Spec-First Development

A structured workflow for LLM-assisted coding that delays implementation until decisions are explicit.

When This Activates

• "Build X" or "Create Y" (new features/projects)
• "Implement..." (non-trivial functionality)
• "Add a feature that..." (multi-step work)
• Any request requiring 3+ files or unclear requirements

When to Skip

• Single-file changes under 50 lines
• Typo fixes, log additions, config tweaks
• User explicitly says "just do it" or "quick fix"

Core Principles

1. Delay implementation until tradeoffs are explicit — Use conversation to clarify constraints, compare options, surface risks. Only then write code.

2. Treat the model like a junior engineer with infinite typing speed — Provide structure: clear interfaces, small tasks, explicit acceptance criteria. Code is cheap; understanding and correctness are scarce.

3. Specs beat prompts — For anything non-trivial, create a durable artifact (spec file) that can be re-fed, diffed, and reused across sessions.

4. Generated code is disposable; tests are not — Assume rewrites. Design for easy replacement: small modules, minimal coupling, clean seams, strong tests.

5. The model is over-confident; reality is the judge — Everything important gets verified by execution: tests, linters, typecheckers, reproducible builds.

The 6-Stage Workflow

Stage A: Frame the Problem (conversation mode)

Goal: Decide before you implement.

Prompts that work:

• "List 3 viable approaches. Compare on: complexity, failure modes, testability, future change, time to first demo."
• "What assumptions are you making? Which ones are risky?"
• "Propose a minimal version that can be deleted later without regret."

Output: Decision notes for decisions.md

Stage B: Write spec.md (freeze decisions)

Goal: Turn decisions into unambiguous requirements.

# spec.md

## Purpose
One paragraph: what this is for.

## Non-Goals
Explicitly state what you are NOT building.

## Interfaces
Inputs/outputs, data types, file formats, API endpoints, CLI commands.

## Key Decisions
Libraries, architecture, persistence choices, constraints.

## Edge Cases and Failure Modes
Timeouts, retries, partial failures, invalid input, concurrency, idempotency.

## Acceptance Criteria
Bullet list of testable statements. Avoid "should be fast."
Prefer: "processes 1k items under 2s on M1 Mac."

## Test Plan
Unit/integration boundaries, fixtures, golden files, what must be mocked.

Stage C: Generate todo.md (planning mode)

Goal: Stepwise checklist where each step has a verification command.

# todo.md

- [ ] Add project scaffolding (build/run/test commands)
  Verify: `npm run build && npm test`

- [ ] Implement module X with interface Y
  Verify: `npm test -- --grep "module X"`

- [ ] Add tests for edge cases A/B/C
  Verify: `npm test -- --grep "edge cases"`

- [ ] Wire integration
  Verify: `npm run integration`

- [ ] Add docs
  Verify: `npm run docs && open docs/index.html`

Each item must be independently checkable. This prevents "looks right" progress.

Stage D: Execute Changes (implementation mode)

Goal: Small diffs, frequent verification, controlled context.

Rules:

• One logical change per step
• Keep focus on one interface at a time
• After each change: run verification command, paste actual output back
• Commit early and often

For large codebases:

• Provide only relevant files plus spec/todo
• If summarizing repo, do it once and keep as reusable artifact

Stage E: Verify and Review (adversarial mode)

Goal: Force the model to try to break its own work.

Prompts:

• "Act as a hostile reviewer. Find correctness bugs, not style nits. List concrete failing scenarios."
• "Given these acceptance criteria, which are not actually satisfied? Be specific."
• "Propose 5 tests that would fail if the implementation is wrong."

Stage F: Decide What Lasts

Goal: Keep the system easy to delete and rewrite.

Heuristics:

• Keep "policy" (business rules) separate from "mechanism" (I/O, DB, HTTP)
• Prefer shallow abstractions that can be removed without cascade
• Invest in tests and fixtures more than clever architecture

The Three-File Convention

Keep at the root of every project:

spec.md      # what/why/constraints
todo.md      # steps + verification commands
decisions.md # tradeoffs, rejected options, assumptions

Agent Readiness Checklist (IMPACT)

Before running autonomous/agentic execution, verify:

Approve at meaningful checkpoints (end of todo item, after test suite passes), not every micro-step.

Prompt Patterns

Authoritarian (for correctness):

Edit these files: [paths]
Interface: [exact signatures]
Acceptance criteria: [list]
Required tests: [list]
Don't change anything else.

Options and tradeoffs (for design):

Give me 3 options and a recommendation.
Make the recommendation conditional on constraints A/B/C.

Context discipline (for large codebases):

Only use the files I provided.
If you need more context, ask for a specific file and explain why.

Make it provable:

Add a test that fails on the buggy version and passes on the correct one.

Output Format

When this skill activates, produce:

SPEC-FIRST WORKFLOW

STAGE A - FRAMING:
[3 approaches with tradeoffs]
[Recommendation]

STAGE B - SPEC:
[Draft spec.md content]

STAGE C - TODO:
[Draft todo.md with verification commands]

Ready to proceed to Stage D (execution)?