PRDs capture intent. Test suites verify assertions. But between those two, there's no artifact that tracks what the product promises to do — grounded in code, confirmed by humans. That's the PBC layer.
AI coding agents now have AGENTS.md, memory banks, harnesses, evals, and monitors. They still lack product context: what the product promises to do and which behaviors must stay true.
Your coding agent ships correct-looking code that breaks product promises. The problem isn't capability — it's a missing context layer that AGENTS.md and memory banks were never designed to provide.
Your repo has workflow instructions, session context, and feature specs. None of them answer: what does the product promise to do? That's a different layer — and it needs its own artifact.
A .pbc.md file opens in VS Code, renders on GitHub, and reads like any other Markdown document. Drop it into pbc.stewie.sh and the same file becomes navigable structured UI. One source of truth, two reading experiences.