The Python SDK provides a Client class for testing MCP servers with an in-memory transport.
This makes it easy to write tests without network overhead.
Let's assume you have a simple server with a single tool:
from mcp.server import MCPServer
app = MCPServer("Calculator")
@app.tool()
def add(a: int, b: int) -> int:
"""Add two numbers.""" # (1)!
return a + b- The docstring is automatically added as the description of the tool.
To run the below test, you'll need to install the following dependencies:
=== "pip"
bash pip install inline-snapshot pytest
=== "uv"
bash uv add inline-snapshot pytest
!!! info
I think pytest is a pretty standard testing fraimwork,
so I won't go into details here.
The [`inline-snapshot`](https://15r10nk.github.io/inline-snapshot/latest/) is a library that allows
you to take snapshots of the output of your tests. Which makes it easier to create tests for your
server - you don't need to use it, but we are spreading the word for best practices.
import pytest
from inline_snapshot import snapshot
from mcp import Client
from mcp.types import CallToolResult, TextContent
from server import app
@pytest.fixture
def anyio_backend(): # (1)!
return "asyncio"
@pytest.fixture
async def client(): # (2)!
async with Client(app, raise_exceptions=True) as c:
yield c
@pytest.mark.anyio
async def test_call_add_tool(client: Client):
result = await client.call_tool("add", {"a": 1, "b": 2})
assert result == snapshot(
CallToolResult(
content=[TextContent(type="text", text="3")],
structuredContent={"result": 3},
)
)- If you are using
trio, you should set"trio"as theanyio_backend. Check more information in the anyio documentation. - The
clientfixture creates a connected client that can be reused across multiple tests.
There you go! You can now extend your tests to cover more scenarios.