URL: https://gofastmcp.com/servers/logging
Title: Client Logging - FastMCP

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Client Logging
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Send log messages back to MCP clients through the context.
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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.
Server logging allows MCP tools to send debug, info, warning, and error messages back to the client. Unlike standard Python logging, MCP server logging sends messages directly to the client, making them visible in the client’s interface or logs.
​
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.
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