ts-json-schema-generator

This looks interesting but I already have TypeScript types for my APIs so I developed https://github.com/vega/ts-json-schema-generator which lets me generate JSON schema from the sources directly. Yes, it does have some oddities because the two languages have slightly different feature sets but it’s been working well for us for a few years. If I didn’t have TypeScript or a smaller API surface I’d be okay with typing again I would look at TypeSpec though. It definitely beats writing JSON schema by hand.

Not OP, but we have a very similar setup. We use https://github.com/vega/ts-json-schema-generator in a prebuild hook to generate a schema from Typescript. The schema is then fed into Fastify, and referenced in route configurations for incoming (validation) and outgoing (fast serialization) data, as well as auto-generated Swagger. Took some custom code to simplify attaching definition references to our route configs, but it works quite well. As an extra, our frontend uses the same shared TS code for data transfer objects, this way there's an extra level of type safety at the API.

This is why I originally started a project to convert ts to json schema; https://github.com/YousefED/typescript-json-schema (and also check out a good alternative https://github.com/vega/ts-json-schema-generator)

ts-json-schema-generator[1] generates a schema from an annotated interface definition. [1] https://github.com/vega/ts-json-schema-generator