New in version: 2.0.0

MCP servers can emit log messages to clients. The client can handle these logs through a log handler callback.

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Log Handler

Provide a log_handler function when creating the client:

from fastmcp import Client
from fastmcp.client.logging import LogMessage

async def log_handler(message: LogMessage):
    level = message.level.upper()
    logger = message.logger or 'server'
    data = message.data
    print(f"[{level}] {logger}: {data}")

client = Client(
    "my_mcp_server.py",
    log_handler=log_handler,
)

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Handler Parameters

The log_handler is called every time a log message is received. It receives a LogMessage object:

Log Handler Parameters

​
LogMessage
Log Message Object
async def detailed_log_handler(message: LogMessage):
    if message.level == "error":
        print(f"ERROR: {message.data}")
    elif message.level == "warning":
        print(f"WARNING: {message.data}")
    else:
        print(f"{message.level.upper()}: {message.data}")

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Default Log Handling

If you don’t provide a custom log_handler, FastMCP uses a default handler that emits a DEBUG-level FastMCP log for every log message received from the server, which is useful for visibility without polluting your own logs.

client = Client("my_mcp_server.py")

async with client:
    # Server logs will be emitted at DEBUG level automatically
    await client.call_tool("some_tool")