diff --git a/api/routes.py b/api/routes.py
index 6a538b20..3b548304 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -335,17 +335,20 @@ def _clear_stale_stream_state(session) -> bool:
         stream_alive = stream_id in STREAMS
     if stream_alive:
         return False
-    session.active_stream_id = None
-    if hasattr(session, "pending_user_message"):
-        session.pending_user_message = None
-    if hasattr(session, "pending_attachments"):
-        session.pending_attachments = []
-    if hasattr(session, "pending_started_at"):
-        session.pending_started_at = None
-    try:
-        session.save()
-    except Exception:
-        pass
+    with _get_session_agent_lock(session.session_id):
+        if getattr(session, "active_stream_id", None) != stream_id:
+            return False
+        session.active_stream_id = None
+        if hasattr(session, "pending_user_message"):
+            session.pending_user_message = None
+        if hasattr(session, "pending_attachments"):
+            session.pending_attachments = []
+        if hasattr(session, "pending_started_at"):
+            session.pending_started_at = None
+        try:
+            session.save()
+        except Exception:
+            pass
     return True
 
 # ── CSRF: validate Origin/Referer on POST ────────────────────────────────────
diff --git a/tests/test_stale_stream_cleanup.py b/tests/test_stale_stream_cleanup.py
index fe117d01..5f294789 100644
--- a/tests/test_stale_stream_cleanup.py
+++ b/tests/test_stale_stream_cleanup.py
@@ -1,11 +1,48 @@
+import queue
+import threading
 from pathlib import Path
 
+import api.config as config
+import api.routes as routes
+
 REPO = Path(__file__).resolve().parents[1]
 ROUTES_SRC = (REPO / "api" / "routes.py").read_text(encoding="utf-8")
 SESSIONS_SRC = (REPO / "static" / "sessions.js").read_text(encoding="utf-8")
 SW_SRC = (REPO / "static" / "sw.js").read_text(encoding="utf-8")
 
 
+class _GateLock:
+    def __init__(self):
+        self._lock = threading.Lock()
+        self.lookup_finished = threading.Event()
+        self.writer_finished = threading.Event()
+
+    def __enter__(self):
+        self._lock.acquire()
+        return self
+
+    def __exit__(self, exc_type, exc, tb):
+        self._lock.release()
+        if not self.lookup_finished.is_set():
+            self.lookup_finished.set()
+            assert self.writer_finished.wait(2), "writer did not finish race setup"
+        return False
+
+
+class _FakeSession:
+    session_id = "issue1533-session"
+
+    def __init__(self):
+        self.active_stream_id = "stale-stream"
+        self.pending_user_message = "old prompt"
+        self.pending_attachments = ["old.txt"]
+        self.pending_started_at = 123
+        self.saved_stream_ids = []
+
+    def save(self):
+        self.saved_stream_ids.append(self.active_stream_id)
+
+
 def test_stale_stream_cleanup_helper_exists():
     assert "def _clear_stale_stream_state(session)" in ROUTES_SRC
     assert "stream_id in STREAMS" in ROUTES_SRC
@@ -30,6 +67,53 @@ def test_chat_start_clears_stale_pending_state_not_only_active_id():
     assert stale_comment_pos < cleanup_pos < stream_id_pos
 
 
+def test_stale_stream_cleanup_does_not_clobber_concurrent_chat_start(monkeypatch):
+    """Regression for #1533: stale cleanup must not erase a new stream id.
+
+    The gate lock pauses the cleanup thread after it has decided that the old
+    stream id is stale, then lets a chat_start-like writer register and persist
+    a new active_stream_id for the same session.
+    """
+    config.STREAMS.clear()
+    config.SESSION_AGENT_LOCKS.clear()
+    gate_lock = _GateLock()
+    session = _FakeSession()
+    new_stream_id = "new-stream"
+    result = {}
+
+    monkeypatch.setattr(routes, "STREAMS_LOCK", gate_lock)
+
+    def cleanup_stale_stream():
+        result["cleared"] = routes._clear_stale_stream_state(session)
+
+    def start_new_stream():
+        assert gate_lock.lookup_finished.wait(2), "cleanup did not reach race point"
+        with routes.STREAMS_LOCK:
+            routes.STREAMS[new_stream_id] = queue.Queue()
+        with routes._get_session_agent_lock(session.session_id):
+            session.active_stream_id = new_stream_id
+            session.pending_user_message = "new prompt"
+            session.pending_attachments = ["new.txt"]
+            session.pending_started_at = 456
+            session.save()
+        gate_lock.writer_finished.set()
+
+    cleanup_thread = threading.Thread(target=cleanup_stale_stream)
+    writer_thread = threading.Thread(target=start_new_stream)
+    cleanup_thread.start()
+    writer_thread.start()
+    cleanup_thread.join(2)
+    writer_thread.join(2)
+
+    assert not cleanup_thread.is_alive()
+    assert not writer_thread.is_alive()
+    assert result["cleared"] is False
+    assert session.active_stream_id == new_stream_id
+    assert session.pending_user_message == "new prompt"
+    assert session.pending_attachments == ["new.txt"]
+    assert session.pending_started_at == 456
+
+
 def test_frontend_drops_inflight_cache_when_server_session_is_idle():
     marker = "If the server says the session is idle, discard any browser-side inflight"
     marker_pos = SESSIONS_SRC.index(marker)