quicktype

quicktype takes a blob of JSON and hands back real typed models in the language you actually work in. Paste an API response, pick Go or Swift or Kotlin or Rust, and you get structs plus the serialization code to parse them safely, instead of hand-writing map lookups and finding out in production that a field was a string. It also reads JSON Schema, TypeScript, and GraphQL queries, and targets more than twenty languages. Apache 2.0, free, no account.

There is nothing to self-host. The browser app at app.quicktype.io does the work locally and never uploads your sample data, which matters when the JSON you are pasting is a real customer record. For repeatable work there is a CLI on npm you can wire into a build step so types regenerate when the schema moves. Needs Node 20 or newer.

Use it. Solo developers get the biggest win, because this is exactly the tedious work you would otherwise skip. Teams should put the CLI in CI rather than passing generated files around by hand. There is no paid tier and no upsell.

The catch is that generated types are only as good as the sample you feed it. One response where an optional field happens to be null and quicktype will confidently tell you that field is always null. Give it several samples, or give it the schema.