Tutorial: Refactor Legacy Code Safely With Claude Code

Legacy refactors kill more projects than any other category of work. Claude Code changes the calculus: the agent can read a 200K-line codebase in one session and refactor safely behind a strong test net.

Updated: April 24, 2026

Before you touch code

Make sure you have tests. If the legacy codebase has none, first session is "write characterization tests for the three most critical paths". No refactor without a safety net.

Strangle, don't rewrite

Refactor by module, with a feature flag. Each refactored module goes to production behind the flag, watched on metrics for a week, then the old module is deleted. Claude Code manages the flag flip across the codebase in one PR.

Scale up the agent

For very large codebases use the Task tool to spawn sub-agents that each own a module. Main session coordinates; sub-agents do the surgical work. Context stays clean, refactor runs in parallel.

Frequently asked questions

Can Claude Code refactor across languages?

Yes. We've migrated PHP to Go and COBOL to Java. Pairing with a human expert on the source language is still required.