Add ergonomic Client class for testing MCP servers (#1870)
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Felix Weinberger
GitHub
parent
8adb5bdce8
commit
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1 changed files with 1298 additions and 526 deletions
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@ -3,9 +3,10 @@ from collections.abc import Callable
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import pytest
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import mcp.types as types
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from mcp.client._memory import InMemoryTransport
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from mcp.client.session import ClientSession
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from mcp.server import Server
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from mcp.server.fastmcp import FastMCP
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from mcp.shared.memory import create_connected_server_and_client_session as create_session
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from mcp.types import ListToolsRequest, ListToolsResult
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from .conftest import StreamSpyCollection
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@ -18,30 +19,28 @@ async def full_featured_server():
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"""Create a server with tools, resources, prompts, and templates."""
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server = FastMCP("test")
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@server.tool(name="test_tool_1")
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async def test_tool_1() -> str: # pragma: no cover
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"""First test tool"""
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return "Result 1"
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@server.tool(name="test_tool_2")
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async def test_tool_2() -> str: # pragma: no cover
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"""Second test tool"""
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return "Result 2"
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@server.resource("resource://test/data")
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async def test_resource() -> str: # pragma: no cover
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"""Test resource"""
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return "Test data"
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@server.prompt()
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async def test_prompt(name: str) -> str: # pragma: no cover
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"""Test prompt"""
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# pragma: no cover on handlers below - these exist only to register items with the
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# server so list_* methods return results. The handlers themselves are never called
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# because these tests only verify pagination/cursor behavior, not tool/resource invocation.
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@server.tool()
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def greet(name: str) -> str: # pragma: no cover
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"""Greet someone by name."""
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return f"Hello, {name}!"
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@server.resource("resource://test/{name}")
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async def test_template(name: str) -> str: # pragma: no cover
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"""Test resource template"""
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return f"Data for {name}"
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@server.resource("test://resource")
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def test_resource() -> str: # pragma: no cover
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"""A test resource."""
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return "Test content"
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@server.resource("test://template/{id}")
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def test_template(id: str) -> str: # pragma: no cover
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"""A test resource template."""
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return f"Template content for {id}"
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@server.prompt()
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def greeting_prompt(name: str) -> str: # pragma: no cover
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"""A greeting prompt."""
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return f"Please greet {name}."
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return server
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@ -61,78 +60,82 @@ async def test_list_methods_params_parameter(
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method_name: str,
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request_method: str,
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):
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"""Test that the params parameter works correctly for list methods.
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"""Test that the params parameter is accepted and correctly passed to the server.
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Covers: list_tools, list_resources, list_prompts, list_resource_templates
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This tests the new params parameter API (non-deprecated) to ensure
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it correctly handles all parameter combinations.
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See: https://modelcontextprotocol.io/specification/2025-03-26/server/utilities/pagination#request-format
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"""
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async with create_session(full_featured_server._mcp_server) as client_session:
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spies = stream_spy()
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method = getattr(client_session, method_name)
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transport = InMemoryTransport(full_featured_server)
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async with transport.connect() as (read_stream, write_stream):
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async with ClientSession(read_stream, write_stream) as session:
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await session.initialize()
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spies = stream_spy()
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# Test without params parameter (omitted)
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_ = await method()
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requests = spies.get_client_requests(method=request_method)
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assert len(requests) == 1
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assert requests[0].params is None
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# Test without params (omitted)
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method = getattr(session, method_name)
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_ = await method()
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requests = spies.get_client_requests(method=request_method)
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assert len(requests) == 1
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assert requests[0].params is None
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spies.clear()
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spies.clear()
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# Test with params=None
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_ = await method(params=None)
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requests = spies.get_client_requests(method=request_method)
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assert len(requests) == 1
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assert requests[0].params is None
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# Test with params containing cursor
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_ = await method(params=types.PaginatedRequestParams(cursor="from_params"))
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requests = spies.get_client_requests(method=request_method)
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assert len(requests) == 1
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assert requests[0].params is not None
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assert requests[0].params["cursor"] == "from_params"
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spies.clear()
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spies.clear()
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# Test with empty params (for strict servers)
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_ = await method(params=types.PaginatedRequestParams())
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requests = spies.get_client_requests(method=request_method)
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assert len(requests) == 1
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assert requests[0].params is not None
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assert requests[0].params.get("cursor") is None
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spies.clear()
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# Test with params containing cursor
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_ = await method(params=types.PaginatedRequestParams(cursor="some_cursor_value"))
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requests = spies.get_client_requests(method=request_method)
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assert len(requests) == 1
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assert requests[0].params is not None
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assert requests[0].params["cursor"] == "some_cursor_value"
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# Test with empty params
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_ = await method(params=types.PaginatedRequestParams())
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requests = spies.get_client_requests(method=request_method)
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assert len(requests) == 1
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# Empty params means no cursor
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assert requests[0].params is None or "cursor" not in requests[0].params
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async def test_list_tools_with_strict_server_validation():
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"""Test that list_tools works with strict servers require a params field,
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even if it is empty.
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async def test_list_tools_with_strict_server_validation(
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full_featured_server: FastMCP,
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):
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"""Test pagination with a server that validates request format strictly."""
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transport = InMemoryTransport(full_featured_server)
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async with transport.connect() as (read_stream, write_stream):
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async with ClientSession(read_stream, write_stream) as session:
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await session.initialize()
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result = await session.list_tools(params=types.PaginatedRequestParams())
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assert isinstance(result, ListToolsResult)
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assert len(result.tools) > 0
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Some MCP servers may implement strict JSON-RPC validation that requires
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the params field to always be present in requests, even if empty {}.
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This test ensures such servers are supported by the client SDK for list_resources
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requests without a cursor.
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"""
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server = Server("strict_server")
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async def test_list_tools_with_lowlevel_server():
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"""Test that list_tools works with a lowlevel Server using params."""
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server = Server("test-lowlevel")
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@server.list_tools()
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async def handle_list_tools(request: ListToolsRequest) -> ListToolsResult: # pragma: no cover
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"""Strict handler that validates params field exists"""
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async def handle_list_tools(request: ListToolsRequest) -> ListToolsResult:
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# Echo back what cursor we received in the tool description
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cursor = request.params.cursor if request.params else None
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return ListToolsResult(
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tools=[
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types.Tool(
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name="test_tool",
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description=f"cursor={cursor}",
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input_schema={},
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)
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]
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)
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# Simulate strict server validation
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if request.params is None:
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raise ValueError(
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"Strict server validation failed: params field must be present. "
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"Expected params: {} for requests without cursor."
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)
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transport = InMemoryTransport(server)
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async with transport.connect() as (read_stream, write_stream):
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async with ClientSession(read_stream, write_stream) as session:
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await session.initialize()
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# Return empty tools list
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return ListToolsResult(tools=[])
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result = await session.list_tools(params=types.PaginatedRequestParams())
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assert result.tools[0].description == "cursor=None"
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async with create_session(server) as client_session:
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# Use params to explicitly send params: {} for strict server compatibility
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result = await client_session.list_tools(params=types.PaginatedRequestParams())
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assert result is not None
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result = await session.list_tools(params=types.PaginatedRequestParams(cursor="page2"))
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assert result.tools[0].description == "cursor=page2"
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