Add get_server_capabilities() to ClientSession (#1588)

src/mcp/client/session.py

@@ -134,6 +134,7 @@ class ClientSession(
         self._logging_callback = logging_callback or _default_logging_callback
         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
 
     async def initialize(self) -> types.InitializeResult:
         sampling = types.SamplingCapability() if self._sampling_callback is not _default_sampling_callback else None
@@ -170,10 +171,19 @@ class ClientSession(
         if result.protocolVersion not in SUPPORTED_PROTOCOL_VERSIONS:
             raise RuntimeError(f"Unsupported protocol version from the server: {result.protocolVersion}")
 
+        self._server_capabilities = result.capabilities
+
         await self.send_notification(types.ClientNotification(types.InitializedNotification()))
 
         return result
 
+    def get_server_capabilities(self) -> types.ServerCapabilities | None:
+        """Return the server capabilities received during initialization.
+
+        Returns None if the session has not been initialized yet.
+        """
+        return self._server_capabilities
+
     async def send_ping(self) -> types.EmptyResult:
         """Send a ping request."""
         return await self.send_request(

tests/client/test_session.py

@@ -504,6 +504,78 @@ async def test_client_capabilities_with_custom_callbacks():
     assert received_capabilities.roots.listChanged is True
 
 
+@pytest.mark.anyio
+async def test_get_server_capabilities():
+    """Test that get_server_capabilities returns None before init and capabilities 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_capabilities = ServerCapabilities(
+        logging=types.LoggingCapability(),
+        prompts=types.PromptsCapability(listChanged=True),
+        resources=types.ResourcesCapability(subscribe=True, listChanged=True),
+        tools=types.ToolsCapability(listChanged=False),
+    )
+
+    async def mock_server():
+        session_message = await client_to_server_receive.receive()
+        jsonrpc_request = session_message.message
+        assert isinstance(jsonrpc_request.root, JSONRPCRequest)
+        request = ClientRequest.model_validate(
+            jsonrpc_request.model_dump(by_alias=True, mode="json", exclude_none=True)
+        )
+        assert isinstance(request.root, InitializeRequest)
+
+        result = ServerResult(
+            InitializeResult(
+                protocolVersion=LATEST_PROTOCOL_VERSION,
+                capabilities=expected_capabilities,
+                serverInfo=Implementation(name="mock-server", version="0.1.0"),
+            )
+        )
+
+        async with server_to_client_send:
+            await server_to_client_send.send(
+                SessionMessage(
+                    JSONRPCMessage(
+                        JSONRPCResponse(
+                            jsonrpc="2.0",
+                            id=jsonrpc_request.root.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_capabilities() is None
+
+        tg.start_soon(mock_server)
+        await session.initialize()
+
+        capabilities = session.get_server_capabilities()
+        assert capabilities is not None
+        assert capabilities == expected_capabilities
+        assert capabilities.logging is not None
+        assert capabilities.prompts is not None
+        assert capabilities.prompts.listChanged is True
+        assert capabilities.resources is not None
+        assert capabilities.resources.subscribe is True
+        assert capabilities.tools is not None
+        assert capabilities.tools.listChanged is False
+
+
 @pytest.mark.anyio
 @pytest.mark.parametrize(argnames="meta", argvalues=[None, {"toolMeta": "value"}])
 async def test_client_tool_call_with_meta(meta: dict[str, Any] | None):