simply nesting handling

src/fastmcp/client/client.py

@@ -44,10 +44,9 @@ class Client:
         read_timeout_seconds: datetime.timedelta | None = None,
     ):
         self.transport = infer_transport(transport)
-        # stack to record nested context manager, None is pushed if reuse existing session
-        self._session_cms: list[
-            tuple[AbstractAsyncContextManager[ClientSession] | None, ClientSession]
-        ] = []
+        self._session: ClientSession | None = None
+        self._session_cms: AbstractAsyncContextManager[ClientSession] | None = None
+        self._nesting_counter: int = 0
 
         self._session_kwargs: SessionKwargs = {
             "sampling_callback": None,
@@ -66,12 +65,11 @@ class Client:
     @property
     def session(self) -> ClientSession:
         """Get the current active session. Raises RuntimeError if not connected."""
-        if not self._session_cms:
+        if not self._session:
             raise RuntimeError(
                 "Client is not connected. Use 'async with client:' context manager first."
             )
-        _, session = self._session_cms[-1]
-        return session
+        return self._session
 
     def set_roots(self, roots: RootsList | RootsHandler) -> None:
         """Set the roots for the client. This does not automatically call `send_roots_list_changed`."""
@@ -85,24 +83,24 @@ class Client:
 
     def is_connected(self) -> bool:
         """Check if the client is currently connected."""
-        return len(self._session_cms) > 0
+        return self._session is not None
 
     async def __aenter__(self):
-        if self._session_cms:
-            # share the current session, push a None as context manager to avoid close it in aexit
-            _, session = self._session_cms[-1]
-            self._session_cms.append((None, session))
-        else:
+        if self._nesting_counter == 0:
             # create new session
-            session_cm = self.transport.connect_session(**self._session_kwargs)
-            session = await session_cm.__aenter__()
-            self._session_cms.append((session_cm, session))
+            self._session_cm = self.transport.connect_session(**self._session_kwargs)
+            self._session = await self._session_cm.__aenter__()
+
+        self._nesting_counter += 1
         return self
 
     async def __aexit__(self, exc_type, exc_val, exc_tb):
-        cm, _ = self._session_cms.pop()
-        if cm is not None:
-            await cm.__aexit__(exc_type, exc_val, exc_tb)
+        self._nesting_counter -= 0
+
+        if self._nesting_counter == 0 and self._session_cms is not None:
+            await self._session_cms.__aexit__(exc_type, exc_val, exc_tb)
+            self._session_cms = None
+            self._session = None
 
     # --- MCP Client Methods ---
     async def ping(self) -> None: