refactor: Enhance GenerationJobManager with improved subscriber handling

- Updated RuntimeJobState to include allSubscribersLeftHandlers for managing client disconnections without affecting subscriber count.
- Refined createJob and subscribe methods to ensure generation starts only when the first real client connects.
- Added detailed documentation for methods and properties to clarify the synchronization of job generation with client readiness.
- Improved logging for subscriber checks and event handling to facilitate debugging and monitoring.

packages/api/src/stream/GenerationJobManager.ts

@@ -10,6 +10,17 @@ import { InMemoryJobStore } from './implementations/InMemoryJobStore';
 /**
  * Runtime state for active jobs - not serializable, kept in-memory per instance.
  * Contains AbortController, ready promise, and other non-serializable state.
+ *
+ * @property abortController - Controller to abort the generation
+ * @property readyPromise - Resolves when first real subscriber connects (used to sync generation start)
+ * @property resolveReady - Function to resolve readyPromise
+ * @property finalEvent - Cached final event for late subscribers
+ * @property syncSent - Whether sync event was sent (reset when all subscribers leave)
+ * @property allSubscribersLeftHandlers - Internal handlers for disconnect events.
+ *   These are stored separately from eventTransport subscribers to avoid being counted
+ *   in subscriber count. This is critical: if these were registered via subscribe(),
+ *   they would count as subscribers, causing isFirstSubscriber() to return false
+ *   when the real client connects, which would prevent readyPromise from resolving.
  */
 interface RuntimeJobState {
   abortController: AbortController;
@@ -17,6 +28,7 @@ interface RuntimeJobState {
   resolveReady: () => void;
   finalEvent?: t.ServerSentEvent;
   syncSent: boolean;
+  allSubscribersLeftHandlers?: Array<(...args: unknown[]) => void>;
 }
 
 /**
@@ -65,13 +77,30 @@ class GenerationJobManagerClass {
 
   /**
    * Create a new generation job.
+   *
+   * This sets up:
+   * 1. Serializable job data in the job store
+   * 2. Runtime state including readyPromise (resolves when first SSE client connects)
+   * 3. allSubscribersLeft callback for handling client disconnections
+   *
+   * The readyPromise mechanism ensures generation doesn't start before the client
+   * is ready to receive events. The controller awaits this promise (with a short timeout)
+   * before starting LLM generation.
+   *
+   * @param streamId - Unique identifier for this stream
+   * @param userId - User who initiated the request
+   * @param conversationId - Optional conversation ID for lookup
    * @returns A facade object compatible with the old GenerationJob interface
    */
   createJob(streamId: string, userId: string, conversationId?: string): t.GenerationJob {
     // Create serializable job data (sync for in-memory)
     const jobData = this.jobStore.createJobSync(streamId, userId, conversationId);
 
-    // Create runtime state
+    /**
+     * Create runtime state with readyPromise.
+     * readyPromise is resolved in subscribe() when isFirstSubscriber() returns true.
+     * This synchronizes generation start with client connection.
+     */
     let resolveReady: () => void;
     const readyPromise = new Promise<void>((resolve) => {
       resolveReady = resolve;
@@ -85,17 +114,28 @@ class GenerationJobManagerClass {
     };
     this.runtimeState.set(streamId, runtime);
 
-    // Set up all-subscribers-left callback
+    /**
+     * Set up all-subscribers-left callback.
+     * When all SSE clients disconnect, this:
+     * 1. Resets syncSent so reconnecting clients get sync event
+     * 2. Calls any registered allSubscribersLeft handlers (e.g., to save partial responses)
+     */
     this.eventTransport.onAllSubscribersLeft(streamId, () => {
       const currentRuntime = this.runtimeState.get(streamId);
       if (currentRuntime) {
         currentRuntime.syncSent = false;
+        // Call registered handlers (from job.emitter.on('allSubscribersLeft', ...))
+        const content = this.contentState.getContentParts(streamId) ?? [];
+        if (currentRuntime.allSubscribersLeftHandlers) {
+          for (const handler of currentRuntime.allSubscribersLeftHandlers) {
+            try {
+              handler(content);
+            } catch (err) {
+              logger.error(`[GenerationJobManager] Error in allSubscribersLeft handler:`, err);
+            }
+          }
+        }
       }
-      const content = this.contentState.getContentParts(streamId) ?? [];
-      this.eventTransport.emitChunk(streamId, {
-        _internal: 'allSubscribersLeft',
-        content,
-      });
       logger.debug(`[GenerationJobManager] All subscribers left ${streamId}, reset syncSent`);
     });
 
