fix(gateway): enqueue SSE EOS sentinel on task completion

tests/gateway/test_api_server.py

@@ -681,6 +681,56 @@ class TestChatCompletionsEndpoint:
                 assert "[DONE]" in body
                 assert "Hello!" in body
 
+    @pytest.mark.asyncio
+    async def test_stream_task_done_callback_enqueues_eos_for_chat_completions(self, adapter):
+        """Regression guard for #24451: completion callback must signal SSE EOS."""
+        app = _create_app(adapter)
+        async with TestClient(TestServer(app)) as cli:
+            class _FakeTask:
+                def __init__(self):
+                    self.callbacks = []
+
+                def add_done_callback(self, cb):
+                    self.callbacks.append(cb)
+
+            fake_task = _FakeTask()
+
+            def _fake_ensure_future(coro):
+                # We short-circuit task scheduling in this unit test.
+                coro.close()
+                return fake_task
+
+            with (
+                patch.object(
+                    adapter,
+                    "_run_agent",
+                    new=AsyncMock(
+                        return_value=(
+                            {"final_response": "ok", "messages": [], "api_calls": 1},
+                            {"input_tokens": 1, "output_tokens": 1, "total_tokens": 2},
+                        )
+                    ),
+                ),
+                patch("gateway.platforms.api_server.asyncio.ensure_future", side_effect=_fake_ensure_future),
+                patch.object(adapter, "_write_sse_chat_completion", new_callable=AsyncMock) as mock_write_sse,
+            ):
+                mock_write_sse.return_value = web.Response(status=200, text="ok")
+                resp = await cli.post(
+                    "/v1/chat/completions",
+                    json={
+                        "model": "test",
+                        "messages": [{"role": "user", "content": "hi"}],
+                        "stream": True,
+                    },
+                )
+                assert resp.status == 200
+
+            assert len(fake_task.callbacks) == 1
+            stream_q = mock_write_sse.call_args.args[4]
+            assert stream_q.empty()
+            fake_task.callbacks[0](fake_task)
+            assert stream_q.get_nowait() is None
+
     @pytest.mark.asyncio
     async def test_stream_sends_keepalive_during_quiet_tool_gap(self, adapter):
         """Idle SSE streams should send keepalive comments while tools run silently."""
@@ -1676,6 +1726,52 @@ class TestResponsesStreaming:
                 assert "Hello" in body
                 assert " world" in body
 
+    @pytest.mark.asyncio
+    async def test_stream_task_done_callback_enqueues_eos_for_responses(self, adapter):
+        """Regression guard for #24451 on /v1/responses streaming path."""
+        app = _create_app(adapter)
+        async with TestClient(TestServer(app)) as cli:
+            class _FakeTask:
+                def __init__(self):
+                    self.callbacks = []
+
+                def add_done_callback(self, cb):
+                    self.callbacks.append(cb)
+
+            fake_task = _FakeTask()
+
+            def _fake_ensure_future(coro):
+                # We short-circuit task scheduling in this unit test.
+                coro.close()
+                return fake_task
+
+            with (
+                patch.object(
+                    adapter,
+                    "_run_agent",
+                    new=AsyncMock(
+                        return_value=(
+                            {"final_response": "ok", "messages": [], "api_calls": 1},
+                            {"input_tokens": 1, "output_tokens": 1, "total_tokens": 2},
+                        )
+                    ),
+                ),
+                patch("gateway.platforms.api_server.asyncio.ensure_future", side_effect=_fake_ensure_future),
+                patch.object(adapter, "_write_sse_responses", new_callable=AsyncMock) as mock_write_sse,
+            ):
+                mock_write_sse.return_value = web.Response(status=200, text="ok")
+                resp = await cli.post(
+                    "/v1/responses",
+                    json={"model": "hermes-agent", "input": "hi", "stream": True},
+                )
+                assert resp.status == 200
+
+            assert len(fake_task.callbacks) == 1
+            stream_q = mock_write_sse.call_args.kwargs["stream_q"]
+            assert stream_q.empty()
+            fake_task.callbacks[0](fake_task)
+            assert stream_q.get_nowait() is None
+
     @pytest.mark.asyncio
     async def test_stream_emits_function_call_and_output_items(self, adapter):
         app = _create_app(adapter)
@@ -3061,4 +3157,3 @@ class TestSessionKeyHeader:
             assert resp.status == 200
             data = await resp.json()
             assert data["features"]["session_key_header"] == "X-Hermes-Session-Key"
-