diff --git a/api/routes.py b/api/routes.py
index 7d30ab90..b73ec9af 100644
--- a/api/routes.py
+++ b/api/routes.py
@@ -22810,6 +22810,30 @@ def _prepare_chat_start_session_for_stream(
         s.save()
 
 
+def _cleanup_chat_start_launch_failure(session, stream_id: str) -> None:
+    """Release state registered before a worker thread successfully starts."""
+    clear_session_writeback_owner_if_owned(session.session_id, stream_id)
+    unregister_stream_owner(stream_id)
+    with STREAMS_LOCK:
+        STREAMS.pop(stream_id, None)
+    STREAM_GOAL_RELATED.pop(stream_id, None)
+    if getattr(session, "active_stream_id", None) != stream_id:
+        return
+    session.active_stream_id = None
+    session.pending_user_message = None
+    session.pending_attachments = []
+    session.pending_started_at = None
+    session.pending_user_source = None
+    try:
+        session.save()
+    except Exception:
+        logger.debug(
+            "Failed to persist chat-start cleanup after worker launch failure for %s",
+            stream_id,
+            exc_info=True,
+        )
+
+
 def _is_hidden_empty_session(s) -> bool:
     return (
         getattr(s, "title", "Untitled") == "Untitled"
@@ -23393,6 +23417,7 @@ def _start_chat_stream_for_session(
                 _clear_gateway_run_starting(stream_id)
             except Exception:
                 logger.debug("Failed to record gateway run-start failure for stream %s", stream_id, exc_info=True)
+        _cleanup_chat_start_launch_failure(s, stream_id)
         raise
     response = {
         "stream_id": stream_id,
diff --git a/tests/test_issue6869_gateway_launch_failure.py b/tests/test_issue6869_gateway_launch_failure.py
new file mode 100644
index 00000000..7d8c6fe0
--- /dev/null
+++ b/tests/test_issue6869_gateway_launch_failure.py
@@ -0,0 +1,93 @@
+"""Regression tests for #6869: failed Gateway worker launch cleanup."""
+
+from types import SimpleNamespace
+
+import api.config as config
+import api.routes as routes
+from api import turn_journal
+
+
+def test_gateway_thread_start_failure_releases_writeback_owner_and_stream_state(monkeypatch):
+    """A thread-start exception must not leave one owner per failed Gateway launch."""
+    class FailingThread:
+        def __init__(self, *args, **kwargs):
+            pass
+
+        def start(self):
+            raise RuntimeError("thread launch failed")
+
+    def fake_prepare(session, *, stream_id, **kwargs):
+        session.active_stream_id = stream_id
+        session.pending_user_message = kwargs["msg"]
+        session.pending_started_at = 1.0
+        config.register_session_writeback_owner(session.session_id, stream_id)
+
+    monkeypatch.setattr(routes.threading, "Thread", FailingThread)
+    monkeypatch.setattr(routes, "_prepare_chat_start_session_for_stream", fake_prepare)
+    monkeypatch.setattr(routes, "set_last_workspace", lambda _workspace: None)
+    monkeypatch.setattr(routes, "_is_hidden_empty_session", lambda _session: False)
+    monkeypatch.setattr(
+        turn_journal,
+        "append_turn_journal_event",
+        lambda *_args, **_kwargs: {},
+    )
+
+    for index in range(3):
+        session = SimpleNamespace(
+            session_id=f"session-launch-failure-{index}",
+            title="Existing",
+            active_stream_id=None,
+            pending_user_message=None,
+            pending_attachments=[],
+            pending_started_at=None,
+            pending_user_source=None,
+            messages=[{"role": "user", "content": "old"}],
+            workspace="/tmp",
+            model="old-model",
+            model_provider=None,
+            worktree_path=None,
+            profile=None,
+            save=lambda: None,
+        )
+
+        try:
+            routes._start_chat_stream_for_session(
+                session,
+                msg="start gateway turn",
+                attachments=[],
+                workspace="/tmp",
+                model="test-model",
+                external_runtime_owned=True,
+            )
+        except RuntimeError as exc:
+            assert str(exc) == "thread launch failed"
+        else:
+            raise AssertionError("thread-start failure must propagate")
+
+    assert config.SESSION_WRITEBACK_OWNERS == {}
+    assert config.STREAM_SESSION_OWNERS == {}
+    assert config.STREAMS == {}
+
+
+def test_gateway_launch_failure_cleanup_does_not_clear_successor_owner():
+    """Cleanup must be compare-and-clear when a successor already took over."""
+    session = SimpleNamespace(
+        session_id="session-launch-successor",
+        active_stream_id="new-stream",
+        pending_user_message="new prompt",
+        pending_attachments=["new.txt"],
+        pending_started_at=2.0,
+        pending_user_source="webui",
+        save=lambda: None,
+    )
+    config.register_session_writeback_owner(session.session_id, "old-stream")
+    config.register_session_writeback_owner(session.session_id, "new-stream")
+    config.register_stream_owner("old-stream", session.session_id)
+    config.STREAMS["old-stream"] = object()
+
+    routes._cleanup_chat_start_launch_failure(session, "old-stream")
+
+    assert config.session_writeback_owner(session.session_id) == "new-stream"
+    assert session.active_stream_id == "new-stream"
+    assert session.pending_user_message == "new prompt"
+    assert "old-stream" not in config.STREAMS