feat: expose sampling_capabilities parameter for SEP-1577 tool support

Author: jlowin

src/mcp/client/session.py

@@ -115,6 +115,7 @@ def __init__(
         write_stream: MemoryObjectSendStream[SessionMessage],
         read_timeout_seconds: timedelta | None = None,
         sampling_callback: SamplingFnT | None = None,
+        sampling_capabilities: types.SamplingCapability | None = None,
         elicitation_callback: ElicitationFnT | None = None,
         list_roots_callback: ListRootsFnT | None = None,
         logging_callback: LoggingFnT | None = None,
@@ -132,6 +133,7 @@ def __init__(
         )
         self._client_info = client_info or DEFAULT_CLIENT_INFO
         self._sampling_callback = sampling_callback or _default_sampling_callback
+        self._sampling_capabilities = sampling_capabilities
         self._elicitation_callback = elicitation_callback or _default_elicitation_callback
         self._list_roots_callback = list_roots_callback or _default_list_roots_callback
         self._logging_callback = logging_callback or _default_logging_callback
@@ -144,7 +146,11 @@ def __init__(
         self._task_handlers = experimental_task_handlers or ExperimentalTaskHandlers()
 
     async def initialize(self) -> types.InitializeResult:
-        sampling = types.SamplingCapability() if self._sampling_callback is not _default_sampling_callback else None
+        sampling = (
+            (self._sampling_capabilities or types.SamplingCapability())
+            if self._sampling_callback is not _default_sampling_callback
+            else None
+        )
         elicitation = (
             types.ElicitationCapability(
                 form=types.FormElicitationCapability(),

tests/client/test_session.py

@@ -497,13 +497,93 @@ async def mock_server():
     # Custom sampling callback provided
     assert received_capabilities.sampling is not None
     assert isinstance(received_capabilities.sampling, types.SamplingCapability)
+    # Default sampling capabilities (no tools)
+    assert received_capabilities.sampling.tools is None
     # Custom list_roots callback provided
     assert received_capabilities.roots is not None
     assert isinstance(received_capabilities.roots, types.RootsCapability)
     # Should be True for custom callback
     assert received_capabilities.roots.listChanged is True
 
 
+@pytest.mark.anyio
+async def test_client_capabilities_with_sampling_tools():
+    """Test that sampling capabilities with tools are properly advertised"""
+    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)
+
+    received_capabilities = None
+
+    async def custom_sampling_callback(  # pragma: no cover
+        context: RequestContext["ClientSession", Any],
+        params: types.CreateMessageRequestParams,
+    ) -> types.CreateMessageResult | types.ErrorData:
+        return types.CreateMessageResult(
+            role="assistant",
+            content=types.TextContent(type="text", text="test"),
+            model="test-model",
+        )
+
+    async def mock_server():
+        nonlocal received_capabilities
+
+        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)
+        received_capabilities = request.root.params.capabilities
+
+        result = ServerResult(
+            InitializeResult(
+                protocolVersion=LATEST_PROTOCOL_VERSION,
+                capabilities=ServerCapabilities(),
+                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),
+                        )
+                    )
+                )
+            )
+            # Receive initialized notification
+            await client_to_server_receive.receive()
+
+    async with (
+        ClientSession(
+            server_to_client_receive,
+            client_to_server_send,
+            sampling_callback=custom_sampling_callback,
+            sampling_capabilities=types.SamplingCapability(tools=types.SamplingToolsCapability()),
+        ) as session,
+        anyio.create_task_group() as tg,
+        client_to_server_send,
+        client_to_server_receive,
+        server_to_client_send,
+        server_to_client_receive,
+    ):
+        tg.start_soon(mock_server)
+        await session.initialize()
+
+    # Assert that sampling capabilities with tools are properly advertised
+    assert received_capabilities is not None
+    assert received_capabilities.sampling is not None
+    assert isinstance(received_capabilities.sampling, types.SamplingCapability)
+    # Tools capability should be present
+    assert received_capabilities.sampling.tools is not None
+    assert isinstance(received_capabilities.sampling.tools, types.SamplingToolsCapability)
+
+
 @pytest.mark.anyio
 async def test_get_server_capabilities():
     """Test that get_server_capabilities returns None before init and capabilities after"""