feat(companion): add platform event subscription count passthrough

README.md

@@ -1200,7 +1200,7 @@ Companion runtime helper:
   - `createHeartbeatLoop()` convenience helper that returns a bound `CompanionHeartbeatLoop`
   - optional `defaultSessionId` for canvas helper calls so `sessionId` can be omitted per call
   - `dispose()` lifecycle helper for unified runtime teardown
-  - stream passthrough helpers (`subscribeEvents`, `clearEventSubscriptions`, `listKnownEventNames`, `subscribeAgentStream/Typing/ContextWarning`, `waitForAnyEvent`, `waitForAgentStream/Typing/ContextWarning`)
+  - stream passthrough helpers (`subscribeEvents`, `clearEventSubscriptions`, `listKnownEventNames`, `eventSubscriptionCount`, `subscribeAgentStream/Typing/ContextWarning`, `waitForAnyEvent`, `waitForAgentStream/Typing/ContextWarning`)
 - `src/companion/heartbeatLoop.ts` provides `CompanionHeartbeatLoop` for periodic heartbeat scheduling (`publishHeartbeat`) with start/stop safety, optional interval jitter (`jitterRatio`) to spread load, `tickNow()` for manual sends, success/error hooks, failure observability (`failureCount`, `lastFailure`, `getState()`), and optional auto-stop after repeated failures.
 
 ## Canvas / A2UI Foundation

docs/plans/state.json

@@ -673,6 +673,19 @@
       ],
       "test_status": "pnpm test:run src/companion/runtimeClient.test.ts src/companion/platformClients.test.ts src/companion/heartbeatLoop.test.ts src/companion/platformClients.integration.test.ts + pnpm typecheck passing"
     },
+    "companion-platform-event-subscription-count-passthrough": {
+      "status": "completed",
+      "date": "2026-02-17",
+      "updated": "2026-02-17",
+      "summary": "Added `eventSubscriptionCount` passthrough on platform companion clients so wrappers expose active runtime event-listener cardinality for observability and debugging.",
+      "files_modified": [
+        "src/companion/platformClients.ts",
+        "src/companion/platformClients.test.ts",
+        "README.md",
+        "docs/plans/state.json"
+      ],
+      "test_status": "pnpm test:run src/companion/platformClients.test.ts src/companion/runtimeClient.test.ts src/companion/heartbeatLoop.test.ts src/companion/platformClients.integration.test.ts + pnpm typecheck passing"
+    },
     "browser-tools-activation-clarity": {
       "status": "completed",
       "date": "2026-02-17",

src/companion/platformClients.test.ts

@@ -35,6 +35,7 @@ function createRuntimeMock(): {
   waitForAgentTyping: ReturnType<typeof vi.fn>;
   waitForContextWarning: ReturnType<typeof vi.fn>;
   waitForAnyEvent: ReturnType<typeof vi.fn>;
+  eventSubscriptionCount: number;
 } {
   const connect = vi.fn(async () => undefined);
   const disconnect = vi.fn(() => undefined);
@@ -70,6 +71,7 @@ function createRuntimeMock(): {
   const waitForAgentTyping = vi.fn(async () => ({ active: true }));
   const waitForContextWarning = vi.fn(async () => ({ thresholdPct: 75, estimatedPct: 90 }));
   const waitForAnyEvent = vi.fn(async () => ({ event: 'agent.stream', data: { token: 'any' } }));
+  const eventSubscriptionCount = 3;
 
   const runtime = {
     connect,
@@ -99,6 +101,9 @@ function createRuntimeMock(): {
     waitForAgentTyping,
     waitForContextWarning,
     waitForAnyEvent,
+    get eventSubscriptionCount() {
+      return eventSubscriptionCount;
+    },
   } as unknown as CompanionRuntimeClient;
 
   return {
@@ -130,6 +135,7 @@ function createRuntimeMock(): {
     waitForAgentTyping,
     waitForContextWarning,
     waitForAnyEvent,
+    eventSubscriptionCount,
   };
 }
 
@@ -244,6 +250,13 @@ describe('platform companion clients', () => {
     expect(events).toEqual(['agent.stream', 'agent.typing', 'context_warning']);
   });
 
+  it('platform eventSubscriptionCount forwards runtime getter value', async () => {
+    const mock = createRuntimeMock();
+    const client = new IOSCompanionClient({ runtime: mock.runtime, nodeId: 'ios-node' });
+
+    expect(client.eventSubscriptionCount).toBe(mock.eventSubscriptionCount);
+  });
+
   it('macOS client forwards canvas methods to runtime client', async () => {
     const mock = createRuntimeMock();
     const client = new MacOSCompanionClient({ runtime: mock.runtime, nodeId: 'mac-node' });

src/companion/platformClients.ts

@@ -260,6 +260,10 @@ export class MacOSCompanionClient {
     return this.runtime.listKnownEventNames();
   }
 
+  get eventSubscriptionCount(): number {
+    return this.runtime.eventSubscriptionCount;
+  }
+
   waitForAnyEvent<TData = unknown>(
     eventNames: readonly string[],
     options?: {
@@ -476,6 +480,10 @@ export class IOSCompanionClient {
     return this.runtime.listKnownEventNames();
   }
 
+  get eventSubscriptionCount(): number {
+    return this.runtime.eventSubscriptionCount;
+  }
+
   waitForAnyEvent<TData = unknown>(
     eventNames: readonly string[],
     options?: {
@@ -690,6 +698,10 @@ export class AndroidCompanionClient {
     return this.runtime.listKnownEventNames();
   }
 
+  get eventSubscriptionCount(): number {
+    return this.runtime.eventSubscriptionCount;
+  }
+
   waitForAnyEvent<TData = unknown>(
     eventNames: readonly string[],
     options?: {