How to use get_http_headers when headers are dynamic in nature for each api calls when registering tools using from_openapi

[ksajan]

I read the issues and few places it is mentioned could get the thing to work or maybe i don't understand it well enough on how to use this feature of get_http_headers dynamically for each request that comes into my MCP server that got started from openapi spec. Can someone help in adding any insight, demo code on how it works? Thanks in advance

[venkatkalluru]

+1 on this.
in the below sample code from here, we can't hard code the Authorization header value as the token can have expiry time on it and it has to be taken from the request headers and use it to further when making calls to the underlying server in this case api.example.com

import httpx
from fastmcp import FastMCP

# Bearer token authentication
api_client = httpx.AsyncClient(
    base_url="https://api.example.com",
    headers={"Authorization": "Bearer YOUR_TOKEN"}
)

# Create MCP server with authenticated client
mcp = FastMCP.from_openapi(..., client=api_client)

[venkatkalluru]

After further looking into the code and testing, I found that if you pass the headers it is taking from the request and passing the headers to downstream API server call and it works.

[ksajan]

Can you share a sample code for me to try please.

[venkatkalluru]

Currently in my use-case, I configure the MCP Client with my token (in Authorization header) for the server configuration where it is hard-coded and when the call is made it to the MCP Server, the server automatically passes the headers to the underlying http client that is used to make the OpenAPI calls. For my current use-case this works.

I know the hard-coding is now shifted to the client side, but if you are interested in not hard-coding also on the client side, I think it should be done in some oAuth mechanism flow as described here which I haven't implemented or tested.