diff --git a/src/live/authz.ts b/src/live/authz.ts
new file mode 100644
index 0000000..10eb470
--- /dev/null
+++ b/src/live/authz.ts
@@ -0,0 +1,90 @@
+/**
+ * Per-event authorization client.
+ *
+ * Checks whether a user has access to a specific event by delegating to
+ * Directus's REST API with the user's cookie. Directus enforces row-level
+ * security; if Directus returns 200 the user has access. If 403, they don't.
+ *
+ * Authorization is checked ONCE at subscribe time. The hot fan-out path has
+ * zero Directus calls — it operates entirely on in-memory subscription state.
+ *
+ * Spec: docs/wiki/synthesis/processor-ws-contract.md §Subscription model
+ */
+
+import type { Config } from '../config/load.js';
+import type { Metrics } from '../shared/types.js';
+import type { Logger } from 'pino';
+
+// ---------------------------------------------------------------------------
+// Types
+// ---------------------------------------------------------------------------
+
+/**
+ * Result of an authorization check.
+ * `allowed: true` → user may subscribe to the topic.
+ * `allowed: false` → user is rejected; `reason` tells the client why.
+ */
+export type AuthzResult =
+  | { readonly allowed: true }
+  | { readonly allowed: false; readonly reason: 'forbidden' | 'not-found' | 'error' };
+
+export type AuthzClient = {
+  /**
+   * Checks whether the user identified by `cookieHeader` can access
+   * the event with `eventId`.
+   *
+   * Delegates to `GET /items/events/<eventId>?fields=id` with the user's
+   * cookie. Directus's row-level security does the org-membership check.
+   *
+   * Never throws. Returns `{ allowed: false, reason: 'error' }` on any
+   * transient failure.
+   */
+  readonly canAccessEvent: (
+    cookieHeader: string,
+    eventId: string,
+  ) => Promise<AuthzResult>;
+};
+
+// ---------------------------------------------------------------------------
+// Factory
+// ---------------------------------------------------------------------------
+
+export function createAuthzClient(
+  config: Config,
+  logger: Logger,
+  metrics: Metrics,
+): AuthzClient {
+  async function canAccessEvent(
+    cookieHeader: string,
+    eventId: string,
+  ): Promise<AuthzResult> {
+    const start = performance.now();
+    try {
+      const res = await fetch(
+        `${config.DIRECTUS_BASE_URL}/items/events/${eventId}?fields=id`,
+        {
+          method: 'GET',
+          headers: { cookie: cookieHeader },
+          signal: AbortSignal.timeout(config.DIRECTUS_AUTHZ_TIMEOUT_MS),
+        },
+      );
+
+      if (res.status === 200) return { allowed: true };
+      if (res.status === 403) return { allowed: false, reason: 'forbidden' };
+      if (res.status === 404) return { allowed: false, reason: 'not-found' };
+
+      logger.warn(
+        { status: res.status, eventId },
+        'directus /items/events returned unexpected status',
+      );
+      return { allowed: false, reason: 'error' };
+    } catch (err) {
+      logger.warn({ err, eventId }, 'directus authz call failed');
+      return { allowed: false, reason: 'error' };
+    } finally {
+      metrics.observe('processor_live_authz_latency_ms', performance.now() - start);
+    }
+  }
+
+  return { canAccessEvent };
+}
diff --git a/src/live/protocol.ts b/src/live/protocol.ts
index 49baf67..5758725 100644
--- a/src/live/protocol.ts
+++ b/src/live/protocol.ts
@@ -130,7 +130,9 @@ export type ErrorCode =
   | 'unknown-topic'
   | 'protocol-violation'
   | 'not-implemented'
-  | 'rate-limited';
+  | 'rate-limited'
+  /** Transient server-side error (e.g. Directus authz call failed). Retry. */
+  | 'error';
 
 /**
  * An error response from the server, scoped to a topic or connection-level.
diff --git a/src/live/registry.ts b/src/live/registry.ts
new file mode 100644
index 0000000..dc8ccf7
--- /dev/null
+++ b/src/live/registry.ts
@@ -0,0 +1,324 @@
+/**
+ * Subscription registry — manages the bidirectional mapping between WebSocket
+ * connections and topics, and handles per-event authorization at subscribe time.
+ *
+ * Data structures:
+ *  - connectionTopics: WeakMap<LiveConnection, Set<string>>  (conn → topics)
+ *    WeakMap allows GC cleanup if a connection somehow leaks the onConnectionClose call.
+ *  - topicConnections: Map<string, Set<LiveConnection>>      (topic → conns)
+ *    Standard Map keyed by topic string; cleaned up by onConnectionClose.
+ *
+ * Authorization:
+ *  - Checked ONCE per subscribe, via the authz client (Directus /items/events/<id>).
+ *  - Zero Directus calls in the fan-out hot path.
+ *
+ * Snapshot:
+ *  - Task 1.5.3 sends an empty snapshot with `subscribed`. Task 1.5.5 wires in
+ *    the real snapshot provider to populate the array.
