Improve event stream resilience

packages/ui/src/sync/__tests__/event-pipeline.test.js

@@ -667,6 +667,85 @@ describe('createEventPipeline', () => {
     ]);
   });
 
+  it('passes the last websocket event id when falling back to SSE', async () => {
+    installDomStubs();
+    globalThis.WebSocket = FakeWebSocket;
+    const originalConsoleError = console.error;
+    console.error = () => {};
+
+    let releaseStream;
+    const hold = new Promise((resolve) => {
+      releaseStream = resolve;
+    });
+
+    const eventOptions = [];
+    const received = [];
+    const sdk = {
+      global: {
+        event: async (options) => {
+          eventOptions.push(options);
+          return {
+            stream: (async function* () {
+              yield {
+                payload: {
+                  type: 'server.connected',
+                  properties: {},
+                },
+              };
+              await hold;
+            })(),
+          };
+        },
+      },
+    };
+
+    const delivered = new Promise((resolve) => {
+      const { cleanup } = createEventPipeline({
+        sdk,
+        transport: 'auto',
+        reconnectDelayMs: 0,
+        wsReadyTimeoutMs: 20,
+        onEvent: (directory, payload) => {
+          received.push({ directory, payload });
+          if (payload.type !== 'server.connected') {
+            return;
+          }
+          cleanup();
+          releaseStream();
+          resolve();
+        },
+      });
+    });
+
+    try {
+      await Promise.resolve();
+
+      const firstSocket = FakeWebSocket.instances[0];
+      firstSocket.emitOpen();
+      firstSocket.emitMessage({ type: 'ready', scope: 'global' });
+      firstSocket.emitMessage({
+        type: 'event',
+        eventId: 'evt-1',
+        directory: '/tmp/project',
+        payload: {
+          type: 'session.status',
+          properties: {
+            sessionID: 'session-1',
+          },
+        },
+      });
+      firstSocket.emitClose();
+
+      await new Promise((resolve) => setTimeout(resolve, 40));
+      await delivered;
+
+      expect(eventOptions[0]?.headers?.['Last-Event-ID']).toBe('evt-1');
+      expect(received.some((entry) => entry.payload.type === 'server.connected')).toBe(true);
+    } finally {
+      console.error = originalConsoleError;
+    }
+  });
+
   it('marks the pipeline disconnected on heartbeat timeout and recovers on the next websocket connect', async () => {
     installDomStubs();
     globalThis.WebSocket = FakeWebSocket;

packages/ui/src/sync/event-pipeline.ts

@@ -339,6 +339,7 @@ export function createEventPipeline(input: EventPipelineInput) {
   const runSseAttempt = async (signal: AbortSignal) => {
     const events = await sdk.global.event({
       signal,
+      ...(lastEventId && lastEventId.length > 0 ? { headers: { "Last-Event-ID": lastEventId } } : {}),
       onSseError: (error: unknown) => {
         if (isAbortError(error)) return
         if (streamErrorLogged) return