Making substituter failure noisier

I have a custom S3 substituter. If I enter a nix develop, it may want to build a derivation for one of two reasons:

  1. The substituter does not have that derivation.
  2. The substituter is busted (e.g. I’ve broken my AWS configuration and S3 returns Access Denied)

If it’s because of #2, the failure is effectively silent. Is there a way to make that failure more noisy?

I think you want to set nix’ fallback setting to false.