About the rubber duck agent

Copilot CLI has a built-in critic. The rubber duck agent reviews plans, code and tests, and sorts what it finds into blocking issues, non-blocking issues and suggestions. It is scoped tightly: real problems only, so no comments on style, formatting, naming, comment grammar or minor refactors.

The design decision worth noting is that it deliberately runs on a different model from the one driving your session. If you are on Claude the critic might be GPT, and the other way round. The stated reason is to avoid inheriting the same blind spots and failure modes as the model being reviewed.

Copilot consults it automatically at what the docs call high-leverage moments: after planning a non-trivial change, during complex implementation, after writing tests, and when it keeps failing at something. You can also call it directly with /rubber-duck. The different-model argument is made more forcefully in self-audit is blind to its own blind spots.