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Modernize the system without guessing what it does

Legacy modernization rarely fails because a team cannot generate enough new code. It fails because nobody can prove which behavior matters, who still depends on it, or whether a replacement is safe to operate.

Claude Code for Legacy Modernization is a practical field handbook for changing critical systems without pretending that the repository is a complete specification. It shows you how to recover hidden business rules, reconstruct runtime behavior, identify unknown consumers, and turn those findings into evidence that engineers and decision owners can review.

The method follows one bounded modernization slice from discovery to retirement.

You will learn how to:

recover business rules from code, data, runtime flows, side effects, and operational knowledge; establish a trustworthy baseline before asking AI to change anything;
build characterization tests, reviewed golden masters, contract tests, invariants, and differential comparisons;
find or introduce a safe implementation seam;
create a reversible strangler path;
compare legacy and replacement behavior before cutover;
define acceptance criteria and rollback triggers in advance;
cut over under named human authority;
measure accepted outcomes and retire the old path without losing the evidence trail.

Claude Code assists with repository discovery, test preparation, bounded implementation, old-and-new comparison, contradiction checks, and evidence assembly. It does not decide which behavior is correct or authorize a production change. Architects, domain owners, security reviewers, and operational leaders keep those decisions.

The book follows Northstar Settlements, a fictional financial-services system, through a complete modernization thread. The companion lab lets you reproduce the baseline, inspect the legacy behavior, run the tests, expose an intentional replacement mismatch, and complete the exercises in your own branch.

What is included

more than 790 pages of practical guidance;
26 connected modernization chapters;
86 architecture, workflow, and evidence diagrams;
a runnable GitHub exercise repository;
synthetic fixtures, a retained golden master, and differential tests;
Claude Code project controls and compatibility wrappers;
reusable schemas, checklists, evidence records, and decision templates.

Companion lab

Northstar Modernization Lab

This book is for software engineers, architects, technical leads, quality engineers, and platform or operations professionals responsible for systems that are valuable, difficult to understand, and dangerous to change.

### Continue with the companion guides

Need the wider production-agent engineering foundation? Claude Code: Building Production Agents That Actually Scale covers tools, MCP, hooks, permissions, evals, observability, cost engineering, and multi-agent operations.

Need both books? The Claude Code Production Engineering bundle combines that production-agent guide with this complete legacy-modernization handbook.

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Kick off your book project, get started with GhostAI, get better at marketing, or spend the day doing all three! Free live workshops on Zoom. Saturday, June 27, 2026.

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