I Didn't Let Claude Upgrade My Java App. I Let It Work for Me.

Description and use

Migrating a small Spring Boot 2.x app to Java 21 and Spring Boot 3.3.5 using two AI agents—one for code generation, one for verification—following a checklist and ownership rules to ensure compile‑clean progress.

Problem

Upgrading a legacy Spring Boot 2.x Java 8 service to Java 21 and Spring Boot 3.3.5 was complex, with many breaking changes and a risk of compounding errors.

Outcome

The migration succeeded with a structured, agent‑driven workflow that minimized manual effort, avoided unnecessary version hops, and ensured compile‑clean per‑item progress.

Workflow

• Plan migration with a checklist in MIGRATION.md
• Use two Claude agents: Migrator (writes code) and Verifier (read‑only review)
• Iterate per checklist item: Migrator marks [x], Verifier checks and marks [v], developer approves to [a] and commits
• Re‑run Verifier after each approval to catch regressions
• Ensure final build passes all tests and API replay before committing

Evidence

I asked Claude how to do this properly. The answer was very correct and very by the book: hop through Java 11 and 17, run OpenRewrite recipes, plus five specialist reviewer agents.

Source

The rule: the agent that writes the code never grades it.

Source

The build may be red, but an item isn't done while its files have compile errors or removal warnings that it, or an earlier item, owns.

Source