diff --git a/agent/dashboard/routes.py b/agent/dashboard/routes.py
index 2f78bcbe..721351ad 100644
--- a/agent/dashboard/routes.py
+++ b/agent/dashboard/routes.py
@@ -996,19 +996,15 @@ async def api_thread_stream_events(
     session: dict[str, Any] = _SESSION_DEP,
 ) -> StreamingResponse:
     body = await request.body()
-
-    async def event_generator():
-        async for chunk in proxy_dashboard_thread_stream_events(
-            thread_id,
-            session["sub"],
-            body,
-            email=session.get("email"),
-            content_type=request.headers.get("content-type", "application/json"),
-        ):
-            yield chunk
-
+    stream = await proxy_dashboard_thread_stream_events(
+        thread_id,
+        session["sub"],
+        body,
+        email=session.get("email"),
+        content_type=request.headers.get("content-type", "application/json"),
+    )
     return StreamingResponse(
-        event_generator(),
+        stream,
         media_type="text/event-stream",
         headers={"Cache-Control": "no-cache", "Connection": "keep-alive"},
     )
diff --git a/agent/dashboard/thread_api.py b/agent/dashboard/thread_api.py
index 5954c3cd..b0e0bfa1 100644
--- a/agent/dashboard/thread_api.py
+++ b/agent/dashboard/thread_api.py
@@ -1032,8 +1032,18 @@ async def proxy_dashboard_thread_stream_events(
     email: str | None = None,
     content_type: str = "application/json",
 ) -> AsyncIterator[bytes]:
+    # Preflight here (not in the generator) so auth/content-type failures
+    # surface as real HTTP errors before the SSE response starts streaming.
     _require_json_content_type(content_type)
     await _authorized_thread_metadata(thread_id, login, email=email)
+    return _stream_thread_events(thread_id, body, content_type)
+
+
+async def _stream_thread_events(
+    thread_id: str,
+    body: bytes,
+    content_type: str,
+) -> AsyncIterator[bytes]:
     url = f"{langgraph_url().rstrip('/')}/threads/{thread_id}/stream/events"
     headers = _langgraph_proxy_headers(content_type=content_type, accept="text/event-stream")
 
diff --git a/tests/test_dashboard_thread_api.py b/tests/test_dashboard_thread_api.py
index 3dbb3c9c..54072e9a 100644
--- a/tests/test_dashboard_thread_api.py
+++ b/tests/test_dashboard_thread_api.py
@@ -424,7 +424,7 @@ async def test_proxy_endpoints_enforce_thread_ownership(monkeypatch) -> None:
     assert exc_info.value.status_code == 404
 
     with pytest.raises(HTTPException) as exc_info:
-        await anext(thread_api.proxy_dashboard_thread_stream_events("tid", "intruder", b"{}"))
+        await thread_api.proxy_dashboard_thread_stream_events("tid", "intruder", b"{}")
     assert exc_info.value.status_code == 404
 
 
diff --git a/ui/src/lib/agents/queries.ts b/ui/src/lib/agents/queries.ts
index f21d401e..f9279fb3 100644
--- a/ui/src/lib/agents/queries.ts
+++ b/ui/src/lib/agents/queries.ts
@@ -31,7 +31,12 @@ export function useSeedAgentThreadDetails(
   useEffect(() => {
     for (const thread of threads) {
       if (thread.id === activeThreadId) continue
-      queryClient.setQueryData(agentThreadKeys.detail(thread.id), thread)
+      // Seed as already-stale: the detail GET is what marks a thread viewed
+      // server-side, so opening a seeded entry must still refetch despite the
+      // detail query's `staleTime` (which exists for the optimistic seed).
+      queryClient.setQueryData(agentThreadKeys.detail(thread.id), thread, {
+        updatedAt: 0,
+      })
     }
   }, [activeThreadId, queryClient, threads])
 }
@@ -51,6 +56,10 @@ export function useAgentThread(threadId: string) {
   return useQuery({
     queryKey: agentThreadKeys.detail(threadId),
     queryFn: () => agentsApi.getThread(threadId),
+    // Lets the optimistic detail seeded by `AgentsHome` survive until the
+    // proxied run.start stamps the server-side thread; an immediate refetch
+    // would 404 and bounce the route back to /agents.
+    staleTime: 30_000,
   })
 }