Summary

Explain that .next contains the development server metadata and incremental compilation caches used across edits and reloads. Resetting it should be a recovery step for corrupt or incompatible generated output, not routine troubleshooting.

Add the same guidance to the next-dev-loop Skill so agents preserve useful Next.js development state while diagnosing source problems.

Add an agentic regression eval that first populates .next from two working development routes, then introduces an invalid generateStaticParams signature. The resulting production error points into .next/types, but deleting .next only regenerates the same error. The eval checks that the agent fixes the source problem, preserves .next, verifies both routes in development, and completes a production build.

Eval results

The eval covers a behavior observed in real agent sessions. These samples all preserved .next, so the results are regression coverage rather than evidence of lift from the Guide or Skill. Reusing the same development server process is not part of the score.

Verification

• pnpm build-all
• pnpm --filter=next build
• pnpm --dir evals/evals/agent-055-stale-build-error build
• Sonnet 4.6 eval: baseline, bundled Guide, and next-dev-loop Skill
• Haiku 4.5 eval: baseline
• Prettier and ESLint checks