typeof createRealtimeService>,
+  actorId: string,
+  interests: readonly InterestCell[] = [FLOOR],
+  subject = `auth-subject-for-${actorId}`,
+) {
+  const result = service.join({
+    requestId: `request-${actorId}`,
+    actorId,
+    subject,
+    role: "member",
+    interests,
+  });
+  assert.equal(result.ok, true);
+  if (!result.ok) throw new Error("realtime fixture could not join");
+  return result.value;
+}
+
+describe("authoritative realtime service", () => {
+  it("stores only hashed credentials and authenticates opaque sessions", () => {
+    const service = createRealtimeService();
+    const session = joined(service, "member-a");
+    assert.notEqual(session.sessionId, session.token);
+    assert.equal(service.authenticate(session.sessionId, session.token).ok, true);
+    assert.equal(service.authenticate(session.sessionId, `${session.token}x`).ok, false);
+    assert.equal(session.grant.resumeToken, session.token);
+    assert.deepEqual(session.grant.initial, []);
+  });
+
+  it("keeps auth subjects private while reserving each opaque actor id", () => {
+    const service = createRealtimeService();
+    const first = service.join({
+      requestId: "request",
+      actorId: "page-f4f9445d",
+      subject: "private-auth-subject",
+      role: "member",
+      interests: [FLOOR],
+    });
+    assert.equal(first.ok, true);
+    if (!first.ok) return;
+    assert.equal(JSON.stringify(first.value.grant).includes("private-auth-subject"), false);
+    const view = service.authenticate(first.value.sessionId, first.value.token);
+    assert.equal(view.ok, true);
+    if (view.ok) assert.equal(JSON.stringify(view.value).includes("private-auth-subject"), false);
+
+    const collision = service.join({
+      requestId: "request-2",
+      actorId: "page-f4f9445d",
+      subject: "another-private-subject",
+      role: "member",
+      interests: [FLOOR],
+    });
+    assert.deepEqual(collision, {
+      ok: false,
+      code: "conflict",
+      message: "Realtime actor id is already active.",
+    });
+  });
+
+  it("broadcasts poses only to listeners sharing the exact interest cell", () => {
+    let now = 10_000;
+    const service = createRealtimeService({ now: () => now });
+    const a = joined(service, "member-a");
+    const b = joined(service, "member-b");
+    const c = joined(service, "member-c", [OTHER_FLOOR]);
+    const receivedA: unknown[] = [];
+    const receivedB: unknown[] = [];
+    const receivedC: unknown[] = [];
+    service.connect(a.sessionId, a.token, (message) => receivedA.push(message));
+    service.connect(b.sessionId, b.token, (message) => receivedB.push(message));
+    service.connect(c.sessionId, c.token, (message) => receivedC.push(message));
+    const submitted = service.submitPose(
+      a.sessionId,
+      a.token,
+      actorPose("member-a", 1, now, 0),
+    );
+    assert.equal(submitted.ok, true);
+    assert.equal(receivedA.length, 1);
+    assert.equal(receivedB.length, 1);
+    assert.equal(receivedC.length, 0);
+    now += 1_000;
+    const second = service.submitPose(a.sessionId, a.token, actorPose("member-a", 2, now, 5));
+    assert.equal(second.ok, true);
+    const late = joined(service, "member-late");
+    assert.equal(late.grant.initial.length, 1, "a late join receives the current cell snapshot");
+  });
+
+  it("allows geographic city poses and scopes them to the exact detailed city", () => {
+    const now = 15_000;
+    const service = createRealtimeService({ now: () => now });
+    const bay = { kind: "city" as const, cityId: "bay-area" as const };
+    const socal = { kind: "city" as const, cityId: "socal" as const };
+    const source = joined(service, "page-bay-source", [bay]);
+    const bayPeer = joined(service, "page-bay-peer", [bay]);
+    const socalPeer = joined(service, "page-socal-peer", [socal]);
+    const bayMessages: unknown[] = [];
+    const socalMessages: unknown[] = [];
+    service.connect(bayPeer.sessionId, bayPeer.token, (message) => bayMessages.push(message));
+    service.connect(socalPeer.sessionId, socalPeer.token, (message) => socalMessages.push(message));
+    const sent = service.submitPose(
+      source.sessionId,
+      source.token,
+      geographicActor("page-bay-source", 1, now),
+    );
+    assert.equal(sent.ok, true);
+    assert.equal(bayMessages.length, 1);
+    assert.equal(socalMessages.length, 0);
+  });
+
+  it("removes the previous entity when one session switches actor to vehicle", () => {
+    let now = 17_000;
+    const service = createRealtimeService({ now: () => now });
+    const source = joined(service, "page-switch");
+    const peer = joined(service, "page-peer");
+    const received: PoseDelta[] = [];
+    service.connect(peer.sessionId, peer.token, (message) => {
+      if (message.type === "pose-delta") received.push(message);
+    });
+    service.submitPose(source.sessionId, source.token, actorPose("page-switch", 1, now, 0));
+    now += 100;
+    const switched = service.submitPose(
+      source.sessionId,
+      source.token,
+      vehiclePose("page-switch", 2, now),
+    );
+    assert.equal(switched.ok, true);
+    if (!switched.ok) return;
+    assert.deepEqual(switched.value.removedEntityIds, ["actor:page-switch"]);
+    assert.equal(switched.value.updates[0]?.entity, "vehicle");
+    assert.ok(switched.value.sequence > (received[0]?.sequence ?? -1));
+    assert.deepEqual(received.at(-1)?.removedEntityIds, ["actor:page-switch"]);
+  });
+
+  it("broadcasts removals to old-cell peers on interest move and leave", () => {
+    let now = 18_000;
+    const service = createRealtimeService({ now: () => now });
+    const source = joined(service, "page-moving");
+    const oldPeer = joined(service, "page-old-peer");
+    const received: PoseDelta[] = [];
+    service.connect(oldPeer.sessionId, oldPeer.token, (message) => {
+      if (message.type === "pose-delta") received.push(message);
+    });
+    service.submitPose(source.sessionId, source.token, actorPose("page-moving", 1, now, 0));
+    const moved = service.resume({
+      type: "resume-request",
+      protocolVersion: 1,
+      requestId: "move-request",
+      sessionId: source.sessionId,
+      resumeToken: source.token,
+      serverEpoch: source.grant.serverEpoch,
+      lastReceivedSequence: received.at(-1)?.sequence ?? 0,
+      interests: [OTHER_FLOOR],
+    }, "auth-subject-for-page-moving");
+    assert.equal(moved.ok, true);
+    assert.deepEqual(received.at(-1)?.removedEntityIds, ["actor:page-moving"]);
+
+    const leaving = joined(service, "page-leaving");
+    now += 100;
+    service.submitPose(leaving.sessionId, leaving.token, actorPose("page-leaving", 1, now, 0));
+    const beforeLeave = received.at(-1)?.sequence ?? -1;
+    assert.equal(service.leave(leaving.sessionId, leaving.token).ok, true);
+    assert.deepEqual(received.at(-1)?.removedEntityIds, ["actor:page-leaving"]);
+    assert.ok((received.at(-1)?.sequence ?? -1) > beforeLeave);
+  });
+
+  it("rejects stale, unowned, off-interest and impossible motion", () => {
+    let now = 20_000;
+    const service = createRealtimeService({ now: () => now });
+    const session = joined(service, "member-a");
+    assert.equal(service.submitPose(
+      session.sessionId,
+      session.token,
+      actorPose("member-b", 1, now, 0),
+    ).ok, false);
+    assert.equal(service.submitPose(
+      session.sessionId,
+      session.token,
+      actorPose("member-a", 1, now, 0, OTHER_FLOOR),
+    ).ok, false);
+    assert.equal(service.submitPose(
+      session.sessionId,
+      session.token,
+      actorPose("member-a", 1, now, 0),
+    ).ok, true);
+    assert.equal(service.submitPose(
+      session.sessionId,
+      session.token,
+      actorPose("member-a", 1, now, 1),
+    ).ok, false);
+    now += 100;
+    const hacked = service.submitPose(
+      session.sessionId,
+      session.token,
+      actorPose("member-a", 2, now, 1_000),
+    );
+    assert.equal(hacked.ok, false);
+    if (!hacked.ok) assert.equal(hacked.code, "motion");
+  });
+
+  it("caps session/cell occupancy and pose rate", () => {
+    let now = 30_000;
+    const service = createRealtimeService({
+      now: () => now,
+      maximumSessionsPerCell: 1,
+      maximumPoseUpdatesPerWindow: 1,
+    });
+    const session = joined(service, "member-a");
+    const refused = service.join({
+      requestId: "request-b",
+      actorId: "member-b",
+      subject: "member-b",
+      role: "member",
+      interests: [FLOOR],
+    });
+    assert.equal(refused.ok, false);
+    assert.equal(service.submitPose(
+      session.sessionId,
+      session.token,
+      actorPose("member-a", 1, now, 0),
+    ).ok, true);
+    now += 50;
+    const limited = service.submitPose(
+      session.sessionId,
+      session.token,
+      actorPose("member-a", 2, now, 0),
+    );
+    assert.equal(limited.ok, false);
+    if (!limited.ok) assert.equal(limited.code, "rate-limited");
+  });
+
+  it("honours disconnect grace, absolute TTL, leave and membership revocation", () => {
+    let now = 40_000;
+    const service = createRealtimeService({
+      now: () => now,
+      disconnectGraceMs: 500,
+      sessionTtlMs: 2_000,
+    });
+    const session = joined(service, "member-a");
+    const messages: unknown[] = [];
+    const connected = service.connect(session.sessionId, session.token, (message) => messages.push(message));
+    assert.equal(connected.ok, true);
+    if (!connected.ok) return;
+    connected.value.disconnect();
+    now += 400;
+    assert.equal(service.cleanup(), 0);
+    assert.equal(service.authenticate(session.sessionId, session.token).ok, true);
+    now += 600;
+    assert.equal(service.cleanup(), 1);
+
+    const revoked = joined(service, "member-b");
+    service.connect(revoked.sessionId, revoked.token, (message) => messages.push(message));
+    assert.equal(service.revokeSubject("auth-subject-for-member-b"), 1);
+    assert.equal(messages.length, 1);
+    assert.equal(service.authenticate(revoked.sessionId, revoked.token).ok, false);
+
+    const left = joined(service, "member-c");
+    assert.equal(service.leave(left.sessionId, left.token).ok, true);
+    assert.equal(service.sessionCount(), 0);
+  });
+
+  it("strictly resumes, changes interest, and rotates the usable token", () => {
+    const service = createRealtimeService({ now: () => 50_000 });
+    const session = joined(service, "member-a");
+    const resumed = service.resume({
+      type: "resume-request",
+      protocolVersion: 1,
+      requestId: "resume-1",
+      sessionId: session.sessionId,
+      resumeToken: session.token,
+      serverEpoch: session.grant.serverEpoch,
+      lastReceivedSequence: session.grant.nextSequence === 0 ? 0 : session.grant.nextSequence - 1,
+      interests: [OTHER_FLOOR],
+    }, "auth-subject-for-member-a");
+    assert.equal(resumed.ok, true);
+    if (!resumed.ok) return;
+    assert.equal(resumed.value.type, "resume-grant");
+    assert.equal(resumed.value.requestId, "resume-1");
+    assert.notEqual(resumed.value.resumeToken, session.token);
+    assert.equal(service.authenticate(session.sessionId, session.token).ok, false);
+    const current = service.authenticate(session.sessionId, resumed.value.resumeToken);
+    assert.equal(current.ok, true);
+    if (current.ok) assert.deepEqual(current.value.interests, [OTHER_FLOOR]);
+  });
+});
+
+const SECRET = "realtime-test-secret-that-is-long-enough";
+
+function jwt(subject: string): string {
+  const encode = (value: unknown): string => Buffer.from(JSON.stringify(value)).toString("base64url");
+  const signed = `${encode({ alg: "HS256", typ: "JWT" })}.${encode({
+    sub: subject,
+    exp: Math.floor(Date.now() / 1_000) + 3_600,
+  })}`;
+  return `${signed}.${createHmac("sha256", SECRET).update(signed).digest("base64url")}`;
+}
+
+function app() {
+  const config = loadConfig({ TERA_AUTH_MODE: "jwt", TERA_AUTH_JWT_SECRET: SECRET });
+  config.logLevel = "silent";
+  return buildApp(config);
+}
+
+function joinBody(actorId: string, interests: readonly InterestCell[]) {
+  return {
+    type: "join-request",
+    protocolVersion: 1,
+    requestId: `request-${actorId}`,
+    actorId,
+    resumeToken: null,
+    lastReceivedSequence: null,
+    interests,
+  };
+}
+
+describe("realtime HTTP routes", () => {
+  it("declares exactly the three public bundled office fallbacks", () => {
+    assert.deepEqual([...BUNDLED_REALTIME_OFFICE_IDS].sort(), [
+      "frontier-valley",
+      "lumbridge-hq",
+      "mateo-court",
+    ]);
+  });
+
+  it("never accepts stream credentials in a URL", async () => {
+    const server = app();
+    after(() => server.close());
+    const response = await server.inject({
+      method: "GET",
+      url: "/api/v1/realtime/events?sessionId=leak&token=must-not-be-logged",
+    });
+    assert.equal(response.statusCode, 404);
+  });
+
+  it("authenticates before joining and rejects opaque actor-id collisions", async () => {
+    const server = app();
+    after(() => server.close());
+    const anonymous = await server.inject({
+      method: "POST",
+      url: "/api/v1/realtime/join",
+      payload: joinBody("member-a", [{ kind: "city", cityId: "bay-area" }]),
+    });
+    assert.equal(anonymous.statusCode, 401);
+    const opaque = await server.inject({
+      method: "POST",
+      url: "/api/v1/realtime/join",
+      headers: { authorization: `Bearer ${jwt("member-a")}` },
+      payload: joinBody("page-random-opaque", [{ kind: "city", cityId: "bay-area" }]),
+    });
+    assert.equal(opaque.statusCode, 201);
+    assert.equal(JSON.stringify(opaque.json()).includes("member-a"), false);
+    const collision = await server.inject({
+      method: "POST",
+      url: "/api/v1/realtime/join",
+      headers: { authorization: `Bearer ${jwt("member-b")}` },
+      payload: joinBody("page-random-opaque", [{ kind: "city", cityId: "bay-area" }]),
+    });
+    assert.equal(collision.statusCode, 409);
+    const absentOffice = await server.inject({
+      method: "POST",
+      url: "/api/v1/realtime/join",
+      headers: { authorization: `Bearer ${jwt("member-a")}` },
+      payload: joinBody("member-a", [{ kind: "office", officeId: "invented-private-office" }]),
+    });
+    assert.equal(absentOffice.statusCode, 400);
+    assert.deepEqual(absentOffice.json(), {
