fix: drain terminal streamable HTTP responses

Author: he-yufeng

src/mcp/client/streamable_http.py

@@ -240,16 +240,19 @@ async def _handle_resumption_request(self, ctx: RequestContext) -> None:
             event_source.response.raise_for_status()
             logger.debug("Resumption GET SSE connection established")
 
+            response_complete = False
             async for sse in event_source.aiter_sse():  # pragma: no branch
+                if response_complete:
+                    continue
+
                 is_complete = await self._handle_sse_event(
                     sse,
                     ctx.read_stream_writer,
                     original_request_id,
                     ctx.metadata.on_resumption_token_update if ctx.metadata else None,
                 )
                 if is_complete:
-                    await event_source.response.aclose()
-                    break
+                    response_complete = True
 
     async def _handle_post_request(self, ctx: RequestContext) -> None:
         """Handle a POST request with response processing."""
@@ -342,7 +345,11 @@ async def _handle_sse_response(
 
         try:
             event_source = EventSource(response)
+            response_complete = False
             async for sse in event_source.aiter_sse():  # pragma: no branch
+                if response_complete:
+                    continue
+
                 # Track last event ID for potential reconnection
                 if sse.id:
                     last_event_id = sse.id
@@ -359,13 +366,15 @@ async def _handle_sse_response(
                     is_initialization=is_initialization,
                 )
                 # If the SSE event indicates completion, like returning response/error
-                # break the loop
+                # keep draining the response to EOF so the HTTP connection can be reused.
                 if is_complete:
-                    await response.aclose()
-                    return  # Normal completion, no reconnect needed
+                    response_complete = True
         except Exception:
             logger.debug("SSE stream ended", exc_info=True)  # pragma: no cover
 
+        if response_complete:
+            return  # Normal completion, no reconnect needed
+
         # Stream ended without response - reconnect if we received an event with ID
         if last_event_id is not None:  # pragma: no branch
             logger.info("SSE stream disconnected, reconnecting...")
@@ -405,7 +414,11 @@ async def _handle_reconnection(
                 reconnect_last_event_id: str = last_event_id
                 reconnect_retry_ms = retry_interval_ms
 
+                response_complete = False
                 async for sse in event_source.aiter_sse():
+                    if response_complete:
+                        continue
+
                     if sse.id:  # pragma: no branch
                         reconnect_last_event_id = sse.id
                     if sse.retry is not None:
@@ -418,8 +431,10 @@ async def _handle_reconnection(
                         ctx.metadata.on_resumption_token_update if ctx.metadata else None,
                     )
                     if is_complete:
-                        await event_source.response.aclose()
-                        return
+                        response_complete = True
+
+                if response_complete:
+                    return
 
                 # Stream ended again without response - reconnect again (reset attempt counter)
                 logger.info("SSE stream disconnected, reconnecting...")

tests/shared/test_streamable_http.py

@@ -27,6 +27,7 @@
 from starlette.requests import Request
 from starlette.routing import Mount
 
+import mcp.client.streamable_http as streamable_http
 from mcp import MCPError, types
 from mcp.client.session import ClientSession
 from mcp.client.streamable_http import StreamableHTTPTransport, streamable_http_client
@@ -139,6 +140,38 @@ async def replay_events_after(  # pragma: no cover
         return target_stream_id
 
 
+class FakeStreamResponse:
+    def __init__(self) -> None:
+        self.close_count = 0
+
+    def raise_for_status(self) -> None:
+        pass
+
+    async def aclose(self) -> None:
+        self.close_count += 1
+
+
+class FakeEventSource:
+    def __init__(self, events: list[ServerSentEvent]) -> None:
+        self.response = FakeStreamResponse()
+        self.events = events
+        self.seen = 0
+
+    async def aiter_sse(self) -> AsyncIterator[ServerSentEvent]:
+        for event in self.events:
+            self.seen += 1
+            yield event
+
+
+def jsonrpc_response_event(request_id: str, event_id: str) -> ServerSentEvent:
+    return ServerSentEvent(
+        event="message",
+        data=json.dumps({"jsonrpc": "2.0", "id": request_id, "result": {}}),
+        id=event_id,
+        retry=None,
+    )
+
+
 @dataclass
 class ServerState:
     lock: anyio.Event = field(default_factory=anyio.Event)
@@ -1803,6 +1836,88 @@ async def test_handle_sse_event_skips_empty_data():
         await read_stream.aclose()
 
 
+@pytest.mark.anyio
+async def test_handle_sse_response_drains_after_terminal_event(monkeypatch: pytest.MonkeyPatch):
+    transport = StreamableHTTPTransport(url="http://localhost:8000/mcp")
+    response = FakeStreamResponse()
+    event_source = FakeEventSource(
+        [
+            jsonrpc_response_event("request-1", "event-1"),
+            ServerSentEvent(event="message", data="", id="event-2", retry=None),
+        ]
+    )
+    monkeypatch.setattr(streamable_http, "EventSource", lambda _response: event_source)
+
+    write_stream, read_stream = create_context_streams[SessionMessage | Exception](1)
+    try:
+        async with httpx.AsyncClient() as client:
+            ctx = streamable_http.RequestContext(
+                client=client,
+                session_id=None,
+                session_message=SessionMessage(
+                    JSONRPCRequest(jsonrpc="2.0", id="request-1", method="tools/call", params={})
+                ),
+                metadata=None,
+                read_stream_writer=write_stream,
+            )
+
+            await transport._handle_sse_response(response, ctx)
+
+        received = await read_stream.receive()
+        assert isinstance(received.message, types.JSONRPCResponse)
+        assert received.message.id == "request-1"
+        assert event_source.seen == 2
+        assert response.close_count == 0
+    finally:
+        await write_stream.aclose()
+        await read_stream.aclose()
+
+
+@pytest.mark.anyio
+async def test_reconnection_drains_after_terminal_event(monkeypatch: pytest.MonkeyPatch):
+    transport = StreamableHTTPTransport(url="http://localhost:8000/mcp")
+    event_source = FakeEventSource(
+        [
+            jsonrpc_response_event("request-1", "event-2"),
+            ServerSentEvent(event="message", data="", id="event-3", retry=None),
+        ]
+    )
+
+    async def sleep_noop(_delay: float) -> None:
+        pass
+
+    @asynccontextmanager
+    async def connect_sse(*args: Any, **kwargs: Any) -> AsyncIterator[FakeEventSource]:
+        yield event_source
+
+    monkeypatch.setattr(streamable_http.anyio, "sleep", sleep_noop)
+    monkeypatch.setattr(streamable_http, "aconnect_sse", connect_sse)
+
+    write_stream, read_stream = create_context_streams[SessionMessage | Exception](1)
+    try:
+        async with httpx.AsyncClient() as client:
+            ctx = streamable_http.RequestContext(
+                client=client,
+                session_id=None,
+                session_message=SessionMessage(
+                    JSONRPCRequest(jsonrpc="2.0", id="request-1", method="tools/call", params={})
+                ),
+                metadata=None,
+                read_stream_writer=write_stream,
+            )
+
+            await transport._handle_reconnection(ctx, last_event_id="event-1")
+
+        received = await read_stream.receive()
+        assert isinstance(received.message, types.JSONRPCResponse)
+        assert received.message.id == "request-1"
+        assert event_source.seen == 2
+        assert event_source.response.close_count == 0
+    finally:
+        await write_stream.aclose()
+        await read_stream.aclose()
+
+
 @pytest.mark.anyio
 async def test_priming_event_not_sent_for_old_protocol_version():
     """Test that _maybe_send_priming_event skips for old protocol versions (backwards compat)."""