authoring-feature-spec — SKILL.md

Variant: standard · When to use: elaborating the features a PRD names into a comprehensive, testable feature specification — to a bar the downstream build can be planned and verified from. Greenfield, or a versioned amend of an approved spec.

Overview

This skill is the how-to of writing a strong feature specification — the layer below the PRD. The PRD says what the product does and why, and names its features; the feature spec says how each named feature behaves, in enough detail to build and test. This skill carries the producer's judgment — the research method and the quality bar — not the section list. It assumes two collaborators: a feature-spec template tool that supplies the section structure, and a deep-research capability to ground each feature in evidence. The producer is handed the approved PRD (and any upstream context) and must decompose and elaborate its features — never emit generic boilerplate. The bar to clear: each feature is implementable and testable — an engineer can build it and a tester can verify it without asking the author, and planning can cut tasks from it.

The method spine is unchanged from a strong feature spec; this version deepens how you write unambiguous behavior (EARS, use-case flows, the requirements-smell catalog), makes the state-transition table and decision table explicit, adds a per-feature non-functional-requirement discipline and archetype overlays (a feature's emphasis shifts by its nature), handles probabilistic/ML features (threshold criteria, not "works"), and adds an amend method for the common case of changing an approved spec.

When to activate

• Authoring a feature spec from an approved PRD that names a set of features.
• Elaborating one feature (or a clearly-bounded set) into testable behavior, I/O, states, edge cases, and acceptance criteria.
• Amending an approved feature spec (a delta: a feature/behavior/AC/state changed or added) — you are handed the existing spec + a change request.
• Filling a feature-spec template with researched, decision-complete per-feature content.

Do NOT activate when: