fix(streamable-http): avoid startup race after initialize (#1675)

Author: Epochex

src/mcp/client/streamable_http.py

@@ -10,7 +10,7 @@
 
 import anyio
 import httpx
-from anyio.abc import TaskGroup
+from anyio.abc import TaskGroup, TaskStatus
 from httpx_sse import EventSource, ServerSentEvent, aconnect_sse
 from pydantic import ValidationError
 
@@ -437,10 +437,13 @@ async def post_writer(
         write_stream: ContextSendStream[SessionMessage],
         start_get_stream: Callable[[], None],
         tg: TaskGroup,
+        *,
+        task_status: TaskStatus[None] = anyio.TASK_STATUS_IGNORED,
     ) -> None:
         """Handle writing requests to the server."""
         try:
             async with write_stream_reader, read_stream_writer, write_stream:
+                task_status.started()
 
                 async def _handle_message(session_message: SessionMessage) -> None:
                     message = session_message.message
@@ -570,7 +573,7 @@ async def streamable_http_client(
             def start_get_stream() -> None:
                 tg.start_soon(transport.handle_get_stream, client, read_stream_writer)
 
-            tg.start_soon(
+            await tg.start(
                 transport.post_writer,
                 client,
                 write_stream_reader,

tests/shared/test_streamable_http.py

@@ -1071,6 +1071,45 @@ async def test_streamable_http_client_basic_connection(basic_server: None, basic
             assert result.server_info.name == SERVER_NAME
 
 
+@pytest.mark.anyio
+async def test_streamable_http_client_no_race_on_consecutive_requests(basic_server: None, basic_server_url: str):
+    """Regression test for a start-up race immediately after initialize().
+
+    In some cases, the first request after initialize() (e.g. list_tools())
+    could behave inconsistently. This test runs multiple short-lived sessions
+    to reliably catch any start-up race.
+    """
+    for iteration in range(10):  # pragma: no branch
+        async with streamable_http_client(f"{basic_server_url}/mcp") as (read_stream, write_stream):
+            async with ClientSession(read_stream, write_stream) as session:
+                await session.initialize()
+
+                tools = await session.list_tools()
+                assert len(tools.tools) == 10, f"Iteration {iteration}: expected 10 tools, got {len(tools.tools)}"
+                assert tools.tools[0].name == "test_tool"
+
+                tools2 = await session.list_tools()
+                assert len(tools2.tools) == 10
+
+                resource = await session.read_resource(uri="foobar://test-iteration")
+                assert len(resource.contents) == 1
+
+
+@pytest.mark.anyio
+async def test_streamable_http_client_rapid_request_sequence(basic_server: None, basic_server_url: str):
+    """Stress test for rapid sequences of requests."""
+    async with streamable_http_client(f"{basic_server_url}/mcp") as (read_stream, write_stream):
+        async with ClientSession(read_stream, write_stream) as session:
+            await session.initialize()
+
+            for i in range(20):
+                tools = await session.list_tools()
+                assert len(tools.tools) == 10, f"Request {i}: expected 10 tools, got {len(tools.tools)}"
+
+            resource = await session.read_resource(uri="foobar://final-test")
+            assert len(resource.contents) == 1
+
+
 @pytest.mark.anyio
 async def test_streamable_http_client_resource_read(initialized_client_session: ClientSession):
     """Test client resource read functionality."""