diff --git a/frontend-modern/src/api/__tests__/aiChat.test.ts b/frontend-modern/src/api/__tests__/aiChat.test.ts
index 3e5b14194..01381f393 100644
--- a/frontend-modern/src/api/__tests__/aiChat.test.ts
+++ b/frontend-modern/src/api/__tests__/aiChat.test.ts
@@ -1146,6 +1146,35 @@ describe('AIChatAPI', () => {
     clearTimeoutSpy.mockRestore();
   });
 
+  it('rejects stalled chat stream reads instead of emitting synthetic completion', async () => {
+    vi.useFakeTimers({ toFake: ['setTimeout', 'clearTimeout'] });
+    const read = vi.fn(
+      () => new Promise<ReadableStreamReadResult<Uint8Array>>(() => undefined),
+    );
+    const releaseLock = vi.fn();
+    const onEvent = vi.fn();
+
+    apiFetchMock.mockResolvedValueOnce({
+      ok: true,
+      body: {
+        getReader: () => ({ read, releaseLock }),
+      },
+    } as unknown as Response);
+
+    const streamPromise = AIChatAPI.chat('hello', undefined, undefined, onEvent);
+    const expectedRejection = expect(streamPromise).rejects.toThrow(
+      'Pulse Assistant stream timed out waiting for provider data.',
+    );
+    await flushMicrotasks();
+    await vi.advanceTimersByTimeAsync(300000);
+
+    await expectedRejection;
+    expect(read).toHaveBeenCalledTimes(1);
+    expect(releaseLock).toHaveBeenCalledTimes(1);
+    expect(onEvent).not.toHaveBeenCalledWith({ type: 'done' });
+    expect(logger.warn).toHaveBeenCalledWith('[AI Chat] Stream timeout');
+  });
+
   it('ignores invalid chat stream events through the shared JSON-text helper', async () => {
     const encoder = new TextEncoder();
     const read = vi
diff --git a/frontend-modern/src/api/__tests__/streaming.test.ts b/frontend-modern/src/api/__tests__/streaming.test.ts
index 0039441a9..2ec1edffe 100644
--- a/frontend-modern/src/api/__tests__/streaming.test.ts
+++ b/frontend-modern/src/api/__tests__/streaming.test.ts
@@ -235,4 +235,39 @@ describe('consumeJSONEventStream', () => {
     await streamPromise;
     expect(events).toEqual(['tool_start', 'tool_progress', 'done']);
   });
+
+  it('surfaces stalled read timeouts without reporting normal completion', async () => {
+    vi.useFakeTimers();
+    const read = vi.fn(
+      () => new Promise<ReadableStreamReadResult<Uint8Array>>(() => undefined),
+    );
+    const releaseLock = vi.fn();
+    const onEvent = vi.fn();
+    const onTimeout = vi.fn();
+    const onComplete = vi.fn();
+
+    const streamPromise = consumeJSONEventStream<{ type: string }>(
+      {
+        body: {
+          getReader: () => ({ read, releaseLock }),
+        },
+      } as unknown as Response,
+      {
+        onEvent,
+        onTimeout,
+        onComplete,
+        timeoutMs: 1000,
+      },
+    );
+
+    await flushMicrotasks();
+    await vi.advanceTimersByTimeAsync(1000);
+    await streamPromise;
+
+    expect(read).toHaveBeenCalledTimes(1);
+    expect(onTimeout).toHaveBeenCalledTimes(1);
+    expect(onComplete).not.toHaveBeenCalled();
+    expect(onEvent).not.toHaveBeenCalled();
+    expect(releaseLock).toHaveBeenCalledTimes(1);
+  });
 });
diff --git a/frontend-modern/src/api/aiChat.ts b/frontend-modern/src/api/aiChat.ts
index 915bb1a91..ea5ae2a4f 100644
--- a/frontend-modern/src/api/aiChat.ts
+++ b/frontend-modern/src/api/aiChat.ts
@@ -408,6 +408,7 @@ export class AIChatAPI {
       },
       onTimeout: () => {
         logger.warn('[AI Chat] Stream timeout');
+        throw new Error('Pulse Assistant stream timed out waiting for provider data.');
       },
       onComplete: () => {
         onEvent({ type: 'done' });
diff --git a/frontend-modern/src/api/streaming.ts b/frontend-modern/src/api/streaming.ts
index 2effd8214..13b73edb5 100644
--- a/frontend-modern/src/api/streaming.ts
+++ b/frontend-modern/src/api/streaming.ts
@@ -65,6 +65,12 @@ export async function consumeJSONEventStream<T>(
   let buffer = '';
   const timeoutMs = options.timeoutMs ?? 300000;
   let lastEventTime = Date.now();
+  let timedOut = false;
+
+  const markTimedOut = () => {
+    timedOut = true;
+    options.onTimeout?.();
+  };
 
   const readWithTimeout = async (): Promise<ReadableStreamReadResult<Uint8Array>> => {
     let timeoutId: ReturnType<typeof setTimeout> | undefined;
@@ -122,7 +128,7 @@ export async function consumeJSONEventStream<T>(
   try {
     for (;;) {
       if (Date.now() - lastEventTime > timeoutMs) {
-        options.onTimeout?.();
+        markTimedOut();
         break;
       }
 
@@ -131,6 +137,7 @@ export async function consumeJSONEventStream<T>(
         result = await readWithTimeout();
       } catch (error) {
         if ((error as Error).message === 'Read timeout') {
+          markTimedOut();
           break;
         }
         throw error;
@@ -148,6 +155,10 @@ export async function consumeJSONEventStream<T>(
       }
     }
 
+    if (timedOut) {
+      return;
+    }
+
     const trailing = buffer.trim();
     if (trailing.startsWith('data: ')) {
       const event = parseJSONTextSafe<T>(trailing.slice(6));