Store server_info on ClientSession during initialization

Adds `_server_info` attribute to ClientSession that is populated from
InitializeResult.server_info after initialize() completes. Also adds
a `get_server_info()` accessor method matching get_server_capabilities().

This lets clients access the server's name and version without needing
to capture the initialize() return value.

Fixes #1018

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: kxbnb

src/mcp/client/session.py

@@ -133,6 +133,7 @@ def __init__(
         self._message_handler = message_handler or _default_message_handler
         self._tool_output_schemas: dict[str, dict[str, Any] | None] = {}
         self._server_capabilities: types.ServerCapabilities | None = None
+        self._server_info: types.Implementation | None = None
         self._experimental_features: ExperimentalClientFeatures | None = None
 
         # Experimental: Task handlers (use defaults if not provided)
@@ -190,6 +191,7 @@ async def initialize(self) -> types.InitializeResult:
             raise RuntimeError(f"Unsupported protocol version from the server: {result.protocol_version}")
 
         self._server_capabilities = result.capabilities
+        self._server_info = result.server_info
 
         await self.send_notification(types.InitializedNotification())
 
@@ -202,6 +204,13 @@ def get_server_capabilities(self) -> types.ServerCapabilities | None:
         """
         return self._server_capabilities
 
+    def get_server_info(self) -> types.Implementation | None:
+        """Return the server info received during initialization.
+
+        Returns None if the session has not been initialized yet.
+        """
+        return self._server_info
+
     @property
     def experimental(self) -> ExperimentalClientFeatures:
         """Experimental APIs for tasks and other features.

tests/client/test_session.py

@@ -607,6 +607,63 @@ async def mock_server():
         assert capabilities.tools.list_changed is False
 
 
+@pytest.mark.anyio
+async def test_get_server_info():
+    """Test that get_server_info returns None before init and server info after"""
+    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)
+
+    expected_server_info = Implementation(name="test-server", version="1.2.3")
+
+    async def mock_server():
+        session_message = await client_to_server_receive.receive()
+        jsonrpc_request = session_message.message
+        assert isinstance(jsonrpc_request, JSONRPCRequest)
+        request = client_request_adapter.validate_python(
+            jsonrpc_request.model_dump(by_alias=True, mode="json", exclude_none=True)
+        )
+        assert isinstance(request, InitializeRequest)
+
+        result = InitializeResult(
+            protocol_version=LATEST_PROTOCOL_VERSION,
+            capabilities=ServerCapabilities(),
+            server_info=expected_server_info,
+        )
+
+        async with server_to_client_send:
+            await server_to_client_send.send(
+                SessionMessage(
+                    JSONRPCResponse(
+                        jsonrpc="2.0",
+                        id=jsonrpc_request.id,
+                        result=result.model_dump(by_alias=True, mode="json", exclude_none=True),
+                    )
+                )
+            )
+            await client_to_server_receive.receive()
+
+    async with (
+        ClientSession(
+            server_to_client_receive,
+            client_to_server_send,
+        ) as session,
+        anyio.create_task_group() as tg,
+        client_to_server_send,
+        client_to_server_receive,
+        server_to_client_send,
+        server_to_client_receive,
+    ):
+        assert session.get_server_info() is None
+
+        tg.start_soon(mock_server)
+        await session.initialize()
+
+        server_info = session.get_server_info()
+        assert server_info is not None
+        assert server_info.name == "test-server"
+        assert server_info.version == "1.2.3"
+
+
 @pytest.mark.anyio
 @pytest.mark.parametrize(argnames="meta", argvalues=[None, {"toolMeta": "value"}])
 async def test_client_tool_call_with_meta(meta: RequestParamsMeta | None):