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12 changes: 9 additions & 3 deletions src/mcp/client/session.py
Original file line number Diff line number Diff line change
Expand Up @@ -25,7 +25,7 @@ async def __call__(
self,
context: RequestContext["ClientSession", Any],
params: types.CreateMessageRequestParams,
) -> types.CreateMessageResult | types.ErrorData: ... # pragma: no branch
) -> types.CreateMessageResult | types.CreateMessageResultWithTools | types.ErrorData: ... # pragma: no branch


class ElicitationFnT(Protocol):
Expand Down Expand Up @@ -65,7 +65,7 @@ async def _default_message_handler(
async def _default_sampling_callback(
context: RequestContext["ClientSession", Any],
params: types.CreateMessageRequestParams,
) -> types.CreateMessageResult | types.ErrorData:
) -> types.CreateMessageResult | types.CreateMessageResultWithTools | types.ErrorData:
return types.ErrorData(
code=types.INVALID_REQUEST,
message="Sampling not supported",
Expand Down Expand Up @@ -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,
Expand All @@ -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
Expand All @@ -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(),
Expand Down
1 change: 1 addition & 0 deletions src/mcp/types.py
Original file line number Diff line number Diff line change
Expand Up @@ -1928,6 +1928,7 @@ class ElicitationRequiredErrorData(BaseModel):
ClientResultType: TypeAlias = (
EmptyResult
| CreateMessageResult
| CreateMessageResultWithTools
| ListRootsResult
| ElicitResult
| GetTaskResult
Expand Down
80 changes: 80 additions & 0 deletions tests/client/test_session.py
Original file line number Diff line number Diff line change
Expand Up @@ -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"""
Expand Down