fix: serialize session lifecycle transitions

backend/test/sse-route.test.ts

@@ -55,6 +55,17 @@ async function captureReplay(options: { query?: string; lastEventId?: string }):
   }
 }
 
+async function expectStreamEnd(reader: ReadableStreamDefaultReader<Uint8Array>): Promise<void> {
+  const result = await Promise.race([
+    reader.read(),
+    new Promise<{ timeout: true }>((resolve) =>
+      setTimeout(() => resolve({ timeout: true }), 1_000),
+    ),
+  ]);
+  expect(result).not.toEqual({ timeout: true });
+  expect("done" in result && result.done).toBe(true);
+}
+
 test("SSE emits ids and honors the native Last-Event-ID replay cursor", async () => {
   const body = await captureReplay({ lastEventId: "1" });
 
@@ -80,3 +91,54 @@ test("SSE uses the newer valid cursor when header and query are both present", a
   expect(body).not.toContain('"two"');
   expect(body).toContain("id: 3\nevent: info\n");
 });
+
+test("clear ends every live SSE response and a cursor reconnect replays post-clear events", async () => {
+  const hub = new SseHub();
+  const app = buildApp(loadConfig({ THT_HARNESS_DIR: "/tmp/h" }), {
+    hub,
+    thtRunner: {} as any,
+  });
+  await app.listen({ port: 0, host: "127.0.0.1" });
+  const port = (app.server.address() as { port: number }).port;
+  const controllers = [new AbortController(), new AbortController(), new AbortController()];
+
+  try {
+    const responses = await Promise.all(controllers.slice(0, 2).map((controller) => fetch(
+      `http://127.0.0.1:${port}/sessions/s1/events`,
+      { signal: controller.signal },
+    )));
+    const readers = responses.map((response) => response.body!.getReader());
+
+    expect(hub.publish("s1", "info", { type: "info", text: "before" })).toBe(1);
+    const initial = await Promise.all(readers.map((reader) =>
+      readUntil(reader, (text) => text.includes('"before"'))));
+    expect(initial.every((body) => body.includes("id: 1\nevent: info\n"))).toBe(true);
+
+    hub.clear("s1");
+    await Promise.all(readers.map(expectStreamEnd));
+
+    expect(hub.publish("s1", "info", { type: "info", text: "after" })).toBe(2);
+    expect(hub.publish("s1", "ui_request", {
+      type: "ui_request",
+      ui_request: { id: "gate-1", widget: "select" },
+    })).toBe(3);
+
+    const reconnected = await fetch(
+      `http://127.0.0.1:${port}/sessions/s1/events`,
+      {
+        headers: { "Last-Event-ID": "1" },
+        signal: controllers[2].signal,
+      },
+    );
+    const reconnectReader = reconnected.body!.getReader();
+    const replay = await readUntil(reconnectReader, (text) => text.includes('"gate-1"'));
+    expect(replay).toContain("id: 2\nevent: info\n");
+    expect(replay).toContain('data: {"type":"info","text":"after"}');
+    expect(replay).toContain("id: 3\nevent: ui_request\n");
+    expect(replay).toContain('"ui_request":{"id":"gate-1","widget":"select"}');
+    await reconnectReader.cancel();
+  } finally {
+    for (const controller of controllers) controller.abort();
+    await app.close();
+  }
+});