Testing your MCP servers thoroughly is essential for ensuring they work correctly when deployed. FastMCP makes this easy through a variety of testing patterns.

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In-Memory Testing

The most efficient way to test an MCP server is to pass your FastMCP server instance directly to a Client. This enables in-memory testing without having to start a separate server process, which is particularly useful because managing an MCP server programmatically can be challenging.

Here is an example of using a Client to test a server with pytest:

import pytest
from fastmcp import FastMCP, Client

@pytest.fixture
def mcp_server():
    server = FastMCP("TestServer")
    
    @server.tool()
    def greet(name: str) -> str:
        return f"Hello, {name}!"
        
    return server

async def test_tool_functionality(mcp_server):
    # Pass the server directly to the Client constructor
    async with Client(mcp_server) as client:
        result = await client.call_tool("greet", {"name": "World"})
        assert "Hello, World!" in str(result[0])

This pattern creates a direct connection between the client and server, allowing you to test your server’s functionality efficiently.