| name | spec-driven-development |
| description | Creates specs before coding. Use when starting a new project, feature, or significant change and no specification exists yet. Use when requirements are unclear, ambiguous, or only exist as a vague idea. |
Spec-Driven Development
Overview
Write a structured specification before writing any code. The spec is the shared source of truth between you and the human engineer — it defines what we're building, why, and how we'll know it's done. Code without a spec is guessing.
When to Use
- Starting a new project or feature
- Requirements are ambiguous or incomplete
- The change touches multiple files or modules
- You're about to make an architectural decision
- The task would take more than 30 minutes to implement
When NOT to use: Single-line fixes, typo corrections, or changes where requirements are unambiguous and self-contained.
The Workflow
Spec-driven development has four phases. Do not advance to the next phase until the current one is validated against its self-check criteria (no human gate — this runs in CI).
SPECIFY ──→ PLAN ──→ TASKS ──→ IMPLEMENT
│ │ │ │
▼ ▼ ▼ ▼
Self- Self- Self- Self-
validate validate validate validate
Phase 1: Specify
Start with a high-level vision. Derive requirements from the issue body, comments, and codebase context. If requirements are ambiguous, surface assumptions explicitly and proceed with the most reasonable interpretation — do not block on human input (CI mode).
Surface assumptions immediately. Before writing any spec content, list what you're assuming:
ASSUMPTIONS I'M MAKING:
1. This is a web application (not native mobile)
2. Authentication uses session-based cookies (not JWT)
3. The database is PostgreSQL (based on existing Prisma schema)
4. We're targeting modern browsers only (no IE11)
→ Proceeding with these assumptions. Document any contrary evidence in the spec's Open Questions section.
Don't silently fill in ambiguous requirements. The spec's entire purpose is to surface misunderstandings before code gets written — assumptions are the most dangerous form of misunderstanding.
Write a spec document covering these six core areas:
-
Objective — What are we building and why? Who is the user? What does success look like?
-
Commands — Full executable commands with flags, not just tool names.
Build: npm run build
Test: npm test -- --coverage
Lint: npm run lint --fix
Dev: npm run dev
-
Project Structure — Where source code lives, where tests go, where docs belong.
src/ → Application source code
src/components → React components
src/lib → Shared utilities
tests/ → Unit and integration tests
e2e/ → End-to-end tests
docs/ → Documentation
-
Code Style — One real code snippet showing your style beats three paragraphs describing it. Include naming conventions, formatting rules, and examples of good output.
-
Testing Strategy — What framework, where tests live, coverage expectations, which test levels for which concerns.
-
Boundaries — Three-tier system:
- Always do: Run tests before commits, follow naming conventions, validate inputs
- Ask first: Database schema changes, adding dependencies, changing CI config
- Never do: Commit secrets, edit vendor directories, remove failing tests without approval
Spec template:
# Spec: [Project/Feature Name]
## Objective
[What we're building and why. User stories or acceptance criteria.]
## Tech Stack
[Framework, language, key dependencies with versions]
## Commands
[Build, test, lint, dev — full commands]
## Project Structure
[Directory layout with descriptions]
## Code Style
[Example snippet + key conventions]
## Testing Strategy
[Framework, test locations, coverage requirements, test levels]
## Boundaries
- Always: [...]
- Ask first: [...]
- Never: [...]
## Success Criteria
[How we'll know this is done — specific, testable conditions]
## Open Questions
[Anything unresolved that needs human input]
Reframe instructions as success criteria. When receiving vague requirements, translate them into concrete conditions:
REQUIREMENT: "Make the dashboard faster"
REFRAMED SUCCESS CRITERIA:
- Dashboard LCP < 2.5s on 4G connection
- Initial data load completes in < 500ms
- No layout shift during load (CLS < 0.1)
→ These targets are derived from industry benchmarks. Adjust in the spec if the issue specifies different targets.
This lets you loop, retry, and problem-solve toward a clear goal rather than guessing what "faster" means.
Phase 2: Plan
With the validated spec, generate a technical implementation plan:
- Identify the major components and their dependencies
- Determine the implementation order (what must be built first)
- Note risks and mitigation strategies
- Identify what can be built in parallel vs. what must be sequential
- Define verification checkpoints between phases
Follow planning-and-task-breakdown for the dependency-graph mapping and vertical-slicing mechanics behind these steps; it is the canonical source. The bullets above are a lightweight summary; if they ever diverge, planning-and-task-breakdown takes precedence.
Output convention: Save the plan to tasks/plan.md and the task list to tasks/todo.md, per the /plan command convention. Create tasks/ if it does not exist. Downstream commands (/build, etc.) expect these paths.
The plan should be self-consistent: verify that every spec requirement maps to at least one task, and that task dependencies form a valid DAG with no cycles.
Phase 3: Tasks
Break the plan into discrete, implementable tasks:
- Each task should be completable in a single focused session
- Each task has explicit acceptance criteria
- Each task includes a verification step (test, build, manual check)
- Tasks are ordered by dependency, not by perceived importance
- No task should require changing more than ~5 files
Follow planning-and-task-breakdown for the full task-sizing and dependency-ordering mechanics; it is the canonical source. The template below is a lightweight inline form; if they ever diverge, planning-and-task-breakdown takes precedence.
Task template:
- [ ] Task: [Description]
- Acceptance: [What must be true when done]
- Verify: [How to confirm — test command, build, manual check]
- Files: [Which files will be touched]
Phase 4: Implement
Execute tasks one at a time following skills/incremental-implementation/SKILL.md (incremental-implementation) and skills/test-driven-development/SKILL.md (test-driven-development).
Keeping the Spec Alive
The spec is a living document, not a one-time artifact:
- Update when decisions change — If you discover the data model needs to change, update the spec first, then implement.
- Update when scope changes — Features added or cut should be reflected in the spec.
- Commit the spec — The spec belongs in version control alongside the code.
- Reference the spec in PRs — Link back to the spec section that each PR implements.
Common Rationalizations
| Rationalization | Reality |
|---|
| "This is simple, I don't need a spec" | Simple tasks don't need long specs, but they still need acceptance criteria. A two-line spec is fine. |
| "I'll write the spec after I code it" | That's documentation, not specification. The spec's value is in forcing clarity before code. |
| "The spec will slow us down" | A 15-minute spec prevents hours of rework. Waterfall in 15 minutes beats debugging in 15 hours. |
| "Requirements will change anyway" | That's why the spec is a living document. An outdated spec is still better than no spec. |
| "The user knows what they want" | Even clear requests have implicit assumptions. The spec surfaces those assumptions. |
Red Flags
- Starting to write code without any written requirements
- Asking "should I just start building?" before clarifying what "done" means
- Implementing features not mentioned in any spec or task list
- Making architectural decisions without documenting them
- Skipping the spec because "it's obvious what to build"
Verification
Before proceeding to implementation, confirm:
