Server Logging - Fastmcp

import { VersionBadge } from '/snippets/version-badge.mdx'

Use this when you need to capture or process log messages sent by the server.

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

Log Handler

Provide a log_handler function when creating the client:

python

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

logging.basicConfig(
    level=logging.INFO,
    format='%(asctime)s - %(name)s - %(levelname)s - %(message)s'
)

logger = logging.getLogger(__name__)
LOGGING_LEVEL_MAP = logging.getLevelNamesMapping()

async def log_handler(message: LogMessage):
    """Forward MCP server logs to Python's logging system."""
    msg = message.data.get('msg')
    extra = message.data.get('extra')

    level = LOGGING_LEVEL_MAP.get(message.level.upper(), logging.INFO)
    logger.log(level, msg, extra=extra)

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

The handler receives a LogMessage object:

<Card icon="code" title="LogMessage"> <ResponseField name="level" type='Literal["debug", "info", "notice", "warning", "error", "critical", "alert", "emergency"]'> The log level </ResponseField> <ResponseField name="logger" type="str | None"> The logger name (may be None) </ResponseField> <ResponseField name="data" type="dict"> The log payload, containing `msg` and `extra` keys </ResponseField> </Card>

Structured Logs

The message.data attribute is a dictionary containing the log payload. This enables structured logging with rich contextual information.

python

async def detailed_log_handler(message: LogMessage):
    msg = message.data.get('msg')
    extra = message.data.get('extra')

    if message.level == "error":
        print(f"ERROR: {msg} | Details: {extra}")
    elif message.level == "warning":
        print(f"WARNING: {msg} | Details: {extra}")
    else:
        print(f"{message.level.upper()}: {msg}")

This structure is preserved even when logs are forwarded through a FastMCP proxy, making it useful for debugging multi-server applications.

Default Behavior

If you do not provide a custom log_handler, FastMCP's default handler routes server logs to Python's logging system at the appropriate severity level. The MCP levels map as follows: notice becomes INFO; alert and emergency become CRITICAL.

python

client = Client("my_mcp_server.py")

async with client:
    # Server logs are forwarded at proper severity automatically
    await client.call_tool("some_tool")