TL;DR: PRDs capture why you're building something. Test suites verify code behavior. But neither one records what the product promises to do right now — confirmed by humans, grounded in code evidence, evolving with every deploy. A Product Behavior Contract (PBC) fills that gap. These three artifacts aren't competing — they're stages in a product knowledge lifecycle, and most teams are missing the middle one.
Code does. PBCs promise. The gap is where bugs live.
Most teams treat PRDs, specs, and test suites as independent documents. They're not. They're a pipeline: intent flows into promises, promises inform verification. The problem is the middle stage — the one that records what the product commits to doing — usually has no artifact at all.
Three artifacts, one lifecycle
Think of it like an API:
- An API design doc explains why the endpoint exists and what problem it solves
- An OpenAPI spec declares what the endpoint promises to accept and return
- Integration tests verify the implementation matches
Nobody would argue these compete. They're layers. If you skip the OpenAPI spec, you can still build and test the API — but you've lost the contract. Any drift between intent and implementation becomes invisible until something breaks in production.
Product behavior works the same way.
What does each artifact actually capture?
| PRD | PBC | Test Suite | |
|---|---|---|---|
| Captures | Intent — why we're building this | Promises — what the product commits to doing | Correctness — does code match assertions? |
| Written by | PM / stakeholder | Extracted from code, confirmed by humans | Engineers |
| Lifespan | Snapshot — point in time | Living — evolves with code | Living — but only covers what's been asserted |
| Grounded in | Business context, strategy | Code evidence + human confirmation | Code execution |
| Drift means | Business context changed | Code broke a product promise | An assertion failed |
How do they feed each other?
A PRD says: "We're adding a 14-day refund window for the Pro tier."
The team builds it. A PBC confirms: "Refund window is 14 days, Pro tier only — confirmed, evidence: billing/refund.ts:42."
A test suite asserts: expect(refundWindow).toBe(14) for Pro users.
Six months later, the business decides to extend refunds to 30 days. A new PRD captures that intent. The PBC gets updated when someone confirms the new behavior in code. The test catches if the old assertion still runs against the new logic.
Without the PBC layer: the PRD says 30 days, the test still asserts 14, and nobody knows which reflects the current product promise until a customer files a support ticket.
PRDs are starters, not sources of truth
PRDs are invaluable. They capture reasoning, trade-offs, and stakeholder alignment. But they're snapshots. The moment code ships, the PRD starts drifting from reality.
Most teams know this. They just don't have anything that replaces the PRD as the living record of product promises. So they either:
- Treat the PRD as truth — it isn't. It reflects what someone believed at a point in time.
- Treat tests as truth — they verify behavior, but don't explain what the product promises or why.
- Ask the engineer who built it — tribal knowledge. Doesn't scale, doesn't survive team changes.
A PBC doesn't replace the PRD. It's where the PRD's intent lands after the code is written and a human confirms: "Yes, this is what the product actually commits to."
Tests prove code works. They don't explain what the product promises.
A passing test suite tells you code does what someone asserted. It doesn't tell you:
- Whether that assertion reflects a confirmed product decision or a guess
- Why that behavior exists
- Whether the promise should still hold after the last pivot
- Which behaviors are load-bearing business rules and which are implementation details
PBCs are the contract layer — like OpenAPI for your product logic. They declare what the product promises, with evidence. When code drifts from those promises, the gap is visible. When a test verifies behavior that no longer matches a product promise, the mismatch surfaces before it reaches production.
You probably need all three. Which one are you missing?
Quick practical guidance:
- Writing a PRD? Good — it seeds the PBC. Include the specific behaviors you expect, not just feature descriptions. Those behaviors become candidates for confirmation.
- Have a PBC? Your confirmed promises are your highest-confidence test candidates. Test suites can be derived from them. "Confirmed" means someone verified this is intentional — that's worth protecting with a test.
- Only have tests? You have verification without explanation. You know what passes, but not what the product promises or why it should.
The lifecycle isn't optional. Most teams just skip the middle step and pay for it in misalignment, rework, and knowledge that walks out the door when people leave.
Code does. PBCs promise. The gap is where bugs live.
For the format behind this: What is a Product Behavior Contract?. If you're a PM trying to verify what shipped: Stewie for Product Managers. If you're an engineer tired of being the living documentation: Stewie for Engineering Teams.