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Use RFC 9457 Problem Details as the common HTTP error format, then add two application-defined fields: code for a stable error identifier and request_id for correlation with server-side logs. The result is a seven-field profile—not a schema mandated by the RFC—and it can be implemented with centralized error handling in FastAPI and Express 5.
What the seven fields mean
RFC 9457 defines five standard Problem Details members. The profile below retains those members and adds two extensions for application-level error identity and request correlation. RFC 9457 describes its purpose as carrying “machine-readable details of errors in HTTP response content to avoid the need to define new error response formats for HTTP APIs,” as Mark Nottingham, an RFC co-author, explains in the RFC abstract.
| Field | Role in the response | Status |
|---|---|---|
type |
URI identifying the category of problem. | RFC 9457 member |
title |
Short human-readable summary of the problem type. | RFC 9457 member |
status |
HTTP status associated with this occurrence. | RFC 9457 member |
detail |
Human-readable explanation specific to this occurrence. | RFC 9457 member |
instance |
URI identifying this particular occurrence. | RFC 9457 member |
code |
Stable, application-specific identifier clients can use to handle a known error category. | Application extension |
request_id |
Correlation value clients can include in a support report and operators can look up in controlled logs. | Application extension |
The standard members are the protocol baseline; the seven-field arrangement is an application profile. RFC 9457 allows extensions and obsoletes RFC 7807. Keep status and code distinct: the former communicates HTTP semantics, while the latter identifies an application error consistently across relevant endpoints.
How to make request correlation useful
Generate or accept a request identifier according to an explicit application policy, make it available to downstream handlers and logging, and return that same value in request_id. A client can then report the identifier without needing access to internal logs. The reviewed framework and RFC sources do not prescribe a header name or trust policy, so document those choices for your own API.
#1 Best Overall
instance and request_id are not automatically interchangeable. The RFC member identifies a problem occurrence; the extension is a practical correlation value for a request and its associated logs. Decide whether your system maps one to the other or keeps them separate, and preserve the distinction in clients and telemetry.
Implement the profile in FastAPI
Centralize exception handling
FastAPI has default handlers for HTTPException and invalid request data, and supports custom exception handlers for application exceptions. A centralized handler can translate these paths into the same Problem Details profile rather than returning unrelated error shapes. See FastAPI’s error-handling guide.
- Define the application’s mapping from exception categories to HTTP status, stable
code, safetitle, and safedetail. - Establish the request correlation value early enough that handlers and logging can access it. FastAPI supports ASGI middleware via
app.add_middleware(); middleware is one suitable place to attach the value. See FastAPI’s middleware guide. - Register custom handlers for the exception paths that need the shared response format, including application exceptions and request-validation failures.
- Return the five RFC members plus
codeandrequest_id, keeping the HTTP response status consistent with thestatusvalue.
Shape validation errors without leaking internals
FastAPI validation details include a location, message, and type. These can help classify an invalid request, but they should not be copied indiscriminately into a public response. FastAPI warns that converting validation exceptions directly to strings and returning them can expose information about the system. Use a client-safe explanation and retain any necessary diagnostic detail in appropriately controlled logs. The disclosure warning appears in FastAPI’s error-handling documentation.
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Forward asynchronous errors to the central handler
Express error middleware receives errors passed through the middleware chain. In Express 5, a route handler or middleware that returns a Promise automatically forwards thrown errors and rejected promises to error handling. A promise that is not returned does not get that automatic treatment; route it explicitly. The current Express error-handling guide covers this behavior.
- Choose a consistent mapping from errors to HTTP status, application
code, and safe public text. - Make the request correlation value available throughout the request path and in the logs, using an application-defined policy for inbound values.
- Use centralized error middleware to construct the shared response, including the five standard members and the two documented extensions.
- Keep status handling aligned with the response: Express’s default handler uses
err.statusorerr.statusCodewhen appropriate and falls back to 500 for values outside the 4xx/5xx ranges.
Account for response state and production disclosure
If response headers have already been sent, custom Express error middleware should delegate onward rather than try to replace the response. Express’s default handler omits stack traces in production. Your own handler should likewise keep stack traces and other internal details out of public responses. See the Express 5 error-handling documentation.
Do not assume Express 5’s automatic promise forwarding applies to Express 4. The Express 4 guide says rejected promises are not automatically passed to next; use explicit forwarding or an appropriate wrapper on that version: Express 4 error handling.
Quick Recap
Check the contract across both frameworks
- Contract alignment: Keep the five RFC members recognizable and define
codeandrequest_idas application extensions. - Consistent client behavior: Ensure
statusmatches the HTTP response and thatcoderemains stable for the same application error category. - Complete error propagation: Confirm that synchronous and asynchronous failures reach the central handler in the framework version you deploy.
- Useful correlation: Check that the returned
request_idcan locate the corresponding request records without exposing those logs publicly. - Controlled disclosure: Review validation messages, exception text, stack traces, and request data before including any of them in client-facing responses.
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