Aider

A developer used Aider to refactor a Python utility library in a monorepo. Aider made the changes cleanly within the library itself — renaming functions, changing return types, removing deprecated methods. It ran the library's own test suite, which passed. What it didn't check was that 34 other microservices in the monorepo imported from this library. The changes were committed and merged. CI for the downstream services caught 28 of the 34 failures, but 6 services had no tests for the affected code paths and broke silently in production.