---
title: logging
sidebarTitle: logging
---

# `fastmcp.server.middleware.logging`


Comprehensive logging middleware for FastMCP servers.

## Functions

### `default_serializer` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/middleware/logging.py#L14" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
default_serializer(data: Any) -> str
```


The default serializer for Payloads in the logging middleware.


## Classes

### `LoggingMiddleware` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/middleware/logging.py#L19" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>


Middleware that provides comprehensive request and response logging.

Logs all MCP messages with configurable detail levels. Useful for debugging,
monitoring, and understanding server usage patterns.


**Methods:**

#### `on_message` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/middleware/logging.py#L91" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
on_message(self, context: MiddlewareContext[Any], call_next: CallNext[Any, Any]) -> Any
```

Log all messages.


### `StructuredLoggingMiddleware` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/middleware/logging.py#L114" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>


Middleware that provides structured JSON logging for better log analysis.

Outputs structured logs that are easier to parse and analyze with log
aggregation tools like ELK stack, Splunk, or cloud logging services.


**Methods:**

#### `on_message` <sup><a href="https://github.com/jlowin/fastmcp/blob/main/src/fastmcp/server/middleware/logging.py#L185" target="_blank"><Icon icon="github" style="width: 14px; height: 14px;" /></a></sup>

```python
on_message(self, context: MiddlewareContext[Any], call_next: CallNext[Any, Any]) -> Any
```

Log structured message information.

