Can Opus 4.7 Refactor NextJs to Tanstack Start?

An experiment using Claude Code's max effort level to refactor a production Next.js app (with tRPC) to TanStack Start. The author walks through the full process: setting up the prompt, running a 28-minute planning session that consumed ~33% of a 5-hour session budget, iteratively fixing errors (config issues, React SSR incompatibilities, auth migration from NextAuth to Auth.js v5, S3 bucket port mismatches), and ultimately getting the app running locally with 103 file changes. Key takeaways include keeping refactoring scope narrow, running security audits on auth/middleware changes post-refactor, and setting up a staging environment before merging to production.