diff --git a/webui/src/hooks/useNanobotStream.ts b/webui/src/hooks/useNanobotStream.ts
index 9b1d7ac2f..dc6f0cc01 100644
--- a/webui/src/hooks/useNanobotStream.ts
+++ b/webui/src/hooks/useNanobotStream.ts
@@ -643,6 +643,28 @@ export function useNanobotStream(
     return () => document.removeEventListener("visibilitychange", flushOnReturn);
   }, [flushPendingStreamEvents]);
 
+  useEffect(() => {
+    if (!chatId) return;
+    return client.onRunStatus((runChatId, startedAt) => {
+      if (runChatId !== chatId) return;
+      if (startedAt !== null) {
+        setRunStartedAt(startedAt);
+        setIsStreaming(true);
+        return;
+      }
+      flushPendingStreamEvents();
+      buffer.current = null;
+      activeAssistantRef.current = null;
+      closedAssistantStreamIdsRef.current.clear();
+      clearActivitySegment();
+      setMessages((prev) => prev.map((message) => (
+        message.isStreaming ? { ...message, isStreaming: false } : message
+      )));
+      setRunStartedAt(null);
+      setIsStreaming(false);
+    });
+  }, [chatId, client, clearActivitySegment, flushPendingStreamEvents]);
+
   // Reset local state when switching chats. Do not reset on every
   // ``initialMessages`` update: a brand-new chat can receive an empty/404
   // history response after the optimistic first message has already rendered.
diff --git a/webui/src/tests/thread-shell.test.tsx b/webui/src/tests/thread-shell.test.tsx
index 7a16aef9b..ed75437c3 100644
--- a/webui/src/tests/thread-shell.test.tsx
+++ b/webui/src/tests/thread-shell.test.tsx
@@ -21,6 +21,7 @@ function makeClient() {
     (modelName: string | null, modelPreset?: string | null) => void
   >();
   const sessionUpdateHandlers = new Set<(chatId: string, scope?: string) => void>();
+  const runStatusHandlers = new Set<(chatId: string, startedAt: number | null) => void>();
   const runStartedAtByChatId = new Map<string, number>();
   const runGenerationByChatId = new Map<string, number>();
   const latestRunTurnIdByChatId = new Map<string, string>();
@@ -98,6 +99,13 @@ function makeClient() {
         statusHandlers.delete(handler);
       };
     },
+    onRunStatus: (handler: (chatId: string, startedAt: number | null) => void) => {
+      runStatusHandlers.add(handler);
+      for (const [chatId, startedAt] of runStartedAtByChatId) handler(chatId, startedAt);
+      return () => {
+        runStatusHandlers.delete(handler);
+      };
+    },
     onRuntimeModelUpdate: (
       handler: (modelName: string | null, modelPreset?: string | null) => void,
     ) => {
@@ -157,11 +165,13 @@ function makeClient() {
       ) {
         advanceRunGeneration(chatId, ev.turn_id);
         runStartedAtByChatId.set(chatId, ev.started_at);
+        for (const h of runStatusHandlers) h(chatId, ev.started_at);
       } else if (
         (ev.event === "goal_status" && ev.status === "idle")
         || ev.event === "turn_end"
       ) {
         runStartedAtByChatId.delete(chatId);
+        for (const h of runStatusHandlers) h(chatId, null);
       }
       if (ev.event === "goal_state") {
         goalStateByChatId.set(chatId, ev.goal_state);
diff --git a/webui/src/tests/useNanobotStream.test.tsx b/webui/src/tests/useNanobotStream.test.tsx
index 85c4056f2..c3a2d3931 100644
--- a/webui/src/tests/useNanobotStream.test.tsx
+++ b/webui/src/tests/useNanobotStream.test.tsx
@@ -70,6 +70,7 @@ function normalizeProjection(messages: UIMessage[]): Array<Record<string, unknow
 function fakeClient() {
   const handlers = new Map<string, Set<(ev: InboundEvent) => void>>();
   const statusHandlers = new Set<(status: ConnectionStatus) => void>();
+  const runStatusHandlers = new Set<(chatId: string, startedAt: number | null) => void>();
   const errorHandlers = new Set<(error: StreamError) => void>();
   const runStartedAtByChatId = new Map<string, number>();
   const unsettledRunByChatId = new Map<string, boolean>();
@@ -111,6 +112,11 @@ function fakeClient() {
         handler(status);
         return () => statusHandlers.delete(handler);
       },
+      onRunStatus(handler: (chatId: string, startedAt: number | null) => void) {
+        runStatusHandlers.add(handler);
+        for (const [chatId, startedAt] of runStartedAtByChatId) handler(chatId, startedAt);
+        return () => runStatusHandlers.delete(handler);
+      },
       onError(handler: (error: StreamError) => void) {
         errorHandlers.add(handler);
         return () => errorHandlers.delete(handler);
@@ -154,6 +160,11 @@ function fakeClient() {
       status = nextStatus;
       statusHandlers.forEach((handler) => handler(status));
     },
+    emitRunStatus(chatId: string, startedAt: number | null) {
+      if (startedAt === null) runStartedAtByChatId.delete(chatId);
+      else runStartedAtByChatId.set(chatId, startedAt);
+      runStatusHandlers.forEach((handler) => handler(chatId, startedAt));
+    },
     emitError(error: StreamError) {
       errorHandlers.forEach((handler) => handler(error));
     },
@@ -327,6 +338,39 @@ describe("useNanobotStream", () => {
     });
   });
 
+  it("clears stale stream state when the transport resets a run", async () => {
+    const fake = fakeClient();
+    const { result } = renderHook(
+      () => useNanobotStream("chat-reconnect-reset", EMPTY_MESSAGES),
+      { wrapper: wrap(fake.client) },
+    );
+
+    act(() => {
+      fake.emit("chat-reconnect-reset", {
+        event: "goal_status",
+        chat_id: "chat-reconnect-reset",
+        status: "running",
+        started_at: 1_700,
+      });
+      fake.emit("chat-reconnect-reset", {
+        event: "delta",
+        chat_id: "chat-reconnect-reset",
+        text: "partial",
+      });
+    });
+    await flushStreamFrame();
+    expect(result.current.isStreaming).toBe(true);
+
+    act(() => fake.emitRunStatus("chat-reconnect-reset", null));
+
+    expect(result.current.runStartedAt).toBeNull();
+    expect(result.current.isStreaming).toBe(false);
+    expect(result.current.messages[0]).toMatchObject({
+      content: "partial",
+      isStreaming: false,
+    });
+  });
+
   it("flushes pending delta text before turn_end finalizes the turn", () => {
     const fake = fakeClient();
     const { result } = renderHook(() => useNanobotStream("chat-flush", EMPTY_MESSAGES), {