Add client handling for sampling, list roots, ping (#218)

Adds sampling and list roots callbacks to the ClientSession, allowing the client to handle requests from the server.

Co-authored-by: TerminalMan <84923604+SecretiveShell@users.noreply.github.com>
Co-authored-by: David Soria Parra <davidsp@anthropic.com>

README.md

@@ -476,9 +476,21 @@ server_params = StdioServerParameters(
     env=None # Optional environment variables
 )
 
+# Optional: create a sampling callback
+async def handle_sampling_message(message: types.CreateMessageRequestParams) -> types.CreateMessageResult:
+    return types.CreateMessageResult(
+        role="assistant",
+        content=types.TextContent(
+            type="text",
+            text="Hello, world! from model",
+        ),
+        model="gpt-3.5-turbo",
+        stopReason="endTurn",
+    )
+
 async def run():
     async with stdio_client(server_params) as (read, write):
-        async with ClientSession(read, write) as session:
+        async with ClientSession(read, write, sampling_callback=handle_sampling_message) as session:
             # Initialize the connection
             await session.initialize()
 

src/mcp/client/session.py

@@ -1,13 +1,51 @@
 from datetime import timedelta
+from typing import Any, Protocol
 
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
-from pydantic import AnyUrl
+from pydantic import AnyUrl, TypeAdapter
 
 import mcp.types as types
-from mcp.shared.session import BaseSession
+from mcp.shared.context import RequestContext
+from mcp.shared.session import BaseSession, RequestResponder
 from mcp.shared.version import SUPPORTED_PROTOCOL_VERSIONS
 
 
+class SamplingFnT(Protocol):
+    async def __call__(
+        self,
+        context: RequestContext["ClientSession", Any],
+        params: types.CreateMessageRequestParams,
+    ) -> types.CreateMessageResult | types.ErrorData: ...
+
+
+class ListRootsFnT(Protocol):
+    async def __call__(
+        self, context: RequestContext["ClientSession", Any]
+    ) -> types.ListRootsResult |  types.ErrorData: ...    
+
+
+async def _default_sampling_callback(
+    context: RequestContext["ClientSession", Any],
+    params: types.CreateMessageRequestParams,
+) -> types.CreateMessageResult | types.ErrorData:
+    return types.ErrorData(
+        code=types.INVALID_REQUEST,
+        message="Sampling not supported",
+    )
+
+
+async def _default_list_roots_callback(
+    context: RequestContext["ClientSession", Any],
+) -> types.ListRootsResult |  types.ErrorData:
+    return types.ErrorData(
+        code=types.INVALID_REQUEST,
+        message="List roots not supported",
+    )
+
+
+ClientResponse = TypeAdapter(types.ClientResult | types.ErrorData)
+
+
 class ClientSession(
     BaseSession[
         types.ClientRequest,
@@ -22,6 +60,8 @@ class ClientSession(
         read_stream: MemoryObjectReceiveStream[types.JSONRPCMessage | Exception],
         write_stream: MemoryObjectSendStream[types.JSONRPCMessage],
         read_timeout_seconds: timedelta | None = None,
+        sampling_callback: SamplingFnT | None = None,
+        list_roots_callback: ListRootsFnT | None = None,
     ) -> None:
         super().__init__(
             read_stream,
@@ -30,8 +70,24 @@ class ClientSession(
             types.ServerNotification,
             read_timeout_seconds=read_timeout_seconds,
         )
+        self._sampling_callback = sampling_callback or _default_sampling_callback
+        self._list_roots_callback = list_roots_callback or _default_list_roots_callback
 
