fix: fail fast when session is not started

Author: Epochex

src/mcp/client/session.py

@@ -155,6 +155,7 @@ def __init__(
         self._tool_output_schemas: dict[str, dict[str, Any] | None] = {}
         self._initialize_result: types.InitializeResult | None = None
         self._task_group: anyio.abc.TaskGroup | None = None
+        self._entered = False
         if dispatcher is not None:
             if read_stream is not None or write_stream is not None:
                 raise ValueError("pass read_stream/write_stream or dispatcher, not both")
@@ -168,6 +169,8 @@ def __init__(
             )
 
     async def __aenter__(self) -> Self:
+        if self._entered:
+            raise RuntimeError("Session is already running")
         self._task_group = anyio.create_task_group()
         await self._task_group.__aenter__()
         try:
@@ -184,6 +187,7 @@ async def __aenter__(self) -> Self:
             task_group.cancel_scope.shield = True
             await task_group.__aexit__(None, None, None)
             raise
+        self._entered = True
         return self
 
     async def __aexit__(
@@ -195,9 +199,12 @@ async def __aexit__(
         # Exit must not block: cancel the dispatcher and in-flight callbacks.
         assert self._task_group is not None
         self._task_group.cancel_scope.cancel()
-        result = await self._task_group.__aexit__(exc_type, exc_val, exc_tb)
-        await resync_tracer()
-        return result
+        try:
+            result = await self._task_group.__aexit__(exc_type, exc_val, exc_tb)
+            await resync_tracer()
+            return result
+        finally:
+            self._entered = False
 
     async def send_request(
         self,
@@ -216,6 +223,11 @@ async def send_request(
             MCPError: Error response, read timeout, or connection closed.
             RuntimeError: Called before entering the context manager.
         """
+        if self._task_group is None:
+            raise RuntimeError(
+                "Session is not running. Use it as an async context manager "
+                "(e.g. `async with ClientSession(...) as session:`)."
+            )
         data = request.model_dump(by_alias=True, mode="json", exclude_none=True)
         method: str = data["method"]
         opts: CallOptions = {}

tests/client/test_session.py

@@ -149,6 +149,23 @@ async def message_handler(  # pragma: no cover
     assert isinstance(initialized_notification, InitializedNotification)
 
 
+@pytest.mark.anyio
+async def test_client_session_requires_context_manager():
+    client_to_server_send, _client_to_server_receive = anyio.create_memory_object_stream[SessionMessage](1)
+    _server_to_client_send, server_to_client_receive = anyio.create_memory_object_stream[SessionMessage](1)
+
+    async with (
+        client_to_server_send,
+        _client_to_server_receive,
+        _server_to_client_send,
+        server_to_client_receive,
+    ):
+        session = ClientSession(server_to_client_receive, client_to_server_send)
+
+        with pytest.raises(RuntimeError, match="async context manager"):
+            await session.initialize()
+
+
 @pytest.mark.anyio
 async def test_client_session_custom_client_info():
     client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage](1)