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[v2] Dispatcher/ServerRunner receive-path swap — replaces BaseSession (#2710)

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tests/server/test_session.py
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@ -1,535 +1,160 @@
from typing import Any
"""Tests for `ServerSession`.
`ServerSession` is a thin proxy over a dispatcher and a `Connection`. Tested
with a stub dispatcher so we can assert what reaches the wire (method, params,
`CallOptions`, related-request-id) without standing up a full transport.
"""
from collections.abc import Mapping
from typing import Any, cast
import anyio
import pytest
from mcp import types
from mcp.client.session import ClientSession
from mcp.server import Server, ServerRequestContext
from mcp.server.lowlevel import NotificationOptions
from mcp.server.models import InitializationOptions
from mcp.server.connection import Connection
from mcp.server.session import ServerSession
from mcp.shared.exceptions import MCPError
from mcp.shared.message import SessionMessage
from mcp.shared.session import RequestResponder
from mcp.shared.dispatcher import CallOptions
from mcp.shared.exceptions import NoBackChannelError
from mcp.shared.jsonrpc_dispatcher import JSONRPCDispatcher
from mcp.shared.message import ServerMessageMetadata
from mcp.types import (
ClientNotification,
CompletionsCapability,
InitializedNotification,
PromptsCapability,
ResourcesCapability,
ServerCapabilities,
LATEST_PROTOCOL_VERSION,
ClientCapabilities,
Implementation,
InitializeRequestParams,
SamplingCapability,
SamplingToolsCapability,
)
@pytest.mark.anyio
async def test_server_session_initialize():
server_to_client_send, server_to_client_receive = anyio.create_memory_object_stream[SessionMessage](1)
client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage](1)
class StubDispatcher:
"""Records `send_raw_request` / `notify` calls and returns a canned result."""
# Create a message handler to catch exceptions
async def message_handler( # pragma: no cover
message: RequestResponder[types.ServerRequest, types.ClientResult] | types.ServerNotification | Exception,
) -> None:
if isinstance(message, Exception):
raise message
def __init__(self, result: dict[str, Any] | None = None) -> None:
self.requests: list[tuple[str, Mapping[str, Any] | None, CallOptions | None, Any]] = []
self.result = result if result is not None else {}
received_initialized = False
async def send_raw_request(
self,
method: str,
params: Mapping[str, Any] | None,
opts: CallOptions | None = None,
*,
_related_request_id: Any = None,
) -> dict[str, Any]:
self.requests.append((method, params, opts, _related_request_id))
return self.result
async def run_server():
nonlocal received_initialized
async def notify(self, method: str, params: Mapping[str, Any] | None) -> None:
raise NotImplementedError
async with ServerSession(
client_to_server_receive,
server_to_client_send,
InitializationOptions(
server_name="mcp",
server_version="0.1.0",
capabilities=ServerCapabilities(),
),
) as server_session:
async for message in server_session.incoming_messages: # pragma: no branch
if isinstance(message, Exception): # pragma: no cover
raise message
if isinstance(message, ClientNotification) and isinstance(
message, InitializedNotification
): # pragma: no branch
received_initialized = True
return
try:
async with (
ClientSession(
server_to_client_receive,
client_to_server_send,
message_handler=message_handler,
) as client_session,
anyio.create_task_group() as tg,
):
tg.start_soon(run_server)
await client_session.initialize()
except anyio.ClosedResourceError: # pragma: no cover
pass
assert received_initialized
def _make_session(
dispatcher: StubDispatcher,
*,
capabilities: ClientCapabilities | None = None,
has_standalone_channel: bool = True,
) -> ServerSession:
conn = Connection(dispatcher, has_standalone_channel=has_standalone_channel)
if capabilities is not None:
conn.client_params = InitializeRequestParams(
protocol_version=LATEST_PROTOCOL_VERSION,
capabilities=capabilities,
client_info=Implementation(name="c", version="0"),
)
# cast: `ServerSession` is typed to take `JSONRPCDispatcher` but only ever
# calls `send_raw_request` / `notify`, so the stub is structurally sufficient.
return ServerSession(cast("JSONRPCDispatcher[Any]", dispatcher), conn)
@pytest.mark.anyio
async def test_check_client_capability():
"""check_client_capability reflects the capabilities sent by the client at initialize."""
