Mechanical enforcement of input-schema invariants — every "just don't do X" rule that is detectable gets detected and raised, upgrading conventions to invariants.

The runner invokes assert_valid_input_schema!/2 automatically on an action's first run (memoized per action). The memoization lives in :persistent_term and survives dev code reloads, so editing an input schema in a running server does not re-trigger the check until restart — another reason host apps should also call it from a CI test on every action's input module for compile-adjacent feedback:

for action <- MyApp.Actions.all(), input = action.input() do
  Enact.Guardrails.assert_valid_input_schema!(input,
    mode: Keyword.get(action.config(), :mode, :create)
  )
end