+      error: "invalid",
+      message: "Realtime interest is unavailable.",
+    });
+  });
+
+  it("allows bundled demo office envelopes without disk packs but not invented floors", async () => {
+    const server = app();
+    after(() => server.close());
+    const authorization = `Bearer ${jwt("member-office")}`;
+    const office = await server.inject({
+      method: "POST",
+      url: "/api/v1/realtime/join",
+      headers: { authorization },
+      payload: joinBody("member-office", [{ kind: "office", officeId: "lumbridge-hq" }]),
+    });
+    assert.equal(office.statusCode, 201);
+    const floor = await server.inject({
+      method: "POST",
+      url: "/api/v1/realtime/join",
+      headers: { authorization },
+      payload: joinBody("member-office", [{
+        kind: "floor",
+        officeId: "lumbridge-hq",
+        floorId: "invented-floor",
+      }]),
+    });
+    assert.equal(floor.statusCode, 400);
+  });
+
+  it("joins, accepts an owned pose, and leaves without persistence", async () => {
+    const server = app();
+    after(() => server.close());
+    const joinedResponse = await server.inject({
+      method: "POST",
+      url: "/api/v1/realtime/join",
+      headers: { authorization: `Bearer ${jwt("member-a")}` },
+      payload: joinBody("member-a", [{ kind: "california-tile", level: 6, x: 1, y: 2 }]),
+    });
+    assert.equal(joinedResponse.statusCode, 201);
+    const grant = joinedResponse.json<{ sessionId: string; resumeToken: string }>();
+    const timestampMs = Date.now();
+    const pose = await server.inject({
+      method: "POST",
+      url: "/api/v1/realtime/pose",
+      payload: {
+        sessionId: grant.sessionId,
+        token: grant.resumeToken,
+        snapshot: {
+          entity: "actor",
+          actorId: "member-a",
+          kind: "humanoid",
+          sequence: 1,
+          timestampMs,
+          pose: {
+            space: "geographic",
+            lat: 37.7,
+            lng: -122.4,
+            altitudeM: 10,
+            headingDeg: 0,
+            pitchDeg: 0,
+          },
+          velocity: { xMps: 0, yMps: 0, zMps: 0, yawDegPerSec: 0 },
+        },
+      },
+    });
+    assert.equal(pose.statusCode, 202);
+    assert.equal(pose.json<{ type: string }>().type, "pose-delta");
+    const left = await server.inject({
+      method: "POST",
+      url: "/api/v1/realtime/leave",
+      payload: { sessionId: grant.sessionId, token: grant.resumeToken },
+    });
+    assert.equal(left.statusCode, 204);
+    const gone = await server.inject({
+      method: "POST",
+      url: "/api/v1/realtime/pose",
+      payload: {
+        sessionId: grant.sessionId,
+        token: grant.resumeToken,
+        snapshot: {
+          entity: "actor",
+          actorId: "member-a",
+          kind: "humanoid",
+          sequence: 2,
+          timestampMs: timestampMs + 100,
+          pose: {
+            space: "geographic",
+            lat: 37.7,
+            lng: -122.4,
+            altitudeM: 10,
+            headingDeg: 0,
+            pitchDeg: 0,
+          },
+          velocity: { xMps: 0, yMps: 0, zMps: 0, yawDegPerSec: 0 },
+        },
+      },
+    });
+    assert.equal(gone.statusCode, 401);
+  });
+
+  it("streams the immediate snapshot from a credential-body POST", async () => {
+    const server = app();
+    const address = await server.listen({ host: "127.0.0.1", port: 0 });
+    const controller = new AbortController();
+    try {
+      const joinResponse = await fetch(`${address}/api/v1/realtime/join`, {
+        method: "POST",
+        headers: {
+          authorization: `Bearer ${jwt("member-stream")}`,
+          "content-type": "application/json",
+        },
+        body: JSON.stringify(joinBody("member-stream", [{ kind: "city", cityId: "bay-area" }])),
+      });
+      assert.equal(joinResponse.status, 201);
+      const grant = await joinResponse.json() as {
+        sessionId: string;
+        resumeToken: string;
+        serverEpoch: string;
+        nextSequence: number;
+      };
+      const events = await fetch(`${address}/api/v1/realtime/events`, {
+        method: "POST",
+        headers: {
+          authorization: `Bearer ${jwt("member-stream")}`,
+          "content-type": "application/json",
+        },
+        body: JSON.stringify({
+          type: "resume-request",
+          protocolVersion: 1,
+          requestId: "stream-resume",
+          sessionId: grant.sessionId,
+          resumeToken: grant.resumeToken,
+          serverEpoch: grant.serverEpoch,
+          lastReceivedSequence: grant.nextSequence === 0 ? 0 : grant.nextSequence - 1,
+          interests: [{ kind: "city", cityId: "bay-area" }],
+        }),
+        signal: controller.signal,
+      });
+      assert.equal(events.status, 200);
+      assert.match(events.headers.get("content-type") ?? "", /^text\/event-stream/);
+      const reader = events.body?.getReader();
+      assert.ok(reader);
+      const first = await reader.read();
+      assert.equal(first.done, false);
+      const event = new TextDecoder().decode(first.value);
+      assert.match(event, /event: resume-grant/);
+      assert.match(event, /"type":"resume-grant"/);
+      controller.abort();
+      await reader.cancel().catch(() => undefined);
+    } finally {
+      controller.abort();
+      await server.close();
+    }
+  });
+
+  it("accepts the root client's exact move-interest resume shape on join", async () => {
+    const server = app();
+    after(() => server.close());
+    const authorization = `Bearer ${jwt("member-client")}`;
+    const first = await server.inject({
+      method: "POST",
+      url: "/api/v1/realtime/join",
+      headers: { authorization },
+      payload: joinBody("member-client", [{ kind: "city", cityId: "bay-area" }]),
+    });
+    assert.equal(first.statusCode, 201);
+    const grant = first.json<{
+      sessionId: string;
+      resumeToken: string;
+      serverEpoch: string;
+      nextSequence: number;
+    }>();
+    const moved = await server.inject({
+      method: "POST",
+      url: "/api/v1/realtime/join",
+      headers: { authorization },
+      payload: {
+        type: "resume-request",
+        protocolVersion: 1,
+        requestId: "tera-client-2",
+        sessionId: grant.sessionId,
+        resumeToken: grant.resumeToken,
+        serverEpoch: grant.serverEpoch,
+        lastReceivedSequence: grant.nextSequence === 0 ? 0 : grant.nextSequence - 1,
+        interests: [{ kind: "city", cityId: "socal" }],
+      },
+    });
+    assert.equal(moved.statusCode, 200);
+    const resume = moved.json<{ type: string; requestId: string; resumeToken: string }>();
+    assert.equal(resume.type, "resume-grant");
+    assert.equal(resume.requestId, "tera-client-2");
+    assert.notEqual(resume.resumeToken, grant.resumeToken);
+  });
+});
diff --git a/src/engine/scene.ts b/src/engine/scene.ts
index b2bef60..1ff2d45 100644
--- a/src/engine/scene.ts
+++ b/src/engine/scene.ts
@@ -78,6 +78,10 @@ import type {
   AircraftActionSnapshot,
   AircraftControllerSnapshot,
 } from "../aircraft/controller.ts";
+import type { ScenePeers, ScenePeersOptions } from "../realtime/scenePeers.ts";
+import type { EntityPoseSnapshot } from "../realtime/types.ts";
+
+export type CityRealtimePeersOptions = Omit<ScenePeersOptions, "project" | "groundAt">;
 
 export interface SceneOptions {
   city: City;
@@ -99,6 +103,8 @@ export interface SceneOptions {
   actorAnchor?: { lat: number; lng: number };
   /** Optional possessed fixed-wing aircraft; the city supplies its projection and terrain. */
   aircraft?: Omit<SceneAircraftOptions, "project" | "groundAt">;
+  /** Optional remote authoritative entities. Nothing is imported or allocated when absent. */
+  realtimePeers?: CityRealtimePeersOptions;
   flights?: FlightSource;
   /**
    * Element sets to propagate, if this deployment has any.
@@ -205,6 +211,11 @@ export interface SceneHandle {
   aircraftState(): Readonly<AircraftControllerSnapshot> | null;
   setAircraftActive(active: boolean): void;
   aircraftActive(): boolean;
+  /** Canonical remote snapshots only; a no-op when realtime peers were not configured. */
+  upsertRemoteSnapshot(snapshot: EntityPoseSnapshot): boolean;
+  removeRemoteEntity(id: string): boolean;
+  clearRemoteEntities(): void;
+  remoteEntityCount(): number;
   setMarkers(markers: Marker[]): void;
   /** Take this city off the stage and release everything it built. */
   dispose(): void;
@@ -401,6 +412,23 @@ export async function createScene(
       })
     : null;
   if (sceneAircraft) scene.add(sceneAircraft.root);
+  // Dynamic on purpose: the offline/default entry must not download four peer
+  // prototypes merely because `createScene` can optionally host them.
+  let realtimePeers: ScenePeers | null = null;
+  if (options.realtimePeers) {
+    const { createScenePeers } = await import("../realtime/scenePeers.ts");
+    realtimePeers = createScenePeers({
+      ...options.realtimePeers,
+      project: (lat, lng) => world.project(lat, lng),
+      groundAt: (lat, lng) => world.groundAt(lat, lng),
+      geographicSceneUnitsPerMetre:
+        options.realtimePeers.geographicSceneUnitsPerMetre ?? 1 / world.metresPerUnit,
+      geographicVisualScale:
+        options.realtimePeers.geographicVisualScale ??
+        (city.id === "california" ? 0.025 : 1 / world.metresPerUnit),
+    });
+    scene.add(realtimePeers.root);
+  }
 
   let flightLayer: FlightLayer | null = null;
   let flightTimer = 0;
@@ -521,6 +549,7 @@ export async function createScene(
       if (actorPlaying && sceneActor) kit.setPose(sceneActor.followPose());
       sceneAircraft?.tick(dt);
       if (aircraftPlaying && sceneAircraft) kit.setPose(sceneAircraft.followPose());
+      realtimePeers?.tick(Date.now());
       roadTraffic?.tick(dt);
       clouds.tick(dt);
       if (options.flights && flightLayer) {
@@ -572,6 +601,7 @@ export async function createScene(
       markerLayer.dispose();
       roadTraffic?.dispose();
       sceneAircraft?.dispose();
+      realtimePeers?.dispose();
       kit.dispose();
       scene.traverse((obj) => {
         const mesh = obj as THREE.Mesh;
@@ -644,12 +674,26 @@ export async function createScene(
       kit.camera.updateProjectionMatrix();
     },
     aircraftActive: () => sceneAircraft?.active() ?? false,
+    upsertRemoteSnapshot: (snapshot) => {
+      if (snapshot.pose.space === "local") {
+        // Local office coordinates have no meaning on a geographic board.
+        // Local city coordinates are accepted only for this detailed board;
+        // California actors use geographic poses.
+        const expected = city.id === "sf" ? "bay-area" : city.id === "socal" ? "socal" : null;
+        if (snapshot.pose.cell.kind !== "city" || snapshot.pose.cell.cityId !== expected) return false;
+      }
+      return realtimePeers?.upsert(snapshot) ?? false;
+    },
+    removeRemoteEntity: (id) => realtimePeers?.remove(id) ?? false,
+    clearRemoteEntities: () => { realtimePeers?.clear(); },
+    remoteEntityCount: () => realtimePeers?.count() ?? 0,
     setMarkers(markers) {
       markerLayer.setMarkers(markers);
     },
     dispose() {
       sceneActor?.dispose();
       sceneAircraft?.dispose();
+      realtimePeers?.dispose();
       /**
        * Off the stage, then released — and the stage itself is left running.
        *
diff --git a/src/interiors/officeScene.ts b/src/interiors/officeScene.ts
index ebcbbeb..22d783b 100644
--- a/src/interiors/officeScene.ts
+++ b/src/interiors/officeScene.ts
@@ -99,6 +99,13 @@ import {
   type RobotSpec,
   type RobotView,
 } from "./robots.ts";
+import {
+  createScenePeers,
+  type ScenePeersOptions,
+} from "../realtime/scenePeers.ts";
+import type { EntityPoseSnapshot } from "../realtime/types.ts";
+
+export type OfficeRealtimePeersOptions = Omit<ScenePeersOptions, "project" | "groundAt">;
 
 /** Shared empty, so a pack with no robots does not allocate one per call. */
 const NO_ROBOTS: readonly RobotView[] = [];
@@ -222,6 +229,8 @@ export interface OfficeSceneOptions {
   robots?: readonly RobotSpec[];
   /** Optional local walk actor. Constructed inactive unless `walker.active` says otherwise. */
   walker?: OfficeWalkerOptions;
+  /** Full-depth-only authoritative remote actors/vehicles in local metre coordinates. */
+  realtimePeers?: OfficeRealtimePeersOptions;
   /** Defaults to false — the lid comes off, because that is the whole view. */
   showCeilings?: boolean;
   /** Fade the walls you are looking through. Defaults to true. */
@@ -306,6 +315,10 @@ export interface OfficeScene extends StageScene {
    * The plan panel and the ceiling lights both consume it that way.
    */
   robots(): readonly RobotView[];
+  upsertRemoteSnapshot(snapshot: EntityPoseSnapshot): boolean;
+  removeRemoteEntity(id: string): boolean;
+  clearRemoteEntities(): void;
+  remoteEntityCount(): number;
 }
 
 export function createOfficeScene(office: Office, options: OfficeSceneOptions): OfficeScene {
@@ -509,6 +522,18 @@ export function createOfficeScene(office: Office, options: OfficeSceneOptions):
       : null;
   const officeWalker = options.walker ? createOfficeWalker(plan, options.walker) : null;
   if (officeWalker) scene.add(officeWalker.root);
+  // Live occupancy is excluded from the public-depth build rather than hidden.
+  // Geographic callbacks are required by the generic adapter but harmless here:
+  // an office subscription sends validated local poses in metre coordinates.
+  const realtimePeers = depth === "full" && options.realtimePeers
+    ? createScenePeers({
+        ...options.realtimePeers,
+        project: () => [0, 0],
+        groundAt: () => 0,
+        localSceneUnitsPerMetre: options.realtimePeers.localSceneUnitsPerMetre ?? 1,
+      })
+    : null;
+  if (realtimePeers) scene.add(realtimePeers.root);
   if (robots) {
     scene.add(robots.group);
     /**
@@ -830,6 +855,18 @@ export function createOfficeScene(office: Office, options: OfficeSceneOptions):
     setWalkers(walkers) {
       luminaires.setWalkers(walkers);
     },
+    upsertRemoteSnapshot: (snapshot) => {
+      // The generic peer adapter can also project geographic poses, but an
+      // office must never render a stale city stream or another office's local
+      // coordinates at its origin during an interest handoff.
