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1---
2description: "Generate multiple radically different interface designs for a module using parallel sub-agents, then compare trade-offs. Based on \"Design It Twice\" from A Philosophy of Software Design. Use when user wants to design an API, explore interface options, compare module shapes, or mentions \"design it twice\"."
3alwaysApply: false
4---
5 
6# Design Interface
7 
8Based 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.
9 
10> **HARD GATE** — Multiple design options must be explored. Do NOT settle on first idea. Compare trade-offs (UX, complexity, extensibility, performance) before committing.
11 
12## Workflow
13 
14### 1. Gather Requirements
15 
16Before designing, understand:
17 
18- [ ] What problem does this module solve?
19- [ ] Who are the callers? (other modules, external users, tests)
20- [ ] What are the key operations?
21- [ ] Any constraints? (performance, compatibility, existing patterns)
22- [ ] What should be hidden inside vs exposed?
23 
24Ask: "What does this module need to do? Who will use it?"
25 
26### 2. Generate Designs (Parallel Sub-Agents)
27 
28Spawn 3+ sub-agents simultaneously using Task tool. Each must produce a **radically different** approach.
29 
30```
31Prompt template for each sub-agent:
32 
33Design an interface for: [module description]
34 
35Requirements: [gathered requirements]
36 
37Constraints for this design: [assign a different constraint to each agent]
38- Agent 1: "Minimize method count - aim for 1-3 methods max"
39- Agent 2: "Maximize flexibility - support many use cases"
40- Agent 3: "Optimize for the most common case"
41- Agent 4: "Take inspiration from [specific paradigm/library]"
42 
43Output format:
441. Interface signature (types/methods)
452. Usage example (how caller uses it)
463. What this design hides internally
474. Trade-offs of this approach
48```
49 
50### 3. Present Designs
51 
52Show each design with:
53 
541. **Interface signature** — types, methods, params
552. **Usage examples** — how callers actually use it in practice
563. **What it hides** — complexity kept internal
57 
58Present designs sequentially so user can absorb each approach before comparison.
59 
60### 4. Compare Designs
61 
62After showing all designs, compare them on:
63 
64- **Interface simplicity**: fewer methods, simpler params
65- **General-purpose vs specialized**: flexibility vs focus
66- **Implementation efficiency**: does shape allow efficient internals?
67- **Depth**: small interface hiding significant complexity (good) vs large interface with thin implementation (bad)
68- **Ease of correct use** vs **ease of misuse**
69 
70Discuss trade-offs in prose, not tables. Highlight where designs diverge most.
71 
72### 5. Synthesize
73 
74Often the best design combines insights from multiple options. Ask:
75 
76- "Which design best fits your primary use case?"
77- "Any elements from other designs worth incorporating?"
78 
79## Evaluation Criteria
80 
81From "A Philosophy of Software Design":
82 
83**Interface simplicity**: Fewer methods, simpler params = easier to learn and use correctly.
84 
85**General-purpose**: Can handle future use cases without changes. But beware over-generalization.
86 
87**Implementation efficiency**: Does interface shape allow efficient implementation? Or force awkward internals?
88 
89**Depth**: Small interface hiding significant complexity = deep module (good). Large interface with thin implementation = shallow module (avoid).
90 
91## Anti-Patterns
92 
93- Don't let sub-agents produce similar designs — enforce radical difference
94- Don't skip comparison — the value is in contrast
95- Don't implement — this is purely about interface shape
96- Don't evaluate based on implementation effort
97