FastMCP Middleware
The Basic Idea
Middleware is a way to add behaviour that runs around every request your server handles — without touching the individual tools, resources, or prompts themselves. Think of it as a series of gates that every request must pass through on its way in, and pass through again on its way back out.
This is a FastMCP-specific feature and is not part of the official MCP protocol specification.
When a client sends a request to your server, it doesn't go straight to your tool. It first passes through each piece of middleware you've registered, in the order you added them. After the tool runs and produces a result, the response passes back through the same middleware in reverse. This bidirectional flow is the key insight: middleware gets to see both the incoming request and the outgoing response.
The pipeline looks like this:
Practical MCP with FastMCP & LangChain
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