+      if (snapshot.pose.space !== "local") return false;
+      const cell = snapshot.pose.cell;
+      if (!("officeId" in cell) || cell.officeId !== office.id) return false;
+      return realtimePeers?.upsert(snapshot) ?? false;
+    },
+    removeRemoteEntity: (id) => realtimePeers?.remove(id) ?? false,
+    clearRemoteEntities: () => { realtimePeers?.clear(); },
+    remoteEntityCount: () => realtimePeers?.count() ?? 0,
     // Stepping back out to the city should retire the hover with it, or the
     // detail card for whoever the pointer was over survives the journey.
     onExit: () => kit.resetPick(),
@@ -840,6 +877,7 @@ export function createOfficeScene(office: Office, options: OfficeSceneOptions):
       kit.tick(dt);
       officeWalker?.tick(dt);
       if (walking && officeWalker) kit.setPose(officeWalker.followPose());
+      realtimePeers?.tick(Date.now());
       updateOcclusion();
       // Robots first: the lights above them should respond to where they are
       // *now*, not to where they were last frame.
@@ -849,6 +887,7 @@ export function createOfficeScene(office: Office, options: OfficeSceneOptions):
     dispose() {
       mediaSurfaces.dispose();
       officeWalker?.dispose();
+      realtimePeers?.dispose();
       robots?.dispose();
       luminaires.dispose();
       if (horizonPlane) {
diff --git a/src/main.ts b/src/main.ts
index 957e5dc..0b0f28a 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -114,6 +114,13 @@ import type { MaterialRegistry } from "./assets/materials.ts";
 import type { OfficeMinimap } from "./engine/officeMinimap.ts";
 import type { Godmode, GodmodeHouseLights, GodmodePlace } from "./tools/index.ts";
 import type { PoseEditor } from "./tools/poseEditor.ts";
+import type {
+  EntityPoseSnapshot,
+  InterestCell,
+  RealtimeClient,
+  ServerRealtimeMessage,
+} from "./realtime/index.ts";
+import type { PresenceIndicator } from "./realtime/presenceIndicator.ts";
 
 const CITIES: { id: string; label: string; city: City }[] = [
   { id: "california", label: "California", city: CALIFORNIA },
@@ -247,6 +254,15 @@ let wantedCity = "california";
 let office: OfficeScene | null = null;
 let inside = false;
 let markers: Marker[] = SAMPLE_MARKERS;
+let realtimeClient: RealtimeClient | null = null;
+let presenceIndicator: PresenceIndicator | null = null;
+let stopRealtimeSubscription: (() => void) | null = null;
+let realtimeOperation = 0;
+let lastRealtimePublishAt = 0;
+let realtimePageActive = true;
+/** Page-scoped wire identities; auth subject never enters the spatial protocol. */
+const realtimeActorId = crypto.randomUUID();
+const realtimeVehicleId = crypto.randomUUID();
 
 /**
  * The buildings you can walk into, as procedural glyphs on the city.
@@ -921,6 +937,9 @@ async function mountCity(id: string) {
     onProgress: (p) => {
       if (!mount.signal.aborted) bootProgress(entry.label, p.fraction, p.onMainThread);
     },
+    ...(access.subject !== null
+      ? { realtimePeers: { localSceneUnitsPerMetre: entry.city.latScale / 111_320 } }
+      : {}),
   });
   if (!handle) {
     /**
@@ -934,6 +953,7 @@ async function mountCity(id: string) {
     return;
   }
   city = handle;
+  moveRealtimePresence();
   // A new board builds a new layer, and a new layer starts visible. Reapply
   // whatever the panel last said, or the setting silently undoes itself on the
   // first city switch.
@@ -1086,6 +1106,7 @@ requestAnimationFrame(function pumpMinimap() {
   // everyone who is not god, so this is one property read per frame on a
   // public page.
   poseEditor?.tick();
+  publishRealtimePresence(performance.now());
   pollLiveness();
 });
 
@@ -1231,12 +1252,14 @@ async function enterOffice() {
       ...(depth === "full"
         ? { onPresencePick: (p) => showDetail(p ? p.label : null) }
         : { onPlacePick: (place) => showDetail(place ? place.label : null) }),
+      ...(access.subject !== null ? { realtimePeers: {} } : {}),
     });
     office.onViewChange(() => renderLegend());
     officePlan = buildOfficePlan(createOfficeMinimap, office);
   }
   city.stage.setScene(office);
   inside = true;
+  moveRealtimePresence();
   const desiredCity: JourneyCity = officeId === "mateo-court" ? "socal" : "bay-area";
   journeyToCity(desiredCity);
   dispatchJourney({ type: "enter-office", officeId: officeId as "lumbridge-hq" | "frontier-valley" | "mateo-court" });
@@ -1362,6 +1385,7 @@ function leaveOffice() {
   dispatchJourney({ type: "leave-office" });
   city.stage.setScene(city.stageScene);
   inside = false;
+  moveRealtimePresence();
   showPlan();
   showDetail(null);
   refreshGodmodePlace();
@@ -1512,6 +1536,7 @@ const source = document.querySelector<HTMLElement>("#source");
 const minimapFrame = document.querySelector<HTMLElement>("#minimap .minimap-frame");
 const minimapReadout = document.querySelector<HTMLElement>("#minimap-readout");
 const tierBadge = document.querySelector<HTMLElement>("#tier");
+const presenceHost = document.querySelector<HTMLElement>("#presence-host");
 const officeBadge = document.querySelector<HTMLElement>("#office-badge");
 const panelToggle = document.querySelector<HTMLButtonElement>("#panel-toggle");
 const panelToggleLabel = document.querySelector<HTMLElement>("#panel-toggle-label");
@@ -1927,6 +1952,256 @@ function renderTierBadge() {
   tierBadge.hidden = false;
 }
 
+// ---- Hosted realtime presence --------------------------------------------
+
+function realtimeInterest(): InterestCell {
+  if (inside) return { kind: "office", officeId };
+  if (cityId === "california") return { kind: "california-tile", x: 0, y: 0, level: 0 };
+  return { kind: "city", cityId: cityId === "socal" ? "socal" : "bay-area" };
+}
+
+function degrees(radians: number): number {
+  return ((radians * 180 / Math.PI + 540) % 360) - 180;
+}
+
+function realtimePose(): EntityPoseSnapshot | null {
+  if (access.subject === null) return null;
+  const timestampMs = Date.now();
+
+  if (inside) {
+    const walker = office?.walker;
+    if (!walker) return null;
+    const state = walker.state();
+    const headingDeg = -degrees(Math.atan2(-state.facing.x, -state.facing.z));
+    return {
+      entity: "actor",
+      actorId: realtimeActorId,
+      kind: state.actor === "anonymous-dog" ? "dog" : "humanoid",
+      sequence: 0,
+      timestampMs,
+      pose: {
+        space: "local",
+        cell: { kind: "office", officeId },
+        xM: state.position.x,
+        yM: office?.plan.level(state.levelId)?.floorY ?? 0,
+        zM: state.position.z,
+        headingDeg,
+        pitchDeg: 0,
+      },
+      velocity: {
+        xMps: state.action.x * 1.6,
+        yMps: 0,
+        zMps: state.action.z * 1.6,
+        yawDegPerSec: 0,
+      },
+    };
+  }
+
+  const vehicle = journey.vehicle !== null ? city?.vehicleState() : null;
+  if (vehicle) {
+    const heading = vehicle.headingDeg * Math.PI / 180;
+    return {
+      entity: "vehicle",
+      vehicleId: realtimeVehicleId,
+      kind: "model-x",
+      driverActorId: realtimeActorId,
+      sequence: 0,
+      timestampMs,
+      pose: {
+        space: "geographic",
+        lat: vehicle.lat,
+        lng: vehicle.lng,
+        altitudeM: 0,
+        headingDeg: vehicle.headingDeg,
+        pitchDeg: 0,
+      },
+      velocity: {
+        xMps: Math.sin(heading) * vehicle.speedMps,
+        yMps: 0,
+        zMps: Math.cos(heading) * vehicle.speedMps,
+        yawDegPerSec: 0,
+      },
+      steering: vehicle.steering,
+      wheelRadians: vehicle.wheelRadians,
+    };
+  }
+
+  const actor = city?.actorState();
+  if (!actor || !city) return null;
+  const headingDeg = -degrees(actor.yaw);
+  const velocity = {
+    xMps: -Math.sin(actor.yaw) * actor.speedMps,
+    yMps: actor.verticalSpeedMps,
+    // Geographic velocity is east/up/north; scene +Z is south.
+    zMps: Math.cos(actor.yaw) * actor.speedMps,
+    yawDegPerSec: 0,
+  };
+  const origin = cityId === "california" ? { lat: 35.5, lng: -119.5 } : city.world.city.center;
+  const [originX, originZ] = city.world.project(origin.lat, origin.lng);
+  const [lat, lng] = city.world.unproject(
+    originX + actor.x / city.world.metresPerUnit,
+    originZ + actor.z / city.world.metresPerUnit,
+  );
+  return {
+    entity: "actor",
+    actorId: realtimeActorId,
+    kind: actor.kind,
+    sequence: 0,
+    timestampMs,
+    pose: {
+      space: "geographic",
+      lat,
+      lng,
+      altitudeM: actor.y,
+      headingDeg,
+      pitchDeg: degrees(actor.pitch),
+    },
+    velocity,
+  };
+}
+
+function ownRealtimeEntity(snapshot: EntityPoseSnapshot): boolean {
+  if (snapshot.entity === "actor") return snapshot.actorId === realtimeClient?.state().actorId;
+  return snapshot.driverActorId === realtimeClient?.state().actorId || snapshot.vehicleId === realtimeVehicleId;
+}
+
+function clearRealtimePeers(): void {
+  city?.clearRemoteEntities();
+  office?.clearRemoteEntities();
+  updatePresenceIndicator();
+}
+
+function applyRealtimeMessage(message: ServerRealtimeMessage): void {
+  if (message.type === "membership-revoked") {
+    clearRealtimePeers();
+    return;
+  }
+  const updates = message.type === "pose-delta"
+    ? message.updates
+    : message.type === "join-grant"
+      ? message.initial
+      : message.type === "resume-grant"
+        ? message.snapshot
+        : [];
+  const target = inside ? office : city;
+  for (const snapshot of updates) {
+    if (!ownRealtimeEntity(snapshot)) target?.upsertRemoteSnapshot(snapshot);
+  }
+  if (message.type === "pose-delta") {
+    for (const id of message.removedEntityIds) {
+      if (id !== `actor:${realtimeClient?.state().actorId}` && id !== `vehicle:${realtimeVehicleId}`) {
+        target?.removeRemoteEntity(id);
+      }
+    }
+  }
+  updatePresenceIndicator();
+}
+
+function updatePresenceIndicator(state = realtimeClient?.state()): void {
+  if (!presenceIndicator || access.subject === null) return;
+  const connection: "connecting" | "live" | "reconnecting" | "offline" =
+    state?.status === "streaming" ? "live"
+      : state?.status === "reconnecting" ? "reconnecting"
+        : state?.status === "left" || state?.status === "disposed" ? "offline"
+          : "connecting";
+  const nearbyPeerCount = connection === "live"
+    ? (inside ? office?.remoteEntityCount() : city?.remoteEntityCount()) ?? 0
+    : 0;
+  presenceIndicator.update({ signedIn: true, connection, nearbyPeerCount });
+}
+
+async function joinRealtimePresence(operation: number): Promise<void> {
+  const client = realtimeClient;
+  const pose = realtimePose();
+  if (!client || !pose) return;
+  try {
+    const grant = await client.join(realtimeInterest(), pose);
+    if (operation !== realtimeOperation) return;
+    applyRealtimeMessage(grant);
+  } catch {
+    if (operation === realtimeOperation) {
+      presenceIndicator?.update({ signedIn: true, connection: "offline", nearbyPeerCount: 0 });
+    }
+  }
+}
+
+function moveRealtimePresence(): void {
+  clearRealtimePeers();
+  const client = realtimeClient;
+  if (!client || access.subject === null) return;
+  const operation = ++realtimeOperation;
+  if (client.state().sessionId === null) {
+    void joinRealtimePresence(operation);
+    return;
+  }
+  updatePresenceIndicator({ ...client.state(), status: "reconnecting" });
+  void client.moveInterest(realtimeInterest())
+    .then((grant) => {
+      if (operation === realtimeOperation) applyRealtimeMessage(grant);
+    })
+    .catch(() => {
+      if (operation === realtimeOperation) {
+        presenceIndicator?.update({ signedIn: true, connection: "offline", nearbyPeerCount: 0 });
+      }
+    });
+}
+
+function publishRealtimePresence(now: number): void {
+  if (!realtimeClient || now - lastRealtimePublishAt < 100) return;
+  const pose = realtimePose();
+  if (!pose || realtimeClient.state().sessionId === null) return;
+  lastRealtimePublishAt = now;
+  try {
+    realtimeClient.publishPose(pose);
+  } catch {
+    presenceIndicator?.update({ signedIn: true, connection: "offline", nearbyPeerCount: 0 });
+  }
+}
+
+async function initializeRealtimePresence(): Promise<void> {
+  if (!realtimePageActive || access.subject === null || !presenceHost || realtimeClient) return;
+  const [{ createRealtimeClient }, { createPresenceIndicator }] = await Promise.all([
+    import("./realtime/client.ts"),
+    import("./realtime/presenceIndicator.ts"),
+  ]);
+  // `pagehide` can overtake the lazy chunks. Never construct a fresh client
+  // after the page has already run its terminal cleanup.
+  if (!realtimePageActive || access.subject === null || realtimeClient) return;
+  presenceHost.hidden = false;
+  document.body.classList.add("presence-on");
+  presenceIndicator = createPresenceIndicator({
+    container: presenceHost,
+    onRetry: () => moveRealtimePresence(),
+  });
+  presenceIndicator.update({ signedIn: true, connection: "connecting", nearbyPeerCount: 0 });
+  realtimeClient = createRealtimeClient({
+    actorId: realtimeActorId,
+    authenticatedFetch: authFetch,
+    sendIntervalMs: 100,
+  });
+  stopRealtimeSubscription = realtimeClient.subscribe({
+    onMessage: applyRealtimeMessage,
+    onStateChange: updatePresenceIndicator,
+    onError: () => presenceIndicator?.update({ signedIn: true, connection: "offline", nearbyPeerCount: 0 }),
+  });
+  moveRealtimePresence();
+}
+
+window.addEventListener("pagehide", () => {
+  realtimePageActive = false;
+  realtimeOperation += 1;
+  stopRealtimeSubscription?.();
+  stopRealtimeSubscription = null;
+  clearRealtimePeers();
+  // Page teardown cannot await the leave round trip. Contain a rotated-token
+  // or offline failure so navigation never produces an unhandled rejection;
+  // the server's disconnect grace still removes the ephemeral session.
+  void realtimeClient?.dispose().catch(() => undefined);
+  realtimeClient = null;
+  presenceIndicator?.dispose();
+  presenceIndicator = null;
+});
+
 function applyProfilePreview(profile: LocalProfile): void {
   if (access.subject === null) return;
   if (inside) {
@@ -3165,6 +3440,12 @@ async function boot() {
     mountCity(first?.id ?? "california"),
   );
 
+  // There must be a mounted scene before remote snapshots have anywhere to go.
