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?" The user may also provide links to Figma designs, Linear issues, or Notion documents.

2. Gather external context

If the user provided references to external tools, use the available MCP tools to pull in context:

• Figma: The user may provide a Figma URL. Fetch design context and screenshots to understand the visual and interaction design that the interface must support.
• Linear: The user may provide a ticket code (e.g., EO-1234) or a URL. Fetch issue details for requirements, constraints, and prior discussion.
• Notion: The user may provide a page title or a URL. Search Notion by title if no URL is given. Fetch documents for specs, API contracts, or architectural notes.

Use this context to refine the requirements checklist above. If no external references are provided, skip this step.

3. 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

4. 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.

5. 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.

6. 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