+ *
+ * Spec: docs/wiki/synthesis/processor-ws-contract.md §Subscription model
+ */
+
+import type { Logger } from 'pino';
+import type { Metrics } from '../shared/types.js';
+import type { LiveConnection } from './server.js';
+import { sendOutbound } from './server.js';
+import type { AuthzClient } from './authz.js';
+import type { Config } from '../config/load.js';
+import type { PositionSnapshotEntry } from './protocol.js';
+
+// ---------------------------------------------------------------------------
+// Topic parsing
+// ---------------------------------------------------------------------------
+
+const EVENT_TOPIC_REGEX =
+  /^event:([0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12})$/i;
+
+type ParsedTopic = { readonly kind: 'event'; readonly eventId: string };
+
+function parseTopic(topic: string): ParsedTopic | null {
+  const match = EVENT_TOPIC_REGEX.exec(topic);
+  if (match?.[1]) return { kind: 'event', eventId: match[1] };
+  return null;
+}
+
+// ---------------------------------------------------------------------------
+// Snapshot provider type (injected from task 1.5.5)
+// ---------------------------------------------------------------------------
+
+/**
+ * Pluggable snapshot provider. Task 1.5.3 uses the stub (empty array).
+ * Task 1.5.5 injects the real Postgres-backed provider.
+ */
+export type SnapshotProvider = {
+  readonly forEvent: (eventId: string) => Promise<PositionSnapshotEntry[]>;
+};
+
+const STUB_SNAPSHOT_PROVIDER: SnapshotProvider = {
+  forEvent: () => Promise.resolve([]),
+};
+
+// ---------------------------------------------------------------------------
+// Public interface
+// ---------------------------------------------------------------------------
+
+export type SubscriptionRegistry = {
+  /** Subscribe `conn` to `topic`. Authorizes, then sends `subscribed` or `error`. */
+  readonly subscribe: (
+    conn: LiveConnection,
+    topic: string,
+    correlationId?: string,
+  ) => Promise<void>;
+  /** Unsubscribe `conn` from `topic`. Always sends `unsubscribed` (idempotent). */
+  readonly unsubscribe: (
+    conn: LiveConnection,
+    topic: string,
+    correlationId?: string,
+  ) => void;
+  /** Remove all subscriptions for a closed connection (called on ws close). */
+  readonly onConnectionClose: (conn: LiveConnection) => void;
+  /** Iterates all connections currently subscribed to `topic`. Used by fan-out. */
+  readonly connectionsForTopic: (topic: string) => Iterable<LiveConnection>;
+  /** Iterates all topics the given connection is subscribed to. */
+  readonly topicsForConnection: (conn: LiveConnection) => Iterable<string>;
+  /** Aggregate stats for monitoring and sanity checks. */
+  readonly stats: () => { connections: number; topics: number; subscriptions: number };
+};
+
+// ---------------------------------------------------------------------------
+// Factory
+// ---------------------------------------------------------------------------
+
+export function createSubscriptionRegistry(
+  authzClient: AuthzClient,
+  config: Config,
+  logger: Logger,
+  metrics: Metrics,
+  snapshotProvider: SnapshotProvider = STUB_SNAPSHOT_PROVIDER,
+): SubscriptionRegistry {
+  // conn → Set of topic strings the connection is subscribed to.
+  // WeakMap: if a connection object is somehow not cleaned up via onConnectionClose,
+  // the GC will reclaim the Set when the connection is collected.
+  const connectionTopics = new WeakMap<LiveConnection, Set<string>>();
+
+  // topic string → Set of connections subscribed to that topic.
+  const topicConnections = new Map<string, Set<LiveConnection>>();
+
+  // Total active subscriptions counter (kept in sync with topicConnections).
+  let totalSubscriptions = 0;
+
+  // -------------------------------------------------------------------------
+  // Subscribe
+  // -------------------------------------------------------------------------
+
+  async function subscribe(
+    conn: LiveConnection,
+    topic: string,
+    correlationId?: string,
+  ): Promise<void> {
+    const parsed = parseTopic(topic);
+    if (!parsed) {
+      sendOutbound(
+        conn,
+        {
+          type: 'error',
+          topic,
+          id: correlationId,
+          code: 'unknown-topic',
+          message: 'Unknown topic format. Supported: event:<uuid>',
+        },
+        metrics,
+        config.LIVE_WS_BACKPRESSURE_THRESHOLD_BYTES,
+      );
+      metrics.inc('processor_live_subscribe_attempts_total', { result: 'unknown-topic' });
+      return;
+    }
+
+    // Idempotent: if already subscribed, re-send `subscribed` with a fresh snapshot.
+    const existing = connectionTopics.get(conn);
+    if (existing?.has(topic)) {
+      const snapshot = await fetchSnapshot(parsed.eventId);
+      sendOutbound(
+        conn,
+        { type: 'subscribed', topic, id: correlationId, snapshot },
+        metrics,
+        config.LIVE_WS_BACKPRESSURE_THRESHOLD_BYTES,
+      );
+      // Do not double-count in subscriptions gauge.
+      return;
+    }
+
+    // Authorization check — one Directus call per subscribe.
