This documentation covers MCP client logging—sending messages from your server to MCP clients. For standard server-side logging (e.g., writing to files, console), use fastmcp.utilities.logging.get_logger() or Python’s built-in logging module.
Basic Usage
Use the context logging methods within any tool function:
from fastmcp import FastMCP, Context
mcp = FastMCP("LoggingDemo")
@mcp.tool
async def analyze_data(data: list[float], ctx: Context) -> dict:
"""Analyze numerical data with comprehensive logging."""
await ctx.debug("Starting analysis of numerical data")
await ctx.info(f"Analyzing {len(data)} data points")
try:
if not data:
await ctx.warning("Empty data list provided")
return {"error": "Empty data list"}
result = sum(data) / len(data)
await ctx.info(f"Analysis complete, average: {result}")
return {"average": result, "count": len(data)}
except Exception as e:
await ctx.error(f"Analysis failed: {str(e)}")
raise
Log Levels
| Level | Use Case |
|---|
ctx.debug() | Detailed execution information for diagnosing problems |
ctx.info() | General information about normal program execution |
ctx.warning() | Potentially harmful situations that don’t prevent execution |
ctx.error() | Error events that might still allow the application to continue |
Structured Logging
All logging methods accept an extra parameter for sending structured data to the client. This is useful for creating rich, queryable logs.
@mcp.tool
async def process_transaction(transaction_id: str, amount: float, ctx: Context):
await ctx.info(
f"Processing transaction {transaction_id}",
extra={
"transaction_id": transaction_id,
"amount": amount,
"currency": "USD"
}
)
Server-Side Logs
Messages sent to clients via ctx.log() and its convenience methods are also logged to the server’s log at DEBUG level. Enable debug logging on the fastmcp.server.context.to_client logger to see these messages:
import logging
from fastmcp.utilities.logging import get_logger
to_client_logger = get_logger(name="fastmcp.server.context.to_client")
to_client_logger.setLevel(level=logging.DEBUG)
Client Handling
Log messages are sent to the client through the MCP protocol. How clients handle these messages depends on their implementation—development clients may display logs in real-time, production clients may store them for analysis, and integration clients may forward them to external logging systems.
See Client Logging for details on how clients handle server log messages.
