fix: harden resume and SSE replay

frontend/src/stream/useSessionStream.test.tsx

@@ -70,16 +70,46 @@ test("closes the stream on unmount", () => {
   expect(es.closed).toBe(true);
 });
 
-test("reconnects the same session when the generation changes", () => {
+test("a same-session generation reconnect includes the last consumed SSE id", () => {
   const { rerender } = renderHook(
     ({ generation }) => useSessionStream("s1", generation),
     { initialProps: { generation: 0 } },
   );
   const first = FakeEventSource.instances[0];
+  act(() => first.emitNamed("text_delta", { type: "text_delta", text: "hello" }, "7"));
 
   rerender({ generation: 1 });
 
   expect(first.closed).toBe(true);
   expect(FakeEventSource.instances).toHaveLength(2);
-  expect(FakeEventSource.instances[1].url).toBe(first.url);
+  expect(FakeEventSource.instances[1].url).toBe(
+    "http://localhost:8787/sessions/s1/events?lastEventId=7",
+  );
+
+  act(() =>
+    FakeEventSource.instances[1].emitNamed(
+      "text_delta",
+      { type: "text_delta", text: " world" },
+      "8",
+    ),
+  );
+  expect(useSessionStore.getState().transcript).toEqual([
+    { role: "assistant", text: "hello world" },
+  ]);
+  expect(useSessionStore.getState().activityLog.filter((entry) => entry.kind === "assistant"))
+    .toEqual([{ kind: "assistant", phase: null, text: "hello world" }]);
+});
+
+test("changing the session resets the manual reconnect cursor", () => {
+  const { rerender } = renderHook(
+    ({ sessionId }) => useSessionStream(sessionId),
+    { initialProps: { sessionId: "s1" as string | null } },
+  );
+  const first = FakeEventSource.instances[0];
+  act(() => first.emitNamed("info", { type: "info", text: "ready" }, "12"));
+
+  rerender({ sessionId: "s2" });
+
+  expect(first.closed).toBe(true);
+  expect(FakeEventSource.instances[1].url).toBe("http://localhost:8787/sessions/s2/events");
 });

frontend/src/stream/useSessionStream.ts

@@ -1,4 +1,4 @@
-import { useEffect, useState } from "react";
+import { useEffect, useRef, useState } from "react";
 import { BASE } from "../api/client";
 import { joinBackendPath } from "../api/runtime-config";
 import { useSessionStore } from "../store/sessionStore";
@@ -7,15 +7,24 @@ import type { StreamEvent } from "../api/types";
 export function useSessionStream(sessionId: string | null, generation = 0) {
   const [connected, setConnected] = useState(false);
   const applyEvent = useSessionStore((s) => s.applyEvent);
+  const cursor = useRef({ sessionId: null as string | null, lastEventId: "" });
+
+  if (cursor.current.sessionId !== sessionId) {
+    cursor.current = { sessionId, lastEventId: "" };
+  }
 
   useEffect(() => {
     if (!sessionId) return;
 
-    const es = new EventSource(joinBackendPath(BASE, `/sessions/${sessionId}/events`));
+    const query = cursor.current.lastEventId
+      ? `?lastEventId=${encodeURIComponent(cursor.current.lastEventId)}`
+      : "";
+    const es = new EventSource(joinBackendPath(BASE, `/sessions/${sessionId}/events${query}`));
     es.onopen = () => setConnected(true);
     es.onerror = () => setConnected(false);
 
     const handle = (ev: MessageEvent) => {
+      if (ev.lastEventId) cursor.current.lastEventId = ev.lastEventId;
       try {
         applyEvent(JSON.parse(ev.data) as StreamEvent);
       } catch {