+    const verdict = await authzClient.canAccessEvent(conn.cookieHeader, parsed.eventId);
+    if (!verdict.allowed) {
+      sendOutbound(
+        conn,
+        {
+          type: 'error',
+          topic,
+          id: correlationId,
+          code: verdict.reason,
+          message: buildForbiddenMessage(verdict.reason),
+        },
+        metrics,
+        config.LIVE_WS_BACKPRESSURE_THRESHOLD_BYTES,
+      );
+      metrics.inc('processor_live_subscribe_attempts_total', { result: verdict.reason });
+      return;
+    }
+
+    // Fetch snapshot (fails open — snapshot failure does not block the subscribe).
+    const snapshot = await fetchSnapshot(parsed.eventId);
+
+    // Insert into both indexes.
+    if (!existing) connectionTopics.set(conn, new Set());
+    connectionTopics.get(conn)!.add(topic);
+
+    if (!topicConnections.has(topic)) topicConnections.set(topic, new Set());
+    topicConnections.get(topic)!.add(conn);
+
+    totalSubscriptions += 1;
+    metrics.observe('processor_live_subscriptions', totalSubscriptions);
+    metrics.inc('processor_live_subscribe_attempts_total', { result: 'success' });
+
+    logger.debug(
+      { connId: conn.id, topic, userId: conn.user.id },
+      'subscribed',
+    );
+
+    sendOutbound(
+      conn,
+      { type: 'subscribed', topic, id: correlationId, snapshot },
+      metrics,
+      config.LIVE_WS_BACKPRESSURE_THRESHOLD_BYTES,
+    );
+  }
+
+  // -------------------------------------------------------------------------
+  // Unsubscribe
+  // -------------------------------------------------------------------------
+
+  function unsubscribe(
+    conn: LiveConnection,
+    topic: string,
+    correlationId?: string,
+  ): void {
+    const topics = connectionTopics.get(conn);
+    const wasSubscribed = topics?.has(topic) ?? false;
+
+    topics?.delete(topic);
+
+    const conns = topicConnections.get(topic);
+    if (conns) {
+      conns.delete(conn);
+      if (conns.size === 0) topicConnections.delete(topic);
+    }
+
+    if (wasSubscribed) {
+      totalSubscriptions -= 1;
+      metrics.observe('processor_live_subscriptions', totalSubscriptions);
+    }
+
+    logger.debug({ connId: conn.id, topic }, 'unsubscribed');
+
+    // Always reply, even if not subscribed (idempotent).
+    sendOutbound(
+      conn,
+      { type: 'unsubscribed', topic, id: correlationId },
+      metrics,
+      config.LIVE_WS_BACKPRESSURE_THRESHOLD_BYTES,
+    );
+  }
+
+  // -------------------------------------------------------------------------
+  // onConnectionClose
+  // -------------------------------------------------------------------------
+
+  function onConnectionClose(conn: LiveConnection): void {
+    const topics = connectionTopics.get(conn);
+    if (!topics) return;
+
+    for (const topic of topics) {
+      const conns = topicConnections.get(topic);
+      if (conns) {
+        conns.delete(conn);
+        if (conns.size === 0) topicConnections.delete(topic);
+      }
+      totalSubscriptions -= 1;
+    }
+
+    connectionTopics.delete(conn);
+    metrics.observe('processor_live_subscriptions', totalSubscriptions);
+
+    logger.debug(
+      { connId: conn.id, removedTopics: topics.size },
+      'connection closed — subscriptions cleaned up',
+    );
+  }
+
+  // -------------------------------------------------------------------------
+  // Query
+  // -------------------------------------------------------------------------
+
+  function connectionsForTopic(topic: string): Iterable<LiveConnection> {
+    return topicConnections.get(topic) ?? new Set<LiveConnection>();
+  }
+
+  function topicsForConnection(conn: LiveConnection): Iterable<string> {
+    return connectionTopics.get(conn) ?? new Set<string>();
+  }
+
+  function stats(): { connections: number; topics: number; subscriptions: number } {
+    return {
+      connections: topicConnections.size > 0
+        ? [...topicConnections.values()].reduce((acc, s) => acc + s.size, 0)
+        : 0,
+      topics: topicConnections.size,
+      subscriptions: totalSubscriptions,
+    };
+  }
+
+  // -------------------------------------------------------------------------
+  // Snapshot helper
+  // -------------------------------------------------------------------------
+
+  async function fetchSnapshot(eventId: string): Promise<PositionSnapshotEntry[]> {
+    const start = performance.now();
+    try {
+      const snapshot = await snapshotProvider.forEvent(eventId);
+      metrics.observe('processor_live_snapshot_query_latency_ms', performance.now() - start);
+      metrics.observe('processor_live_snapshot_size', snapshot.length);
+      return snapshot;
+    } catch (err) {
+      logger.warn(
+        { err, eventId },
+        'snapshot query failed; sending empty snapshot',
+      );
+      return [];
+    }
+  }
+
+  return {
+    subscribe,
+    unsubscribe,
+    onConnectionClose,
+    connectionsForTopic,
+    topicsForConnection,
+    stats,
+  };
+}
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+function buildForbiddenMessage(reason: 'forbidden' | 'not-found' | 'error'): string {
+  switch (reason) {
+    case 'forbidden':
+      return 'User does not have access to this event.';
+    case 'not-found':
+      return 'Event not found.';
+    case 'error':
+      return 'Authorization check failed. Please try again.';
+  }
+}
diff --git a/src/main.ts b/src/main.ts
index fc1a707..99e46c1 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -16,10 +16,12 @@ import { connectRedis, createConsumer } from './core/consumer.js';
 import type { ConsumedRecord } from './core/consumer.js';
 import { createDeviceStateStore } from './core/state.js';
 import { createWriter } from './core/writer.js';
-import { createLiveServer, sendOutbound } from './live/server.js';
+import { createLiveServer } from './live/server.js';
 import type { LiveServer, LiveConnection } from './live/server.js';
 import type { InboundMessage } from './live/protocol.js';
 import { createAuthClient } from './live/auth.js';
+import { createAuthzClient } from './live/authz.js';
+import { createSubscriptionRegistry } from './live/registry.js';
 
 // -------------------------------------------------------------------------
 // Startup: validate config (fail fast on bad env), build logger
@@ -132,29 +134,28 @@ async function main(): Promise<void> {
     return ackIds;
   };
 
-  // 10. Build the live WebSocket server (task 1.5.2 adds auth).
