GitHub Copilot CLI GA + Rubber Duck: Cross-Model Second-Opinion Agent Closes 74.7% of Sonnet→Opus Gap

GitHub Blog·high signal

Copilot CLI reaches general availability with agentic workflows, Autopilot mode, and GPT-5.4 support. The headline feature is Rubber Duck (experimental): a second LLM from a different model family reviews the primary agent's plan at checkpoints — after plan drafting, complex implementation, or pre-test execution. Claude Sonnet + GPT-5.4 Rubber Duck closes 74.7% of the performance gap to Opus alone on multi-file tasks.