server_to_client_send, server_to_client_receive = anyio.create_memory_object_stream[SessionMessage](1)
client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage](1)
async def test_send_request_forwards_timeout_and_progress_callback_as_call_options():
dispatcher = StubDispatcher(result={"roots": []})
session = _make_session(dispatcher)
initialized = anyio.Event()
async def list_roots_callback(context: Any) -> types.ListRootsResult: # pragma: no cover
return types.ListRootsResult(roots=[])
async def run_server(server_session: ServerSession):
async for message in server_session.incoming_messages: # pragma: no branch
if isinstance(message, ClientNotification) and isinstance(
message, InitializedNotification
): # pragma: no branch
initialized.set()
return
async with (
ServerSession(
client_to_server_receive,
server_to_client_send,
InitializationOptions(server_name="mcp", server_version="0.1.0", capabilities=ServerCapabilities()),
) as server_session,
ClientSession(
server_to_client_receive,
client_to_server_send,
list_roots_callback=list_roots_callback,
) as client_session,
anyio.create_task_group() as tg,
):
tg.start_soon(run_server, server_session)
await client_session.initialize()
with anyio.fail_after(5):
await initialized.wait()
# ClientSession advertises roots when a list_roots_callback is provided.
assert server_session.check_client_capability(types.ClientCapabilities(roots=types.RootsCapability()))
# ClientSession does not advertise sampling without a sampling_callback.
assert not server_session.check_client_capability(types.ClientCapabilities(sampling=types.SamplingCapability()))
@pytest.mark.anyio
async def test_server_capabilities():
notification_options = NotificationOptions()
experimental_capabilities: dict[str, Any] = {}
async def noop_list_prompts(
ctx: ServerRequestContext, params: types.PaginatedRequestParams | None
) -> types.ListPromptsResult:
async def on_progress(progress: float, total: float | None, message: str | None) -> None:
raise NotImplementedError
async def noop_list_resources(
ctx: ServerRequestContext, params: types.PaginatedRequestParams | None
) -> types.ListResourcesResult:
raise NotImplementedError
async def noop_completion(ctx: ServerRequestContext, params: types.CompleteRequestParams) -> types.CompleteResult:
raise NotImplementedError
# No capabilities
server = Server("test")
caps = server.get_capabilities(notification_options, experimental_capabilities)
assert caps.prompts is None
assert caps.resources is None
assert caps.completions is None
# With prompts handler
server = Server("test", on_list_prompts=noop_list_prompts)
caps = server.get_capabilities(notification_options, experimental_capabilities)
assert caps.prompts == PromptsCapability(list_changed=False)
assert caps.resources is None
assert caps.completions is None
# With prompts + resources handlers
server = Server("test", on_list_prompts=noop_list_prompts, on_list_resources=noop_list_resources)
caps = server.get_capabilities(notification_options, experimental_capabilities)
assert caps.prompts == PromptsCapability(list_changed=False)
assert caps.resources == ResourcesCapability(subscribe=False, list_changed=False)
assert caps.completions is None
# With prompts + resources + completion handlers
server = Server(
"test",
on_list_prompts=noop_list_prompts,
on_list_resources=noop_list_resources,
on_completion=noop_completion,
result = await session.send_request(
types.ListRootsRequest(),
types.ListRootsResult,
request_read_timeout_seconds=2.5,
metadata=ServerMessageMetadata(related_request_id=7),
progress_callback=on_progress,
)
caps = server.get_capabilities(notification_options, experimental_capabilities)
assert caps.prompts == PromptsCapability(list_changed=False)
assert caps.resources == ResourcesCapability(subscribe=False, list_changed=False)
assert caps.completions == CompletionsCapability()
assert isinstance(result, types.ListRootsResult)
method, _params, opts, related = dispatcher.requests[0]
assert method == "roots/list"
assert opts == {"timeout": 2.5, "on_progress": on_progress}
assert related == 7
@pytest.mark.anyio
async def test_server_session_initialize_with_older_protocol_version():
"""Test that server accepts and responds with older protocol (2024-11-05)."""