-  //     The stub message handler replies with `error/not-implemented` until
-  //     task 1.5.3 wires in the real subscription-registry handler.
+  // 10. Build the live WebSocket server (tasks 1.5.2 and 1.5.3).
   const authClient = createAuthClient(config, logger, metrics);
+  const authzClient = createAuthzClient(config, logger, metrics);
+  const registry = createSubscriptionRegistry(authzClient, config, logger, metrics);
 
-  const stubMessageHandler = async (
+  const messageHandler = async (
     conn: LiveConnection,
-    _message: InboundMessage,
+    message: InboundMessage,
   ): Promise<void> => {
-    sendOutbound(
-      conn,
-      { type: 'error', code: 'not-implemented' },
-      metrics,
-      config.LIVE_WS_BACKPRESSURE_THRESHOLD_BYTES,
-    );
+    if (message.type === 'subscribe') {
+      await registry.subscribe(conn, message.topic, message.id);
+    } else if (message.type === 'unsubscribe') {
+      registry.unsubscribe(conn, message.topic, message.id);
+    }
   };
 
   const liveServer: LiveServer = createLiveServer(
     config,
     logger,
     metrics,
-    stubMessageHandler,
-    undefined, // onClose: wired in task 1.5.3
+    messageHandler,
+    (conn) => registry.onConnectionClose(conn),
     authClient,
   );
   await liveServer.start();
diff --git a/test/live-authz.test.ts b/test/live-authz.test.ts
new file mode 100644
index 0000000..7fa7dc8
--- /dev/null
+++ b/test/live-authz.test.ts
@@ -0,0 +1,148 @@
+/**
+ * Unit tests for src/live/authz.ts — per-event authorization.
+ *
+ * Covers:
+ *  - canAccessEvent returns { allowed: true } when /items/events/:id returns 200.
+ *  - Returns { allowed: false, reason: 'forbidden' } on 403.
+ *  - Returns { allowed: false, reason: 'not-found' } on 404.
+ *  - Returns { allowed: false, reason: 'error' } on network failure (never throws).
+ *  - Returns { allowed: false, reason: 'error' } on 500.
+ *  - Authz latency histogram is observed on every call.
