FastAPI Docs 403'ing when running on AWS APIGW/Lambda with Mangum

Question

Trying to follow https://fastapi.tiangolo.com/advanced/behind-a-proxy/?h=proxy#proxy-with-a-stripped-path-prefix but keep getting 403 when I trying to access /docs or /redoc:

I get the 403 error(image attached) if I do the below:

app = FastAPI(
        servers=servers,
        root_path="/latest", # Same 403 error without the root_path or omitting it
        root_path_in_servers=True, # Same issue when setting to False or omitting it

• /docs and /redoc works locally, just not once I push it to AWS APIGW/Lambda
• All other paths work without issue both locally and in APIGW/Lambda
• I am using Mangum when it's deployed to Lambda, locally I use uvicorn
• I tried variations of no root_path and root_path_in_servers=True or False and had the same issue
• I have an APIGW GET /docs path setup to route to the Lambda
• I have API GW DNS set up so my DNS goes from api.example.com > API GW CloudFront URL which is mapped to my APIGW stage named /latest, then it sends to Lambda

I also reviewed this post with a similar/same issue, and tried:

docs_url='/latest/docs',
redoc_url='/latest/redoc',
openapi_url='/latest/openapi.json'

but encountered the same 403 error(image attached) or {"detail":"Not Found"}

Answer 1

app = FastAPI(openapi_prefix='/dev/')

This worked for me. Here, dev is the API stage name