the good PRD: what should the agent build?

why is the PRD the first lesson?

The PRD (Product Requirements Document – the product specification: what we build, for whom, and what counts as done) is the step beginners skip most often – and the most common reason an agent build falls apart. The agent builds everything you ask for. The trouble starts with what you don't ask for: it builds that too, just according to its own defaults.

This site's PRD is public – throughout this lesson we work from it, with live examples.

the three mandatory layers of a PRD

An agent-friendly PRD is not a long document but three sharp boundary lines:

1. What we build and why – one paragraph, with numbers. Not "a modern webshop", but "a one-page order form that sends the order by email, with 0 Ft of monthly infrastructure cost".
2. What is explicitly EXCLUDED – this layer is missing from most specifications, yet with an agent it's worth the most. Whatever you don't rule out, the agent will build one day.
3. The definition of done – verifiable statements: "the build runs without errors", "it doesn't break on mobile at 375px", "the form cannot be submitted with an empty field".

the decision log: the living part of the PRD

A specification isn't written once. Every mid-flight decision (D1, D2, …) is numbered and kept in the PRD – so the agent's next session doesn't reopen questions that are already settled. This pattern lives in this very site's PRD too: every D number is a closed debate.

what it looks like in production

This site was built with exactly this way of working. The replay below is a real session from building the lesson engine (2026-07-14): notice that the agent's first step is not writing code but reading the specification.

exercise

❯ exercise — your first own PRD

Write the PRD of your own first production project with the three layers, in one page at most. In the next lesson we hand this PRD to the agent.

• ○one paragraph on what we build and why — with numbers, not adjectives
• ○at least five items on the EXCLUDED list
• ○every statement in the definition of done is verifiable (runnable or inspectable)
• ○the whole thing fits on one page

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