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Testing MCP Servers

If you call yourself a developer, you will want to test your MCP server. The Python SDK offers the create_connected_server_and_client_session function to create a session using an in-memory transport. I know, I know, the name is too long... We are working on improving it.

Anyway, let's assume you have a simple server with a single tool:

server.py
from mcp.server import FastMCP

app = FastMCP("Calculator")

@app.tool()
def add(a: int, b: int) -> int:
    """Add two numbers."""  # (1)!
    return a + b
  1. 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 install inline-snapshot pytest

uv add inline-snapshot pytest

Info

I think pytest is a pretty standard testing framework, so I won't go into details here.

The inline-snapshot 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.

test_server.py
from collections.abc import AsyncGenerator

import pytest
from inline_snapshot import snapshot
from mcp.client.session import ClientSession
from mcp.shared.memory import create_connected_server_and_client_session
from mcp.types import CallToolResult, TextContent

from server import app


@pytest.fixture
def anyio_backend():  # (1)!
    return "asyncio"


@pytest.fixture
async def client_session() -> AsyncGenerator[ClientSession]:
    async with create_connected_server_and_client_session(app, raise_exceptions=True) as _session:
        yield _session


@pytest.mark.anyio
async def test_call_add_tool(client_session: ClientSession):
    result = await client_session.call_tool("add", {"a": 1, "b": 2})
    assert result == snapshot(
        CallToolResult(
            content=[TextContent(type="text", text="3")],
            structuredContent={"result": 3},
        )
    )
  1. If you are using trio, you should set "trio" as the anyio_backend. Check more information in the anyio documentation.

There you go! You can now extend your tests to cover more scenarios.