+  // Anonymous and zero-server builds retain their existing local-only path.
+  // Hosted presence is an enhancement: a failed lazy chunk or unavailable API
+  // must not stop profile/tools initialization after the world is already live.
+  await initializeRealtimePresence().catch(() => undefined);
+
   // The instruments, after the first board, because the panel reads a live
   // stage and there is not one before this line.
   await mountGodmode();
diff --git a/src/realtime/client.ts b/src/realtime/client.ts
new file mode 100644
index 0000000..7930f4e
--- /dev/null
+++ b/src/realtime/client.ts
@@ -0,0 +1,567 @@
+import {
+  isEntityPoseSnapshot,
+  isInterestCell,
+  validateClientRealtimeMessage,
+  validateServerRealtimeMessage,
+} from "./protocol.ts";
+import type {
+  EntityPoseSnapshot,
+  InterestCell,
+  JoinGrant,
+  JoinRequest,
+  ResumeGrant,
+  ResumeRequest,
+  ServerRealtimeMessage,
+} from "./types.ts";
+
+const MIN_SEND_INTERVAL_MS = 1_000 / 15;
+const MAX_SEND_INTERVAL_MS = 1_000 / 10;
+const UINT32_MAX = 4_294_967_295;
+
+type Fetch = typeof globalThis.fetch;
+type TimerHandle = ReturnType<typeof globalThis.setTimeout>;
+
+export interface RealtimeClientTimers {
+  setTimeout(callback: () => void, delayMs: number): TimerHandle;
+  clearTimeout(handle: TimerHandle): void;
+}
+
+export interface RealtimeClientEndpoints {
+  join: string;
+  pose: string;
+  events: string;
+  leave: string;
+}
+
+export type RealtimeClientStatus =
+  | "idle"
+  | "joined"
+  | "streaming"
+  | "reconnecting"
+  | "left"
+  | "disposed";
+
+export interface RealtimeClientState {
+  status: RealtimeClientStatus;
+  sessionId: string | null;
+  actorId: string;
+  interests: readonly InterestCell[];
+  serverEpoch: string | null;
+  lastReceivedSequence: number | null;
+  nextPoseSequence: number;
+  posePending: boolean;
+  subscribed: boolean;
+  reconnectAttempt: number;
+}
+
+export interface RealtimeSubscriptionCallbacks {
+  onMessage?(message: ServerRealtimeMessage): void;
+  onStateChange?(state: RealtimeClientState): void;
+  /** Receives protocol/parser errors without the untrusted payload or credentials. */
+  onMalformedEvent?(error: Error): void;
+  onError?(error: Error): void;
+}
+
+export interface RealtimeClientOptions {
+  actorId: string;
+  /**
+   * Optional SSO bearer, kept only in this closure and never placed in a URL.
+   * Omit it when the deployment uses an HttpOnly same-origin session cookie.
+   */
+  accessToken?: string;
+  /**
+   * Preferred request adapter for hosted builds. Passing `authFetch` keeps SSO
+   * token lookup at request time and also supports HttpOnly cookie sessions.
+   */
+  authenticatedFetch?: Fetch;
+  endpoints?: Partial<RealtimeClientEndpoints>;
+  fetch?: Fetch;
+  timers?: RealtimeClientTimers;
+  sendIntervalMs?: number;
+  reconnectBaseMs?: number;
+  reconnectMaximumMs?: number;
+  reconnectJitter?: number;
+  random?: () => number;
+}
+
+export interface RealtimeClient {
+  join(
+    interest: InterestCell | readonly InterestCell[],
+    entity: EntityPoseSnapshot,
+  ): Promise<JoinGrant>;
+  publishPose(entity: EntityPoseSnapshot): void;
+  subscribe(callbacks: RealtimeSubscriptionCallbacks): () => void;
+  moveInterest(interest: InterestCell | readonly InterestCell[]): Promise<ResumeGrant>;
+  rejoin(): Promise<ResumeGrant>;
+  state(): RealtimeClientState;
+  leave(): Promise<void>;
+  dispose(): Promise<void>;
+}
+
+const DEFAULT_ENDPOINTS: RealtimeClientEndpoints = {
+  join: "/api/v1/realtime/join",
+  pose: "/api/v1/realtime/pose",
+  events: "/api/v1/realtime/events",
+  leave: "/api/v1/realtime/leave",
+};
+
+const DEFAULT_TIMERS: RealtimeClientTimers = {
+  setTimeout: (callback, delayMs) => globalThis.setTimeout(callback, delayMs),
+  clearTimeout: (handle) => globalThis.clearTimeout(handle),
+};
+
+function requireNonEmpty(value: string, name: string): void {
+  if (value.length === 0 || value.length > 256) throw new TypeError(`${name} must be 1-256 characters`);
+}
+
+function normalizeInterests(value: InterestCell | readonly InterestCell[]): readonly InterestCell[] {
+  const values = Array.isArray(value) ? value : [value];
+  if (values.length === 0 || values.length > 128 || !values.every(isInterestCell)) {
+    throw new TypeError("realtime interests are invalid");
+  }
+  return values.map((interest) => ({ ...interest }));
+}
+
+function errorFrom(reason: unknown, fallback: string): Error {
+  return reason instanceof Error ? reason : new Error(fallback);
+}
+
+function isTerminalStatus(status: RealtimeClientStatus): boolean {
+  return status === "disposed" || status === "left";
+}
+
+/**
+ * Browser transport for the server-authoritative realtime protocol.
+ *
+ * Streaming uses fetch + ReadableStream instead of EventSource. This permits a
+ * normal Authorization header and POST body, keeping both the access token and
+ * resume token out of URLs, referrers, history, and intermediary URL logs.
+ */
+export function createRealtimeClient(options: RealtimeClientOptions): RealtimeClient {
+  requireNonEmpty(options.actorId, "actorId");
+  if (options.accessToken !== undefined) requireNonEmpty(options.accessToken, "accessToken");
+  if (options.fetch && options.authenticatedFetch) {
+    throw new TypeError("provide either fetch or authenticatedFetch, not both");
+  }
+  const fetcher = options.authenticatedFetch ?? options.fetch ?? globalThis.fetch;
+  if (typeof fetcher !== "function") throw new TypeError("fetch is required");
+  const timers = options.timers ?? DEFAULT_TIMERS;
+  const endpoints = { ...DEFAULT_ENDPOINTS, ...options.endpoints };
+  for (const [name, endpoint] of Object.entries(endpoints)) requireNonEmpty(endpoint, `${name} endpoint`);
+
+  const sendIntervalMs = options.sendIntervalMs ?? MAX_SEND_INTERVAL_MS;
+  if (!Number.isFinite(sendIntervalMs) || sendIntervalMs < MIN_SEND_INTERVAL_MS || sendIntervalMs > MAX_SEND_INTERVAL_MS) {
+    throw new RangeError("sendIntervalMs must be between 66.67ms (15Hz) and 100ms (10Hz)");
+  }
+  const reconnectBaseMs = options.reconnectBaseMs ?? 500;
+  const reconnectMaximumMs = options.reconnectMaximumMs ?? 30_000;
+  const reconnectJitter = options.reconnectJitter ?? 0.2;
+  if (!Number.isFinite(reconnectBaseMs) || reconnectBaseMs <= 0) throw new RangeError("reconnectBaseMs must be positive");
+  if (!Number.isFinite(reconnectMaximumMs) || reconnectMaximumMs < reconnectBaseMs) {
+    throw new RangeError("reconnectMaximumMs must be at least reconnectBaseMs");
+  }
+  if (!Number.isFinite(reconnectJitter) || reconnectJitter < 0 || reconnectJitter > 1) {
+    throw new RangeError("reconnectJitter must be between zero and one");
+  }
+  const random = options.random ?? Math.random;
+
+  let status: RealtimeClientStatus = "idle";
+  let interests: readonly InterestCell[] = [];
+  let sessionId: string | null = null;
+  let serverEpoch: string | null = null;
+  let resumeToken: string | null = null;
+  let lastReceivedSequence: number | null = null;
+  let nextPoseSequence = 0;
+  let pendingPose: EntityPoseSnapshot | null = null;
+  let poseTimer: TimerHandle | null = null;
+  let reconnectTimer: TimerHandle | null = null;
+  let reconnectAttempt = 0;
+  let subscription: RealtimeSubscriptionCallbacks | null = null;
+  let streamAbort: AbortController | null = null;
+  // Resume rotates the credential. Do not race a pose against the first grant
+  // and send the token the server has just retired.
+  let streamReady = false;
+  let streamGeneration = 0;
+  // Scene changes can overtake join/resume HTTP calls. Only the newest
+  // transition may commit credentials or interest cells.
+  let transitionGeneration = 0;
+  let requestCounter = 0;
+
+  function snapshot(): RealtimeClientState {
+    return {
+      status,
+      sessionId,
+      actorId: options.actorId,
+      interests: interests.map((interest) => ({ ...interest })),
+      serverEpoch,
+      lastReceivedSequence,
+      nextPoseSequence,
+      posePending: pendingPose !== null,
+      subscribed: subscription !== null,
+      reconnectAttempt,
+    };
+  }
+
+  function changed(): void {
+    subscription?.onStateChange?.(snapshot());
+  }
+
+  function requestId(): string {
+    requestCounter += 1;
+    return `tera-client-${requestCounter}`;
+  }
+
+  function authHeaders(accept = "application/json"): HeadersInit {
+    const headers: Record<string, string> = {
+      Accept: accept,
+      "Content-Type": "application/json",
+    };
+    if (options.accessToken !== undefined) headers.Authorization = `Bearer ${options.accessToken}`;
+    return headers;
+  }
+
+  async function post(endpoint: string, body: unknown, accept?: string, signal?: AbortSignal): Promise<Response> {
+    return fetcher(endpoint, {
+      method: "POST",
+      credentials: "same-origin",
+      cache: "no-store",
+      redirect: "error",
+      headers: authHeaders(accept),
+      body: JSON.stringify(body),
+      signal,
+    });
+  }
+
+  async function json(response: Response): Promise<unknown> {
+    if (!response.ok) throw new Error(`realtime request failed (${response.status})`);
+    return response.json() as Promise<unknown>;
+  }
+
+  function cancelPose(): void {
+    if (poseTimer !== null) timers.clearTimeout(poseTimer);
+    poseTimer = null;
+    pendingPose = null;
+  }
+
+  function cancelStream(): void {
+    streamGeneration += 1;
+    streamAbort?.abort();
+    streamAbort = null;
+    streamReady = false;
+    if (reconnectTimer !== null) timers.clearTimeout(reconnectTimer);
+    reconnectTimer = null;
+  }
+
+  function acceptMessage(message: ServerRealtimeMessage): boolean {
+    if (serverEpoch !== null && "serverEpoch" in message && message.serverEpoch !== serverEpoch) {
+      subscription?.onMalformedEvent?.(new Error("realtime event belongs to a different server epoch"));
+      return false;
+    }
+    if (message.type === "pose-delta" || message.type === "membership-revoked") {
+      if (lastReceivedSequence !== null && message.sequence <= lastReceivedSequence) {
+        subscription?.onMalformedEvent?.(new Error("realtime event sequence is not monotonic"));
+        return false;
+      }
+      lastReceivedSequence = message.sequence;
+    } else if (message.type === "resume-grant") {
+      resumeToken = message.resumeToken;
+      serverEpoch = message.serverEpoch;
+      lastReceivedSequence = message.nextSequence === 0 ? 0 : message.nextSequence - 1;
+      streamReady = true;
+      status = "streaming";
+    }
+    reconnectAttempt = 0;
+    subscription?.onMessage?.(message);
+    if (message.type === "membership-revoked" && !message.reconnectAllowed) {
+      cancelStream();
+      status = "left";
+      resumeToken = null;
+    }
+    changed();
+    return true;
+  }
+
+  function parseEvent(data: string): void {
+    if (data.length === 0) return;
+    let raw: unknown;
+    try {
+      raw = JSON.parse(data);
+    } catch {
+      subscription?.onMalformedEvent?.(new Error("realtime event is not valid JSON"));
+      return;
+    }
+    const result = validateServerRealtimeMessage(raw);
+    if (!result.ok) {
+      subscription?.onMalformedEvent?.(new Error(result.error));
+      return;
+    }
+    acceptMessage(result.value);
+  }
+
+  async function readSse(response: Response, generation: number): Promise<void> {
+    if (!response.ok) throw new Error(`realtime stream failed (${response.status})`);
+    if (!response.body) throw new Error("realtime stream response has no body");
+    const reader = response.body.getReader();
+    const decoder = new TextDecoder();
+    let buffer = "";
+    let dataLines: string[] = [];
+    const line = (value: string): void => {
+      if (value === "") {
+        if (dataLines.length > 0) parseEvent(dataLines.join("\n"));
+        dataLines = [];
+      } else if (value.startsWith("data:")) {
+        dataLines.push(value.slice(5).replace(/^ /, ""));
+      }
+    };
+    try {
+      while (generation === streamGeneration) {
+        const chunk = await reader.read();
+        // An abort and an already-queued chunk may settle together. Never
+        // parse the old cell after another transition has won that race.
