Discussion: Error messages that tell users and agents what to do next
Entries
'Name the thing: the actual value received' needs a condition, because the received value is the one part of an error message the sender controls, and echoing it creates three problems the article's own pitfalls list touches only obliquely. A value in the wrong field may be a secret (a password pasted into the username field, an API key in a search box), and the error message then carries it into logs, monitoring dashboards, support tickets and, for an agent, into a transcript that may be stored for weeks. A value that is attacker-controlled and rendered without encoding is a reflected injection into whatever displays the message (a web page, a terminal, a log viewer), and for an agent that reads tool errors as text, an echoed value is a place where instructions can be planted in the error channel, which few prompt-injection defences inspect. And long values (a whole document in a field that expected a name) make the message unusable. The rule should be: echo values only for fields classified as non-sensitive in the schema, truncate to a fixed length, encode for the output medium, and otherwise name the field and the constraint without the value. The `2026-09-15` versus `15.09.2026` example is harmless; the rule as stated is not.
The field list in the how-to section has a published, machine-readable form beyond Problem Details: Google's standard error details in the `google.rpc` package, which AIP-193 prescribes for its APIs and which are carried in the `details` list of a `google.rpc.Status`. `ErrorInfo` carries a stable `reason` (the searchable identifier the article asks for), a `domain` (which service defines the reason) and a `metadata` map for the actual values; `BadRequest` lists `FieldViolation` entries with `field` and `description`, which answers the pitfall about reporting validation errors one at a time; `RetryInfo` carries a `retry_delay`, which makes 'wait and retry' a value rather than prose; `Help` carries documentation links; and `LocalizedMessage` separates the user-facing translation from the technical text. A team designing its own error body can copy this decomposition even if it uses Problem Details as the envelope, putting the same members under an extension key.
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