Design

Make technical decisions when the "how" is the problem.

When to Use

• Algorithms: Non-trivial logic that needs to be thought through
• Data structures: How things relate, what to store, how to index
• System boundaries: Where does this live? What owns what?
• Performance-sensitive code: Choices that affect speed/memory
• Security-sensitive code: Choices that affect attack surface

When to Skip

Design is overhead when the implementation is obvious:

• UI changes where spec describes the outcome
• CRUD operations
• Wiring existing pieces together
• Configuration changes

The 100 Forks Test

If you ran /prep-plan 100 times with current context:

• Forks diverge: AI invents different solutions. You need more context. Design provides it.
• Forks converge: AI finds the obvious solution. Spec is enough. Skip design.

Process

1. Search for related prior work: Use the Task tool to invoke the lore-researcher agent with the technical problem description. Do not run in background. Wait for the result before continuing. Include findings in the Context section.
2. Review any relevant .lore/research/, .lore/brainstorm/, or .lore/specs/ context
3. Clarify the technical problem - what exactly needs deciding?
4. Explore approaches - at least 2-3 options with trade-offs
5. Decide: Pick an approach and document why
6. Define the interface/contract - how will other code interact?
7. Document edge cases
8. Confirm with user before saving
9. Save to .lore/design/
10. Offer fresh-eyes review (see below)

Output

Save to .lore/design/[topic].md

Use kebab-case for filenames. Match spec naming where a spec exists (e.g., if spec is history-sync.md, design is history-sync.md or history-sync-dedup-algorithm.md).

Document Structure

Before writing: Load ${CLAUDE_PLUGIN_ROOT}/shared/frontmatter-schema.md to get frontmatter field definitions and status values for design documents.

---
[frontmatter per schema]
---

# Design: [Topic]

## Problem
What technical problem are we solving? Link to spec if one exists.

## Constraints
- Technical constraints
- Performance requirements
- Integration points
- Security considerations

## Approaches Considered

### Option 1: [Name]
Description of the approach.

**Pros:**
- Pro 1
- Pro 2

**Cons:**
- Con 1
- Con 2

### Option 2: [Name]
Description of the approach.

**Pros:**
- Pro 1
- Pro 2

**Cons:**
- Con 1
- Con 2

## Decision
Which approach and why. **This section is required.** A design without a decision is just research.

## Interface/Contract
How other code will interact with this:
- Function signatures
- Data structures
- Protocols
- APIs

## Edge Cases
Known edge cases and how they're handled:
- Edge case 1: Handled by...
- Edge case 2: Handled by...

## Open Questions
(Optional) Things still TBD that don't block implementation.

What vs How

Design sits between spec and plan:

Document	Answers	Example
Spec	What are we building?	"Deduplicate history entries"
Design	How does it work?	"Use content hashing with LRU eviction"
Plan	How do we build it?	"Add HashIndex class in src/index.ts"

Design is "how it works" in the abstract. Algorithms, data structures, protocols. Implementation-agnostic where possible.

Plan is "how to build it" in the concrete. Files, functions, dependencies. Implementation-specific.

Research vs Design

Both explore options. The difference is commitment:

Document	Output	Decision Required?
Research	"Here are the options"	No
Design	"Here's what we're doing and why"	Yes

If you're documenting options without picking one, that's research. Design requires the Decision section.

After Saving: Fresh-Eyes Review

After the design is saved, run a fresh-eyes review. Designs written in conversation accumulate assumptions. A reviewer with fresh context reads only what's on the page, catching what the author can't see.

Invoke the design-reviewer agent on the saved design using the Task tool. The agent evaluates designs through four lenses: decision quality, trade-off clarity, interface implementability, and edge case coverage. Present the findings and offer to address critical issues before moving on.

Specialized Agents

If .lore/lore-agents.md exists, consult it for specialized agents that can help with domain-specific concerns. Security, performance, or architecture experts can identify trade-offs you might miss. Invoke relevant agents via Task tool and incorporate their insights.

Linking to Specs

Design documents should reference their parent spec when one exists:

• In frontmatter: related: [.lore/specs/history-sync.md]
• In Problem section: "See Spec: history-sync for requirements."

Design documents can also stand alone for technical problems that don't have user-facing requirements.