When a container on App Platform returns any 5xx response, the ingress replaces it with a generic HTML error page and a 504 status. The original status survives only as a response header: for a 503 my API sent, clients receive HTTP/2 504 content-type: text/html x-do-orig-status: 503 with the body dropped. Support confirmed this is enforced by the Cloudflare integration and cannot be changed per app. Why this matters: Structured API errors. My API returns 503 with a JSON body (an error code and a user-facing message) when a downstream dependency such as the database is unavailable. The API itself is healthy. Mobile clients render that message; with the substitution they receive HTML they cannot parse and fall back to a generic error, so the app cannot tell users what actually happened or whether it is safe to retry. Health and status endpoints. A /health endpoint that answers 503 with a JSON report of which internal check failed is standard practice. Monitors still see "down", but the report never leaves the platform, so operators cannot read it from outside. Standard error contracts. RFC 9457 problem details, GraphQL over HTTP and most SDKs assume the origin's status and body are delivered as sent. Requested: a per-app (or per-component) setting to pass origin 5xx responses through unchanged, or at minimum to preserve the origin status code and content-type. The platform page could remain the default and still apply when the origin is unreachable, which is the case it exists for.
