The official Python SDK for Model Context Protocol servers and clients
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# Testing MCP Servers
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If you call yourself a developer, you will want to test your MCP server.
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The Python SDK offers the `create_connected_server_and_client_session` function to create a session
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using an in-memory transport. I know, I know, the name is too long... We are working on improving it.
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Anyway, 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 FastMCP
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app = FastMCP("Calculator")
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@app.tool()
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def add(a: int, b: int) -> int:
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"""Add two numbers.""" # (1)!
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return a + b
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```
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1. The docstring is automatically added as the description of the tool.
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To run the below test, you'll need to install the following dependencies:
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=== "pip"
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```bash
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pip install inline-snapshot pytest
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```
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=== "uv"
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```bash
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uv add inline-snapshot pytest
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```
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!!! info
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I think [`pytest`](https://docs.pytest.org/en/stable/) is a pretty standard testing framework,
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so I won't go into details here.
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The [`inline-snapshot`](https://15r10nk.github.io/inline-snapshot/latest/) is a library that allows
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you to take snapshots of the output of your tests. Which makes it easier to create tests for your
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server - you don't need to use it, but we are spreading the word for best practices.
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```python title="test_server.py"
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from collections.abc import AsyncGenerator
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import pytest
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from inline_snapshot import snapshot
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from mcp.client.session import ClientSession
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from mcp.shared.memory import create_connected_server_and_client_session
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from mcp.types import CallToolResult, TextContent
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from server import app
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@pytest.fixture
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def anyio_backend(): # (1)!
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return "asyncio"
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@pytest.fixture
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async def client_session() -> AsyncGenerator[ClientSession]:
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async with create_connected_server_and_client_session(app, raise_exceptions=True) as _session:
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yield _session
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@pytest.mark.anyio
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async def test_call_add_tool(client_session: ClientSession):
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result = await client_session.call_tool("add", {"a": 1, "b": 2})
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assert result == snapshot(
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CallToolResult(
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content=[TextContent(type="text", text="3")],
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structuredContent={"result": 3},
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)
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)
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```
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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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There you go! You can now extend your tests to cover more scenarios.
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