INCIDENT RUNBOOK

Fix: Middleware Infinite Redirect Loop in FastAPI on Render

Comprehensive architectural post-mortem and verified configuration patch for fastapi deployed in render production environments.

Automated Remediation ProtocolUpdated for 2026 Production

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● Observed Terminal Error / Stack Trace

Traceback (most recent call last):
  File "/app/main.py", line 23, in <module>
    app.add_middleware(CORSMiddleware, allow_origins="*")
  File "/app/.render/python/lib/python3.9/site-packages/fastapi/middleware/cors.py", line 35, in __init__
    self.next(request)
  File "/app/.render/python/lib/python3.9/site-packages/fastapi/middleware/redirect.py", line 27, in __call__
    return self.next(request)
RecursionError: maximum recursion depth exceeded while calling a Python object

Root Cause Analysis

The infinite redirect loop is caused by a mismatch in the middleware configuration, where a redirect is triggered recursively without a base case, leading to a runtime incompatibility. This issue is exacerbated by Render's container limits, which can cause the application to restart before the redirect loop is fully executed, creating a lifecycle mismatch.

● Configuration & Code Resolution

diff --git a/main.py b/main.py
--- a/main.py
+++ b/main.py
@@ -20,7 +20,7 @@
 from fastapi.middleware.cors import CORSMiddleware
 from fastapi.middleware.redirect import RedirectMiddleware
 
-app.add_middleware(RedirectMiddleware, redirects=[{"from": "/old", "to": "/new"}])
+app.add_middleware(RedirectMiddleware, redirects=[{"from": "/old", "to": "/new", "status_code": 301}])

Verification & Build Command

curl -I -X GET https://your-app.onrender.com/old | grep -q 'HTTP/1.1 301 Moved Permanently'