Overview
OpenSpec is a lightweight, brownfield-first CLI tool focused on change management. It separates current truth (specs/) from proposed changes (changes/).
specs.md is a full lifecycle methodology covering Inception through Operations, with support for both greenfield and brownfield development.
Side-by-Side Comparison
When to Choose Each
Choose specs.md (AI-DLC) if:
- You need full project lifecycle support
- You’re building new features (greenfield)
- You want DDD integration
- You need Mob rituals for team alignment
- You want formal methodology structure
- You want a VS Code extension to track progress visually
Choose OpenSpec if:
- You primarily make incremental changes
- You’re working with existing codebases
- You want maximum token efficiency
- You prefer minimal overhead
- You want the simplest possible setup
- You make high-volume, small-scope changes
Key Differences
1. Design Philosophy
OpenSpec: Brownfield-first. Designed for modifying existing systems with a clear separation between current state and proposed changes.2. Token Efficiency
OpenSpec: Excellent token efficiency. Only sends “spec deltas” (proposed changes) to the LLM, not the full codebase context. specs.md: Memory Bank provides structured context. You can achieve similar change-focused efficiency by creating Units specifically for changes—keeping context scoped to what matters.3. Brownfield Handling
OpenSpec: Primary design focus. Every change is explicit with clear before/after separation. specs.md: Supported via “model elevation” - AI first converts existing code to semantic models (domain components, relationships) before making changes. More advanced brownfield process coming soon.4. Methodology vs Tool
OpenSpec: A tool for managing specs and changes. No prescribed methodology. specs.md: A complete methodology with:- Defined phases (Inception → Construction → Operations)
- Rituals (Mob Elaboration, Mob Construction)
- Design integration (DDD as core)
- Role definitions (AI drives, human validates)
Use Case Comparison
Summary
Bottom Line: OpenSpec excels at brownfield changes with minimal overhead. specs.md provides full lifecycle support for complex systems.For brownfield work within a larger project, consider using both together.