Facing Issues Handling Custom Headers using Middleware

[kskarthik]

Hy! I am using the latest version of fastmcp to create a server with http transport

The issue is that i need to handle few custom headers coming from the client for authorization (api keys) which are required by all tools & prompts

I followed this example in the middleware docs https://gofastmcp.com/servers/middleware#custom-middleware-example

from fastmcp.server.middleware import Middleware, MiddlewareContext
from fastmcp.server.dependencies import get_http_headers

class ValidateHeaders(Middleware):
    async def on_request(self, context: MiddlewareContext, call_next):
        # Add custom logic here
        print(f"Processing {context.method}")
        
        # added my custom http headers parsing and validation here using the get_http_headers()
        # and error handling
        
        # then
        result = await call_next(context)
        
        print(f"Completed {context.method}")
        return result

mcp.add_middleware(ValidateHeaders())

After adding my custom header validation in this class, The client is unable to access the tools on my server. As my tools are not async, I tried again by converting this class to sync, Despite that attempt, it didn't help.

Am i missing something here? you help is greatly appreciated

[jaimeescano]

Hi @kskarthik ,

I hope this helps you.
I have just done a test .. and I can see my headers: here an example:

from fastmcp.client.transports import StreamableHttpTransport

def static_token_client(token: str, server_url: str = "http://127.0.0.1:8000/mcp", headers:dict = {}):
    transport = StreamableHttpTransport(
        server_url, 
        auth=token,
        headers={
            **headers,
            "X-Test": "test-value", 
        },
    )

    return Client(transport)



///// MIDDLEWARE AS YOU DEFINED //////
from fastmcp.server.middleware import Middleware, MiddlewareContext
from fastmcp.server.dependencies import get_http_headers

class ValidateHeaders(Middleware):
    async def on_request(self, context: MiddlewareContext, call_next):
        # Add custom logic here    
        print (f"HTTP headers: {get_http_headers()}", file=sys.stderr)
        # added my custom http headers parsing and validation here using the get_http_headers()
        # and error handling
        
        # then
        result = await call_next(context)
        
        print(f"Completed {context.method}")
        return result

mcp.add_middleware(ValidateHeaders())

Then in the console ... I can see my header "x-test".

[kskarthik]

Yes, they are shown but these headers need to be consumed by tools & prompts. I fixed that by exposing those variables in global scope. Maybe there could be a better way. But it works out for now :)