+ */
+
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import type { Logger } from 'pino';
+import type { Config } from '../src/config/load.js';
+import type { Metrics } from '../src/core/types.js';
+import { createAuthzClient } from '../src/live/authz.js';
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+function makeSilentLogger(): Logger {
+  return {
+    debug: vi.fn(), info: vi.fn(), warn: vi.fn(), error: vi.fn(),
+    fatal: vi.fn(), trace: vi.fn(), child: vi.fn().mockReturnThis(),
+    level: 'silent', silent: vi.fn(),
+  } as unknown as Logger;
+}
+
+type TestMetrics = Metrics & {
+  readonly observeCalls: Array<{ name: string; value: number }>;
+};
+
+function makeMetrics(): TestMetrics {
+  const observeCalls: Array<{ name: string; value: number }> = [];
+  return {
+    observeCalls,
+    inc: vi.fn(),
+    observe(name, value) { observeCalls.push({ name, value }); },
+  };
+}
+
+function makeConfig(): Config {
+  return {
+    NODE_ENV: 'test',
+    INSTANCE_ID: 'test-1',
+    LOG_LEVEL: 'silent',
+    REDIS_URL: 'redis://localhost:6379',
+    POSTGRES_URL: 'postgres://localhost:5432/test',
+    REDIS_TELEMETRY_STREAM: 'telemetry:t',
+    REDIS_CONSUMER_GROUP: 'processor',
+    REDIS_CONSUMER_NAME: 'test-consumer',
+    METRICS_PORT: 0,
+    BATCH_SIZE: 100,
+    BATCH_BLOCK_MS: 500,
+    WRITE_BATCH_SIZE: 50,
+    DEVICE_STATE_LRU_CAP: 10_000,
+    LIVE_WS_PORT: 8081,
+    LIVE_WS_HOST: '0.0.0.0',
+    LIVE_WS_PING_INTERVAL_MS: 30_000,
+    LIVE_WS_DRAIN_TIMEOUT_MS: 5_000,
+    LIVE_WS_BACKPRESSURE_THRESHOLD_BYTES: 1_048_576,
+    DIRECTUS_BASE_URL: 'http://directus.test',
+    DIRECTUS_AUTH_TIMEOUT_MS: 5_000,
+    DIRECTUS_AUTHZ_TIMEOUT_MS: 5_000,
+    LIVE_BROADCAST_GROUP_PREFIX: 'live-broadcast',
+    LIVE_BROADCAST_BATCH_SIZE: 100,
+    LIVE_BROADCAST_BATCH_BLOCK_MS: 1_000,
+    LIVE_DEVICE_EVENT_REFRESH_MS: 30_000,
+  };
+}
+
+const EVENT_ID = 'ada60b3d-b29f-4017-b702-cd6b700f9f6c';
+
+function makeStatusFetch(status: number): typeof fetch {
+  return vi.fn().mockResolvedValue({
+    status,
+    ok: status >= 200 && status < 300,
+    json: () => Promise.resolve({ data: { id: EVENT_ID } }),
+  } as unknown as Response);
+}
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+describe('createAuthzClient.canAccessEvent', () => {
+  let originalFetch: typeof globalThis.fetch;
+
+  beforeEach(() => { originalFetch = globalThis.fetch; });
+  afterEach(() => {
+    globalThis.fetch = originalFetch;
+    vi.restoreAllMocks();
+  });
+
+  it('returns { allowed: true } when Directus returns 200', async () => {
+    globalThis.fetch = makeStatusFetch(200);
+    const client = createAuthzClient(makeConfig(), makeSilentLogger(), makeMetrics());
+    const result = await client.canAccessEvent('cookie=abc', EVENT_ID);
+    expect(result.allowed).toBe(true);
+  });
+
+  it('returns { allowed: false, reason: "forbidden" } on 403', async () => {
+    globalThis.fetch = makeStatusFetch(403);
+    const client = createAuthzClient(makeConfig(), makeSilentLogger(), makeMetrics());
+    const result = await client.canAccessEvent('cookie=abc', EVENT_ID);
+    expect(result.allowed).toBe(false);
+    if (!result.allowed) expect(result.reason).toBe('forbidden');
+  });
+
+  it('returns { allowed: false, reason: "not-found" } on 404', async () => {
+    globalThis.fetch = makeStatusFetch(404);
+    const client = createAuthzClient(makeConfig(), makeSilentLogger(), makeMetrics());
+    const result = await client.canAccessEvent('cookie=abc', EVENT_ID);
+    expect(result.allowed).toBe(false);
+    if (!result.allowed) expect(result.reason).toBe('not-found');
+  });
+
+  it('returns { allowed: false, reason: "error" } on 500', async () => {
+    globalThis.fetch = makeStatusFetch(500);
+    const client = createAuthzClient(makeConfig(), makeSilentLogger(), makeMetrics());
+    const result = await client.canAccessEvent('cookie=abc', EVENT_ID);
+    expect(result.allowed).toBe(false);
+    if (!result.allowed) expect(result.reason).toBe('error');
+  });
+
+  it('returns { allowed: false, reason: "error" } when fetch throws (never throws itself)', async () => {
+    globalThis.fetch = vi.fn().mockRejectedValue(new Error('ECONNREFUSED'));
+    const client = createAuthzClient(makeConfig(), makeSilentLogger(), makeMetrics());
+    const result = await client.canAccessEvent('cookie=abc', EVENT_ID);
+    expect(result.allowed).toBe(false);
+    if (!result.allowed) expect(result.reason).toBe('error');
+  });
+
+  it('observes authz latency on every call', async () => {
+    globalThis.fetch = makeStatusFetch(200);
+    const metrics = makeMetrics();
+    const client = createAuthzClient(makeConfig(), makeSilentLogger(), metrics);
+    await client.canAccessEvent('cookie=abc', EVENT_ID);
+
+    const latencyCalls = metrics.observeCalls.filter(
+      (c) => c.name === 'processor_live_authz_latency_ms',
+    );
+    expect(latencyCalls.length).toBeGreaterThanOrEqual(1);
+    expect(latencyCalls[0]!.value).toBeGreaterThanOrEqual(0);
+  });
+});
diff --git a/test/live-registry.test.ts b/test/live-registry.test.ts
new file mode 100644
index 0000000..fa37e38
--- /dev/null
+++ b/test/live-registry.test.ts
@@ -0,0 +1,345 @@
+/**
+ * Unit tests for src/live/registry.ts — subscription registry.
