FastMCP Middleware
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Hooks: Choosing the Right Level
FastMCP provides hooks at three levels of specificity, and multiple hooks fire for a single request in a chain from general to specific. When a client calls a tool, FastMCP processes on_message first, then on_request, then on_call_tool. Each hook in the chain calls call_next to reach the next one, so if you override both on_message and on_call_tool in the same middleware, both will run.
| Hook | Fires when… | Typical use case |
|---|---|---|
on_message | Any MCP message arrives | Observability, broad logging |
on_request | A request expecting a response arrives | Authentication, authorization |
on_notification | A fire-and-forget notification arrives | Async side effects, event logging |
on_initialize | A client connects and initialises |
Practical MCP with FastMCP & LangChain
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