GitHub Copilot CLI combines model families for a second opinion (opens in new tab)
GitHub Copilot CLI’s experimental Rubber Duck feature adds an independent reviewer from a different AI model family to catch mistakes before they compound. When Claude models orchestrate a task, GPT-5.4 reviews plans, implementations, and tests at key checkpoints. On SWE-Bench Pro, Claude Sonnet 4.6 with Rubber Duck closed 74.7% of the performance gap with Claude Opus 4.6 alone, particularly on complex, multi-file tasks. ## The Problem with Self-Review - Coding agents typically assess a task, plan, implement, test, and iterate. - Early assumptions can create downstream dependencies and make small mistakes expensive to fix. - Self-reflection helps, but a model reviewing its own work may retain the same training biases and blind spots. ## Cross-Family Review with Rubber Duck - Rubber Duck is a focused review agent powered by a complementary model family. - Claude orchestrators currently use GPT-5.4 as the reviewer. - It produces a short list of high-value concerns, including: - Missed details - Questionable assumptions - Architectural risks - Relevant edge cases ## Evaluation Results - On SWE-Bench Pro, Sonnet 4.6 plus Rubber Duck approached the resolution rate of Opus 4.6 running alone. - Benefits were strongest for problems involving at least three files and 70 or more steps. - Sonnet plus Rubber Duck scored: - 3.8% above the Sonnet baseline on difficult tasks - 4.8% higher on the hardest tasks across three trials - Examples included detecting: - A scheduler that would start and immediately exit - A loop overwriting one dictionary key and dropping Solr facet categories - Cross-file Redis references that would silently break email confirmation flows ## When Reviews Happen Rubber Duck can be invoked automatically, reactively, or on request: - After a plan is drafted, to prevent flawed decisions from spreading. - After complex implementation work, to identify edge cases. - After tests are written but before they run, to expose coverage gaps or weak assertions. - When the primary agent is stuck or repeating an unproductive loop. - Any time the user asks Copilot to critique its work. Copilot incorporates the feedback and explains what changed. Reviews are intentionally infrequent and targeted at checkpoints where they provide the most value. ## Availability and Use Cases - Rubber Duck is available in Copilot CLI’s experimental mode through `/experimental`. - It works with Claude Opus, Sonnet, and Haiku as orchestrator models, provided the user has GPT-5.4 access. - It is especially suited to: - Complex refactors and architectural changes - High-stakes coding tasks - Test coverage review - Getting a second opinion before committing to a plan Rubber Duck is a practical way to reduce model-specific blind spots by combining different AI families. Developers can enable it experimentally in Copilot CLI and use automatic or on-demand critiques for difficult work.