@@ -107,26 +147,43 @@ class GenerationJobManagerClass {
 
   /**
    * Build a GenerationJob facade from job data and runtime state.
-   * This maintains backwards compatibility with existing code.
+   * This maintains backwards compatibility with existing code that expects
+   * job.emitter, job.abortController, etc.
+   *
+   * IMPORTANT: The emitterProxy.on('allSubscribersLeft') handler registration
+   * does NOT use eventTransport.subscribe(). This is intentional:
+   *
+   * If we used subscribe() for internal handlers, those handlers would count
+   * as subscribers. When the real SSE client connects, isFirstSubscriber()
+   * would return false (because internal handler was "first"), and readyPromise
+   * would never resolve - causing a 5-second timeout delay before generation starts.
+   *
+   * Instead, allSubscribersLeft handlers are stored in runtime.allSubscribersLeftHandlers
+   * and called directly from the onAllSubscribersLeft callback in createJob().
+   *
+   * @param streamId - The stream identifier
+   * @param jobData - Serializable job metadata from job store
+   * @param runtime - Non-serializable runtime state (abort controller, promises, etc.)
+   * @returns A GenerationJob facade object
    */
   private buildJobFacade(
     streamId: string,
     jobData: SerializableJobData,
     runtime: RuntimeJobState,
   ): t.GenerationJob {
-    // Create a proxy emitter that delegates to eventTransport
+    /**
+     * Proxy emitter that delegates to eventTransport for most operations.
+     * Exception: allSubscribersLeft handlers are stored separately to avoid
+     * incrementing subscriber count (see class JSDoc above).
+     */
     const emitterProxy = {
       on: (event: string, handler: (...args: unknown[]) => void) => {
         if (event === 'allSubscribersLeft') {
-          // Subscribe to internal event
+          // Store handler for internal callback - don't use subscribe() to avoid counting as a subscriber
-          this.eventTransport.subscribe(streamId, {
+          if (!runtime.allSubscribersLeftHandlers) {
-            onChunk: (e) => {
+            runtime.allSubscribersLeftHandlers = [];
-              const evt = e as Record<string, unknown>;
+          }
-              if (evt._internal === 'allSubscribersLeft') {
+          runtime.allSubscribersLeftHandlers.push(handler);
-                handler(evt.content);
-              }
-            },
-          });
         }
       },
       emit: () => {
@@ -285,6 +342,20 @@ class GenerationJobManagerClass {
 
   /**
    * Subscribe to a job's event stream.
+   *
+   * This is called when an SSE client connects to /chat/stream/:streamId.
+   * On first subscription, it resolves readyPromise to signal that generation can start.
+   *
+   * The subscriber count is critical for the readyPromise mechanism:
+   * - isFirstSubscriber() returns true when subscriber count is exactly 1
+   * - This happens when the first REAL client connects (not internal handlers)
+   * - Internal allSubscribersLeft handlers are stored separately to avoid being counted
+   *
+   * @param streamId - The stream to subscribe to
+   * @param onChunk - Handler for chunk events (streamed tokens, run steps, etc.)
+   * @param onDone - Handler for completion event (includes final message)
+   * @param onError - Handler for error events
+   * @returns Subscription object with unsubscribe function, or null if job not found
    */
   subscribe(
     streamId: string,
@@ -323,9 +394,15 @@ class GenerationJobManagerClass {
     });
 
     // Signal ready on first subscriber
-    if (this.eventTransport.isFirstSubscriber(streamId)) {
+    const isFirst = this.eventTransport.isFirstSubscriber(streamId);
+    logger.debug(
+      `[GenerationJobManager] subscribe check: streamId=${streamId}, isFirst=${isFirst}`,
+    );
+    if (isFirst) {
       runtime.resolveReady();
-      logger.debug(`[GenerationJobManager] First subscriber ready for ${streamId}`);
+      logger.debug(
+        `[GenerationJobManager] First subscriber ready, resolving promise for ${streamId}`,
+      );
     }
 
     return subscription;

packages/api/src/stream/implementations/InMemoryEventTransport.ts

@@ -43,6 +43,10 @@ export class InMemoryEventTransport implements IEventTransport {
     state.emitter.on('done', doneHandler);
     state.emitter.on('error', errorHandler);
 
+    logger.debug(
+      `[InMemoryEventTransport] subscribe ${streamId}: listeners=${state.emitter.listenerCount('chunk')}`,
+    );
+
     return {
       unsubscribe: () => {
         const currentState = this.streams.get(streamId);
@@ -90,7 +94,9 @@ export class InMemoryEventTransport implements IEventTransport {
    */
   isFirstSubscriber(streamId: string): boolean {
     const state = this.streams.get(streamId);
-    return state?.emitter.listenerCount('chunk') === 1;
+    const count = state?.emitter.listenerCount('chunk') ?? 0;
+    logger.debug(`[InMemoryEventTransport] isFirstSubscriber ${streamId}: count=${count}`);
+    return count === 1;
   }
 
   /**

packages/api/src/stream/interfaces/IJobStore.ts

@@ -112,6 +112,9 @@ export interface IEventTransport {
   /** Get subscriber count for a stream */
   getSubscriberCount(streamId: string): number;
 
+  /** Check if this is the first subscriber (for ready signaling) */
+  isFirstSubscriber(streamId: string): boolean;
+
   /** Listen for all subscribers leaving */
   onAllSubscribersLeft(streamId: string, callback: () => void): void;
 }