Banks add columns without warning. ai-bookkeeper now has a test that inserts an unused column at the front and in the middle of every rectangular, headered CSV fixture and checks the rows read are identical. A second test rewrites a single signed Amount column as Debit and Credit columns and checks the same.
Neither found a fault, so they pin behaviour. The debit/credit check only applies to one fixture today, because files with a type column carry their sign there and are skipped. Real exports are still the missing input: these checks use the repository's own fixtures. Estimates only, not accounting advice.