     async def initialize(self) -> types.InitializeResult:
+        sampling = (
+            types.SamplingCapability() if self._sampling_callback is not None else None
+        )
+        roots = (
+            types.RootsCapability(
+                # TODO: Should this be based on whether we
+                # _will_ send notifications, or only whether
+                # they're supported?
+                listChanged=True,
+            )
+            if self._list_roots_callback is not None
+            else None
+        )
+
         result = await self.send_request(
             types.ClientRequest(
                 types.InitializeRequest(
@@ -39,14 +95,9 @@ class ClientSession(
                     params=types.InitializeRequestParams(
                         protocolVersion=types.LATEST_PROTOCOL_VERSION,
                         capabilities=types.ClientCapabilities(
-                            sampling=None,
+                            sampling=sampling,
                             experimental=None,
-                            roots=types.RootsCapability(
+                            roots=roots,
-                                # TODO: Should this be based on whether we
-                                # _will_ send notifications, or only whether
-                                # they're supported?
-                                listChanged=True
-                            ),
                         ),
                         clientInfo=types.Implementation(name="mcp", version="0.1.0"),
                     ),
@@ -243,3 +294,32 @@ class ClientSession(
                 )
             )
         )
+
+    async def _received_request(
+        self, responder: RequestResponder[types.ServerRequest, types.ClientResult]
+    ) -> None:
+        ctx = RequestContext[ClientSession, Any](
+            request_id=responder.request_id,
+            meta=responder.request_meta,
+            session=self,
+            lifespan_context=None,
+        )
+
+        match responder.request.root:
+            case types.CreateMessageRequest(params=params):
+                with responder:
+                    response = await self._sampling_callback(ctx, params)
+                    client_response = ClientResponse.validate_python(response)
+                    await responder.respond(client_response)
+
+            case types.ListRootsRequest():
+                with responder:
+                    response = await self._list_roots_callback(ctx)
+                    client_response = ClientResponse.validate_python(response)
+                    await responder.respond(client_response)
+                    
+            case types.PingRequest():
+                with responder:
+                    return await responder.respond(
+                        types.ClientResult(root=types.EmptyResult())
+                    )

src/mcp/shared/memory.py

@@ -9,7 +9,7 @@ from typing import AsyncGenerator
 import anyio
 from anyio.streams.memory import MemoryObjectReceiveStream, MemoryObjectSendStream
 
-from mcp.client.session import ClientSession
+from mcp.client.session import ClientSession, ListRootsFnT, SamplingFnT
 from mcp.server import Server
 from mcp.types import JSONRPCMessage
 
@@ -54,6 +54,8 @@ async def create_client_server_memory_streams() -> (
 async def create_connected_server_and_client_session(
     server: Server,
     read_timeout_seconds: timedelta | None = None,
+    sampling_callback: SamplingFnT | None = None,
+    list_roots_callback: ListRootsFnT | None = None,
     raise_exceptions: bool = False,
 ) -> AsyncGenerator[ClientSession, None]:
     """Creates a ClientSession that is connected to a running MCP server."""
@@ -80,6 +82,8 @@ async def create_connected_server_and_client_session(
                     read_stream=client_read,
                     write_stream=client_write,
                     read_timeout_seconds=read_timeout_seconds,
+                    sampling_callback=sampling_callback,
+                    list_roots_callback=list_roots_callback,
                 ) as client_session:
                     await client_session.initialize()
                     yield client_session

