A product requirements document describes what a product or feature should do, for whom, and why — at a level of precision that lets engineering build it without inventing the missing parts. It is not a design document, not a project plan, and not a wish list.
What belongs in one
- The problem, stated in terms of the user, not the solution. If this section describes a feature, it is the wrong section.
- Who has it, specifically enough to exclude people.
- Success criteria — what changes if this works, expressed so that it could turn out false.
- Scope, and explicitly what is out of scope. The second half is where most of the value is.
- Requirements — the behaviours, including the unhappy paths.
- Constraints — regulatory, technical, timing.
- Open questions, with owners. A PRD with none is hiding them, not lacking them.
What does not belong
- Implementation choices. "Use Redis" is an architecture decision, and it belongs in an ADR where its reasoning survives.
- Visual design, beyond what constrains behaviour.
- Estimates. Those come after, from the people doing the work.
- Prose written to persuade a stakeholder. That is a different document with a different audience.
The section that prevents the most rework
Out of scope. Most disputes late in a project are not about what was written down — they are about what each side assumed was implied. Listing five things this deliberately does not do is a fifteen-minute exercise that routinely saves weeks, and it is the section most often skipped.
Precision, not length
A good PRD is short and unambiguous. The test is whether two competent engineers reading it independently would build materially the same thing. Length is not a proxy for that — long documents hide ambiguity in volume, and the sections nobody reads are exactly where the assumptions sit.
## Requirement: refund an order
Given an order in state DELIVERED or SHIPPED
When an agent requests a refund with an amount <= order total
Then the refund is issued to the original payment method
And the order moves to REFUNDED
And the customer receives a confirmation email
Not in scope: partial refunds by line item; refunds after 90 days;
refunds to a different payment method.Why this got more important, not less
When implementation is fast and cheap, output quality is bounded by specification quality. A vague requirement no longer produces a clarifying conversation — it produces a plausible, confidently wrong implementation that then has to be reviewed, understood and unpicked. The ambiguity that used to surface as a question now surfaces as rework.
Frequently Asked Questions
How long should a PRD be?
As short as it can be while remaining unambiguous. One or two pages is common for a feature. If it is ten, it is probably several features and should be split.
Who writes it?
Whoever owns the outcome — often a product manager, frequently an engineer in smaller teams. What matters is that engineering reviews it before work starts, not who typed it.
Is a PRD still useful for small changes?
The document is not, but the thinking is. For a small change, the same content fits in a ticket description — problem, behaviour, out of scope.
References
- Documenting Architecture Decisions — Michael Nygard / Cognitect
- The Agentic Engineering Trends Report 2026 — SaaSRise
About Jishu Labs
Jishu Labs is a software development company founded in 2016. We build custom software, AI/ML systems, and full-stack web and mobile applications for clients, and we make eight AI tools for software teams.