Write the specification first, then build to match. Use specs as the single source of truth for design, development, testing, and documentation—eliminating ambiguity and costly rework.
Most bugs and rework come from miscommunication—not coding mistakes. When the spec is the contract between teams, everyone builds the same thing. The spec becomes executable truth.
Spec Driven Development (SDD) is a methodology where you define the complete specification of a feature, API, or system before writing any implementation code. The spec serves as a contract that all parties agree to, and it drives design, development, testing, and documentation.
Define what you're building before how you're building it. The spec is written and reviewed before implementation begins.
The spec is authoritative. Code, tests, and docs are generated from or validated against the spec.
Specs aren't just documents—they generate tests, mocks, and validation. Specs run as part of CI/CD.
Teams agree on the spec contract. Frontend and backend can work in parallel once the contract is set.
Follow these steps to implement spec-first development in your team.
Write a complete, unambiguous specification before any implementation. Include inputs, outputs, edge cases, and error states.
paths:
/users/{id}:
get:
summary: Get user by ID
responses:
200:
description: User found
content:
application/json:
schema:
$ref: '#/components/schemas/User'
404:
description: User not foundAll stakeholders review the spec before implementation. This is where you catch misunderstandings—not in code review.
Pro tip: Use spec review tools like Stoplight, SwaggerHub, or GitHub PRs for OpenAPI specs. Track changes and discussions directly on the spec.
Generate code, tests, mocks, and documentation directly from the spec. This ensures everything stays in sync.
With the contract set, frontend and backend teams can work simultaneously. Frontend uses generated mocks; backend implements to the spec.
Result: Teams unblocked. Integration works on first try because both sides built to the same contract.
Run spec validation in CI/CD. Every API change must pass spec compliance tests. Drift is caught immediately.
Apply spec thinking during the Research to Backlog flow.
Apply spec validation during Backlog to Production flow.
| Approach | When Spec is Written | Pros/Cons |
|---|---|---|
| Spec Driven (Contract First) | Before implementation | ✓ Parallel work, clear contract ✗ Upfront investment in spec |
| Code First | Generated from code after | ✓ Fast initial development ✗ Spec reflects code quirks |
| TDD (Test Driven) | Tests written before code | ✓ High test coverage ✗ Tests ≠ contracts |
| BDD (Behavior Driven) | Behavior specs before code | ✓ Business-readable specs ✗ Focused on behavior, not data |
Use Spec Driven Development for API contracts between services. Use BDD for user-facing behavior. Use TDD for unit-level implementation. They complement each other.
Pick a new API or feature. Write the OpenAPI spec first. Get buy-in by showing how it enables parallel development and catches issues early.
Add openapi-generator or similar to your build. Generate TypeScript types for frontend, interfaces for backend. Make it part of the standard workflow.
Use Spectral for linting, Dredd or Prism for contract testing. Block PRs that break the spec. Make compliance automatic, not optional.
Require spec review before implementation begins. Create a lightweight process: spec PR → review → approve → then start coding.
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