FastAPI middleware loses RapidAPI header on Heroku
I built a FastAPI service and deployed it on Heroku. After that, I listed it on the RapidAPI marketplace. The issue I’m facing is that RapidAPI sends a specific header called x-rapidapi-proxy-secret to ensure requests are routed through their service.
To verify this, I implemented middleware to check for this header:
api = FastAPI()
proxy_secret = os.environ.get("API_SECRET", None)
@api.middleware("http")
async def validate_rapidapi_header(req: Request, proceed):
if proxy_secret:
req_headers = req.headers
print(req_headers)
if (
"X-RapidAPI-Proxy-Secret" not in req_headers
or req_headers["X-RapidAPI-Proxy-Secret"] != proxy_secret
):
return PlainTextResponse(
"Unauthorized direct access", status_code=403
)
result = await proceed(req)
return result
When I check my Heroku logs, the initial request includes all necessary RapidAPI headers, including the proxy secret. However, after Heroku executes a redirect, the subsequent request does not include this header anymore.
The logs indicate a 307 redirect resulting in a 403 error because the secret header is missing.
Is this expected behavior on Heroku? What can I do to ensure that the proxy secret header is forwarded during the redirect? Should I adjust my middleware approach?
The Problem: Your Heroku-deployed FastAPI application loses the x-rapidapi-proxy-secret header after a 307 redirect, leading to unauthorized access errors. This is because HTTP redirects often strip custom headers, and your current middleware approach is unable to prevent this. The solution involves moving the header validation to a pre-processing workflow which avoids the redirect issue entirely.
Understanding the “Why” (The Root Cause): The core issue is not with your FastAPI middleware or Heroku’s behavior itself, but rather the inherent limitations of handling custom headers during HTTP redirects. When a 307 redirect occurs, the browser (or in this case, RapidAPI’s infrastructure) makes a second request to the redirected URL. During this process, many HTTP servers and proxies, including those used by Heroku, strip or don’t forward custom headers like x-rapidapi-proxy-secret for security reasons. Your current middleware only checks the headers after the redirect has happened.
Step-by-Step Guide:
Implement a Pre-Request Validation Workflow: Instead of relying solely on FastAPI middleware, create an external workflow that intercepts requests before they reach your FastAPI application. This workflow will be responsible for validating the x-rapidapi-proxy-secret header. Several workflow automation tools (like Latenode, as mentioned in the original thread) can facilitate this process. This workflow will act as a reverse proxy.
Workflow Configuration (Using a Hypothetical Workflow Tool): The exact configuration will depend on the chosen workflow automation tool. However, the general steps will be similar. Assume a hypothetical workflow tool allowing you to create HTTP request/response nodes.
HTTP Request Node: Set up an HTTP Request node to receive incoming requests from RapidAPI.
Header Validation Node: Create a node that extracts the x-rapidapi-proxy-secret header from the request. Compare this value against the secret stored as an environment variable in your workflow (ensure it’s identical to the one in your Heroku app).
Conditional Logic Node: Use a conditional node to check the validation result. If the secret matches, proceed; otherwise, return a 401 Unauthorized response.
HTTP Request (Forwarding) Node: If the secret is valid, use another HTTP Request node to forward the original request to your Heroku-deployed FastAPI app. This ensures that the header is present during the initial request.
Response Handling Node: Ensure that any response from your FastAPI application is forwarded back to the initial requester (RapidAPI).
Deploy and Configure the Workflow: Deploy the workflow. Update your RapidAPI configuration to point to the URL of your new workflow. This workflow now acts as a proxy, pre-validating requests before they reach your FastAPI app, preventing header stripping during redirects.
Verify Heroku Configuration (FastAPI App): Ensure your FastAPI application remains correctly configured and deployed on Heroku. You might even remove or simplify your FastAPI middleware as the validation is now handled externally.
Test Thoroughly: Test your API extensively. Make requests through RapidAPI and try direct access to your Heroku URL. Direct access should result in a 401 Unauthorized error while RapidAPI requests should function correctly.
Common Pitfalls & What to Check Next:
Workflow Tool Selection: Choose a workflow tool that is well-suited for handling HTTP requests and environment variables.
Secret Management: Ensure secure management of your x-rapidapi-proxy-secret. Never hardcode it directly into your workflow; always use environment variables.
Error Handling: Implement robust error handling in your workflow. Log failed validation attempts for debugging.
Rate Limiting: Be mindful of rate limits imposed by RapidAPI and your workflow tool.
Debugging: Use logging extensively in both your workflow and your FastAPI application to track requests and responses.
Still running into issues? Share your (sanitized) config files, the exact command you ran, and any other relevant details. The community is here to help!
Had this exact problem with my RapidAPI service on Heroku. It’s not Heroku stripping headers - your FastAPI app is redirecting, which kills the headers. Usually happens when RapidAPI hits /api/endpoint but your route expects /api/endpoint/ (or the other way around). FastAPI redirects automatically and drops custom headers. Check your route definitions match exactly what RapidAPI sends. Also verify you don’t have automatic HTTPS redirects in Heroku settings or middleware causing issues. I added logging to my middleware to catch the exact request URL and method before processing. That’s how I figured out it was the trailing slash mismatch, not Heroku doing anything weird.
This happens all the time with HTTP redirects - not just Heroku, but most web servers and proxies do this. The HTTP spec says certain headers get stripped during redirects for security reasons, and custom headers like x-rapidapi-proxy-secret usually get cut. I hit the same issue when I deployed my API service. What fixed it for me was making sure my FastAPI app doesn’t redirect at all. Check if you’ve got trailing slash redirects or domain redirects set up in Heroku. Look at your routes and make sure they match exactly what RapidAPI’s calling. You could also tweak your middleware to be less strict during redirects. Check the request method and path, then skip header validation for certain redirect scenarios while logging it for monitoring. Just don’t make the validation too loose or you’ll create security holes.
Understanding the “Why” (The Root Cause): The core issue is not with your FastAPI middleware or Heroku’s behavior itself, but rather the inherent limitations of handling custom headers during HTTP redirects. When a 307 redirect occurs, the browser (or in this case, RapidAPI’s infrastructure) makes a second request to the redirected URL. During this process, many HTTP servers and proxies, including those used by Heroku, strip or don’t forward custom headers like 
Step-by-Step Guide:
Common Pitfalls & What to Check Next:
Still running into issues? Share your (sanitized) config files, the exact command you ran, and any other relevant details. The community is here to help!