fix(client): respect negotiated capabilities in ClientSessionGroup

Author: Epochex

src/mcp/client/session_group.py

@@ -332,6 +332,8 @@ async def _establish_session(
     async def _aggregate_components(self, server_info: types.Implementation, session: mcp.ClientSession) -> None:
         """Aggregates prompts, resources, and tools from a given session."""
 
+        capabilities = session.initialize_result.capabilities if session.initialize_result else None
+
         # Create a reverse index so we can find all prompts, resources, and
         # tools belonging to this session. Used for removing components from
         # the session group via self.disconnect_from_server.
@@ -345,35 +347,38 @@ async def _aggregate_components(self, server_info: types.Implementation, session
         tool_to_session_temp: dict[str, mcp.ClientSession] = {}
 
         # Query the server for its prompts and aggregate to list.
-        try:
-            prompts = (await session.list_prompts()).prompts
-            for prompt in prompts:
-                name = self._component_name(prompt.name, server_info)
-                prompts_temp[name] = prompt
-                component_names.prompts.add(name)
-        except MCPError as err:  # pragma: no cover
-            logging.warning(f"Could not fetch prompts: {err}")
+        if capabilities is None or capabilities.prompts is not None:
+            try:
+                prompts = (await session.list_prompts()).prompts
+                for prompt in prompts:
+                    name = self._component_name(prompt.name, server_info)
+                    prompts_temp[name] = prompt
+                    component_names.prompts.add(name)
+            except MCPError as err:  # pragma: no cover
+                logging.warning(f"Could not fetch prompts: {err}")
 
         # Query the server for its resources and aggregate to list.
-        try:
-            resources = (await session.list_resources()).resources
-            for resource in resources:
-                name = self._component_name(resource.name, server_info)
-                resources_temp[name] = resource
-                component_names.resources.add(name)
-        except MCPError as err:  # pragma: no cover
-            logging.warning(f"Could not fetch resources: {err}")
+        if capabilities is None or capabilities.resources is not None:
+            try:
+                resources = (await session.list_resources()).resources
+                for resource in resources:
+                    name = self._component_name(resource.name, server_info)
+                    resources_temp[name] = resource
+                    component_names.resources.add(name)
+            except MCPError as err:  # pragma: no cover
+                logging.warning(f"Could not fetch resources: {err}")
 
         # Query the server for its tools and aggregate to list.
-        try:
-            tools = (await session.list_tools()).tools
-            for tool in tools:
-                name = self._component_name(tool.name, server_info)
-                tools_temp[name] = tool
-                tool_to_session_temp[name] = session
-                component_names.tools.add(name)
-        except MCPError as err:  # pragma: no cover
-            logging.warning(f"Could not fetch tools: {err}")
+        if capabilities is None or capabilities.tools is not None:
+            try:
+                tools = (await session.list_tools()).tools
+                for tool in tools:
+                    name = self._component_name(tool.name, server_info)
+                    tools_temp[name] = tool
+                    tool_to_session_temp[name] = session
+                    component_names.tools.add(name)
+            except MCPError as err:  # pragma: no cover
+                logging.warning(f"Could not fetch tools: {err}")
 
         # Clean up exit stack for session if we couldn't retrieve anything
         # from the server.

tests/client/test_session_group.py

@@ -1,4 +1,5 @@
 import contextlib
+import logging
 from unittest import mock
 
 import httpx
@@ -125,6 +126,50 @@ async def test_client_session_group_connect_to_server(mock_exit_stack: contextli
     mock_session.list_prompts.assert_awaited_once()
 
 
+@pytest.mark.anyio
+async def test_client_session_group_connect_with_session_respects_negotiated_capabilities(
+    caplog: pytest.LogCaptureFixture,
+):
+    from mcp import Client
+    from mcp.server import Server, ServerRequestContext
+
+    async def handle_list_tools(
+        ctx: ServerRequestContext, params: types.PaginatedRequestParams | None
+    ) -> types.ListToolsResult:
+        return types.ListToolsResult(
+            tools=[
+                types.Tool(
+                    name="ping",
+                    description="Ping",
+                    input_schema={"type": "object", "properties": {}},
+                )
+            ]
+        )
+
+    async def handle_call_tool(ctx: ServerRequestContext, params: types.CallToolRequestParams) -> types.CallToolResult:
+        return types.CallToolResult(content=[types.TextContent(type="text", text="pong")])
+
+    server = Server(
+        "tools-only-server",
+        on_list_tools=handle_list_tools,
+        on_call_tool=handle_call_tool,
+    )
+
+    group = ClientSessionGroup()
+
+    with caplog.at_level(logging.WARNING):
+        async with Client(server) as client:
+            assert client.initialize_result.capabilities.prompts is None
+            assert client.initialize_result.capabilities.resources is None
+
+            client.session.list_prompts = mock.AsyncMock(side_effect=AssertionError("list_prompts() was called"))
+            client.session.list_resources = mock.AsyncMock(side_effect=AssertionError("list_resources() was called"))
+
+            await group.connect_with_session(client.initialize_result.server_info, client.session)
+
+    assert not caplog.records
+
+
 @pytest.mark.anyio
 async def test_client_session_group_connect_to_server_with_name_hook(mock_exit_stack: contextlib.AsyncExitStack):
     """Test connecting with a component name hook."""