Spec-Driven Development (SDD)

SDD is the professional standard for high-assurance, agentic software engineering in 2026. It reverses the traditional "code-first" approach, elevating specifications to the primary system artifact. By ensuring code is merely a verified expression of precise specs, we eliminate ambiguity, technical debt, and misaligned requirements.

Why SDD in 2026?

As AI agents become the primary contributors to codebases, the bottleneck has shifted from writing code to defining intent. SDD provides:

• Alignment: Ensures AI agents work on the exact problem identified by the human.
• Traceability: Every line of code can be traced back to an acceptance criterion in a spec.
• Verification: Specs serve as the "ground truth" for automated test suites and agent-based verification.
• Efficiency: Reduces the need for costly refactoring by catching logic errors at the specification phase rather than the production phase.

When to Use

• New Features: Implementing non-trivial functionality.
• System Design: Defining architecture, data flows, or API contracts.
• Refactoring: When changing existing systems where behavioral preservation is critical.
• Complex Logic: When business rules are nuanced and prone to ambiguity.
• Agent Orchestration: When directing multiple agents, specs act as the shared project language.

Core Workflow (The SDD Lifecycle)

1. Idea → PRD: Iterate with an AI agent to clarify the problem, user value, and constraints.
2. PRD → Spec: Translate the PRD into an executable feature specification using User Stories and Acceptance Scenarios.
3. Spec → Implementation Plan: Derive the technical approach. Ensure every technical decision is explicitly traced to a requirement in the spec.
4. Plan → Code Generation: Use the plan to generate, review, and integrate code.
5. Feedback → Evolution: Feed production metrics, user behavior, and edge-case discoveries back into the spec to keep the system's "truth" updated.

Spec Template (spec-template.md)

Use this structure to define features:

# Feature: [Name]
## Context
- Business Value: [Why are we building this?]
- Priority: [P1/P2/P3]

## User Stories
- Given [Persona/Condition], When [Action], Then [Expected Result].

## Acceptance Scenarios
1. Scenario: [Title]
   - Given: [Prerequisites]
   - When: [Trigger]
   - Then: [Outcome]

## Constraints & Edge Cases
- [Constraints]
- [Edge Cases]

Implementation Plan Structure (plan-template.md)

# Implementation Plan: [Feature Name]
## Objective
- [Core goal mapped to Spec requirement #]

## Technical Strategy
- [Module/Component Changes]
- [Data Flow/Schema Updates]
- [Dependencies]

## Verification Plan
- [How do we confirm this matches the Spec?]
- [Testing strategy]

Quality Gates

SDD succeeds only when verified. Enforce these gates:

1. The Independence Gate: Every user story must be testable in isolation.
2. The Traceability Gate: No code change may exist without a referenced spec requirement.
3. The Consistency Gate: Continuous validation—every time a plan is generated, check if it contradicts existing spec constraints.

Best Practices

• Spec-First, Code-Last: Never touch an editor until the spec is approved.
• Research-First: Use research agents to gather context, API documentation, and existing codebase constraints before drafting specs.
• Bidirectional Feedback: Treat specs as living documents. If production deviates from the spec, update the spec first.
• Branching for Exploration: If uncertain about technical direction, create multiple implementation plans from the same spec to evaluate trade-offs.

References & Resources

• spec-kit framework
• templates/
• See skills/spec-driven-development/ for local examples.

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SDD: If it isn't specified, it doesn't exist.