Specs, PRDs & RFCs

TL;DR

Writing is the PM's power tool. The documents form a stack: vision (why we exist) → strategy (where we'll win) → roadmap (what, in what order) → PRD (what exactly, for one initiative) → RFC / design doc (how, written by engineering) → tickets (the work itself). Each layer answers questions the layer below shouldn't have to re-litigate. The PRD is yours. A good one nails the problem, goals and non-goals, requirements with acceptance criteria, non-functional requirements, and how we'll measure success — and stays silent on implementation. The RFC is engineering's. Your job there is to be its sharpest reader, checking the design against the product intent it claims to serve.

🎯 For the AI PM

Why it matters — Deterministic features can be specified by enumerating behaviour: "clicking X does Y." An AI feature can't — you can't enumerate every input. The spec has to define quality instead of behaviour.

What it changes in your decisions — Your PRD gains new mandatory sections: example inputs and graded ideal outputs (the seed of an eval set), explicit tolerance for being wrong ("what's the cost of a bad answer, and what does the user see?"), and latency/cost budgets as first-class NFRs.

Ask yourself — "Could an engineer read this spec and know how good is good enough — without asking me?"

Risk if ignored — The team builds to "make the AI answer questions," discovers "good enough" was never defined, and the launch decision becomes a vibes-based argument in week eleven.

The document stack

Two arrows matter most. Acceptance criteria written in the PRD should be traceable all the way into tickets — if a ticket can't be traced to a requirement, ask why it exists. And RFCs feed back: engineering's design work routinely uncovers a constraint ("that data doesn't exist", "that latency is impossible without precomputation") that must flow back up and change the PRD, not get silently absorbed.

Anatomy of a PRD engineers respect

Length is not quality — one to four pages beats twenty. The sections that earn their place:

• Problem & evidence — whose problem, how you know it's real, what it costs today. This is the section engineers actually read to decide if the project is worth caring about. Write it best.
• Goals and non-goals — non-goals are the highest-leverage sentence in the document. "V1 does not support bulk import" kills a hundred Slack threads in advance.
• Requirements with acceptance criteria — each requirement testable: given a context, when the user acts, then this observable result. "The search should be fast" is an opinion. "Search returns first results within 500 ms at p95" is a requirement.
• Non-functional requirements (NFRs) — the ones that get forgotten until they're emergencies: latency and availability targets, scale assumptions, privacy and data residency, accessibility, localization, security review, support/admin tooling. NFRs are where the technical product sense track cashes in.
• Edge cases and unhappy paths — what happens on empty state, failure, retry, abuse. If the PRD only describes the happy path, the engineers design the unhappy ones alone.
• Success metrics — the numbers that will move if this works, and when you'll check (Metrics & experimentation).
• Open questions — an explicit list. Pretending certainty you don't have costs trust. A visible open-questions section builds it.

What a PRD should not contain: database schemas, API designs, technology choices, or a solution disguised as a requirement. ("Build a Redis cache" is a design. "Repeat visits must load in under a second" is the requirement behind it.)

Reading an RFC like a PM

The RFC (design doc) is where engineering proposes how. You won't judge the architecture — but you're the only reader checking it against product intent. Read for:

• Does the design serve the requirements it cites? RFCs sometimes quietly relax a requirement ("we'll refresh nightly instead of real-time"). Nightly might be fine — but that's your call, made in the open.
• The trade-offs section — every honest RFC lists rejected alternatives. Check the rejection reasons against product priorities. Engineers optimize for elegance and operability, which usually aligns with users, but not always.
• The migration/rollout plan — how the world moves from old to new (Launches, rollouts & migrations). Missing rollout plan = risk landing on your launch date.
• New promises being made — an RFC that exposes a new API or data contract is creating something other teams will depend on, which is product surface, whether or not it has a UI.

Comment with questions, not directives. "What happens to requirement 4's p95 target under this design?" moves the conversation. "Use Postgres instead" ends it.

Acceptance criteria for probabilistic features

The AI-specific craft deserves its own pattern. You can't write "the summary is always accurate." You can write:

• Eval-based criteria — "on the 200-example eval set, ≥90% of summaries are graded acceptable or better by the rubric in appendix A; zero examples in the harmful category." The eval set becomes part of the spec.
• Behavioural bounds — "when confidence is low or retrieval returns nothing, the feature says it can't answer rather than guessing; it never fabricates a citation."
• Budgets — "p95 end-to-end latency ≤ 3 s; marginal cost ≤ $0.02 per request at projected volume."

This is the bridge into eval-driven development — the AI capstone builds on exactly this section.

Failure modes

• The write-once PRD — treated as done when circulated, never updated as reality arrives. A stale spec is worse than none. People follow it.
• Solutioneering — specifying implementation in the PRD. You get your design and full blame when it underperforms — and you taught the engineers not to think.
• The unread masterpiece — twenty pages nobody finishes. If the team can't recall the goals and non-goals, the document failed regardless of quality.
• NFRs by default — no latency, privacy, or scale targets stated, so the system inherits whatever the implementation happened to produce.
• Consensus-by-silence — circulating a doc, hearing nothing, calling it alignment. Silence means unread. Review meetings exist for a reason.

Practitioner checklist

• Does my current PRD have explicit non-goals — and have they killed at least one scope argument?
• Is every requirement testable — could QA (or an eval) verify it without asking me?
• Are latency, privacy, scale, and failure behaviour stated, or defaulted?
• Have I read the RFC for my current build and traced each requirement into it?
• For AI features: does the spec include graded examples and a "how good is good enough" threshold?

Related lessons

• Discovery to delivery
• Working with engineering
• Technical product management for AI