server_to_client_send, server_to_client_receive = anyio.create_memory_object_stream[SessionMessage](1)
client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage | Exception](1)
received_initialized = False
received_protocol_version = None
async def run_server():
nonlocal received_initialized
async with ServerSession(
client_to_server_receive,
server_to_client_send,
InitializationOptions(
server_name="mcp",
server_version="0.1.0",
capabilities=ServerCapabilities(),
),
) as server_session:
async for message in server_session.incoming_messages: # pragma: no branch
if isinstance(message, Exception): # pragma: no cover
raise message
if isinstance(message, types.ClientNotification) and isinstance(
message, InitializedNotification
): # pragma: no branch
received_initialized = True
return
async def mock_client():
nonlocal received_protocol_version
# Send initialization request with older protocol version (2024-11-05)
await client_to_server_send.send(
SessionMessage(
types.JSONRPCRequest(
jsonrpc="2.0",
id=1,
method="initialize",
params=types.InitializeRequestParams(
protocol_version="2024-11-05",
capabilities=types.ClientCapabilities(),
client_info=types.Implementation(name="test-client", version="1.0.0"),
).model_dump(by_alias=True, mode="json", exclude_none=True),
)
)
)
# Wait for the initialize response
init_response_message = await server_to_client_receive.receive()
assert isinstance(init_response_message.message, types.JSONRPCResponse)
result_data = init_response_message.message.result
init_result = types.InitializeResult.model_validate(result_data)
# Check that the server responded with the requested protocol version
received_protocol_version = init_result.protocol_version
assert received_protocol_version == "2024-11-05"
# Send initialized notification
await client_to_server_send.send(
SessionMessage(types.JSONRPCNotification(jsonrpc="2.0", method="notifications/initialized"))
)
async with (
client_to_server_send,
client_to_server_receive,
server_to_client_send,
server_to_client_receive,
anyio.create_task_group() as tg,
):
tg.start_soon(run_server)
tg.start_soon(mock_client)
assert received_initialized
assert received_protocol_version == "2024-11-05"
async def test_send_request_omits_call_options_when_none_given():
dispatcher = StubDispatcher(result={"roots": []})
session = _make_session(dispatcher)
await session.send_request(types.ListRootsRequest(), types.ListRootsResult)
_method, _params, opts, related = dispatcher.requests[0]
assert opts is None
assert related is None
@pytest.mark.anyio
async def test_ping_request_before_initialization():
"""Test that ping requests are allowed before initialization is complete."""
server_to_client_send, server_to_client_receive = anyio.create_memory_object_stream[SessionMessage](1)
client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage | Exception](1)
ping_response_received = False
ping_response_id = None
async def run_server():
async with ServerSession(
client_to_server_receive,
server_to_client_send,
InitializationOptions(
server_name="mcp",
server_version="0.1.0",
capabilities=ServerCapabilities(),
),
) as server_session:
async for message in server_session.incoming_messages: # pragma: no branch
if isinstance(message, Exception): # pragma: no cover
raise message
# We should receive a ping request before initialization
if isinstance(message, RequestResponder) and isinstance(
message.request, types.PingRequest
): # pragma: no branch
# Respond to the ping
with message:
await message.respond(types.EmptyResult())
return
async def mock_client():
nonlocal ping_response_received, ping_response_id
# Send ping request before any initialization
await client_to_server_send.send(SessionMessage(types.JSONRPCRequest(jsonrpc="2.0", id=42, method="ping")))
# Wait for the ping response
ping_response_message = await server_to_client_receive.receive()
assert isinstance(ping_response_message.message, types.JSONRPCResponse)
ping_response_received = True
ping_response_id = ping_response_message.message.id
async with (
client_to_server_send,
client_to_server_receive,
server_to_client_send,
server_to_client_receive,
anyio.create_task_group() as tg,
):
tg.start_soon(run_server)
tg.start_soon(mock_client)
assert ping_response_received
assert ping_response_id == 42
async def test_send_request_timeout_zero_means_no_timeout():
"""0 falls through BaseSession's `or`-fallback, so it has always meant
"no timeout"; ClientSession still reads it that way."""
dispatcher = StubDispatcher(result={})
session = _make_session(dispatcher)
await session.send_request(types.PingRequest(), types.EmptyResult, request_read_timeout_seconds=0)
assert dispatcher.requests[0][2] is None
@pytest.mark.anyio
async def test_create_message_tool_result_validation():
"""Test tool_use/tool_result validation in create_message."""
