ReScript Rest V1 has been released and this is the best tool now to build REST API and consume them on the client 
https://x.com/dzakh_dev/status/1854802073914601838
In the version:
- Better Fastify integration with the ability to respond with different statuses
- Out of the box fast-json-stringify integration to make responses 2x faster
- Ability to generate OpenAPI from your Fastify application created with ReScript Rest
- Bindings and documentation on how to host an OpenAPI reference UI using Scalar