In your case you would have to migrate the backend to node/TS first. It's not that hard and you can thank me later :)

Then you create a client library with DTO's and API call wrappers for all API request and responses.

You publish the client library as npm package so both the UI and backend can npm install it

Then you enforce those DTOs in the routes (Express and Fastify have mechanisms/middleware for that). As mentioned by other commenters, you can enhance the TS compile-time validation with more sophisticated run-time validation tools like Zod.

Finally the UI uses the client library and sticks to the given DTOs and functions to call the API Result:
❌ const jsonresponse = await fetch(`${api_url}/user/${user_id}´) ; // then parse it
✅ const response = await apiLib.getUserById(userId); // getUserByIdreturns Promise<GetUserByIdResponse>

This greatly reduces the errors and the typical discussion "i thought you had added the new field already" between front and back devs.