---
name: design-an-interface
description: Generate multiple radically different interface designs for a module using parallel sub-agents. Use when user wants to design an API, explore interface options, compare module shapes, or mentions "design it twice".
---

# Design an Interface

Based on "Design It Twice" from "A Philosophy of Software Design": your first idea is unlikely to be the best. Generate multiple radically different designs, then compare.

## Workflow

### 1. Gather Requirements

Before designing, understand:

- [ ] What problem does this module solve?
- [ ] Who are the callers? (other modules, external users, tests)
- [ ] What are the key operations?
- [ ] Any constraints? (performance, compatibility, existing patterns)
- [ ] What should be hidden inside vs exposed?

Infer these from the request and existing callers; ask only about missing requirements that materially affect the designs.

### 2. Generate Designs (Parallel Sub-Agents)

Compare meaningfully different approaches, honoring the requested count. Two is a useful default for "design it twice"; add options only when they explore a relevant trade-off. Use independent subagents through the available delegation tool when useful and within available slots; otherwise develop the alternatives directly.

```
Prompt template for each sub-agent:

Design an interface for: [module description]

Requirements: [gathered requirements]

Constraints for this design: [assign a different constraint to each agent]
- Option A: "Optimize for the primary caller's workflow"
- Option B: "Expose explicit state transitions for the same requirements"

Choose constraints relevant to the actual problem; do not invent future use cases or a target method count.

Output format:
1. Interface signature (types/methods)
2. Usage example (how caller uses it)
3. What this design hides internally
4. Trade-offs of this approach
```

### 3. Present Designs

Show each design with:

1. **Interface signature** - types, methods, params
2. **Usage examples** - how callers actually use it in practice
3. **What it hides** - complexity kept internal

Present designs sequentially so user can absorb each approach before comparison.

### 4. Compare Designs

After showing all designs, compare them on:

- **Interface simplicity**: fewer methods, simpler params
- **General-purpose vs specialized**: flexibility vs focus
- **Implementation efficiency**: does shape allow efficient internals?
- **Depth**: small interface hiding significant complexity (good) vs large interface with thin implementation (bad)
- **Ease of correct use** vs **ease of misuse**

Use the requested format or the clearest concise comparison. Highlight where designs diverge most.

### 5. Synthesize

Recommend the best fit with reasons. If the user wants to choose interactively, ask:

- "Which design best fits your primary use case?"
- "Any elements from other designs worth incorporating?"

## Evaluation Criteria

From "A Philosophy of Software Design":

**Interface simplicity**: Fewer methods, simpler params = easier to learn and use correctly.

**Scope fit**: Covers known callers and requirements without speculative flexibility.

**Implementation efficiency**: Does interface shape allow efficient implementation? Or force awkward internals?

**Depth**: Small interface hiding significant complexity = deep module (good). Large interface with thin implementation = shallow module (avoid).

## Anti-Patterns

- Don't let sub-agents produce similar designs - enforce radical difference
- Don't skip comparison - the value is in contrast
- Don't implement - this is purely about interface shape
- Include implementation effort and migration risk in the trade-offs without letting them obscure correctness or usability.