Existing codebases

Bring OpenSpec to a codebase with a lot of code and no specs: where to start, what to backfill, and how specs grow from there.

<!-- Skeleton: headings only. Rescoped 2026-08-11: the page is the adoption guide for legacy codebases (a lot of code, no specs), not just the backfill task; "Specs for existing code" is now one section. The opening section owns "you don't need specs first": the quickstart runs on an existing repo as-is, and specs accumulate through changes. Source: docs/existing-projects.md. When prose lands, reshape to the Guides skeleton (one-line what, quickstart link, 80% path, Advanced). -->

Start with a change

Specs for existing code

Working from a PRD

Organizing specs as they grow