DRY Principle for Product Specs: One Rule Definition to Eliminate Drift

Inconsistent wording of the same business rule across multiple specification files can silently introduce conflicting logic, as a refund policy defined differently in billing, checkout, and support specs illustrates. A developer has proposed applying the software engineering 'Don't Repeat Yourself' principle to product specifications by defining each rule once and having all specs cite a single canonical reference. The approach uses a content digest tag so that when a rule changes, a CLI tool instantly flags every outdated citation across the codebase. Integrating this check into a CI pipeline makes specification drift visible and prevents coding agents from treating inconsistent specs as authoritative truth. The method, demonstrated via the open-source prodshape CLI, is part of a broader concept called Product Definition as Code.