How can I resolve the 401 Unauthorized error on Fuel POST requests to my Rails API by setting the proper Content-Type header?

FINALLY SOLVED: NEEDED header("Content-Type", "application/json")

KOTLINCopied
val (request, response, result) = Fuel.post(inputURL)
            .header("Content-Type", "application/json")
            .body(jsonElement.toString())
            .response()

I was experiencing a 401 Unauthorized error when connecting my Android Studio AVD to my Rails 8 API, even though it worked with Postman. I followed the guidelines from https://husseinelgammal.hashnode.dev/implementing-an-authentication-in-a-ruby-on-rails-api-only-project-using-devise-and-jwt#heading-enabling-cors and updated config/devise.rb with config.authentication_keys = [:name].

My MainActivity.kt is as follows:

KOTLINCopied
var baseURL = "http://10.0.2.2:3000/"
var connectURL = baseURL+"users/login"
var rrr: List<Any> = mutableListOf()

//MainClass body


    fun connectButton(view: View) {
        if (binding.loginNameField.text.isNotEmpty() && binding.loginPasswordField.text.isNotEmpty()) {
            Thread{
                rrr = fuelPost(connectURL)
            }.start()
        }
    }

    private fun fuelPost(inputURL: String,
                                 name: String = binding.loginNameField.text.toString(),
                                 password: String = binding.loginPasswordField.text.toString()): List<Any> {

        val json = parseToJsonElement("""{"user":{"name":"$name","password":"$password"}}""")

        val (request, response, result) = Fuel.post(inputURL).body(json.toString()).response()
        Log.i("request", request.toString())
        Log.i("response", response.toString())
        Log.i("result", result.toString())
        return listOf(request, response, result)
    }
}

The Rails console logs show:

RUBYCopied
Started POST "/users/login" for 127.0.0.1 at 2025-01-15 15:18:05 +0200
Processing by Users::SessionsController#create as JSON
Completed 401 Unauthorized in 1ms (ActiveRecord: 0.0ms (0 queries, 0 cached) | GC: 0.0ms)

The Android Studio console displays:

BASHCopied
--> POST http://10.0.2.2:3000/users/login
Body : {"user":{"name":"MUSTDOS","password":"fakepass"}}
...
Body : {"error":"You need to sign in or sign up before continuing."}
.............

Here’s the relevant part of sessions_controller.rb:

RUBYCopied
class Users::SessionsController < Devise::SessionsController
    include RackSessionsFix
  private
  def respond_with(current_user, _opts = {})
  logger.debug "current user: #{current_user.inspect}"
    render "users/sessions/session_success", status: :ok
  end

  def respond_to_on_destroy
    if current_user
      render "users/sessions/session_destroy", status: :ok
    else
      render "users/sessions/session_destroy_errors", status: :unauthorized
    end
  end
end

And the CORS configuration in cors.rb is:

RUBYCopied
Rails.application.config.middleware.insert_before 0, Rack::Cors do
   allow do
     origins "*"

     resource "*",
       headers: :any,
       expose: ["Authorization"],
       methods: [:get, :post, :put, :patch, :delete, :options, :head]
   end
 end

In summary, the root issue was my Fuel POST request missing the required Content-Type: application/json header. Adding it ensured Rails could parse the JSON payload correctly, resolving the 401 Unauthorized error.