Server Logging

Receive and handle log messages from MCP servers.

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.

Log Handler

Provide a log_handler function when creating the client. For robust logging, the log messages can be integrated with Python’s standard logging module.

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

# In a real app, you might configure this in your main entry point
logging.basicConfig(
    level=logging.INFO,
    format='%(asctime)s - %(name)s - %(levelname)s - %(message)s'
)

# Get a logger for the module where the client is used
logger = logging.getLogger(__name__)

# This mapping is useful for converting MCP level strings to Python's levels
LOGGING_LEVEL_MAP = logging.getLevelNamesMapping()

async def log_handler(message: LogMessage):
    """
    Handles incoming logs from the MCP server and forwards them
    to the standard Python logging system.
    """
    msg = message.data.get('msg')
    extra = message.data.get('extra')

    # Convert the MCP log level to a Python log level
    level = LOGGING_LEVEL_MAP.get(message.level.upper(), logging.INFO)

    # Log the message using the standard logging library
    logger.log(level, msg, extra=extra)


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

Handling Structured Logs

The message.data attribute is a dictionary that contains the log payload from the server. This enables structured logging, allowing you to receive rich, contextual information.

The dictionary contains two keys:

• msg: The string log message.
• extra: A dictionary containing any extra data sent from the server.

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

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

Show attributes

level · Literal["debug", "info", "notice", "warning", "error", "critical", "alert", "emergency"]

The log level

logger · str | None

The logger name (optional, may be None)

data · dict

The log payload, containing msg and extra keys.

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}")

Default Log Handling

If you don’t provide a custom log_handler, FastMCP’s default handler routes server logs to the appropriate Python logging levels. The MCP levels are mapped as follows: notice → INFO; alert and emergency → CRITICAL. If the server includes a logger name, it is prefixed in the message, and any extra data is forwarded via the logging extra parameter.

client = Client("my_mcp_server.py")

async with client:
    # Server logs are forwarded at their proper severity (DEBUG/INFO/WARNING/ERROR/CRITICAL)
    await client.call_tool("some_tool")