Custom terraform provider read data

HI, I tried to create my terraform provider.
With POST method looks good.

Read method return empty. However I see, flattening created proper map.
I checked example avenger and got the same issue.
Here is avenger
terraform {
required_providers {
avengers = {
version = “1.0.1”
source = “

provider “avengers” {
host = “http://localhost:8000

data “avengers_datasource” “all” {}

output “all_avang” {
Output is empty, however if I created resource - output is correct.
I checked aws provider, availabilty zone - return proper input.

Could you please provide example read only provider