Android Networking with Retrofit

Modern Retrofit setup for Android using coroutines, kotlinx.serialization, and Hilt. This reference covers the decisions that are easy to get wrong, not the standard boilerplate of service interfaces, Hilt module wiring, and the converter setup.

Service interface

Declare every endpoint as a suspend function. Return the body type directly — Retrofit throws HttpException on non-2xx, giving a clean try/catch at the repository boundary. Use Response<T> only when you need the status code, headers, or error body (e.g. server-side validation messages):

// Direct body — throws HttpException on non-2xx; the common case
@GET("users/{user}/repos")
suspend fun listRepos(@Path("user") user: String): List<Repo>

// Response wrapper — only when you need code/headers/error body
@GET("users/{user}")
suspend fun getUser(@Path("user") user: String): Response<User>

Wrapping every endpoint in Response<T> "just in case" forces callers to check isSuccessful and handle a nullable body even when only the body matters — don't.