+ *
+ * The registry is instantiated with a mocked authz client and a mocked
+ * sendOutbound path. LiveConnection objects are synthetic stubs.
+ *
+ * Covers:
+ *  - Subscribe to event:<uuid> with permitted user → `subscribed` reply,
+ *    registry counts go up.
+ *  - Subscribe with forbidden user → `error/forbidden` reply, no registry change.
+ *  - Subscribe to `device:<imei>` → `error/unknown-topic`, no registry change.
+ *  - Subscribe twice to the same topic → idempotent (single subscription,
+ *    subscribed reply each call, gauge does not double-count).
+ *  - Unsubscribe from a topic → `unsubscribed` reply, gauge decrements.
+ *  - Unsubscribe from a topic not subscribed → `unsubscribed` reply (idempotent),
+ *    gauge unchanged.
+ *  - Connection close removes all subscriptions; gauge returns to pre-connection level.
+ *  - connectionsForTopic returns the correct set.
+ *  - topicsForConnection returns the correct set.
+ */
+
+import { describe, it, expect, vi, beforeEach } from 'vitest';
+import type { Logger } from 'pino';
+import type { Config } from '../src/config/load.js';
+import type { Metrics } from '../src/core/types.js';
+import { createSubscriptionRegistry } from '../src/live/registry.js';
+import type { AuthzClient } from '../src/live/authz.js';
+import type { LiveConnection } from '../src/live/server.js';
+import WebSocket from 'ws';
+
+// ---------------------------------------------------------------------------
+// Helpers
+// ---------------------------------------------------------------------------
+
+function makeSilentLogger(): Logger {
+  return {
+    debug: vi.fn(), info: vi.fn(), warn: vi.fn(), error: vi.fn(),
+    fatal: vi.fn(), trace: vi.fn(), child: vi.fn().mockReturnThis(),
+    level: 'silent', silent: vi.fn(),
+  } as unknown as Logger;
+}
+
+type TestMetrics = Metrics & {
+  readonly incCalls: Array<{ name: string; labels?: Record<string, string> }>;
+  readonly observeCalls: Array<{ name: string; value: number }>;
+};
+
+function makeMetrics(): TestMetrics {
+  const incCalls: Array<{ name: string; labels?: Record<string, string> }> = [];
+  const observeCalls: Array<{ name: string; value: number }> = [];
+  return {
+    incCalls,
+    observeCalls,
+    inc(name, labels) { incCalls.push({ name, labels }); },
+    observe(name, value) { observeCalls.push({ name, value }); },
+  };
+}
+
+function makeConfig(): Config {
+  return {
+    NODE_ENV: 'test',
+    INSTANCE_ID: 'test-1',
+    LOG_LEVEL: 'silent',
+    REDIS_URL: 'redis://localhost:6379',
+    POSTGRES_URL: 'postgres://localhost:5432/test',
+    REDIS_TELEMETRY_STREAM: 'telemetry:t',
+    REDIS_CONSUMER_GROUP: 'processor',
+    REDIS_CONSUMER_NAME: 'test-consumer',
+    METRICS_PORT: 0,
+    BATCH_SIZE: 100,
+    BATCH_BLOCK_MS: 500,
+    WRITE_BATCH_SIZE: 50,
+    DEVICE_STATE_LRU_CAP: 10_000,
+    LIVE_WS_PORT: 8081,
+    LIVE_WS_HOST: '0.0.0.0',
+    LIVE_WS_PING_INTERVAL_MS: 30_000,
+    LIVE_WS_DRAIN_TIMEOUT_MS: 5_000,
+    LIVE_WS_BACKPRESSURE_THRESHOLD_BYTES: 1_048_576,
+    DIRECTUS_BASE_URL: 'http://directus.test',
+    DIRECTUS_AUTH_TIMEOUT_MS: 5_000,
+    DIRECTUS_AUTHZ_TIMEOUT_MS: 5_000,
+    LIVE_BROADCAST_GROUP_PREFIX: 'live-broadcast',
+    LIVE_BROADCAST_BATCH_SIZE: 100,
+    LIVE_BROADCAST_BATCH_BLOCK_MS: 1_000,
+    LIVE_DEVICE_EVENT_REFRESH_MS: 30_000,
+  };
+}
+
+const EVENT_ID = 'ada60b3d-b29f-4017-b702-cd6b700f9f6c';
+const EVENT_TOPIC = `event:${EVENT_ID}`;
+
+/**
+ * Creates a synthetic LiveConnection stub that captures sent messages.