server_to_client_send, server_to_client_receive = anyio.create_memory_object_stream[SessionMessage](1)
client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage | Exception](1)
async with (
client_to_server_send,
client_to_server_receive,
server_to_client_send,
server_to_client_receive,
):
async with ServerSession(
client_to_server_receive,
server_to_client_send,
InitializationOptions(
server_name="test",
server_version="0.1.0",
capabilities=ServerCapabilities(),
),
) as session:
# Set up client params with sampling.tools capability for the test
session._client_params = types.InitializeRequestParams(
protocol_version=types.LATEST_PROTOCOL_VERSION,
capabilities=types.ClientCapabilities(
sampling=types.SamplingCapability(tools=types.SamplingToolsCapability())
),
client_info=types.Implementation(name="test", version="1.0"),
)
tool = types.Tool(name="test_tool", input_schema={"type": "object"})
text = types.TextContent(type="text", text="hello")
tool_use = types.ToolUseContent(type="tool_use", id="call_1", name="test_tool", input={})
tool_result = types.ToolResultContent(type="tool_result", tool_use_id="call_1", content=[])
# Case 1: tool_result mixed with other content
with pytest.raises(ValueError, match="only tool_result content"):
await session.create_message(
messages=[
types.SamplingMessage(role="user", content=text),
types.SamplingMessage(role="assistant", content=tool_use),
types.SamplingMessage(role="user", content=[tool_result, text]), # mixed!
],
max_tokens=100,
tools=[tool],
)
# Case 2: tool_result without previous message
with pytest.raises(ValueError, match="requires a previous message"):
await session.create_message(
messages=[types.SamplingMessage(role="user", content=tool_result)],
max_tokens=100,
tools=[tool],
)
# Case 3: tool_result without previous tool_use
with pytest.raises(ValueError, match="do not match any tool_use"):
await session.create_message(
messages=[
types.SamplingMessage(role="user", content=text),
types.SamplingMessage(role="user", content=tool_result),
],
max_tokens=100,
tools=[tool],
)
# Case 4: mismatched tool IDs
with pytest.raises(ValueError, match="ids of tool_result blocks and tool_use blocks"):
await session.create_message(
messages=[
types.SamplingMessage(role="user", content=text),
types.SamplingMessage(role="assistant", content=tool_use),
types.SamplingMessage(
role="user",
content=types.ToolResultContent(type="tool_result", tool_use_id="wrong_id", content=[]),
),
],
max_tokens=100,
tools=[tool],
)
# Case 5: text-only message with tools (no tool_results) - passes validation
# Covers has_tool_results=False branch.
# We use move_on_after because validation happens synchronously before
# send_request, which would block indefinitely waiting for a response.
# The timeout lets validation pass, then cancels the blocked send.
with anyio.move_on_after(0.01):
await session.create_message(
messages=[types.SamplingMessage(role="user", content=text)],
max_tokens=100,
tools=[tool],
)
# Case 6: valid matching tool_result/tool_use IDs - passes validation
# Covers tool_use_ids == tool_result_ids branch.
# (see Case 5 comment for move_on_after explanation)
with anyio.move_on_after(0.01):
await session.create_message(
messages=[
types.SamplingMessage(role="user", content=text),
types.SamplingMessage(role="assistant", content=tool_use),
types.SamplingMessage(role="user", content=tool_result),
],
max_tokens=100,
tools=[tool],
)
# Case 7: validation runs even without `tools` parameter
# (tool loop continuation may omit tools while containing tool_result)
with pytest.raises(ValueError, match="do not match any tool_use"):
await session.create_message(
messages=[
types.SamplingMessage(role="user", content=text),
types.SamplingMessage(role="user", content=tool_result),
],
max_tokens=100,
# Note: no tools parameter
)
# Case 8: empty messages list - skips validation entirely
# Covers the `if messages:` branch (line 280->302)
with anyio.move_on_after(0.01): # pragma: no branch
await session.create_message(messages=[], max_tokens=100)
async def test_send_request_without_back_channel_or_related_id_fails_fast():
"""No standalone channel and no related request to ride on: raise instead
of parking forever on a response that cannot arrive."""
dispatcher = StubDispatcher(result={})
session = _make_session(dispatcher, has_standalone_channel=False)
with pytest.raises(NoBackChannelError):
await session.send_request(types.PingRequest(), types.EmptyResult)
assert dispatcher.requests == []
# With a related request id the message rides that request's stream.
await session.send_request(
types.PingRequest(), types.EmptyResult, metadata=ServerMessageMetadata(related_request_id=3)
)
assert dispatcher.requests[0][3] == 3
@pytest.mark.anyio
async def test_create_message_without_tools_capability():
"""Test that create_message raises MCPError when tools are provided without capability."""
