fix: drain completed streamable HTTP SSE responses

Author: he-yufeng

src/mcp/client/streamable_http.py

@@ -240,16 +240,18 @@ 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."""
@@ -342,6 +344,7 @@ async def _handle_sse_response(
 
         try:
             event_source = EventSource(response)
+            response_complete = False
             async for sse in event_source.aiter_sse():  # pragma: no branch
                 # Track last event ID for potential reconnection
                 if sse.id:
@@ -351,6 +354,9 @@ async def _handle_sse_response(
                 if sse.retry is not None:
                     retry_interval_ms = sse.retry
 
+                if response_complete:
+                    continue
+
                 is_complete = await self._handle_sse_event(
                     sse,
                     ctx.read_stream_writer,
@@ -359,10 +365,11 @@ 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 stream so the underlying HTTP connection remains reusable.
                 if is_complete:
-                    await response.aclose()
-                    return  # Normal completion, no reconnect needed
+                    response_complete = True
+            if response_complete:
+                return  # Normal completion, no reconnect needed
         except Exception:
             logger.debug("SSE stream ended", exc_info=True)  # pragma: no cover
 
@@ -404,22 +411,27 @@ async def _handle_reconnection(
                 # Track for potential further 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 sse.id:  # pragma: no branch
                         reconnect_last_event_id = sse.id
                     if sse.retry is not None:
                         reconnect_retry_ms = sse.retry
 
+                    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()
-                        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

@@ -27,9 +27,16 @@
 from starlette.requests import Request
 from starlette.routing import Mount
 
+import mcp.client.streamable_http as streamable_http_module
 from mcp import MCPError, types
 from mcp.client.session import ClientSession
-from mcp.client.streamable_http import StreamableHTTPTransport, streamable_http_client
+from mcp.client.streamable_http import (
+    RequestContext as StreamableHTTPClientRequestContext,
+)
+from mcp.client.streamable_http import (
+    StreamableHTTPTransport,
+    streamable_http_client,
+)
 from mcp.server import Server, ServerRequestContext
 from mcp.server.streamable_http import (
     MCP_PROTOCOL_VERSION_HEADER,
@@ -45,7 +52,7 @@
 from mcp.server.streamable_http_manager import StreamableHTTPSessionManager
 from mcp.server.transport_security import TransportSecuritySettings
 from mcp.shared._context import RequestContext
-from mcp.shared._context_streams import create_context_streams
+from mcp.shared._context_streams import ContextSendStream, create_context_streams
 from mcp.shared._httpx_utils import (
     MCP_DEFAULT_SSE_READ_TIMEOUT,
     MCP_DEFAULT_TIMEOUT,
@@ -1803,6 +1810,110 @@ async def test_handle_sse_event_skips_empty_data():
         await read_stream.aclose()
 
 
+class _FakeStreamResponse(httpx.Response):
+    def __init__(self) -> None:
+        super().__init__(200, request=httpx.Request("POST", "http://localhost:8000/mcp"))
+        self.closed_by_transport = False
+
+    async def aclose(self) -> None:
+        self.closed_by_transport = True
+        await super().aclose()
+
+
+def _response_sse(request_id: int | str) -> ServerSentEvent:
+    return ServerSentEvent(
+        event="message",
+        data=json.dumps({"jsonrpc": "2.0", "id": request_id, "result": {}}),
+        id="response-event",
+    )
+
+
+def _make_streamable_http_request_context(
+    request_id: int | str,
+    client: httpx.AsyncClient,
+    write_stream: ContextSendStream[SessionMessage | Exception],
+) -> StreamableHTTPClientRequestContext:
+    return StreamableHTTPClientRequestContext(
+        client=client,
+        session_id=None,
+        session_message=SessionMessage(JSONRPCRequest(jsonrpc="2.0", id=request_id, method="tools/list")),
+        metadata=None,
+        read_stream_writer=write_stream,
+    )
+
+
+@pytest.mark.anyio
+async def test_sse_response_drains_after_terminal_response(monkeypatch: pytest.MonkeyPatch):
+    """Terminal POST SSE responses are drained instead of force-closed."""
+    transport = StreamableHTTPTransport(url="http://localhost:8000/mcp")
+    response = _FakeStreamResponse()
+
+    class FakeEventSource:
+        def __init__(self, response: _FakeStreamResponse) -> None:
+            self.response = response
+
+        async def aiter_sse(self):
+            yield _response_sse(1)
+            yield ServerSentEvent(event="message", data="", id="drained-event")
+
+    async def fail_reconnect(*args: Any, **kwargs: Any) -> None:
+        raise AssertionError("terminal responses should not reconnect after draining")
+
+    monkeypatch.setattr(streamable_http_module, "EventSource", FakeEventSource)
+    monkeypatch.setattr(transport, "_handle_reconnection", fail_reconnect)
+
+    write_stream, read_stream = create_context_streams[SessionMessage | Exception](2)
+    async with httpx.AsyncClient() as client:
+        try:
+            ctx = _make_streamable_http_request_context(1, client, write_stream)
+            await transport._handle_sse_response(response, ctx)
+
+            assert response.closed_by_transport is False
+            message = await read_stream.receive()
+            assert isinstance(message, SessionMessage)
+            assert isinstance(message.message, types.JSONRPCResponse)
+            assert message.message.id == 1
+        finally:
+            await write_stream.aclose()
+            await read_stream.aclose()
+
+
+@pytest.mark.anyio
+async def test_reconnection_drains_after_terminal_response(monkeypatch: pytest.MonkeyPatch):
+    """Resumed GET responses use EOF draining instead of response.aclose()."""
+    transport = StreamableHTTPTransport(url="http://localhost:8000/mcp")
+    response = _FakeStreamResponse()
+
+    class FakeReconnectionEventSource:
+        def __init__(self, response: _FakeStreamResponse) -> None:
+            self.response = response
+
+        async def aiter_sse(self):
+            yield _response_sse("abc")
+            yield ServerSentEvent(event="message", data="", id="drained-event")
+
+    @asynccontextmanager
+    async def fake_aconnect_sse(*args: Any, **kwargs: Any):
+        yield FakeReconnectionEventSource(response)
+
+    monkeypatch.setattr(streamable_http_module, "aconnect_sse", fake_aconnect_sse)
+
+    write_stream, read_stream = create_context_streams[SessionMessage | Exception](2)
+    async with httpx.AsyncClient() as client:
+        try:
+            ctx = _make_streamable_http_request_context("abc", client, write_stream)
+            await transport._handle_reconnection(ctx, "previous-event", retry_interval_ms=0)
+
+            assert response.closed_by_transport is False
+            message = await read_stream.receive()
+            assert isinstance(message, SessionMessage)
+            assert isinstance(message.message, types.JSONRPCResponse)
+            assert message.message.id == "abc"
+        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)."""