Problem

A failure, as RFC 9457 section 3's data model.

Only the data model is adopted. Section 4's application/problem+json waits for derives Api, which will serialize this identical value, so the HTML and JSON paths cannot diverge on errors. A browser hitting a missing widget gets a page, not raw JSON, and eezo does not negotiate on Accept, which is out of scope.

tpe is RFC 9457's type member, spelled the way core's HTML DSL already spells that keyword. It stays about:blank: minting https://eezo.io/problems/... URIs is a documentation obligation taken on before eezo has documentation, and section 4.2.1 makes title the status phrase exactly when type is about:blank.