Write them both in TypeScript and have both the request and response shapes defined as schemas for each API endpoint.

The server validates request bodies and produces responses that match the type signature of the response schema.

The client code has an API where it takes the request body as its input shape. And the client can even validate the server responses to ensure they match the contract.

It’s pretty beautiful in practice as you make one change to the API to say rename a field, and you immediately get all the points of use flagged as type errors.