What should a product spec include?

A good product spec covers: Why (the motivation and problem), Goals (the metric or outcome being targeted), Current state (what exists today and what is wrong with it), Proposal (what is being built and why this approach), and a Decisions log with rationale for each non-obvious choice. The Decisions log is the most underrated section — it prevents decisions from being relitigated in every review meeting.

What is the difference between a PRD and a feature spec?

A PRD tends to be broader — covering the full product or a major initiative — with more emphasis on market requirements and stakeholder alignment. A feature spec is narrower and more implementation-focused: it is written for the team doing the work, captures design decisions, and serves as the reference document during build and review.

How long should a feature spec be?

Long enough to answer the questions that will come up during implementation — no longer. Most feature specs are one to three pages. If it is longer, you are probably over-specifying implementation details that engineering should own. If it is shorter, you are likely missing the decisions section that prevents the spec from becoming obsolete the moment someone asks why you did not do it a different way.

Playbook

Spec idea

Run Spec idea

Turn a feature idea into user stories with acceptance criteria and risky assumptions

Use when

•An idea is moving from discovery to delivery and needs a written design commitment
•Engineering needs clarity on scope before starting implementation
•Multiple teams need to align on what is being built and why before sprint planning
•You want to capture design decisions so future teammates do not relitigate them

What You Get

Feature Spec
A structured markdown spec attached to the parent idea, covering Why, Goals, Current state, Proposal, and a Decisions log with one-line rationales for each non-obvious choice.

How It Works

1
Spec Idea

Why the right spec saves more time than it takes

A spec is not documentation for its own sake — it is a forcing function that surfaces implicit assumptions before implementation exposes them. The most valuable part is the Decisions log: each decision recorded with a one-line rationale so future readers know what was already considered and rejected. Without it, the same decisions get relitigated in every standup, design review, and code review. A good spec stops that cycle by making the thinking visible once and available to everyone who picks up the work.

Who it's for

Most valuable when an idea has passed discovery and the team is ready to commit to a direction. Too early and the spec will be rewritten; too late and the team has been building without alignment.

Product Manager

Technical Product Manager

Founder

Engineering Lead

Frequently asked questions

What should a product spec include?: A good product spec covers: Why (the motivation and problem), Goals (the metric or outcome being targeted), Current state (what exists today and what is wrong with it), Proposal (what is being built and why this approach), and a Decisions log with rationale for each non-obvious choice. The Decisions log is the most underrated section — it prevents decisions from being relitigated in every review meeting.
What is the difference between a PRD and a feature spec?: A PRD tends to be broader — covering the full product or a major initiative — with more emphasis on market requirements and stakeholder alignment. A feature spec is narrower and more implementation-focused: it is written for the team doing the work, captures design decisions, and serves as the reference document during build and review.
How long should a feature spec be?: Long enough to answer the questions that will come up during implementation — no longer. Most feature specs are one to three pages. If it is longer, you are probably over-specifying implementation details that engineering should own. If it is shorter, you are likely missing the decisions section that prevents the spec from becoming obsolete the moment someone asks why you did not do it a different way.