Spec — create, reverse, and update living specs

Create, reverse-engineer, and update living feature specifications. Specs are the permanent source of truth for a feature's architectural intent; code is the source of truth for execution logic. Operate at the spec-anchored maturity level.

Boundaries

• DOES read source code extensively
• DOES create and update files in specs/
• Does NOT write implementation code (dev)
• Does NOT modify source code outside specs/
• Does NOT run build, lint, or test commands (ship)
• Does NOT create issues (lead), but may suggest PBI breakdowns

Mode detection

Determine the mode from the task input:

1. Create (default) — write a new spec from a feature description, requirements, or issue reference
2. Reverse — reverse <feature-domain> or reverse <file-paths> — reverse-engineer a spec from existing code
3. Update — update <feature-domain> — update an existing spec to reflect current reality

Pipeline

Step 1 — Context loading

1. Check for specs/TEMPLATE.md — if it exists, use it as the base template for new specs
2. Read AGENTS.md and ARCHITECTURE.md if they exist (global project context)
3. Load the project's own spec doctrine:
   • src/content/practices/living-specs.md — spec maintenance methodology
   • src/content/patterns/the-spec.md — what a spec IS, structurally
   • src/content/concepts/spec-driven-development.md — SDD maturity levels
   • src/content/config.ts — data schemas
4. If updating, read specs/{feature-domain}/spec.md
5. If an issue reference was provided and an issue tracker MCP is available (Linear, for this project), fetch it

Step 2 — Research

Create:

• Read referenced issues or requirements
• Explore source code in the feature area
• Identify API contracts, data models, dependency directions, integration points
• Check specs/ for related specs that should be cross-referenced

Reverse:

• Read all source files in the feature area systematically
• Trace data flow, imports, dependencies
• Identify API contracts, schemas, error handling, edge cases
• Look for existing tests — these encode unstated contracts
• Reconstruct architectural intent from implementation patterns

Update:

• Read the existing spec
• Review the git diff on the feature area to see what changed
• Identify stale sections
• Check for new constraints the spec doesn't capture

Step 3 — Author

Write the spec to specs/{feature-domain}/spec.md.

Use specs/TEMPLATE.md as the base if it exists. Otherwise use:

# Feature: [Feature Name]

## Blueprint

### Context
[1-2 paragraphs: Why does this feature exist? What problem does it solve?]

### Architecture
- **API Contracts:**
  - `POST /api/v1/[endpoint]` — [Description, request/response shapes]
  - `GET /api/v1/[endpoint]/:id` — [Description]
- **Data Models:** Defined in `[path/to/types]`, validated by `[path/to/schema]`
- **Dependencies:**
  - Depends on: [services, libraries, external APIs]
  - Depended on by: [downstream consumers]
- **Constraints:** [Security, compliance, or architectural boundaries stated as
  facts. Only include rules not already captured by the architecture above.]

## Contract

### Definition of Done
- [ ] [Observable, measurable success criterion]
- [ ] [Each must be independently verifiable by CI or a reviewer]

### Regression Guardrails
- [Invariant that must never break, across all future changes]

### Scenarios
```gherkin
Scenario: [Descriptive name]
  Given [Precondition]
  When [Action]
  Then [Expected outcome]
```

Authoring rules:

• State constraints positively. Do not write an "Anti-Patterns" section. Telling an agent what NOT to do puts the wrong approach in its context window. "All real-time updates use WebSocket" is better than "Don't use polling." Security and compliance rules go under Constraints as facts, not warnings.
• Let Gherkin absorb failure modes. Write scenarios for edge cases instead of anti-pattern lists. Then the system does NOT store credentials in localStorage is a verifiable contract.
• Use file paths, not descriptions. "src/types/User.ts" is actionable. "The user model" is ambiguous.
• Code illustrates, never transcribes. A code block must convey intent (an example algorithm or pseudocode), not mirror the implementation. No real symbol names, import lines, or exact serialization — those drift on the next refactor. If a block would need editing on a pure refactor with no contract change, replace it with a file-path reference. See the Copy-Paste Codebase anti-pattern in src/content/practices/living-specs.md.
• Match depth to complexity. Simple features get simple specs. Omit sections that add no information.
• Assume engineering competence. Document project-specific constraints, not general knowledge.

Step 4 — Cross-reference

• Check specs/ for related specs that should link to this one
• Ensure bidirectional references where features depend on each other
• Verify that file paths named in the spec actually exist (reverse/update modes)

Step 5 — Validate

• Every Definition of Done item is independently verifiable
• Scenarios cover the happy path, at least one error case, and edge cases
• No "Anti-Patterns" section — constraints are stated positively
• Architecture references specific file paths
• Spec depth matches feature complexity
• If updating: stale sections marked [DEPRECATED yyyy-mm-dd] with rationale, not deleted

Step 6 — Reduction pass

Before finishing, subtract. Remove anything a reader could confirm by opening the code — transcribed snippets, real symbol names, field-by-field serialization — and replace each with a file-path reference. Ask of every code block: does this illustrate intent, or mirror the implementation? In reverse and update mode especially, a spec that grew is a red flag: reconciliation should usually make a spec smaller.

Step 7 — Template offer

If specs/TEMPLATE.md does not exist, offer to save it for future consistency.

Principles

• Spec-anchored, not spec-as-source. The spec owns intent and contracts. Code owns execution logic.
• Refinement cycle. Specs are hypotheses. Implementation reveals unknowns. Update the spec in the same commit as the code that revealed them.
• Same-commit rule. If code changes behavior, the spec update ships in the same commit.
• Deprecation over deletion. Mark outdated sections [DEPRECATED] with rationale.
• Positive constraints. State what the system does, not what it shouldn't. Gherkin absorbs failure modes.
• Code illustrates, never transcribes. Example algorithms and pseudocode are welcome; copies of the real implementation are not — they drift. Replace transcription with file-path references.

Task input

Apply the skill to the supplied feature description, reverse <path>, or update <domain> request.