It's designed with compatibility in mind with external services but also keeping the advantages of tightly coupled microservices. For example, a Java microservice can also send requests as usual. It's REST first, so it typechecks HTTP headers, URL parameters, query strings, as well, not just the bodies. Unlike trpc, it also tracks error responses, and even lets you return custom validation errors. It tries to be out of the way as much as possbile, and lets you do everything that is possbile with express.

RPC stands for Remote Rrocedure Call. You define procedures in the server, and you call them from the client. Let it be either backend-backend communication or backend-frontend. RPC usually indicates strict typing of procedures for maximal compatibility and tight coupling.

Please follow the instructions for each side:

Server docs: Server README
Client docs: Server README

Or try the boilerplate: https://github.com/fxdave/react-express-cuple-boilerplate

Examples: ./test/src/examples
Tests: ./test/src

const builder = createBuilder(expressApp);

export const routes = {
  getPost: builder
    .path("/post/:id") // optional for REST compatibility
    .paramsSchema(
      z.object({
        id: z.coerce.number(),
      }),
    )
    .get(async ({ data }) => {
      const post = await getPost(data.params.id);

      if (!post)
        return notFoundError({
          message: "Post is not found",
        });

      return success({
        post,
      });
    }),
};

initRpc(expressApp, {
  path: "/rpc",
  routes,
});

const client = createClient<typeof routes>({
  path: "http://localhost:8080/rpc",
});

async function getPost(id: number) {
  const response = await client.getPosts.get({ params: { id } });

  console.log(postsResponse.post); // type error

  if (response.result === "success") {
    console.log(postsResponse.post); // no type error
  }
}

Boilerplate: https://github.com/fxdave/react-express-cuple-boilerplate
Server docs: Server README
Client docs: Server README
Examples: ./test/src/examples
Tests: ./test/src