+ */
+function makeConn(id = 'conn-1'): LiveConnection & { sentMessages: unknown[] } {
+  const sentMessages: unknown[] = [];
+  const ws = {
+    readyState: WebSocket.OPEN,
+    bufferedAmount: 0,
+    send: vi.fn((data: string) => { sentMessages.push(JSON.parse(data)); }),
+    close: vi.fn(),
+  } as unknown as WebSocket;
+
+  return {
+    id,
+    ws,
+    remoteAddr: '127.0.0.1',
+    openedAt: new Date(),
+    lastSeenAt: new Date(),
+    user: {
+      id: 'user-ada60b3d',
+      email: 'test@example.com',
+      role: null,
+      first_name: 'Test',
+      last_name: 'User',
+    },
+    cookieHeader: 'session=valid',
+    sentMessages,
+  };
+}
+
+function makeAllowedAuthzClient(): AuthzClient {
+  return {
+    canAccessEvent: vi.fn().mockResolvedValue({ allowed: true }),
+  };
+}
+
+function makeForbiddenAuthzClient(): AuthzClient {
+  return {
+    canAccessEvent: vi.fn().mockResolvedValue({ allowed: false, reason: 'forbidden' }),
+  };
+}
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+describe('createSubscriptionRegistry', () => {
+  let metrics: TestMetrics;
+
+  beforeEach(() => {
+    metrics = makeMetrics();
+  });
+
+  it('subscribe to a valid event topic with permitted user → subscribed reply and gauge increment', async () => {
+    const conn = makeConn();
+    const registry = createSubscriptionRegistry(
+      makeAllowedAuthzClient(), makeConfig(), makeSilentLogger(), metrics,
+    );
+
+    await registry.subscribe(conn, EVENT_TOPIC, 'corr-1');
+
+    // Should have sent a `subscribed` message.
+    expect(conn.sentMessages).toHaveLength(1);
+    const msg = conn.sentMessages[0] as Record<string, unknown>;
+    expect(msg['type']).toBe('subscribed');
+    expect(msg['topic']).toBe(EVENT_TOPIC);
+    expect(msg['id']).toBe('corr-1');
+    expect(Array.isArray(msg['snapshot'])).toBe(true);
+
+    // Gauge should have been updated.
+    const subGaugeCalls = metrics.observeCalls.filter(
+      (c) => c.name === 'processor_live_subscriptions',
+    );
+    expect(subGaugeCalls.length).toBeGreaterThanOrEqual(1);
+    expect(subGaugeCalls[subGaugeCalls.length - 1]!.value).toBe(1);
+
+    // Success counter.
+    const successCalls = metrics.incCalls.filter(
+      (c) => c.name === 'processor_live_subscribe_attempts_total' && c.labels?.['result'] === 'success',
+    );
+    expect(successCalls).toHaveLength(1);
+  });
+
+  it('subscribe with forbidden user → error/forbidden reply, no registry change', async () => {
+    const conn = makeConn();
+    const registry = createSubscriptionRegistry(
+      makeForbiddenAuthzClient(), makeConfig(), makeSilentLogger(), metrics,
+    );
+
+    await registry.subscribe(conn, EVENT_TOPIC, 'corr-2');
+
+    const msg = conn.sentMessages[0] as Record<string, unknown>;
+    expect(msg['type']).toBe('error');
+    expect(msg['code']).toBe('forbidden');
+    expect(msg['topic']).toBe(EVENT_TOPIC);
+    expect(msg['id']).toBe('corr-2');
+
+    // Gauge should NOT have changed.
+    const subGaugeCalls = metrics.observeCalls.filter(
+      (c) => c.name === 'processor_live_subscriptions',
+    );
+    // May have been called with 0 for snapshot, but never with a positive value.
+    const positiveGauge = subGaugeCalls.filter((c) => c.value > 0);
+    expect(positiveGauge).toHaveLength(0);
+
+    // connectionsForTopic should return empty.
+    expect([...registry.connectionsForTopic(EVENT_TOPIC)]).toHaveLength(0);
+  });
+
+  it('subscribe to device:<imei> → error/unknown-topic, no registry change', async () => {
+    const conn = makeConn();
+    const authz = makeAllowedAuthzClient();
+    const registry = createSubscriptionRegistry(
+      authz, makeConfig(), makeSilentLogger(), metrics,
+    );
+
+    await registry.subscribe(conn, 'device:356307042441013', 'corr-3');
+
+    const msg = conn.sentMessages[0] as Record<string, unknown>;
+    expect(msg['type']).toBe('error');
+    expect(msg['code']).toBe('unknown-topic');
+
+    // Authz client should NOT have been called.
+    expect(vi.mocked(authz.canAccessEvent)).not.toHaveBeenCalled();
+  });
+
+  it('subscribe twice to the same topic → idempotent (single subscription, subscribed each call)', async () => {
+    const conn = makeConn();
+    const registry = createSubscriptionRegistry(
+      makeAllowedAuthzClient(), makeConfig(), makeSilentLogger(), metrics,
+    );
+
+    await registry.subscribe(conn, EVENT_TOPIC);
+    await registry.subscribe(conn, EVENT_TOPIC); // second call
+
+    // Both calls send `subscribed`.
+    expect(conn.sentMessages).toHaveLength(2);
+    const msgs = conn.sentMessages as Array<Record<string, unknown>>;
+    expect(msgs[0]!['type']).toBe('subscribed');
+    expect(msgs[1]!['type']).toBe('subscribed');
+
+    // Gauge should only count once.
+    const finalGaugeCalls = metrics.observeCalls.filter(
+      (c) => c.name === 'processor_live_subscriptions',
+    );
+    // Last value should be 1, not 2.
