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.
When an outsourcing engagement wraps up, product knowledge walks out the door. A living product contract keeps it in the codebase — not in someone's head.
AI can now extract product logic from your codebase. Stewie builds a living product contract your whole team can read — no code, no stale docs, no waiting on engineers.
PMs confirm behaviors. BAs clarify domain logic. QA knows what to protect. New hires onboard in hours. Vendor teams skip the reverse-engineering phase. Here's how each role contributes to a living product contract.
Strategic leaders shouldn't need three meetings to verify whether a product decision was implemented correctly. A living product contract gives you a direct line to what's running in production.
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.
Shipping fast with AI coding tools is genuinely good. The problem isn't the speed — it's what gets left behind. Product reasoning doesn't survive the vibe.
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.
After running into the same product knowledge gap across multiple SaaS products, I built an open Markdown spec for capturing what your product promises to do. Here's what's in it and why.
A Product Behavior Contract (PBC) is a living, code-grounded spec that describes how your product actually behaves — not how you think it does.
Product knowledge lives in heads, not docs. Here's why that gap compounds — and what a Product Behavior Contract does about it.