From PRD to engineering spec: closing the handoff gap

The PRD and the engineering spec are two documents with a clean division of labor: the PRD owns the what and why (problem, users, requirements, metrics), the spec owns the how (architecture, data models, APIs, technical approach). The trouble is the space between them — the handoff — which is where requirements quietly get lost, reinterpreted, or padded with assumptions nobody wrote down.

The handoff is a conversation, not a wall

The classic failure is the PM finishing a PRD, tossing it over the wall, and an engineer building a spec from it in isolation. Two things go wrong: the engineer fills gaps with guesses (some wrong), and the PM never learns which requirements were expensive or infeasible until it's late. A good handoff is a working session — the engineer asks the clarifying questions, the PM learns where the cost is, and both shape the spec together. The data backs this up: the strongest signal in product interviews is the habit of asking clarifying questions, and it's just as predictive in real handoffs.

Run a gap analysis

Before building, deliberately compare the spec against the PRD and hunt for holes: requirements with no corresponding technical plan, edge cases the PRD implied but didn't state, non-functional needs (performance, security) that got dropped, and assumptions the spec made that the PRD never authorized. This is shift-left in practice — every gap found here costs a fraction of what it costs after engineering has built around the wrong assumption.

Keep traceability

Every significant decision in the spec should trace back to a requirement in the PRD. When it doesn't, you've found either gold-plating (the spec is building something nobody asked for) or a missing PRD requirement (the spec discovered a real need the PRD didn't capture). Both are worth resolving explicitly. Maintaining that thread — requirement to design to test — is what keeps the built thing faithful to the problem you set out to solve.