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# Testing MCP Servers
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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.
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## Basic Usage
Let's assume you have a simple server with a single tool:
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```python title="server.py"
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from mcp.server import MCPServer
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app = MCPServer("Calculator")
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@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`](https://docs.pytest.org/en/stable/) 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.
` ``python title="test_server.py"
import pytest
from inline_snapshot import snapshot
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from mcp import Client
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from mcp.types import CallToolResult, TextContent
from server import app
@pytest.fixture
def anyio_backend(): # (1)!
return "asyncio"
@pytest.fixture
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async def client(): # (2)!
async with Client(app, raise_exceptions=True) as c:
yield c
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@pytest.mark.anyio
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async def test_call_add_tool(client: Client):
result = await client.call_tool("add", {"a": 1, "b": 2})
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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](https://anyio.readthedocs.io/en/stable/testing.html#specifying-the-backends-to-run-on).
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2. The ` client` fixture creates a connected client that can be reused across multiple tests.
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There you go! You can now extend your tests to cover more scenarios.
