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To return the same API error from Python, Go, and JavaScript, define one HTTP contract and translate each language’s local errors into it at the HTTP boundary. RFC 9457 Problem Details is a strong shared format: clients can rely on the same status, media type, problem type, stable title, and documented field meanings without requiring identical internal code or byte-for-byte JSON.
What “the same error response” should mean
Consistency is a public API promise, not a demand that three runtimes handle errors alike internally. Specify the observable response semantics clients can depend on: the HTTP status, Content-Type, problem type, stable title, required fields, and the meaning of each field. Unless your API separately requires canonical serialization, compare parsed JSON rather than whitespace, member order, or other byte-level details.
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RFC 9457 defines Problem Details as a JSON object identified by application/problem+json when sent as JSON. The body adds API-specific context alongside the HTTP status; it does not replace the status code’s HTTP meaning. As the RFC puts it, “HTTP status codes cannot always convey enough information about errors to be helpful.”
Define the wire contract before mapping language errors
Use a shared contract definition or fixture for problem categories, status mappings, stable titles, required fields, and extension policy. Keep it as the source of truth for each service; generating per-language constants or validating implementations against it can reduce drift, though RFC 9457 does not prescribe a particular repository layout or code-generation approach.
| Contract element | Decision to document |
|---|---|
| HTTP status | Choose the status according to HTTP semantics. If the body includes a status member, define that it mirrors the HTTP status line. |
| Media type | Use application/problem+json for JSON Problem Details responses. |
type |
Use a stable identifier for the problem category and document it for clients. |
title |
Use a stable short summary for the problem type; do not vary it for each occurrence. |
detail |
Include occurrence-specific context only when it helps the caller understand or correct the issue. Do not use it as a stack trace. |
instance |
Decide whether to include an identifier for a particular occurrence, such as one that helps with support or investigation. |
| Extensions | Define the names, types, and meanings of API-specific members. Review them for sensitive information before making them public. |
RFC 9457 does not require every optional member in every response. Make clear which members your API always emits and which may be omitted. Problem Details is a natural fit for many 4xx and 5xx responses, but it need not displace a suitable domain-specific format when one better serves the API’s purpose.
Translate each language’s errors at the HTTP boundary
Keep the application’s local error handling idiomatic, then map recognized errors into the shared response contract in the handler, middleware, or equivalent boundary layer. Clients should not have to infer which language produced a response.
Python
Python Packaging Authority’s PEP 847 proposes RFC 9457 Problem Details for errors from HTTP origins serving the Simple Repository API. Its scope is that API, not a general rule for every Python service. For another HTTP API, using the same representation is an implementation choice: map the service’s local exceptions to the documented public problem types and fields.
There is also a serialization edge case to account for in shared contract tests. Python’s JSON encoder allows NaN and infinity values by default, even though those are not valid JSON number tokens. Set allow_nan=False when using json.dumps for strict JSON output so such a value fails serialization rather than producing non-standard JSON.
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Go
Go’s ordinary error flow uses returned error values. The Go Authors’ FAQ explains: “For plain error handling, Go’s multi-value returns make it easy to report an error without overloading the return value.” Convert returned errors into the public Problem Details response at the HTTP handler or another boundary layer. Keep panic and recover for exceptional control flow rather than using them as the normal API error contract.
JavaScript
In JavaScript, throw raises an exception that propagates through the call stack; asynchronous failures may also arrive through rejected promises. Catch or otherwise handle those failures at the HTTP boundary and map them to the same public problem schema. MDN recommends throwing an Error instance or subclass in practice, since caught values may be expected to provide properties such as message. Do not expose a runtime stack trace as the response contract.
Keep diagnostic details useful without leaking internals
A problem response is part of the public HTTP interface, not a debugging channel. Separate stable identifiers and titles from details that differ by occurrence. Decide which detail text and extension values are safe to disclose; retain sensitive diagnostics in appropriately protected server-side logs, and use an occurrence identifier when it supports investigation without publishing those diagnostics.
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The current baseline is RFC 9457. Its predecessor, RFC 7807, was published in March 2016. When referring to guidance from the older RFC, identify it as historical rather than attributing its wording to the current standard.
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Test responses as clients observe them
Build a shared set of request and error scenarios, then compare each service’s externally visible response against the contract. Treat these checks as engineering recommendations; the cited standards do not prescribe a specific testing framework or repository structure.
- Check the HTTP status and
Content-Type. - Check the problem
typeand stabletitle. - Validate the presence and JSON types of contractually required fields.
- Check that
detailand extensions contain only approved, useful information. - Include serialization cases such as non-finite numbers where Python produces JSON, and verify strict output handling.
- For clients, test fallback behavior when the content type is not Problem Details or the body cannot be parsed or validated.
Compare parsed JSON semantics unless byte-for-byte canonical output is an explicit, separately documented API requirement. A client should keep ordinary HTTP error handling available: it can check the content type, parse and validate a Problem Details body when appropriate, present a useful message, and fall back if the response is not in that format or cannot be processed. PEP 847 describes that pattern for its Simple Repository API scope; it does not automatically govern every API client.
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