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To return the same error response from Python, Go, and JavaScript, standardize the HTTP response—not each language’s internal error mechanism. Use RFC 9457 Problem Details as the shared wire contract, then translate each service’s local error at its HTTP boundary. Define consistency as matching status, media type, problem type, stable title, required fields, and field meanings—not byte-for-byte identical JSON.
What “the same error response” should mean
RFC 9457 defines a JSON representation for HTTP errors using the media type application/problem+json. Its purpose is to add API-specific context to an HTTP status code; as the RFC puts it, “HTTP status codes cannot always convey enough information about errors to be helpful.” The status line still carries its HTTP meaning, while the body describes the problem.
For most APIs, this format is most useful for 4xx and 5xx responses. It does not have to replace a domain-specific response format that already serves the API better. The important decision is to document one public contract and apply it consistently wherever that contract is used.
Consistency should be defined semantically. Two services can serialize JSON with different whitespace or member order and still return the same problem. Unless you separately require canonical serialization, compare parsed JSON values and observable HTTP behavior.
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Define the public problem contract
RFC 9457 includes the members below, but does not require every optional member in every response. Decide which your API always sends, which it may omit, and how clients should interpret them. Document extension members as carefully as standard members.
| Element | Contract decision |
|---|---|
| HTTP status | Select a status that accurately represents the HTTP outcome. If the body includes status, make it mirror the status line under a documented policy. |
Content-Type |
Send application/problem+json for JSON Problem Details. |
type |
Use a stable identifier for the problem category and document it for clients. |
title |
Use a stable, short summary for that problem type; do not vary it for each occurrence. |
detail |
Include occurrence-specific context only when it helps the caller understand or correct the problem. Do not use it for a stack trace. |
instance |
Optionally identify an occurrence so it can be investigated or discussed with support. |
| Extension members | Define and document API-specific fields, including their types and meanings. Do not expose secrets or implementation internals. |
Keep stable category information in type and title; reserve detail and occurrence identifiers for information about a particular failure. That separation lets clients recognize a problem reliably without depending on changing prose.
Translate local errors at each language’s HTTP boundary
Python: serialize strictly and map exceptions
Python services can use their ordinary exception handling internally and convert known failures into the shared problem structure in the route handler or middleware. Avoid making exception text itself the API contract: it may change or contain details that should remain private.
There is a notable JSON interoperability edge case. Python’s standard JSON encoder permits NaN and infinity by default, even though these are not valid JSON number tokens. Use allow_nan=False when serializing response data so such values fail serialization rather than producing non-standard JSON, and include them in shared contract tests.
Python packaging’s PEP 847 proposes Problem Details for errors from HTTP origins serving the Simple Repository API. Its scope is that API; it does not make RFC 9457 a universal rule for every Python service.
Go: keep returned errors idiomatic
Go ordinarily reports errors through returned 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.” Keep that internal style, then map recognized errors to the public problem response in the HTTP handler or equivalent boundary. Go’s panic and recover are for exceptional control flow, not a substitute for ordinary error returns.
JavaScript: catch exceptions and rejected operations
JavaScript’s throw propagates an exception through the call stack. MDN recommends throwing an Error instance or subclass, since handlers may expect properties such as message. Convert caught or rejected errors into the same documented HTTP problem schema; do not expose a runtime stack trace as the public response contract.
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Keep clients useful when a problem body cannot be read
A client should not assume every failed request returns a valid Problem Details document. Check the response content type, parse and validate the body when it is the expected media type, and retain ordinary HTTP error handling if the type is different or parsing fails. PEP 847 describes this pattern for clients of the Simple Repository API; it is a useful design model, not a rule that applies automatically to every client.
- Inspect the HTTP status and
Content-Type. - If the content type is
application/problem+json, parse the JSON and validate the fields the client needs. - Present a useful message from valid problem data without assuming optional members are always present.
- If the content type is unexpected, or the body is malformed or invalid for the client’s needs, fall back to ordinary HTTP error handling.
Test observable behavior across implementations
Keep one machine-readable contract definition or fixture for problem types, stable titles, status mappings, required fields, and extension policy. Generating per-language constants or validation from it can reduce drift where the project architecture supports that approach; RFC 9457 does not prescribe a repository layout or code-generation method.
Run the same request and failure scenarios against each implementation. Compare the HTTP behavior and parsed response semantics rather than raw JSON strings unless byte-level canonicalization is an explicit requirement.
- HTTP status and
Content-Type. - Problem
typeand stabletitle. - Presence and data type of every contractually required field.
- Agreement between the status line and body
status, if your contract includes that member. - Whether
detailand extension members contain only approved, safe information. - Client fallback behavior for a different content type, malformed JSON, or invalid required fields.
- Strict JSON serialization, including rejection of non-finite numeric values in Python.
Protect the boundary between useful detail and disclosure
Problem responses are part of the public HTTP interface, not a debugging channel. Choose deliberately what can appear in detail, extensions, and occurrence identifiers. Keep stack traces, credentials, internal paths, and other sensitive implementation diagnostics out of public responses unless a specific field has been reviewed and documented as safe. The predecessor RFC 7807 is historical context, not the current baseline; use RFC 9457 for the current Problem Details standard.
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