---
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".

tags: [mattpocock, design-an-interface, api, experimental-design]
---

# 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


---
Source attribution: mkurman
Original project: https://github.com/mkurman/zorai/blob/d06ee7597049567dab92db22d41b2764cf89094a/skills/mattpocock/design-an-interface/SKILL.md
License: MIT
License information: https://opensource.org/license/mit
Original upstream instructions; downloaded from Mega AI Hub.

## Upstream license notice

MIT License

Copyright (c) 2026 Matt Pocock

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.



# Mega AI Hub resource metadata

Creator / publisher: mkurman
Original project: https://github.com/mkurman/zorai/blob/d06ee7597049567dab92db22d41b2764cf89094a/skills/mattpocock/design-an-interface/SKILL.md
Attribution basis: Source repository record

Fields of work: Design, Development & Engineering, Product Management
Roles: Backend Developer, Design System Lead, Frontend Developer, Product Manager, UX Designer
Specialties: API, Design
Tools: Task Tool
Country applicability: Country not specified
Classification basis: Source catalog labels
Instruction SHA-256: 65366122a2125cb42377cecc254965112a26b7dad334952b1799b50ae0a29f6a