What the source of truth for requirements should actually be

A designer at a 50-person startup laid it out, scared of being scapegoated: sometimes a PRD exists but isn't kept current, sometimes requirements are verbal or in Slack, sometimes they change days after design started, and when work moves to engineering everything gets re-explained instead of referenced. They asked what a healthy team uses as the source of truth.

A solo PM in another thread had the structural version: ideas live in one tool, the PRD is a manual copy of those ideas in another tool, and the two drift apart the moment anything changes. They hated the duplication and the silent divergence. Same disease, different symptom.

The problem isn't which tool. It's having more than one.

"What should the source of truth be" has a trick answer: it doesn't matter which artifact you pick, as long as there's exactly one and everything else points at it. The failure is never that you chose Confluence over Notion. The failure is that requirements live in a doc and Slack and Figma and someone's memory, and none of them is authoritative, so the real spec is whatever the last person remembers from the last meeting.

Why duplication guarantees drift

The moment you copy requirements from one place to another — idea tool to PRD, PRD to tickets, spec to a slide — you've created two things that can disagree. And they will, because only one of them gets updated when the decision changes. The copy you forgot to update becomes a quiet landmine: someone builds from it, and it's wrong, and nobody knows why until it ships.

What a real source of truth looks like

• One artifact is authoritative. Everything else — tickets, slides, the design file — references it instead of restating it.
• It updates in place when decisions change, and keeps the history, so "what did we agree to" has one answer and you can see how it evolved.
• Downstream artifacts are generated from it, not hand-copied. Tickets derived from the spec can't silently drift, because they trace back to it.
• Verbal and Slack decisions get folded back in, not left to evaporate. A decision that only exists in a thread isn't a decision; it's a rumor.

The designer's real fear — getting scapegoated for building the wrong thing — is the direct consequence of no source of truth. When there's one authoritative, current spec, "we built what the spec said" is a defense. When there are four, "you should have known" is the accusation, and it sticks to whoever's most junior.