+        if (generation !== streamGeneration) break;
+        if (chunk.done) break;
+        buffer += decoder.decode(chunk.value, { stream: true }).replace(/\r\n/g, "\n").replace(/\r/g, "\n");
+        let newline = buffer.indexOf("\n");
+        while (newline >= 0) {
+          line(buffer.slice(0, newline));
+          buffer = buffer.slice(newline + 1);
+          newline = buffer.indexOf("\n");
+        }
+      }
+      if (generation !== streamGeneration) return;
+      buffer += decoder.decode();
+      if (buffer.length > 0) line(buffer);
+      if (dataLines.length > 0) parseEvent(dataLines.join("\n"));
+    } finally {
+      reader.releaseLock();
+    }
+  }
+
+  function reconnectDelay(): number {
+    const exponential = Math.min(reconnectMaximumMs, reconnectBaseMs * 2 ** reconnectAttempt);
+    const factor = 1 + (random() * 2 - 1) * reconnectJitter;
+    return Math.max(0, exponential * factor);
+  }
+
+  function scheduleReconnect(): void {
+    if (!subscription || status === "left" || status === "disposed" || reconnectTimer !== null) return;
+    status = "reconnecting";
+    const delay = reconnectDelay();
+    reconnectAttempt += 1;
+    changed();
+    reconnectTimer = timers.setTimeout(() => {
+      reconnectTimer = null;
+      void startStream();
+    }, delay);
+  }
+
+  async function startStream(): Promise<void> {
+    if (!subscription || !sessionId || !resumeToken || !serverEpoch || lastReceivedSequence === null) return;
+    cancelStream();
+    const generation = streamGeneration;
+    const abort = new AbortController();
+    streamAbort = abort;
+    const resume: ResumeRequest = {
+      type: "resume-request",
+      protocolVersion: 1,
+      requestId: requestId(),
+      sessionId,
+      resumeToken,
+      serverEpoch,
+      lastReceivedSequence,
+      interests,
+    };
+    if (!validateClientRealtimeMessage(resume).ok) throw new Error("realtime resume state is invalid");
+    try {
+      const response = await post(endpoints.events, resume, "text/event-stream", abort.signal);
+      if (generation !== streamGeneration) return;
+      await readSse(response, generation);
+      if (generation === streamGeneration) scheduleReconnect();
+    } catch (reason) {
+      if (generation !== streamGeneration || abort.signal.aborted) return;
+      subscription?.onError?.(errorFrom(reason, "realtime stream failed"));
+      scheduleReconnect();
+    }
+  }
+
+  async function sendPose(): Promise<void> {
+    poseTimer = null;
+    const pose = pendingPose;
+    pendingPose = null;
+    if (!pose || !sessionId || status === "left" || status === "disposed") return;
+    if (subscription && !streamReady) {
+      pendingPose = pose;
+      poseTimer = timers.setTimeout(() => void sendPose(), sendIntervalMs);
+      return;
+    }
+    try {
+      const response = await post(endpoints.pose, { sessionId, token: resumeToken, snapshot: pose });
+      if (!response.ok) throw new Error(`realtime pose request failed (${response.status})`);
+    } catch (reason) {
+      subscription?.onError?.(errorFrom(reason, "realtime pose request failed"));
+    }
+    changed();
+  }
+
+  async function join(interest: InterestCell | readonly InterestCell[], entity: EntityPoseSnapshot): Promise<JoinGrant> {
+    if (status === "disposed") throw new Error("realtime client is disposed");
+    const generation = ++transitionGeneration;
+    const nextInterests = normalizeInterests(interest);
+    if (!isEntityPoseSnapshot(entity)) throw new TypeError("realtime entity pose is invalid");
+    const request: JoinRequest = {
+      type: "join-request",
+      protocolVersion: 1,
+      requestId: requestId(),
+      actorId: options.actorId,
+      resumeToken,
+      lastReceivedSequence,
+      interests: nextInterests,
+    };
+    if (!validateClientRealtimeMessage(request).ok) throw new Error("realtime join request is invalid");
+    const parsed = validateServerRealtimeMessage(await json(await post(endpoints.join, { ...request, entity })));
+    if (!parsed.ok || parsed.value.type !== "join-grant") throw new Error("realtime join response is invalid");
+    const grant = parsed.value;
+    if (grant.actorId !== options.actorId || grant.requestId !== request.requestId) {
+      throw new Error("realtime join response identity does not match request");
+    }
+    if (generation !== transitionGeneration || isTerminalStatus(status)) {
+      // A superseded request can still have created a server session before
+      // the browser ignored its answer. Retire it without adopting its token.
+      void post(endpoints.leave, { sessionId: grant.sessionId, token: grant.resumeToken })
+        .catch(() => { /* normal idle expiry is the fallback */ });
+      throw new Error("realtime join was superseded");
+    }
+    interests = grant.interests.map((entry) => ({ ...entry }));
+    sessionId = grant.sessionId;
+    serverEpoch = grant.serverEpoch;
+    resumeToken = grant.resumeToken;
+    lastReceivedSequence = grant.nextSequence === 0 ? 0 : grant.nextSequence - 1;
+    nextPoseSequence = Math.max(entity.sequence + 1, grant.nextSequence);
+    status = "joined";
+    streamReady = subscription === null;
+    reconnectAttempt = 0;
+    changed();
+    if (subscription && lastReceivedSequence !== null) void startStream();
+    return grant;
+  }
+
+  function publishPose(entity: EntityPoseSnapshot): void {
+    if (!sessionId || status === "idle" || status === "left" || status === "disposed") {
+      throw new Error("realtime client must be joined before publishing a pose");
+    }
+    if (!isEntityPoseSnapshot(entity)) throw new TypeError("realtime entity pose is invalid");
+    if (nextPoseSequence > UINT32_MAX) throw new RangeError("realtime pose sequence exhausted");
+    const sequenced = { ...entity, sequence: nextPoseSequence } as EntityPoseSnapshot;
+    if (!isEntityPoseSnapshot(sequenced)) throw new TypeError("realtime entity pose is invalid");
+    nextPoseSequence += 1;
+    pendingPose = sequenced;
+    if (poseTimer === null) poseTimer = timers.setTimeout(() => void sendPose(), sendIntervalMs);
+    changed();
+  }
+
+  function subscribe(callbacks: RealtimeSubscriptionCallbacks): () => void {
+    if (status === "disposed") throw new Error("realtime client is disposed");
+    subscription = callbacks;
+    changed();
+    if (sessionId && lastReceivedSequence !== null) void startStream();
+    let active = true;
+    return () => {
+      if (!active || subscription !== callbacks) return;
+      active = false;
+      subscription = null;
+      cancelStream();
+      if (status === "streaming" || status === "reconnecting") status = sessionId ? "joined" : "idle";
+    };
+  }
+
+  async function resume(nextInterests: readonly InterestCell[]): Promise<ResumeGrant> {
+    if (status === "disposed") throw new Error("realtime client is disposed");
+    if (!sessionId || !resumeToken || !serverEpoch || lastReceivedSequence === null) {
+      throw new Error("realtime client has no resumable session");
+    }
+    const generation = ++transitionGeneration;
+    const expectedSessionId = sessionId;
+    const request: ResumeRequest = {
+      type: "resume-request",
+      protocolVersion: 1,
+      requestId: requestId(),
+      sessionId,
+      resumeToken,
+      serverEpoch,
+      lastReceivedSequence,
+      interests: nextInterests,
+    };
+    if (!validateClientRealtimeMessage(request).ok) throw new Error("realtime resume request is invalid");
+    // The old stream belongs to a different privacy and coordinate cell. Close
+    // it before the handoff request, not after its network round trip.
+    cancelStream();
+    status = "joined";
+    changed();
+    try {
+      const parsed = validateServerRealtimeMessage(await json(await post(endpoints.join, request)));
+      if (!parsed.ok || parsed.value.type !== "resume-grant") throw new Error("realtime resume response is invalid");
+      const grant = parsed.value;
+      if (grant.sessionId !== expectedSessionId || grant.requestId !== request.requestId) {
+        throw new Error("realtime resume response identity does not match request");
+      }
+      if (generation !== transitionGeneration || isTerminalStatus(status)) {
+        throw new Error("realtime resume was superseded");
+      }
+      interests = nextInterests.map((entry) => ({ ...entry }));
+      resumeToken = grant.resumeToken;
+      serverEpoch = grant.serverEpoch;
+      lastReceivedSequence = grant.nextSequence === 0 ? 0 : grant.nextSequence - 1;
+      streamReady = subscription === null;
+      reconnectAttempt = 0;
+      changed();
+      if (subscription && lastReceivedSequence !== null) void startStream();
+      return grant;
+    } catch (error) {
+      if (generation === transitionGeneration && !isTerminalStatus(status)) {
+        status = "joined";
+        changed();
+      }
+      throw error;
+    }
+  }
+
+  async function rejoin(): Promise<ResumeGrant> {
+    return resume(interests);
+  }
+
+  async function moveInterest(interest: InterestCell | readonly InterestCell[]): Promise<ResumeGrant> {
+    return resume(normalizeInterests(interest));
+  }
+
+  async function leave(): Promise<void> {
+    if (status === "disposed") return;
+    transitionGeneration += 1;
+    cancelPose();
+    cancelStream();
+    const leavingSession = sessionId;
+    const leavingToken = resumeToken;
+    status = "left";
+    sessionId = null;
+    serverEpoch = null;
+    resumeToken = null;
+    lastReceivedSequence = null;
+    reconnectAttempt = 0;
+    changed();
+    if (leavingSession && leavingToken) {
+      const response = await post(endpoints.leave, { sessionId: leavingSession, token: leavingToken });
+      if (!response.ok) throw new Error(`realtime leave request failed (${response.status})`);
+    }
+  }
+
+  async function dispose(): Promise<void> {
+    if (status === "disposed") return;
+    try {
+      await leave();
+    } finally {
+      subscription = null;
+      status = "disposed";
+    }
+  }
+
+  return { join, publishPose, subscribe, moveInterest, rejoin, state: snapshot, leave, dispose };
+}
diff --git a/src/realtime/index.ts b/src/realtime/index.ts
index 7dd6bda..f4c7f41 100644
--- a/src/realtime/index.ts
+++ b/src/realtime/index.ts
@@ -8,4 +8,27 @@ export {
   validateServerRealtimeMessage,
 } from "./protocol.ts";
 export { PoseInterpolationBuffer } from "./interpolation.ts";
+export {
+  createRealtimeClient,
+  type RealtimeClient,
+  type RealtimeClientEndpoints,
+  type RealtimeClientOptions,
+  type RealtimeClientState,
+  type RealtimeClientStatus,
+  type RealtimeClientTimers,
+  type RealtimeSubscriptionCallbacks,
+} from "./client.ts";
+export {
+  createScenePeers,
+  scenePeerId,
+  type ScenePeers,
+  type ScenePeersOptions,
+} from "./scenePeers.ts";
+export {
+  createPresenceIndicator,
+  type PresenceConnectionState,
+  type PresenceIndicator,
+  type PresenceIndicatorOptions,
+  type PresenceIndicatorState,
+} from "./presenceIndicator.ts";
 export type * from "./types.ts";
diff --git a/src/realtime/presenceIndicator.ts b/src/realtime/presenceIndicator.ts
new file mode 100644
index 0000000..c12f7a3
--- /dev/null
+++ b/src/realtime/presenceIndicator.ts
@@ -0,0 +1,121 @@
+/** Small, identity-free hosted-presence status for a caller-owned container. */
+
+export type PresenceConnectionState = "off" | "connecting" | "live" | "reconnecting" | "offline";
+
+export interface PresenceIndicatorState {
+  signedIn: boolean;
+  connection: PresenceConnectionState;
+  /** Aggregate only. Individual peer identity never enters this adapter. */
+  nearbyPeerCount: number;
+}
+
+export interface PresenceIndicatorOptions {
+  container: HTMLElement;
+  onRetry?: () => void;
+}
+
+export interface PresenceIndicator {
+  root: HTMLElement;
+  update(state: PresenceIndicatorState): PresenceIndicatorState;
+  state(): PresenceIndicatorState;
+  dispose(): void;
+}
+
+const CONNECTIONS = new Set<PresenceConnectionState>([
+  "off", "connecting", "live", "reconnecting", "offline",
+]);
+
+function copy(state: PresenceIndicatorState): PresenceIndicatorState {
+  return { ...state };
+}
+
+function normalize(input: PresenceIndicatorState): PresenceIndicatorState {
+  if (!input || typeof input !== "object") throw new TypeError("presence indicator: invalid state");
+  if (typeof input.signedIn !== "boolean" || !CONNECTIONS.has(input.connection)) {
+    throw new TypeError("presence indicator: invalid connection state");
+  }
+  if (!Number.isSafeInteger(input.nearbyPeerCount) || input.nearbyPeerCount < 0 || input.nearbyPeerCount > 10_000) {
+    throw new RangeError("presence indicator: nearbyPeerCount must be an integer from 0 to 10000");
+  }
+  if (!input.signedIn) return { signedIn: false, connection: "off", nearbyPeerCount: 0 };
+  return {
+    signedIn: true,
+    connection: input.connection,
+    nearbyPeerCount: input.connection === "live" ? input.nearbyPeerCount : 0,
+  };
+}
+
+function label(state: PresenceIndicatorState): string {
+  if (!state.signedIn || state.connection === "off") return "Hosted presence is off.";
+  switch (state.connection) {
+    case "connecting": return "Hosted presence is connecting.";
+    case "reconnecting": return "Hosted presence is reconnecting.";
+    case "offline": return "Hosted presence is offline.";
+    case "live": {
+      const nearby = state.nearbyPeerCount === 1 ? "1 nearby peer" : `${state.nearbyPeerCount} nearby peers`;
+      return `Hosted presence is live · ${nearby}.`;
+    }
+  }
+}
+
+export function createPresenceIndicator(options: PresenceIndicatorOptions): PresenceIndicator {
+  if (!options.container || typeof options.container.append !== "function") {
+    throw new TypeError("presence indicator: container must be an HTMLElement");
+  }
+  if (options.onRetry !== undefined && typeof options.onRetry !== "function") {
+    throw new TypeError("presence indicator: onRetry must be a function");
+  }
+
+  const doc = options.container.ownerDocument;
+  const root = doc.createElement("div");
+  root.className = "tera-presence";
+  root.setAttribute("data-presence", "off");
+
+  const statusLine = doc.createElement("span");
+  statusLine.className = "tera-presence__status";
+  statusLine.setAttribute("role", "status");
+  statusLine.setAttribute("aria-live", "polite");
+  statusLine.setAttribute("aria-atomic", "true");
+
+  const retry = doc.createElement("button");
+  retry.type = "button";
+  retry.className = "tera-presence__retry";
+  retry.textContent = "Retry presence";
+  retry.hidden = true;
+  root.append(statusLine, retry);
+  options.container.append(root);
+
+  let current: PresenceIndicatorState = { signedIn: false, connection: "off", nearbyPeerCount: 0 };
+  let disposed = false;
+  let onRetry: (() => void) | null = options.onRetry ?? null;
+
+  function render(): void {
+    statusLine.textContent = label(current);
+    root.setAttribute("data-presence", current.connection);
+    retry.hidden = onRetry === null || !current.signedIn ||
+      (current.connection !== "offline" && current.connection !== "reconnecting");
+  }
+
+  retry.addEventListener("click", () => {
+    if (!disposed && !retry.hidden) onRetry?.();
+  });
+  render();
+
+  return {
+    root,
+    update(next) {
+      if (disposed) return copy(current);
+      current = normalize(next);
+      render();
+      return copy(current);
+    },
+    state: () => copy(current),
+    dispose() {
+      if (disposed) return;
+      disposed = true;
+      onRetry = null;
+      current = { signedIn: false, connection: "off", nearbyPeerCount: 0 };
+      root.remove();
+    },
+  };
+}
diff --git a/src/realtime/scenePeers.ts b/src/realtime/scenePeers.ts
new file mode 100644
index 0000000..49e9bfd
--- /dev/null
+++ b/src/realtime/scenePeers.ts
@@ -0,0 +1,359 @@
+/**
+ * Three.js presentation for already-validated remote entity snapshots.
+ *
+ * Networking, trust, auth, and display names stay outside this module. It keeps
+ * only canonical protocol ids in a private Map, buffers authoritative motion,
+ * and gives each peer one stable, identity-free scene root. Procedural clones
+ * share four prototype resource sets for the lifetime of the adapter.