server_to_client_send, server_to_client_receive = anyio.create_memory_object_stream[SessionMessage](1)
client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage | Exception](1)
async with (
client_to_server_send,
client_to_server_receive,
server_to_client_send,
server_to_client_receive,
):
async with ServerSession(
client_to_server_receive,
server_to_client_send,
InitializationOptions(
server_name="test",
server_version="0.1.0",
capabilities=ServerCapabilities(),
),
) as session:
# Set up client params WITHOUT sampling.tools capability
session._client_params = types.InitializeRequestParams(
protocol_version=types.LATEST_PROTOCOL_VERSION,
capabilities=types.ClientCapabilities(sampling=types.SamplingCapability()),
client_info=types.Implementation(name="test", version="1.0"),
)
tool = types.Tool(name="test_tool", input_schema={"type": "object"})
text = types.TextContent(type="text", text="hello")
# Should raise MCPError when tools are provided but client lacks capability
with pytest.raises(MCPError) as exc_info:
await session.create_message(
messages=[types.SamplingMessage(role="user", content=text)],
max_tokens=100,
tools=[tool],
)
assert "does not support sampling tools capability" in exc_info.value.error.message
# Should also raise MCPError when tool_choice is provided
with pytest.raises(MCPError) as exc_info:
await session.create_message(
messages=[types.SamplingMessage(role="user", content=text)],
max_tokens=100,
tool_choice=types.ToolChoice(mode="auto"),
)
assert "does not support sampling tools capability" in exc_info.value.error.message
async def test_send_request_validates_result_alias_only():
"""Peer results validate alias-only; a snake_case key from the wire is
ignored as extra, not populated by Python field name."""
snake = {"role": "assistant", "content": {"type": "text", "text": "x"}, "model": "m", "stop_reason": "endTurn"}
session = _make_session(StubDispatcher(result=snake))
request = types.CreateMessageRequest(params=types.CreateMessageRequestParams(messages=[], max_tokens=1))
result = await session.send_request(request, types.CreateMessageResult)
assert result.stop_reason is None
@pytest.mark.anyio
async def test_other_requests_blocked_before_initialization():
"""Test that non-ping requests are still blocked before initialization."""
server_to_client_send, server_to_client_receive = anyio.create_memory_object_stream[SessionMessage](1)
client_to_server_send, client_to_server_receive = anyio.create_memory_object_stream[SessionMessage | Exception](1)
async def test_create_message_with_tools_returns_with_tools_result():
dispatcher = StubDispatcher(result={"role": "assistant", "content": [{"type": "text", "text": "ok"}], "model": "m"})
session = _make_session(
dispatcher, capabilities=ClientCapabilities(sampling=SamplingCapability(tools=SamplingToolsCapability()))
)
result = await session.create_message(
messages=[types.SamplingMessage(role="user", content=types.TextContent(type="text", text="hi"))],
max_tokens=10,
tools=[types.Tool(name="t", input_schema={"type": "object"})],
)
assert isinstance(result, types.CreateMessageResultWithTools)
method, params, _opts, _related = dispatcher.requests[0]
assert method == "sampling/createMessage"
assert params is not None and params["tools"][0]["name"] == "t"
error_response_received = False
error_code = None
async def run_server():
async with ServerSession(
client_to_server_receive,
server_to_client_send,
InitializationOptions(
server_name="mcp",
server_version="0.1.0",
capabilities=ServerCapabilities(),
),
):
# Server should handle the request and send an error response
# No need to process incoming_messages since the error is handled automatically
await anyio.sleep(0.1) # Give time for the request to be processed
async def mock_client():
nonlocal error_response_received, error_code
# Try to send a non-ping request before initialization
await client_to_server_send.send(
SessionMessage(types.JSONRPCRequest(jsonrpc="2.0", id=1, method="prompts/list"))
)
# Wait for the error response
error_message = await server_to_client_receive.receive()
if isinstance(error_message.message, types.JSONRPCError): # pragma: no branch
error_response_received = True
error_code = error_message.message.error.code
async with (
client_to_server_send,
client_to_server_receive,
server_to_client_send,
server_to_client_receive,
anyio.create_task_group() as tg,
):
tg.start_soon(run_server)
tg.start_soon(mock_client)
assert error_response_received
assert error_code == types.INVALID_PARAMS
def test_check_client_capability_delegates_to_connection():
dispatcher = StubDispatcher()
session = _make_session(dispatcher, capabilities=ClientCapabilities(sampling=SamplingCapability()))
assert session.check_client_capability(ClientCapabilities(sampling=SamplingCapability())) is True
assert session.check_client_capability(ClientCapabilities(experimental={"x": {}})) is False