Spec

Writing Specs

Overview

Create and maintain docs/SPEC.md — the single source of truth for the project's design decisions, architecture, and constraints. This is NOT an implementation plan. It's the "what and why" document that plans will later reference.

Announce at start: "Using spec skill to update the project specification."

Save to: docs/SPEC.md

What the Spec Is

A living document that captures the current state of the project's design. SPEC.md is updated via sync-docs (which merges implemented design docs) or via direct edits through this skill. Think of it as a product brief + technical design that evolves with the project.

Spec Document Structure

# [Project Name]

## Vision
[2-3 sentences. What is this? Why does it exist? What problem does it solve?]

## Goals
- [What success looks like — measurable if possible]
- [Who the users are and what they need]
- [Key outcomes]

## Non-Goals
- [What this project explicitly does NOT cover]
- [Scope boundaries — helps prevent scope creep]

## Architecture
[High-level technical approach. Key technologies, patterns, infrastructure. Not implementation details — just the big structural decisions.]

### Key Components
- [Component 1]: [what it does]
- [Component 2]: [what it does]

### Data Flow
[How data moves through the system. Can be prose, can be a simple diagram.]

## Key Decisions
| Decision | Choice | Rationale |
|----------|--------|-----------|
| [e.g., Database] | [e.g., PostgreSQL] | [Why this over alternatives] |

## Edge Cases & Constraints
- [Known gotchas, edge cases, constraints]
- [Performance requirements]
- [Security considerations]

## Open Questions
- [Things that haven't been decided yet]
- [Things that need user input]

## References
- [Links to docs, inspirations, existing systems, etc.]

Invocation Modes

The spec skill is invoked in two situations.

Mode 1: Direct edit

Triggered when SPEC.md needs changes but no design doc is involved. Common cases:

• User invokes the skill directly to update SPEC.md
• The user wants to explicitly edit the spec outside of sync-docs

1. Read SPEC.md
2. Apply targeted edits from user instructions
3. Present diff to the user for approval
4. Save

Mode 2: Initial project spec (greenfield)

Triggered by brainstorm when no SPEC.md exists yet. The SPEC.md is the design artifact for project initialization.

1. Interview — ask questions one at a time to fill gaps (vision, goals, non-goals, architecture, key decisions)
2. Draft the spec
3. Present for review
4. Revise and save

Note: design doc merging is handled by sync-docs, not by the spec skill. When sync-docs runs, it merges any implemented design docs into SPEC.md and deletes them.

Writing Good Specs

Do:

• Be specific about goals but not about implementation
• Include non-goals — they're as important as goals
• Document key decisions and WHY you made them
• Keep architecture high-level — boxes and arrows, not code
• Call out edge cases and risks early
• Leave open questions as open questions (don't guess)
• Update existing sections when decisions change — don't leave stale info

Don't:

• Include implementation tasks or step-by-step plans (that's for the plan skill)
• Include code snippets (unless illustrating a contract or API shape)
• Get bogged down in edge cases — list them, don't solve them
• Duplicate information across sections

When to Update

Update docs/SPEC.md whenever:

• The user directly invokes the spec skill to make changes (Mode 1)
• Starting a new project with no SPEC.md (brainstorm triggers Mode 2)
• sync-docs merges an implemented design doc into SPEC.md (handled by sync-docs, not this skill)

The spec should always reflect the CURRENT state of the project's design, not historical decisions that were overridden.

Never updated from brainstorm or plan. Brainstorm writes a design doc. Plan reads it. SPEC.md gets updated when the user runs sync-docs or directly invokes the spec skill.

After the Spec

Once SPEC.md is updated:

• Mode 1 (direct edit): "SPEC.md updated."
• Mode 2 (greenfield): "Initial spec saved at docs/SPEC.md. Ready for /plan on the first feature."