A request header can be refreshed progressively using the @Header annotation. If the value is invalid, the header will be overlooked:

// example one
@GET("user")
Call<User> getUser(@Header("Authorization") String authorization)

// example two
@Headers("Accept: application/vnd.github.v3.full+json", "User-Agent: Spring for Android")
@GET("users/{username}")
fun getUser(@Path("username") username: String): Call<Users>