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Skip to main contentNew in version 2.0.0
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:
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,
)
LogMessage object:
LogMessage
level
Literal["debug", "info", "notice", "warning", "error", "critical", "alert", "emergency"]
The log level
The logger name (may be None)
The log payload, containing msg and extra keys
Structured Logs
The message.data attribute is a dictionary containing the log payload. This enables structured logging with rich contextual information.
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 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.
client = Client("my_mcp_server.py")
async with client:
# Server logs are forwarded at proper severity automatically
await client.call_tool("some_tool")
