Plan Mode vs Spec-Driven Development

Overview

Context engineering exists on a spectrum from tactical to strategic. Plan mode and spec-driven development represent different points on this spectrum, each suited for different scenarios and team needs.

Plan mode is a tactical approach where AI agents create implementation plans based on conversational prompts. You describe what you want, the AI proposes a plan, you refine it through dialogue, and implementation proceeds. Context engineering happens through the conversation itself.

Spec-driven development is a strategic approach where you create structured, persistent specifications that serve as the source of truth. Context engineering happens through deliberate specification writing, and these specs guide both AI and human developers over the long term.

Plan Mode: Tactical Context Engineering

Plan mode is built into many AI coding assistants (Claude Code, Cursor, etc.). The workflow:

1. Describe the task: Provide a natural language description of what you want to build
2. AI creates a plan: The AI agent breaks down the task into steps
3. Review and refine: You review the plan and provide clarifications
4. Execute: AI implements the plan, potentially creating files and code
5. Iterate as needed: Adjust based on results

Strengths of Plan Mode

• Low ceremony: Start coding with minimal upfront overhead
• Fast iteration: Quickly explore different approaches
• Good for small features: Effective for isolated, well-scoped tasks
• Learning-friendly: Natural conversation makes it accessible
• Exploration: Great for prototyping when requirements are unclear

Limitations of Plan Mode

• Ephemeral context: Plans exist only in conversation history, not as persistent artifacts
• Difficult to review: Hard for others to understand what was intended without reading full conversation
• No source of truth: When implementation drifts, there's no authoritative reference
• Doesn't scale: Becomes unwieldy for complex, multi-component features
• Limited validation: Hard to verify if implementation matches original intent after time passes

Spec-Driven Development: Strategic Context Engineering

SDD creates persistent, structured specifications as the foundation for development. The workflow:

1. Write specifications: Create structured docs defining requirements and scenarios
2. Review specifications: Validate specs capture your intent (cheapest place to fix issues)
3. Implement: Use specs to guide AI (and human) implementation
4. Validate: Verify implementation matches specification requirements
5. Evolve specs: Keep specifications synchronized as system evolves

Strengths of SDD

• Persistent source of truth: Specs serve as long-term reference for what system should do
• Reviewable: Specs can be reviewed by team members, stakeholders, even AI agents
• Validation: Can verify implementation matches specs at any time
• Scales well: Handles complex, multi-component systems effectively
• Prevents drift: Explicit specs make deviation obvious
• Team alignment: Shared understanding documented in specs

Limitations of SDD

• Higher upfront cost: Writing specs takes time before coding begins
• Learning curve: Requires learning spec frameworks and tools
• Overhead for simple tasks: Can feel excessive for tiny features
• Requires discipline: Team must commit to keeping specs synchronized

Choosing the Right Approach

Use Plan Mode For:

• Quick experiments and throwaway prototypes
• Learning and exploration when requirements are genuinely unclear
• Small, isolated features with minimal complexity
• Personal projects where long-term maintenance isn't critical
• One-off scripts or tools

Use Spec-Driven Development For:

• Production systems requiring long-term maintenance
• Complex features with multiple interactions and edge cases
• Team environments where shared understanding is essential
• Projects where validation and compliance matter
• Systems where context needs to persist beyond conversational memory
• Brownfield projects where understanding existing behavior is critical

Hybrid Approach

Many teams use both: plan mode for initial exploration, then transition to SDD for production implementation. This combines the speed of tactical context engineering with the rigor of strategic approaches. The key is matching the level of formality to the stakes and complexity of what you're building.

Learn More

• Which Spec-Driven Development Tool Should You Choose?

  Comprehensive comparison including tactical vs strategic approaches.