+ */
+import * as THREE from "three";
+import {
+  buildCrow,
+  buildDog,
+  buildHumanoid,
+  cloneCrow,
+  cloneDog,
+  cloneHumanoid,
+  disposeCrow,
+  disposeDog,
+  disposeHumanoid,
+  poseCrowFlight,
+  poseDogAttention,
+  poseDogWalk,
+  poseHumanoid,
+  type CrowRig,
+  type DogRig,
+  type HumanoidRig,
+} from "../assets/actors/index.ts";
+import {
+  buildModelX,
+  cloneModelX,
+  disposeModelX,
+  setModelXSteering,
+  setModelXWheelRotation,
+  type ModelXRig,
+} from "../assets/vehicles/index.ts";
+import { PoseInterpolationBuffer } from "./interpolation.ts";
+import { isEntityPoseSnapshot } from "./protocol.ts";
+import type {
+  BufferedPoseSample,
+  EntityPoseSnapshot,
+  InterpolationOptions,
+  SpatialPose,
+} from "./types.ts";
+
+export interface ScenePeersOptions {
+  /** Geographic map projection. Returned X/Z are already scene units. */
+  project(lat: number, lng: number): readonly [number, number];
+  /** Terrain height in scene units at a geographic position. */
+  groundAt(lat: number, lng: number): number;
+  /** Converts geographic altitude metres and, by default, asset metres to scene units. */
+  geographicSceneUnitsPerMetre?: number;
+  /** Optional presentation exaggeration independent of geographic motion scale. */
+  geographicVisualScale?: number;
+  /** Local office/city metre scale. Defaults to direct 1 metre = 1 scene unit. */
+  localSceneUnitsPerMetre?: number;
+  /** Optional presentation scale independent of local coordinate scale. */
+  localVisualScale?: number;
+  interpolation?: InterpolationOptions;
+  /** Bounded nearby peer count. Defaults to 128, maximum 512. */
+  maximumPeers?: number;
+}
+
+export interface ScenePeers {
+  root: THREE.Group;
+  /** Add a new authoritative sample. Invalid/stale/over-capacity input returns false. */
+  upsert(snapshot: EntityPoseSnapshot): boolean;
+  /** Remove by canonical id from `scenePeerId`/`ids`. */
+  remove(id: string): boolean;
+  /** Apply interpolated state at authoritative server time; returns roots updated. */
+  tick(serverTimeMs: number): number;
+  ids(): readonly string[];
+  count(): number;
+  /** Interest reset: drops peers but retains shared prototype GPU resources. */
+  clear(): void;
+  dispose(): void;
+}
+
+type PeerVisual =
+  | { kind: "humanoid"; rig: HumanoidRig }
+  | { kind: "dog"; rig: DogRig }
+  | { kind: "crow"; rig: CrowRig }
+  | { kind: "model-x"; rig: ModelXRig };
+
+interface Prototypes {
+  humanoid: HumanoidRig;
+  dog: DogRig;
+  crow: CrowRig;
+  vehicle: ModelXRig;
+}
+
+interface Peer {
+  readonly id: string;
+  readonly root: THREE.Group;
+  readonly buffer: PoseInterpolationBuffer;
+  visual: PeerVisual;
+  kind: EntityPoseSnapshot["kind"];
+  spaceKey: string;
+  latest: EntityPoseSnapshot;
+  lastSequence: number;
+  lastTimestampMs: number;
+}
+
+export function scenePeerId(snapshot: EntityPoseSnapshot): string {
+  return snapshot.entity === "actor" ? `actor:${snapshot.actorId}` : `vehicle:${snapshot.vehicleId}`;
+}
+
+function poseSpaceKey(pose: SpatialPose): string {
+  return pose.space === "geographic" ? "geographic" : `local:${JSON.stringify(pose.cell)}`;
+}
+
+function positive(value: number, name: string): number {
+  if (!(value > 0) || !Number.isFinite(value)) throw new RangeError(`${name} must be finite and positive`);
+  return value;
+}
+
+function resolveMaximumPeers(value: number | undefined): number {
+  const resolved = value ?? 128;
+  if (!Number.isInteger(resolved) || resolved < 1 || resolved > 512) {
+    throw new RangeError("maximumPeers must be an integer from 1 to 512");
+  }
+  return resolved;
+}
+
+function makePrototypes(): Prototypes {
+  const vehicle = buildModelX({ detail: "corridor", paint: 0x050607 });
+  vehicle.root.name = "generic-black-ev.prototype";
+  vehicle.root.userData.vehicleModel = "generic-black-ev";
+  return {
+    humanoid: buildHumanoid(),
+    dog: buildDog(),
+    crow: buildCrow(),
+    vehicle,
+  };
+}
+
+function cloneVisual(snapshot: EntityPoseSnapshot, prototypes: Prototypes): PeerVisual {
+  if (snapshot.entity === "vehicle") {
+    const rig = cloneModelX(prototypes.vehicle);
+    rig.root.name = "generic-black-ev";
+    rig.root.userData.vehicleModel = "generic-black-ev";
+    return { kind: "model-x", rig };
+  }
+  if (snapshot.kind === "dog") return { kind: "dog", rig: cloneDog(prototypes.dog) };
+  if (snapshot.kind === "crow") return { kind: "crow", rig: cloneCrow(prototypes.crow) };
+  return { kind: "humanoid", rig: cloneHumanoid(prototypes.humanoid) };
+}
+
+function visualRoot(visual: PeerVisual): THREE.Group {
+  return visual.rig.root;
+}
+
+function replaceVisual(peer: Peer, snapshot: EntityPoseSnapshot, prototypes: Prototypes): void {
+  visualRoot(peer.visual).removeFromParent();
+  peer.visual = cloneVisual(snapshot, prototypes);
+  peer.root.add(visualRoot(peer.visual));
+  peer.kind = snapshot.kind;
+}
+
+function animateActor(
+  visual: Exclude<PeerVisual, { kind: "model-x" }>,
+  sample: BufferedPoseSample,
+): void {
+  const speed = Math.hypot(sample.velocity.xMps, sample.velocity.zMps);
+  const seconds = sample.timestampMs / 1_000;
+  if (visual.kind === "humanoid") {
+    poseHumanoid(visual.rig, {
+      walkPhase: seconds * Math.max(1.2, speed * 5.8),
+      stride: THREE.MathUtils.clamp(speed / 3.2, 0, 0.68),
+    });
+  } else if (visual.kind === "dog") {
+    poseDogWalk(
+      visual.rig,
+      seconds * Math.max(1.4, speed * 7.2),
+      THREE.MathUtils.clamp(speed / 4, 0, 0.68),
+    );
+    poseDogAttention(visual.rig, 0, seconds * 5);
+  } else {
+    poseCrowFlight(
+      visual.rig,
+      seconds * Math.max(3, speed * 1.5),
+      THREE.MathUtils.clamp(0.35 + speed / 14, 0.35, 1),
+    );
+  }
+}
+
+export function createScenePeers(options: ScenePeersOptions): ScenePeers {
+  if (typeof options.project !== "function" || typeof options.groundAt !== "function") {
+    throw new RangeError("scene peers require project and groundAt functions");
+  }
+  const geographicScale = positive(
+    options.geographicSceneUnitsPerMetre ?? 1,
+    "geographicSceneUnitsPerMetre",
+  );
+  const geographicVisualScale = positive(
+    options.geographicVisualScale ?? geographicScale,
+    "geographicVisualScale",
+  );
+  const localScale = positive(options.localSceneUnitsPerMetre ?? 1, "localSceneUnitsPerMetre");
+  const localVisualScale = positive(options.localVisualScale ?? localScale, "localVisualScale");
+  const limit = resolveMaximumPeers(options.maximumPeers);
+  const interpolation = { ...(options.interpolation ?? {}) };
+  const prototypes = makePrototypes();
+  const peers = new Map<string, Peer>();
+  const root = new THREE.Group();
+  root.name = "realtime-scene-peers";
+  root.userData.kind = "remote-presence";
+  let disposed = false;
+
+  function add(snapshot: EntityPoseSnapshot): Peer {
+    const visual = cloneVisual(snapshot, prototypes);
+    const peerRoot = new THREE.Group();
+    // IDs deliberately stay in the private Map, not names/userData inspected by render/debug tooling.
+    peerRoot.name = "remote-peer";
+    peerRoot.userData.entity = snapshot.entity;
+    peerRoot.userData.kind = snapshot.kind;
+    peerRoot.userData.forwardAxis = "-Z";
+    peerRoot.add(visualRoot(visual));
+    root.add(peerRoot);
+    const peer: Peer = {
+      id: scenePeerId(snapshot),
+      root: peerRoot,
+      buffer: new PoseInterpolationBuffer(interpolation),
+      visual,
+      kind: snapshot.kind,
+      spaceKey: poseSpaceKey(snapshot.pose),
+      latest: snapshot,
+      lastSequence: -1,
+      lastTimestampMs: -1,
+    };
+    peers.set(peer.id, peer);
+    return peer;
+  }
+
+  function apply(peer: Peer, sample: BufferedPoseSample): boolean {
+    const pose = sample.pose;
+    let visualScale: number;
+    if (pose.space === "geographic") {
+      const projected = options.project(pose.lat, pose.lng);
+      const ground = options.groundAt(pose.lat, pose.lng);
+      if (
+        projected.length < 2 ||
+        !Number.isFinite(projected[0]) ||
+        !Number.isFinite(projected[1]) ||
+        !Number.isFinite(ground)
+      ) {
+        peer.root.visible = false;
+        return false;
+      }
+      peer.root.position.set(
+        projected[0],
+        ground + pose.altitudeM * geographicScale,
+        projected[1],
+      );
+      visualScale = geographicVisualScale;
+    } else {
+      peer.root.position.set(pose.xM * localScale, pose.yM * localScale, pose.zM * localScale);
+      visualScale = localVisualScale;
+    }
+    peer.root.visible = true;
+    peer.root.scale.setScalar(visualScale);
+    peer.root.rotation.order = "YXZ";
+    peer.root.rotation.set(
+      THREE.MathUtils.degToRad(pose.pitchDeg),
+      -THREE.MathUtils.degToRad(pose.headingDeg),
+      0,
+    );
+    if (peer.visual.kind === "model-x") {
+      const latest = peer.latest.entity === "vehicle" ? peer.latest : null;
+      if (latest) {
+        setModelXSteering(peer.visual.rig, latest.steering * 0.62);
+        // Simulation publishes positive travelled radians; the -Z-forward
+        // wheel geometry rolls forward with negative local-X rotation.
+        setModelXWheelRotation(peer.visual.rig, -latest.wheelRadians);
+      }
+    } else {
+      animateActor(peer.visual, sample);
+    }
+    return true;
+  }
+
+  return {
+    root,
+    upsert(snapshot) {
+      if (disposed || !isEntityPoseSnapshot(snapshot)) return false;
+      const id = scenePeerId(snapshot);
+      let peer = peers.get(id);
+      if (!peer) {
+        if (peers.size >= limit) return false;
+        peer = add(snapshot);
+      } else if (
+        snapshot.sequence <= peer.lastSequence ||
+        snapshot.timestampMs <= peer.lastTimestampMs
+      ) {
+        return false;
+      }
+
+      if (peer.kind !== snapshot.kind) replaceVisual(peer, snapshot, prototypes);
+      const nextSpaceKey = poseSpaceKey(snapshot.pose);
+      if (nextSpaceKey !== peer.spaceKey) {
+        // Interest boundaries are not continuous coordinate frames. Resetting
+        // prevents a floor/city switch from interpolating through empty space.
+        peer.buffer.clear();
+        peer.spaceKey = nextSpaceKey;
+      }
+      if (!peer.buffer.push(snapshot)) {
+        // A failed first push must not consume bounded capacity.