tests/client/test_list_roots_callback.py

@@ -0,0 +1,70 @@
+import pytest
+from pydantic import FileUrl
+
+from mcp.client.session import ClientSession
+from mcp.server.fastmcp.server import Context
+from mcp.shared.context import RequestContext
+from mcp.shared.memory import (
+    create_connected_server_and_client_session as create_session,
+)
+from mcp.types import (
+    ListRootsResult,
+    Root,
+    TextContent,
+)
+
+
+@pytest.mark.anyio
+async def test_list_roots_callback():
+    from mcp.server.fastmcp import FastMCP
+
+    server = FastMCP("test")
+
+    callback_return = ListRootsResult(
+        roots=[
+            Root(
+                uri=FileUrl("file://users/fake/test"),
+                name="Test Root 1",
+            ),
+            Root(
+                uri=FileUrl("file://users/fake/test/2"),
+                name="Test Root 2",
+            ),
+        ]
+    )
+
+    async def list_roots_callback(
+        context: RequestContext[ClientSession, None],
+    ) -> ListRootsResult:
+        return callback_return
+
+    @server.tool("test_list_roots")
+    async def test_list_roots(context: Context, message: str):
+        roots = await context.session.list_roots()
+        assert roots == callback_return
+        return True
+
+    # Test with list_roots callback
+    async with create_session(
+        server._mcp_server, list_roots_callback=list_roots_callback
+    ) as client_session:
+        # Make a request to trigger sampling callback
+        result = await client_session.call_tool(
+            "test_list_roots", {"message": "test message"}
+        )
+        assert result.isError is False
+        assert isinstance(result.content[0], TextContent)
+        assert result.content[0].text == "true"
+
+    # Test without list_roots callback
+    async with create_session(server._mcp_server) as client_session:
+        # Make a request to trigger sampling callback
+        result = await client_session.call_tool(
+            "test_list_roots", {"message": "test message"}
+        )
+        assert result.isError is True
+        assert isinstance(result.content[0], TextContent)
+        assert (
+            result.content[0].text
+            == "Error executing tool test_list_roots: List roots not supported"
+        )

tests/client/test_sampling_callback.py

@@ -0,0 +1,73 @@
+import pytest
+
+from mcp.client.session import ClientSession
+from mcp.shared.context import RequestContext
+from mcp.shared.memory import (
+    create_connected_server_and_client_session as create_session,
+)
+from mcp.types import (
+    CreateMessageRequestParams,
+    CreateMessageResult,
+    SamplingMessage,
+    TextContent,
+)
+
+
+@pytest.mark.anyio
+async def test_sampling_callback():
+    from mcp.server.fastmcp import FastMCP
+
+    server = FastMCP("test")
+
+    callback_return = CreateMessageResult(
+        role="assistant",
+        content=TextContent(
+            type="text", text="This is a response from the sampling callback"
+        ),
+        model="test-model",
+        stopReason="endTurn",
+    )
+
+    async def sampling_callback(
+        context: RequestContext[ClientSession, None],
+        params: CreateMessageRequestParams,
+    ) -> CreateMessageResult:
+        return callback_return
+
+    @server.tool("test_sampling")
+    async def test_sampling_tool(message: str):
+        value = await server.get_context().session.create_message(
+            messages=[
+                SamplingMessage(
+                    role="user", content=TextContent(type="text", text=message)
+                )
+            ],
+            max_tokens=100,
+        )
+        assert value == callback_return
+        return True
+
+    # Test with sampling callback
+    async with create_session(
+        server._mcp_server, sampling_callback=sampling_callback
+    ) as client_session:
+        # Make a request to trigger sampling callback
+        result = await client_session.call_tool(
+            "test_sampling", {"message": "Test message for sampling"}
+        )
+        assert result.isError is False
+        assert isinstance(result.content[0], TextContent)
+        assert result.content[0].text == "true"
+
+    # Test without sampling callback
+    async with create_session(server._mcp_server) as client_session:
+        # Make a request to trigger sampling callback
+        result = await client_session.call_tool(
+            "test_sampling", {"message": "Test message for sampling"}
+        )
+        assert result.isError is True
+        assert isinstance(result.content[0], TextContent)
+        assert (
+            result.content[0].text
+            == "Error executing tool test_sampling: Sampling not supported"
+        )

tests/client/test_stdio.py

@@ -1,12 +1,17 @@
+import shutil
+
 import pytest
 
 from mcp.client.stdio import StdioServerParameters, stdio_client
 from mcp.types import JSONRPCMessage, JSONRPCRequest, JSONRPCResponse
 
+tee: str = shutil.which("tee")  # type: ignore
+
 
 @pytest.mark.anyio
+@pytest.mark.skipif(tee is None, reason="could not find tee command")
 async def test_stdio_client():
-    server_parameters = StdioServerParameters(command="/usr/bin/tee")
+    server_parameters = StdioServerParameters(command=tee)
 
     async with stdio_client(server_parameters) as (read_stream, write_stream):
         # Test sending and receiving messages