Okhttp response callbacks on the main thread

From my understanding, Okhttp callbacks run on the main thread so why do I get this error ?

This is not true. Callbacks run on a background thread. If you want to immediately process something in the UI you will need to post to the main thread.

Since you already have a wrapper around the callback you can do this internally in your helper so that all HttpCallback methods are invoked on the main thread for convenience.

As Jake Wharton suggested, I had to run the callbacks on the main thread explicitly.

So I wrapped the calls to the callbacks with Runnable like this:

private void call(String method, String url, final HttpCallback cb) {
    ...

    client.newCall(request).enqueue(new Callback() {
            Handler mainHandler = new Handler(context.getMainLooper());

            @Override
            public void onFailure(Request request,final Throwable throwable) {
                mainHandler.post(new Runnable() {

                    @Override
                    public void run() {
                        cb.onFailure(null, throwable);
                    }
                });

            }

            @Override
            public void onResponse(final Response response) throws IOException {
                mainHandler.post(new Runnable() {

                    @Override
                    public void run() {
                        if (!response.isSuccessful()) {
                            cb.onFailure(response, null);
                            return;
                        }
                        cb.onSuccess(response);
                    }
                });

            }
        });
 }

I know it's an old question, but recently I encountered the same issue. If you need to update any view, you will need to use runOnUiThread() or post the result back on the main thread.

HttpUtil.get(url, new Callback() { //okhttp3.Callback
   @Override
   public void onFailure(Call call, IOException e) { /* Handle error **/ }

   @Override
   public void onResponse(Call call, Response response) throws IOException {

      String myResponse =  response.body().string();
      //Do something with response
      //...

      MyActivity.this.runOnUiThread(new Runnable() {
            @Override
            public void run() {
               //Handle UI here                        
               findViewById(R.id.loading).setVisibility(View.GONE);                
            }
        });
   }
});

According to Retrofit documentation Callback methods are executed on UI thread by default until you provide a callback executor to Retrofit OR when using custom method return types using CallAdapterFactory