Fix: Prevent hang when ClientSession is initialized without context manager

Author: newbiehwang

src/mcp/client/session.py

@@ -1,5 +1,6 @@
 import logging
 from typing import Any, Protocol, overload
+from types import TracebackType
 
 import anyio.lowlevel
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
@@ -140,11 +141,33 @@ def __init__(
         self._tool_output_schemas: dict[str, dict[str, Any] | None] = {}
         self._server_capabilities: types.ServerCapabilities | None = None
         self._experimental_features: ExperimentalClientFeatures | None = None
+        self._entered = False
 
         # Experimental: Task handlers (use defaults if not provided)
         self._task_handlers = experimental_task_handlers or ExperimentalTaskHandlers()
 
+    async def __aenter__(self) -> "ClientSession":
+        self._entered = True
+        await super().__aenter__()
+        return self
+
+    async def __aexit__(
+        self,
+        exc_type: type[BaseException] | None,
+        exc_value: BaseException | None,
+        traceback: TracebackType | None,
+    ) -> None:
+        self._entered = False
+        await super().__aexit__(exc_type, exc_value, traceback)
+
+    def _check_is_active(self) -> None:
+        if not self._entered:
+            raise RuntimeError(
+                "ClientSession must be used within an 'async with' block."
+            )
+
     async def initialize(self) -> types.InitializeResult:
+        self._check_is_active()
         sampling = (
             (self._sampling_capabilities or types.SamplingCapability())
             if self._sampling_callback is not _default_sampling_callback