Claude Code for Legacy Modernization: Recover Hidden Business Rules, Build a Verification Harness, and Modernize Critical Systems Without Breaking Production

Legacy modernization can go wrong when a team changes code before it knows which behavior production depends on. Calculation rules, edge cases, batch conventions, data quirks, and operational workarounds often exist only in code, data, logs, and staff knowledge. A clean rewrite can improve the architecture and still alter an outcome the business needs. This book gives you a practical method for discovering that behavior before you replace it. You will use Claude Code to investigate unfamiliar repositories, trace business decisions through code and data, and record observed behavior separately from inferences and unresolved questions. Generated explanations remain hypotheses until tests, runtime evidence, or human review support them. The verification harness comes first. You will build characterization tests, golden masters, contract tests, and business invariants around current behavior. These checks expose differences between the legacy system and its replacement. They give reviewers evidence for deciding whether a difference is a defect or an approved change. With those controls in place, you will find safe seams, introduce a strangler path, compare old and new outputs, and keep rollback available. Claude Code supports repository analysis, test creation, rule extraction, and bounded code changes. Schemas, checklists, and review controls define when engineers, QA staff, architects, and operators must review or approve a change. People accountable for production retain cutover authority. The fictional Northstar Settlements system carries the method through the book. The companion lab includes runnable GitHub exercises, synthetic fixtures, a golden master, and differential tests. You can practise the workflow against a realistic modernization problem without production data. Across more than 790 pages and 26 connected chapters, the book includes 86 diagrams, plus reusable schemas, checklists, and templates. It is for software engineers, architects, technical leads, QA professionals, platform engineers, and operations teams responsible for changing critical systems that cannot simply be switched off or replaced in one rewrite.