default-args.rs VS specta

I assume you're using Axum or Actix as your router & web server, in which case I think using OpenAPI specs is your best option. I see that rspc has already been mentioned in this thread, which I would definitely recommend (disclaimer - I work at the company that builds it) if continuing to use REST wasn't a priority, since it can auto-generate all the TS bindings you need and has a React Query integration so you'd feel right at home. It's able to do so much since it's able to sit between your backend and frontend code, but REST is a lot less opinionated, hence the recommendation to continue using OpenAPI. However, if you're willing to lose some type-safety there's the option of using a Rust -> TS generator specifically for your data types, and then manually use React Query. You'd lose the type-level enforcement of your routes, but at least you'd have your structs and enums available in TS. If you were to take this route I'd personally recommend using Specta, since it's the Rust -> TS generator that powers rspc, but there's also ts-rs and Typeshare which could probably do the job (though I don't think they're as good).