+    expect(finalGaugeCalls[finalGaugeCalls.length - 1]!.value).toBe(1);
+
+    // connectionsForTopic should have exactly one connection.
+    expect([...registry.connectionsForTopic(EVENT_TOPIC)]).toHaveLength(1);
+  });
+
+  it('unsubscribe from a subscribed topic → unsubscribed reply and gauge decrement', async () => {
+    const conn = makeConn();
+    const registry = createSubscriptionRegistry(
+      makeAllowedAuthzClient(), makeConfig(), makeSilentLogger(), metrics,
+    );
+
+    await registry.subscribe(conn, EVENT_TOPIC);
+    registry.unsubscribe(conn, EVENT_TOPIC, 'corr-4');
+
+    const msgs = conn.sentMessages as Array<Record<string, unknown>>;
+    expect(msgs[1]!['type']).toBe('unsubscribed');
+    expect(msgs[1]!['topic']).toBe(EVENT_TOPIC);
+    expect(msgs[1]!['id']).toBe('corr-4');
+
+    // Gauge should be back at 0.
+    const finalGaugeCalls = metrics.observeCalls.filter(
+      (c) => c.name === 'processor_live_subscriptions',
+    );
+    expect(finalGaugeCalls[finalGaugeCalls.length - 1]!.value).toBe(0);
+  });
+
+  it('unsubscribe from a topic not subscribed to → unsubscribed reply (idempotent), gauge unchanged', async () => {
+    const conn = makeConn();
+    const registry = createSubscriptionRegistry(
+      makeAllowedAuthzClient(), makeConfig(), makeSilentLogger(), metrics,
+    );
+
+    // Unsubscribe without ever subscribing.
+    registry.unsubscribe(conn, EVENT_TOPIC, 'corr-5');
+
+    const msg = conn.sentMessages[0] as Record<string, unknown>;
+    expect(msg['type']).toBe('unsubscribed');
+
+    // Gauge should still be 0 (not go negative).
+    const finalGaugeCalls = metrics.observeCalls.filter(
+      (c) => c.name === 'processor_live_subscriptions',
+    );
+    const values = finalGaugeCalls.map((c) => c.value);
+    expect(values.every((v) => v >= 0)).toBe(true);
+  });
+
+  it('onConnectionClose removes all subscriptions; gauge returns to 0', async () => {
+    const conn = makeConn();
+    const registry = createSubscriptionRegistry(
+      makeAllowedAuthzClient(), makeConfig(), makeSilentLogger(), metrics,
+    );
+
+    await registry.subscribe(conn, EVENT_TOPIC);
+    await registry.subscribe(conn, `event:f6114c7e-1e94-488a-93c3-41060fcb06bc`);
+
+    registry.onConnectionClose(conn);
+
+    // Gauge should be 0.
+    const finalGaugeCalls = metrics.observeCalls.filter(
+      (c) => c.name === 'processor_live_subscriptions',
+    );
+    expect(finalGaugeCalls[finalGaugeCalls.length - 1]!.value).toBe(0);
+
+    // connectionsForTopic should be empty for both topics.
+    expect([...registry.connectionsForTopic(EVENT_TOPIC)]).toHaveLength(0);
+  });
+
+  it('connectionsForTopic returns only connections subscribed to that topic', async () => {
+    const conn1 = makeConn('conn-1');
+    const conn2 = makeConn('conn-2');
+    const otherTopic = 'event:f6114c7e-1e94-488a-93c3-41060fcb06bc';
+
+    const registry = createSubscriptionRegistry(
+      makeAllowedAuthzClient(), makeConfig(), makeSilentLogger(), metrics,
+    );
+
+    await registry.subscribe(conn1, EVENT_TOPIC);
+    await registry.subscribe(conn2, EVENT_TOPIC);
+    await registry.subscribe(conn1, otherTopic);
+
+    const connsForEvent = [...registry.connectionsForTopic(EVENT_TOPIC)];
+    expect(connsForEvent).toHaveLength(2);
+    expect(connsForEvent.map((c) => c.id).sort()).toEqual(['conn-1', 'conn-2'].sort());
+
+    const connsForOther = [...registry.connectionsForTopic(otherTopic)];
+    expect(connsForOther).toHaveLength(1);
+    expect(connsForOther[0]!.id).toBe('conn-1');
+  });
+
+  it('stats() returns correct counts', async () => {
+    const conn1 = makeConn('conn-1');
+    const conn2 = makeConn('conn-2');
+    const topic2 = 'event:f6114c7e-1e94-488a-93c3-41060fcb06bc';
+
+    const registry = createSubscriptionRegistry(
+      makeAllowedAuthzClient(), makeConfig(), makeSilentLogger(), metrics,
+    );
+
+    await registry.subscribe(conn1, EVENT_TOPIC);
+    await registry.subscribe(conn2, EVENT_TOPIC);
+    await registry.subscribe(conn1, topic2);
+
+    const s = registry.stats();
+    expect(s.subscriptions).toBe(3);
+    expect(s.topics).toBe(2);
+  });
+});