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Add ergonomic Client class for testing MCP servers (#1870)

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Felix Weinberger 2026-01-16 15:49:26 +00:00 • committed by GitHub
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docs/testing.md
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@ -1,10 +1,11 @@
# 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.
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.
Anyway, let's assume you have a simple server with a single tool:
## Basic Usage
Let's assume you have a simple server with a single tool:
```python title="server.py"
from mcp.server import FastMCP
@ -40,12 +41,9 @@ To run the below test, you'll need to install the following dependencies:
server - you don't need to use it, but we are spreading the word for best practices.
```python title="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 import Client
from mcp.types import CallToolResult, TextContent
from server import app
@ -57,14 +55,14 @@ def anyio_backend(): # (1)!
@pytest.fixture
async def client_session() -> AsyncGenerator[ClientSession]:
async with create_connected_server_and_client_session(app, raise_exceptions=True) as _session:
yield _session
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_session: ClientSession):
result = await client_session.call_tool("add", {"a": 1, "b": 2})
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")],
@ -74,5 +72,6 @@ async def test_call_add_tool(client_session: ClientSession):
```
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).
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.