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Testing MCP Servers

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.

Basic Usage

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
  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" bash pip install inline-snapshot pytest

=== "uv" bash 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`](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},
        )
    )
  1. If you are using trio, you should set "trio" as the anyio_backend. Check more information in the anyio documentation.
  2. The client fixture creates a connected client that can be reused across multiple tests.

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