My new book, Claude Code for Legacy Modernization, is now live on Amazon Kindle.
Buy the Kindle edition on Amazon UK or Amazon US.
ASIN: B0HJ1518F1.
Legacy modernization often starts with a deceptively simple request: move the application to a newer stack without changing what it does. The code can tell you part of the story. It cannot tell you why an odd branch exists, who depends on a blank field, or what an operator does after a partial failure.
That missing evidence is where modernization programmes get hurt.
Evidence before replacement
The book treats modernization as an evidence problem before it becomes a code problem. Claude Code can trace dependencies, inspect runtime paths, prepare tests, and help implement a bounded replacement. It cannot decide which unexplained behaviour is safe to remove.
The method starts by recovering business rules and recording the current baseline. You then build a verification harness with characterization tests, golden masters, consumer contracts, replay data, and explicit rollback gates.
Only then do you move a controlled slice.
What is inside
The Kindle edition has 26 connected chapters and 86 diagrams. It follows a fictional financial-services system called Northstar Settlements through discovery, verification, parallel operation, reconciliation, cutover, and retirement.
There is also a runnable companion lab, plus reusable prompts, schemas, checklists, and decision records. The point is to make the modernization decision inspectable. Generated code is useful, but it is not proof that production behaviour has been preserved.
Who I wrote it for
This is for engineers, architects, technical leads, and modernization teams responsible for systems that cannot simply be switched off and rewritten.
It is especially relevant when the application moves money, supports customers, feeds regulatory reporting, or has accumulated years of operational knowledge that never made it into the repository.
Get the Kindle edition
The book is available in both Amazon stores:
You can also read the full book overview before choosing an edition. The Leanpub edition remains available for readers who prefer updates through Leanpub.