v139 · miscellaneous

Spec-compliant JSON MIME type detection

Chromium now recognizes all valid JSON MIME types as defined by the WHATWG mimesniff specification. This includes any MIME type whose subtype ends with “+json”, in addition to the traditional application/json and text/json. This change ensures that web APIs and features relying on JSON detection behave consistently with the web platform s

concepts

  1. MIME Classifier

    Chrome now matches the HTTP spec for JSON MIME detection — any +json suffix triggers JSON handling. Old whitelist-of-a-handful approach gone.

  2. JSON Module Import

    The motivating use case from the explainer: import x from './d.json' with {type:'json'} failed pre-v139 if the server sent application/feed+json. Now it works. Test any MIME string and see the verdict per browser version.

  3. +json Surface Explorer

    Pick a real-world MIME type — vnd.api+json, manifest+json, problem+json — and compare Chrome 138's literal JSON allow-list with Chrome 139's spec-aligned +json recognition. The MIME catalogue, parser status, minimization result, and module-style verdict are all on one page.

  4. JSON MIME Type Tester

    Type any MIME type string and get an instant "JSON? YES/NO" verdict using the spec algorithm: application/json, text/json, or anything ending in +json. Preset buttons cover common types. A Content-Type header parser shows the essence MIME type after stripping parameters like ; charset=utf-8.

why it shipped

Without this change, developers must rely on inconsistent MIME sniffing behavior across browsers, which can lead to incorrect content handling. Aligning with the spec ensures predictable behavior and improves interoperability.

references