Its workflow covers preflight, assessment, dependency and data-flow mapping, extraction and human review of business rules, approval of a phased plan, modernization, and verification. The README says the plugin can target a newer version of the same technology, another language, or a new architecture, and recommends piloting one module rather than starting with an entire estate.
Verification is a distinct stage: the plugin reruns tests from a clean build and, where the legacy implementation can run, compares old and new outputs on the same inputs. The README describes masking only declared varying fields and recording accepted differences. It also says the legacy source is not edited; work is written to analysis and modernized directories. Human approval remains part of the process, including review of extracted rules and acceptance of differences.
The documentation describes intended workflow and checks, not measured migration outcomes or a guarantee of equivalence for every codebase. Public source: https://github.com/anthropics/claude-plugins-official/blob/main/plugins/code-modernization/README.md