+        if (peer.lastSequence < 0) {
+          peer.root.removeFromParent();
+          peers.delete(id);
+        }
+        return false;
+      }
+      peer.latest = snapshot;
+      peer.lastSequence = snapshot.sequence;
+      peer.lastTimestampMs = snapshot.timestampMs;
+      peer.root.userData.entity = snapshot.entity;
+      peer.root.userData.kind = snapshot.kind;
+      return true;
+    },
+    remove(id) {
+      if (disposed) return false;
+      const peer = peers.get(id);
+      if (!peer) return false;
+      peer.root.removeFromParent();
+      peer.buffer.clear();
+      peers.delete(id);
+      return true;
+    },
+    tick(serverTimeMs) {
+      if (disposed || !Number.isFinite(serverTimeMs)) return 0;
+      let updated = 0;
+      for (const peer of peers.values()) {
+        const sample = peer.buffer.sample(serverTimeMs);
+        if (sample && apply(peer, sample)) updated += 1;
+      }
+      return updated;
+    },
+    ids: () => [...peers.keys()],
+    count: () => peers.size,
+    clear() {
+      for (const peer of peers.values()) {
+        peer.root.removeFromParent();
+        peer.buffer.clear();
+      }
+      peers.clear();
+    },
+    dispose() {
+      if (disposed) return;
+      for (const peer of peers.values()) peer.root.removeFromParent();
+      peers.clear();
+      root.removeFromParent();
+      disposeHumanoid(prototypes.humanoid);
+      disposeDog(prototypes.dog);
+      disposeCrow(prototypes.crow);
+      disposeModelX(prototypes.vehicle);
+      disposed = true;
+    },
+  };
+}
diff --git a/src/test/presenceIndicator.test.ts b/src/test/presenceIndicator.test.ts
new file mode 100644
index 0000000..07042ba
--- /dev/null
+++ b/src/test/presenceIndicator.test.ts
@@ -0,0 +1,114 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import { createPresenceIndicator } from "../realtime/index.ts";
+
+type Listener = () => void;
+
+class FakeElement {
+  readonly children: FakeElement[] = [];
+  readonly attributes = new Map<string, string>();
+  readonly listeners = new Map<string, Listener[]>();
+  parentElement: FakeElement | null = null;
+  className = "";
+  textContent = "";
+  hidden = false;
+  type = "";
+  readonly ownerDocument: FakeDocument;
+  constructor(ownerDocument: FakeDocument) { this.ownerDocument = ownerDocument; }
+  append(...nodes: FakeElement[]): void {
+    for (const node of nodes) { node.parentElement = this; this.children.push(node); }
+  }
+  setAttribute(name: string, value: string): void { this.attributes.set(name, value); }
+  getAttribute(name: string): string | null { return this.attributes.get(name) ?? null; }
+  addEventListener(type: string, listener: Listener): void {
+    const listeners = this.listeners.get(type);
+    if (listeners) listeners.push(listener); else this.listeners.set(type, [listener]);
+  }
+  click(): void { for (const listener of this.listeners.get("click") ?? []) listener(); }
+  remove(): void {
+    if (!this.parentElement) return;
+    const index = this.parentElement.children.indexOf(this);
+    if (index >= 0) this.parentElement.children.splice(index, 1);
+    this.parentElement = null;
+  }
+  text(): string { return this.textContent + this.children.map((child) => child.text()).join(""); }
+}
+
+class FakeDocument {
+  createElement(): FakeElement { return new FakeElement(this); }
+}
+
+function setup(onRetry?: () => void) {
+  const document = new FakeDocument();
+  const container = document.createElement();
+  const indicator = createPresenceIndicator({
+    container: container as unknown as HTMLElement,
+    onRetry,
+  });
+  return { container, indicator, root: indicator.root as unknown as FakeElement };
+}
+
+describe("hosted presence indicator", () => {
+  it("is private and off by default with a polite atomic status", () => {
+    const { indicator, root } = setup();
+    assert.deepEqual(indicator.state(), { signedIn: false, connection: "off", nearbyPeerCount: 0 });
+    assert.equal(root.getAttribute("data-presence"), "off");
+    const status = root.children[0]!;
+    assert.equal(status.getAttribute("role"), "status");
+    assert.equal(status.getAttribute("aria-live"), "polite");
+    assert.equal(status.getAttribute("aria-atomic"), "true");
+    assert.equal(root.text(), "Hosted presence is off.Retry presence");
+    assert.equal(root.children[1]?.hidden, true);
+  });
+
+  it("renders only signed-in connection state and aggregate nearby count", () => {
+    const { indicator, root } = setup();
+    const source = { signedIn: true, connection: "live" as const, nearbyPeerCount: 7 };
+    assert.deepEqual(indicator.update(source), source);
+    source.nearbyPeerCount = 99;
+    assert.deepEqual(indicator.state(), { signedIn: true, connection: "live", nearbyPeerCount: 7 });
+    assert.match(root.text(), /live · 7 nearby peers/);
+    assert.doesNotMatch(root.text(), /actor|token|display name/i);
+
+    indicator.update({ signedIn: false, connection: "live", nearbyPeerCount: 400 });
+    assert.deepEqual(indicator.state(), { signedIn: false, connection: "off", nearbyPeerCount: 0 });
+    assert.equal(root.text(), "Hosted presence is off.Retry presence");
+  });
+
+  it("shows retry only while signed-in offline/reconnecting and emits intent only", () => {
+    let retries = 0;
+    const { indicator, root } = setup(() => { retries += 1; });
+    const retry = root.children[1]!;
+    retry.click();
+    assert.equal(retries, 0);
+    indicator.update({ signedIn: true, connection: "connecting", nearbyPeerCount: 0 });
+    assert.equal(retry.hidden, true);
+    indicator.update({ signedIn: true, connection: "reconnecting", nearbyPeerCount: 12 });
+    assert.equal(retry.hidden, false);
+    assert.match(root.text(), /reconnecting/);
+    assert.doesNotMatch(root.text(), /12/);
+    retry.click();
+    indicator.update({ signedIn: true, connection: "offline", nearbyPeerCount: 0 });
+    retry.click();
+    assert.equal(retries, 2);
+    assert.equal("fetch" in indicator, false);
+  });
+
+  it("rejects invalid aggregate state and disposes idempotently", () => {
+    const { container, indicator, root } = setup(() => assert.fail("disposed retry fired"));
+    assert.throws(() => indicator.update({ signedIn: true, connection: "live", nearbyPeerCount: -1 }), /integer/);
+    assert.throws(() => indicator.update({
+      signedIn: true, connection: "secret" as "live", nearbyPeerCount: 0,
+    }), /connection/);
+    indicator.update({ signedIn: true, connection: "offline", nearbyPeerCount: 0 });
+    indicator.dispose();
+    indicator.dispose();
+    root.children[1]!.click();
+    assert.equal(container.children.length, 0);
+    assert.deepEqual(indicator.state(), { signedIn: false, connection: "off", nearbyPeerCount: 0 });
+    assert.deepEqual(
+      indicator.update({ signedIn: true, connection: "live", nearbyPeerCount: 2 }),
+      { signedIn: false, connection: "off", nearbyPeerCount: 0 },
+    );
+  });
+});
diff --git a/src/test/realtimeClient.test.ts b/src/test/realtimeClient.test.ts
new file mode 100644
index 0000000..9f38f73
--- /dev/null
+++ b/src/test/realtimeClient.test.ts
@@ -0,0 +1,412 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import {
+  createRealtimeClient,
+  type ActorPoseSnapshot,
+  type RealtimeClientTimers,
+} from "../realtime/index.ts";
+
+const OFFICE = { kind: "office" as const, officeId: "hq" };
+
+function actor(sequence = 0, xM = 0): ActorPoseSnapshot {
+  return {
+    entity: "actor",
+    actorId: "actor-1",
+    kind: "humanoid",
+    sequence,
+    timestampMs: 1_765_000_000_000 + sequence,
+    pose: {
+      space: "local",
+      cell: OFFICE,
+      xM,
+      yM: 0,
+      zM: 0,
+      headingDeg: 0,
+      pitchDeg: 0,
+    },
+    velocity: { xMps: 0, yMps: 0, zMps: 0, yawDegPerSec: 0 },
+  };
+}
+
+function joinGrant(requestId: string) {
+  return {
+    type: "join-grant",
+    protocolVersion: 1,
+    requestId,
+    sessionId: "session-1",
+    actorId: "actor-1",
+    role: "member",
+    serverEpoch: "epoch-1",
+    serverTimeMs: 1_765_000_000_000,
+    nextSequence: 4,
+    resumeToken: "resume-secret",
+    interests: [OFFICE],
+    initial: [],
+  };
+}
+
+function resumeGrant(requestId: string, interests = [OFFICE]) {
+  return {
+    type: "resume-grant",
+    protocolVersion: 1,
+    requestId,
+    sessionId: "session-1",
+    serverEpoch: "epoch-1",
+    serverTimeMs: 1_765_000_000_001,
+    nextSequence: 5,
+    resumeToken: "rotated-resume-secret",
+    continuous: true,
+    snapshot: [],
+    interests,
+  };
+}
+
+function jsonResponse(value: unknown, status = 200): Response {
+  return new Response(JSON.stringify(value), {
+    status,
+    headers: { "Content-Type": "application/json" },
+  });
+}
+
+class FakeTimers implements RealtimeClientTimers {
+  readonly pending = new Map<number, { callback: () => void; delay: number }>();
+  private next = 1;
+
+  setTimeout(callback: () => void, delay: number): ReturnType<typeof setTimeout> {
+    const id = this.next++;
+    this.pending.set(id, { callback, delay });
+    return id as unknown as ReturnType<typeof setTimeout>;
+  }
+
+  clearTimeout(handle: ReturnType<typeof setTimeout>): void {
+    this.pending.delete(handle as unknown as number);
+  }
+
+  runFirst(): number {
+    const entry = this.pending.entries().next().value as [number, { callback: () => void; delay: number }] | undefined;
+    if (!entry) throw new Error("no timer pending");
+    this.pending.delete(entry[0]);
+    entry[1].callback();
+    return entry[1].delay;
+  }
+}
+
+async function settle(): Promise<void> {
+  await Promise.resolve();
+  await Promise.resolve();
+  await new Promise<void>((resolve) => globalThis.setTimeout(resolve, 0));
+}
+
+describe("realtime browser client", () => {
+  it("uses memory-only bearer auth, same-origin credentials, and validates join identity", async () => {
+    const calls: Array<{ url: string; init: RequestInit; body: Record<string, unknown> }> = [];
+    const fetcher: typeof fetch = async (input, init = {}) => {
+      const body = JSON.parse(String(init.body)) as Record<string, unknown>;
+      calls.push({ url: String(input), init, body });
+      return jsonResponse(joinGrant(String(body.requestId)));
+    };
+    const client = createRealtimeClient({ actorId: "actor-1", accessToken: "access-secret", fetch: fetcher });
+    await client.join(OFFICE, actor());
+
+    assert.equal(calls[0]?.url, "/api/v1/realtime/join");
+    assert.equal(calls[0]?.init.credentials, "same-origin");
+    assert.equal(calls[0]?.init.cache, "no-store");
+    assert.equal(new Headers(calls[0]?.init.headers).get("Authorization"), "Bearer access-secret");
+    assert.equal(calls[0]?.url.includes("access-secret"), false);
+    assert.equal(JSON.stringify(client.state()).includes("secret"), false);
+    assert.throws(() => client.publishPose({
+      ...actor(), velocity: { ...actor().velocity, xMps: Number.NaN },
+    }), /invalid/);
+  });
+
+  it("uses an HttpOnly same-origin session when no browser-readable bearer exists", async () => {
+    const calls: RequestInit[] = [];
+    const fetcher: typeof fetch = async (_input, init = {}) => {
+      const body = JSON.parse(String(init.body)) as Record<string, unknown>;
+      calls.push(init);
+      return jsonResponse(joinGrant(String(body.requestId)), 201);
+    };
+    const client = createRealtimeClient({ actorId: "actor-1", fetch: fetcher });
+    await client.join(OFFICE, actor());
+
+    assert.equal(new Headers(calls[0]?.headers).has("authorization"), false);
+    assert.equal(calls[0]?.credentials, "same-origin");
+  });
+
+  it("accepts one authenticated request adapter without retaining its credential", async () => {
+    let used = false;
+    const authenticatedFetch: typeof fetch = async (_input, init = {}) => {
+      used = true;
+      const body = JSON.parse(String(init.body)) as Record<string, unknown>;
+      return jsonResponse(joinGrant(String(body.requestId)), 201);
+    };
+    const client = createRealtimeClient({ actorId: "actor-1", authenticatedFetch });
+    await client.join(OFFICE, actor());
+    assert.equal(used, true);
+    assert.throws(
+      () => createRealtimeClient({ actorId: "actor-1", authenticatedFetch, fetch }),
+      /either fetch or authenticatedFetch/,
+    );
+  });
+
+  it("coalesces poses at 10-15Hz and assigns monotonic client sequences", async () => {
+    const timers = new FakeTimers();
+    const poses: ActorPoseSnapshot[] = [];
+    const fetcher: typeof fetch = async (input, init = {}) => {
+      const body = JSON.parse(String(init.body)) as Record<string, unknown>;
+      if (String(input).endsWith("/join")) return jsonResponse(joinGrant(String(body.requestId)));
+      if (String(input).endsWith("/pose")) {
+        poses.push(body.snapshot as ActorPoseSnapshot);
+        return new Response(null, { status: 204 });
+      }
+      return new Response(null, { status: 204 });
+    };
+    const client = createRealtimeClient({
+      actorId: "actor-1", accessToken: "token", fetch: fetcher, timers, sendIntervalMs: 80,
+    });
+    await client.join(OFFICE, actor());
+    client.publishPose(actor(0, 1));
+    client.publishPose(actor(0, 2));
+    assert.equal(timers.pending.size, 1);
+    assert.equal(timers.runFirst(), 80);
+    await settle();
+    assert.equal(poses.length, 1);
+    assert.equal(poses[0]?.pose.space === "local" ? poses[0].pose.xM : -1, 2);
+    assert.equal(poses[0]?.sequence, 5, "the superseding pose keeps the later monotonic sequence");
+
+    client.publishPose(actor(0, 3));
+    timers.runFirst();
+    await settle();
+    assert.equal(poses[1]?.sequence, 6);
+    assert.throws(
+      () => createRealtimeClient({ actorId: "actor-1", accessToken: "token", fetch: fetcher, sendIntervalMs: 50 }),
+      /66\.67ms/,
+    );
+  });
+
+  it("holds a pose until a stream resume delivers the rotated credential", async () => {
+    const timers = new FakeTimers();
+    const poseTokens: unknown[] = [];
+    const stream: { current: ReadableStreamDefaultController<Uint8Array> | null } = { current: null };
+    const fetcher: typeof fetch = async (input, init = {}) => {
+      const body = JSON.parse(String(init.body)) as Record<string, unknown>;
+      if (String(input).endsWith("/join")) return jsonResponse(joinGrant(String(body.requestId)));
+      if (String(input).endsWith("/events")) {
+        return new Response(new ReadableStream<Uint8Array>({ start(controller) { stream.current = controller; } }), {
+          headers: { "Content-Type": "text/event-stream" },
+        });
+      }
+      if (String(input).endsWith("/pose")) {
+        poseTokens.push(body.token);
+        return new Response(null, { status: 202 });
+      }
+      return new Response(null, { status: 204 });
+    };
+    const client = createRealtimeClient({
+      actorId: "actor-1", accessToken: "token", fetch: fetcher, timers,
+    });
+    client.subscribe({});
+    await client.join(OFFICE, actor());
+    await settle();
+    client.publishPose(actor(0, 1));
+    timers.runFirst();
+    await settle();
+    assert.equal(poseTokens.length, 0);
+
+    stream.current?.enqueue(new TextEncoder().encode(
+      `data: ${JSON.stringify(resumeGrant("stream-resume"))}\n\n`,
+    ));
+    await settle();
+    timers.runFirst();
+    await settle();
+    assert.deepEqual(poseTokens, ["rotated-resume-secret"]);
+    await client.dispose();
+  });
+
+  it("parses validated POST SSE events, rejects malformed input, and reconnects with backoff", async () => {
+    const timers = new FakeTimers();
+    const malformed: string[] = [];
+    const messages: string[] = [];
