Change a prompt or swap a model without guessing. Repath sends a slice of real traffic to the new version, an LLM judge scores both versions on the same criteria, and a controller advances the rollout (5% → 25% → 100%) or rolls it back to 0% on its own, with the worst answers and the judge's reasons logged. Integration is one base-URL change in the SDK you already use: OpenAI, Anthropic, Gemini, OpenRouter or Vercel AI Gateway. Gateway overhead: ~0.4 ms median.