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+---
+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?
+
+Ask: "What does this module need to do? Who will use it?"
+
+### 2. Generate Designs (Parallel Sub-Agents)
+
+Spawn 3+ sub-agents simultaneously using Task tool. Each must produce a **radically different** approach.
+
+```
+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]
+- Agent 1: "Minimize method count - aim for 1-3 methods max"
+- Agent 2: "Maximize flexibility - support many use cases"
+- Agent 3: "Optimize for the most common case"
+- Agent 4: "Take inspiration from [specific paradigm/library]"
+
+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**
+
+Discuss trade-offs in prose, not tables. Highlight where designs diverge most.
+
+### 5. Synthesize
+
+Often the best design combines insights from multiple options. 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.
+
+**General-purpose**: Can handle future use cases without changes. But beware over-generalization.
+
+**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
+- Don't evaluate based on implementation effort