Retrofit - @Body parameters cannot be used with form or multi-part encoding

maybe this could help some people, if you have this trouble, you should remove @FormUrlEncoded of your interface.Hope this helps.

This post pointed me to the right direction https://stackoverflow.com/a/21423093/1446856.I attached everything in the body and send it as a TypedInput.
So the interface looks something like this

@POST("/api/register")@Headers({ "Content-Type: application/json;charset=UTF-8"})Observable<RegisterResponse> register(    @Header("Authorization") String authorization,    @Body TypedInput body);

and the body looks something like this

String bodyString = jsonBody + "?grant_type=" +     grantType + "&scope=" + scope;TypedInput requestBody = new TypedByteArray(    "application/json", bodyString.getBytes(Charset.forName("UTF-8")));

Adding to Julien's answer, also remove the @Multipart annotation. Here's how I have used it:

@POST("/app/oauth/token")Call<AuthResponse> getAuthToken(@Body RequestBody body);

And, here is how I have constructed the RequestBody:

RequestBody requestBody = new MultipartBody.Builder()                        .setType(MultipartBody.FORM)                        .addFormDataPart("grant_type", "password")                        .addFormDataPart("username", username)                        .addFormDataPart("password", password)                        .build();