[Answered ]-Status: 400; Message: Token validated, but invalid for app <app_id>

Just to elaborate for others. I found this question when my circleci build wasn’t working with the same values.

If you are using a config.yml file, the app_id is treated as an integer. For some reason this confuses the pusher authentication mechanism. You have to wrap your id in quotes, for example:

docker:
  image: ...
  environment:
    PUSHER_APP_ID: "12345"

As opposed to the incorrect version:

docker:
  image: ...
  environment:
    PUSHER_APP_ID: 12345

The other values do not suffer from this, as they contain letters which forces the YAML parser to treat the value as a string.