# Tutorial: Refactor Legacy Code Safely With Claude Code | Alher Tech

> Refactor large legacy codebases safely with Claude Code. Characterization tests, strangler fig pattern, sub-agent orchestration and feature flags that keep production stable.

- Canonical page: https://alhertech.com/en/claude-code/refactor-legacy-code/
- Site: Alher Tech (custom software, AI agents and SEO engineering, https://alhertech.com/)
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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.