I have not found a definitive source as yet and depending on the requirements of the task/project/client I have found the following sources excellent guidelines

I have not found a definitive source as yet and depending on the requirements of the task/project/client I have found the following sources excellent guidelines: * https://stripe.com/docs/api/errors https://stripe.com/docs/api/errors * https://developers.google.com/search-ads/v2/standard-error-responses https://developers.google.com/search-ads/v2/standard-error-r... * https://cloud.google.com/blog/products/api-management/restful-api-design-what-about-errors https://cloud.google.com/blog/products/api-management/restfu... I love the Stripe API documentation.