+    let streamCalls = 0;
+    const fetcher: typeof fetch = async (input, init = {}) => {
+      const body = JSON.parse(String(init.body)) as Record<string, unknown>;
+      if (String(input).endsWith("/join")) return jsonResponse(joinGrant(String(body.requestId)));
+      if (String(input).endsWith("/events")) {
+        streamCalls += 1;
+        const delta = {
+          type: "pose-delta", protocolVersion: 1, serverEpoch: "epoch-1", sequence: 4,
+          timestampMs: 1_765_000_000_004, updates: [], removedEntityIds: [],
+        };
+        return new Response(`data: not-json\n\ndata: ${JSON.stringify(delta)}\n\n`, {
+          headers: { "Content-Type": "text/event-stream" },
+        });
+      }
+      return new Response(null, { status: 204 });
+    };
+    const client = createRealtimeClient({
+      actorId: "actor-1",
+      accessToken: "token",
+      fetch: fetcher,
+      timers,
+      reconnectBaseMs: 200,
+      reconnectMaximumMs: 2_000,
+      reconnectJitter: 0,
+      random: () => 0.5,
+    });
+    client.subscribe({
+      onMalformedEvent: (error) => malformed.push(error.message),
+      onMessage: (message) => messages.push(message.type),
+    });
+    await client.join(OFFICE, actor());
+    await settle();
+    assert.deepEqual(messages, ["pose-delta"]);
+    assert.equal(malformed.length, 1);
+    assert.equal(client.state().status, "reconnecting");
+    assert.equal(timers.runFirst(), 200);
+    await settle();
+    assert.equal(streamCalls, 2);
+  });
+
+  it("moves interest through resume and aborts streaming before leave/dispose", async () => {
+    const requests: Array<{ url: string; body: Record<string, unknown>; signal: AbortSignal | null }> = [];
+    const fetcher: typeof fetch = async (input, init = {}) => {
+      const body = JSON.parse(String(init.body)) as Record<string, unknown>;
+      requests.push({ url: String(input), body, signal: init.signal ?? null });
+      if (String(input).endsWith("/join")) {
+        if (body.type === "resume-request") return jsonResponse(resumeGrant(String(body.requestId)));
+        return jsonResponse(joinGrant(String(body.requestId)));
+      }
+      if (String(input).endsWith("/events")) {
+        return new Response(new ReadableStream({ start() { /* held open until aborted */ } }), {
+          headers: { "Content-Type": "text/event-stream" },
+        });
+      }
+      return new Response(null, { status: 204 });
+    };
+    const client = createRealtimeClient({ actorId: "actor-1", accessToken: "token", fetch: fetcher });
+    client.subscribe({});
+    await client.join(OFFICE, actor());
+    await settle();
+    const firstStream = requests.find((request) => request.url.endsWith("/events"));
+    assert.equal(firstStream?.signal?.aborted, false);
+
+    const moved = { kind: "floor" as const, officeId: "hq", floorId: "two" };
+    await client.moveInterest(moved);
+    assert.deepEqual(client.state().interests, [moved]);
+    assert.equal(firstStream?.signal?.aborted, true);
+    await client.dispose();
+    assert.equal(client.state().status, "disposed");
+    assert.equal(requests.filter((request) => request.url.endsWith("/leave")).length, 1);
+    assert.equal(JSON.stringify(client.state()).includes("resume-secret"), false);
+  });
+
+  it("closes the old-cell stream when an interest handoff fails", async () => {
+    const requests: Array<{ url: string; signal: AbortSignal | null }> = [];
+    let resumes = 0;
+    const fetcher: typeof fetch = async (input, init = {}) => {
+      const url = String(input);
+      const body = JSON.parse(String(init.body)) as Record<string, unknown>;
+      requests.push({ url, signal: init.signal ?? null });
+      if (url.endsWith("/join") && body.type === "resume-request") {
+        resumes += 1;
+        return jsonResponse({ error: "interest" }, 409);
+      }
+      if (url.endsWith("/join")) return jsonResponse(joinGrant(String(body.requestId)));
+      if (url.endsWith("/events")) {
+        return new Response(new ReadableStream({ start() { /* held until abort */ } }), {
+          headers: { "Content-Type": "text/event-stream" },
+        });
+      }
+      return new Response(null, { status: 204 });
+    };
+    const client = createRealtimeClient({ actorId: "actor-1", accessToken: "token", fetch: fetcher });
+    client.subscribe({});
+    await client.join(OFFICE, actor());
+    await settle();
+    const oldStream = requests.find((request) => request.url.endsWith("/events"));
+    await assert.rejects(
+      client.moveInterest({ kind: "city", cityId: "bay-area" }),
+      /request failed/,
+    );
+    assert.equal(resumes, 1);
+    assert.equal(oldStream?.signal?.aborted, true);
+    assert.equal(client.state().status, "joined");
+    assert.deepEqual(client.state().interests, [OFFICE]);
+    await client.dispose();
+  });
+
+  it("closes the old-cell stream before a slow interest handoff settles", async () => {
+    const requests: Array<{ url: string; signal: AbortSignal | null }> = [];
+    let finishResume: ((response: Response) => void) | null = null;
+    let resumeRequestId = "";
+    const fetcher: typeof fetch = async (input, init = {}) => {
+      const url = String(input);
+      const body = JSON.parse(String(init.body)) as Record<string, unknown>;
+      requests.push({ url, signal: init.signal ?? null });
+      if (url.endsWith("/join") && body.type === "resume-request") {
+        resumeRequestId = String(body.requestId);
+        return new Promise<Response>((resolve) => { finishResume = resolve; });
+      }
+      if (url.endsWith("/join")) return jsonResponse(joinGrant(String(body.requestId)));
+      if (url.endsWith("/events")) {
+        return new Response(new ReadableStream({ start() { /* held until abort */ } }), {
+          headers: { "Content-Type": "text/event-stream" },
+        });
+      }
+      return new Response(null, { status: 204 });
+    };
+    const client = createRealtimeClient({ actorId: "actor-1", fetch: fetcher });
+    client.subscribe({});
+    await client.join(OFFICE, actor());
+    await settle();
+    const oldStream = requests.find((request) => request.url.endsWith("/events"));
+    const moved = { kind: "city" as const, cityId: "bay-area" as const };
+    const handoff = client.moveInterest(moved);
+    assert.equal(oldStream?.signal?.aborted, true, "privacy-cell stream closes before HTTP completes");
+    assert.ok(finishResume);
+    (finishResume as (response: Response) => void)(jsonResponse(resumeGrant(resumeRequestId)));
+    await handoff;
+    assert.deepEqual(client.state().interests, [moved]);
+    await client.dispose();
+  });
+
+  it("lets only the newest concurrent join commit client state", async () => {
+    const pending: Array<{
+      body: Record<string, unknown>;
+      resolve: (response: Response) => void;
+    }> = [];
+    const leaves: Record<string, unknown>[] = [];
+    const fetcher: typeof fetch = async (input, init = {}) => {
+      const body = JSON.parse(String(init.body)) as Record<string, unknown>;
+      if (String(input).endsWith("/leave")) {
+        leaves.push(body);
+        return new Response(null, { status: 204 });
+      }
+      return new Promise<Response>((resolve) => pending.push({ body, resolve }));
+    };
+    const client = createRealtimeClient({ actorId: "actor-1", fetch: fetcher });
+    const first = client.join(OFFICE, actor());
+    const latestInterest = { kind: "city" as const, cityId: "socal" as const };
+    const latest = client.join(latestInterest, actor());
+    const secondRequest = pending[1];
+    assert.ok(secondRequest);
+    secondRequest.resolve(jsonResponse({
+      ...joinGrant(String(secondRequest.body.requestId)),
+      sessionId: "session-latest",
+      interests: [latestInterest],
+    }));
+    await latest;
+    const firstRequest = pending[0];
+    assert.ok(firstRequest);
+    firstRequest.resolve(jsonResponse({
+      ...joinGrant(String(firstRequest.body.requestId)),
+      sessionId: "session-stale",
+    }));
+    await assert.rejects(first, /superseded/);
+    await settle();
+    assert.equal(client.state().sessionId, "session-latest");
+    assert.deepEqual(client.state().interests, [latestInterest]);
+    assert.deepEqual(leaves, [{ sessionId: "session-stale", token: "resume-secret" }]);
+    await client.dispose();
+  });
+});
diff --git a/src/test/scenePeers.test.ts b/src/test/scenePeers.test.ts
new file mode 100644
index 0000000..1a20abc
--- /dev/null
+++ b/src/test/scenePeers.test.ts
@@ -0,0 +1,173 @@
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import * as THREE from "three";
+import {
+  createScenePeers,
+  scenePeerId,
+  type ActorPoseSnapshot,
+  type EntityPoseSnapshot,
+  type InterestCell,
+  type ScenePeersOptions,
+  type VehiclePoseSnapshot,
+} from "../realtime/index.ts";
+
+const VELOCITY = { xMps: 0, yMps: 0, zMps: 0, yawDegPerSec: 0 };
+
+function actor(
+  id: string,
+  kind: "humanoid" | "dog" | "crow",
+  sequence: number,
+  timestampMs: number,
+  xM: number,
+  cell: InterestCell = { kind: "floor", officeId: "hq", floorId: "one" },
+): ActorPoseSnapshot {
+  if (cell.kind === "california-tile") throw new Error("local poses cannot use tile cells");
+  return {
+    entity: "actor",
+    actorId: id,
+    kind,
+    sequence,
+    timestampMs,
+    pose: { space: "local", cell, xM, yM: 2, zM: 3, headingDeg: 0, pitchDeg: 0 },
+    velocity: { ...VELOCITY, xMps: 2 },
+  };
+}
+
+function vehicle(sequence: number, timestampMs: number, lat: number): VehiclePoseSnapshot {
+  return {
+    entity: "vehicle",
+    vehicleId: "ev-1",
+    kind: "model-x",
+    driverActorId: null,
+    sequence,
+    timestampMs,
+    pose: { space: "geographic", lat, lng: -120, altitudeM: 10, headingDeg: 90, pitchDeg: 2 },
+    velocity: { ...VELOCITY, xMps: 20 },
+    steering: 0.4,
+    wheelRadians: 3,
+  };
+}
+
+function fixture(over: Partial<ScenePeersOptions> = {}) {
+  return createScenePeers({
+    project: (lat, lng) => [lng * 2, -lat * 3],
+    groundAt: () => 5,
+    geographicSceneUnitsPerMetre: 0.1,
+    localSceneUnitsPerMetre: 1,
+    interpolation: { interpolationDelayMs: 0, maximumExtrapolationMs: 0 },
+    ...over,
+  });
+}
+
+function mesh(root: THREE.Object3D, name: string): THREE.Mesh {
+  const found = root.getObjectByName(name);
+  if (!(found instanceof THREE.Mesh)) throw new Error(`missing mesh ${name}`);
+  return found;
+}
+
+describe("remote scene peers", () => {
+  it("maps local poses directly and keeps one identity-free stable root per peer", () => {
+    const peers = fixture();
+    const first = actor("private-member-id", "humanoid", 1, 1_000, 1);
+    assert.equal(peers.upsert(first), true);
+    assert.deepEqual(peers.ids(), ["actor:private-member-id"]);
+    assert.equal(peers.count(), 1);
+    const root = peers.root.children[0] as THREE.Group;
+    assert.equal(root.name, "remote-peer");
+    assert.equal(JSON.stringify(root.userData).includes("private-member-id"), false);
+    assert.equal(root.getObjectByName("humanoid.face") instanceof THREE.Mesh, true);
+    assert.equal((mesh(root, "humanoid.face").material as THREE.MeshBasicMaterial).map, null);
+    peers.tick(1_000);
+    assert.deepEqual(root.position.toArray(), [1, 2, 3]);
+
+    assert.equal(peers.upsert(actor("private-member-id", "humanoid", 2, 2_000, 5)), true);
+    peers.tick(1_500);
+    assert.equal(peers.root.children[0], root);
+    assert.equal(root.position.x, 3);
+    assert.equal(peers.upsert(actor("private-member-id", "humanoid", 2, 2_100, 8)), false);
+    peers.dispose();
+  });
+
+  it("resets interpolation at local interest-cell boundaries without replacing the root", () => {
+    const peers = fixture();
+    peers.upsert(actor("a", "dog", 1, 1_000, 0));
+    peers.upsert(actor("a", "dog", 2, 2_000, 10));
+    peers.tick(1_500);
+    const root = peers.root.children[0] as THREE.Group;
+    assert.equal(root.position.x, 5);
+    const nextCell = { kind: "floor", officeId: "hq", floorId: "two" } as const;
+    assert.equal(peers.upsert(actor("a", "dog", 3, 3_000, 100, nextCell)), true);
+    peers.tick(2_500);
+    assert.equal(peers.root.children[0], root);
+    assert.equal(root.position.x, 100, "new coordinate frame is held, never mixed with old floor");
+    peers.dispose();
+  });
+
+  it("projects geographic vehicles onto caller terrain and renders a generic black EV", () => {
+    const peers = fixture();
+    const snapshot = vehicle(1, 1_000, 34);
+    assert.equal(scenePeerId(snapshot), "vehicle:ev-1");
+    peers.upsert(snapshot);
+    assert.equal(peers.tick(1_000), 1);
+    const root = peers.root.children[0] as THREE.Group;
+    assert.deepEqual(root.position.toArray(), [-240, 6, -102]);
+    assert.equal(root.scale.x, 0.1);
+    assert.ok(Math.abs(root.rotation.y + Math.PI / 2) < 1e-12);
+    assert.equal(root.getObjectByName("generic-black-ev")?.userData.vehicleModel, "generic-black-ev");
+    assert.equal(root.getObjectByName("frontLeft.spin")?.rotation.x, -snapshot.wheelRadians);
+    const paint = root.getObjectByName("model-x.body:model-x.paint") as THREE.Mesh;
+    assert.equal((paint.material as THREE.MeshStandardMaterial).color.getHex(), 0x050607);
+    peers.dispose();
+  });
+
+  it("shares prototype resources while kind changes retain the entity root", () => {
+    const peers = fixture();
+    peers.upsert(actor("one", "humanoid", 1, 1_000, 0));
+    peers.upsert(actor("two", "humanoid", 1, 1_000, 2));
+    const one = peers.root.children[0] as THREE.Group;
+    const two = peers.root.children[1] as THREE.Group;
+    assert.equal(mesh(one, "humanoid.chest").geometry, mesh(two, "humanoid.chest").geometry);
+    assert.equal(mesh(one, "humanoid.chest").material, mesh(two, "humanoid.chest").material);
+    const oldRig = one.children[0];
+    peers.upsert(actor("one", "crow", 2, 2_000, 1));
+    peers.tick(2_000);
+    assert.equal(peers.root.children[0], one);
+    assert.equal(oldRig?.parent, null);
+    assert.ok(one.getObjectByName("crow"));
+    peers.dispose();
+  });
+
+  it("bounds nearby peers and removes/clears without prematurely disposing shared assets", () => {
+    const peers = fixture({ maximumPeers: 2 });
+    peers.upsert(actor("one", "humanoid", 1, 1_000, 0));
+    peers.upsert(actor("two", "dog", 1, 1_000, 1));
+    assert.equal(peers.upsert(actor("three", "crow", 1, 1_000, 2)), false);
+    const firstPeer = peers.root.children[0];
+    if (!firstPeer) throw new Error("missing first peer");
+    const geometry = mesh(firstPeer, "humanoid.chest").geometry;
+    let disposals = 0;
+    geometry.addEventListener("dispose", () => disposals++);
+    assert.equal(peers.remove("actor:one"), true);
+    assert.equal(peers.remove("actor:one"), false);
+    assert.equal(disposals, 0, "removing one clone cannot release shared prototype resources");
+    peers.clear();
+    assert.equal(peers.count(), 0);
+    assert.deepEqual(peers.ids(), []);
+    assert.equal(disposals, 0);
+    peers.dispose();
+    peers.dispose();
+    assert.equal(disposals, 1);
+    assert.equal(peers.upsert(actor("late", "humanoid", 1, 1_000, 0)), false);
+  });
+
+  it("contains invalid snapshots and projection failures", () => {
+    const peers = fixture({ project: () => [Number.NaN, 0] });
+    assert.equal(peers.upsert({} as EntityPoseSnapshot), false);
+    peers.upsert(vehicle(1, 1_000, 34));
+    assert.equal(peers.tick(1_000), 0);
+    assert.equal(peers.root.children[0]?.visible, false);
+    assert.equal(peers.tick(Number.NaN), 0);
+    peers.dispose();
+    assert.throws(() => fixture({ maximumPeers: 0 }), RangeError);
+  });
+});