design-it-twice

Design

Generate multiple radically different interface designs for a module using parallel sub-agents, then compare and synthesize. Based on Ousterhout's "Design It Twice" principle. Use when the user wants to explore interface options, design an API, compare module shapes, or says "design it twice", "what are my options", or "show me alternatives". Also use when the Architect agent is designing a new module boundary or public interface.

QUICK START

How to use this skill

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I want to install this Agent Skill for this project in Codex.

Source SKILL.md: https://github.com/bdfinst/agentic-dev-team/blob/HEAD/plugins/dev-team/skills/design-it-twice/SKILL.md

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Copying this prompt does not install or run the skill. Review third-party files before use. Codex skill guide

Design It Twice

Overview

Your first design idea is rarely your best. This skill generates multiple radically different interface designs for a module by dispatching parallel sub-agents with divergent constraints, then compares them so the user can make an informed choice. Based on "Design It Twice" from John Ousterhout's A Philosophy of Software Design.

This skill is about interface shape, not implementation. Don't write code — design the contract.

When to Use

Designing a new module, service, or API boundary
Refactoring an existing interface that feels wrong
Any time there's a non-obvious choice about how to expose functionality
When the Architect agent identifies a new module boundary during planning

Process

1. Gather Requirements

Before designing, understand the constraints:

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

Explore the codebase to find existing patterns and conventions. Ask the user only for what you can't determine from the code.

2. Generate Designs (Parallel Sub-Agents)

Spawn 3+ sub-agents simultaneously using the Agent tool. Each must produce a radically different approach — not variations on a theme.

Assign each agent a different design constraint:

Agent	Constraint	Optimizes for
1	"Minimize the interface — aim for 1-3 methods max"	Simplicity, deep module
2	"Maximize flexibility — support many use cases and extension"	Generality, future-proofing
3	"Optimize for the most common caller — make the default case trivial"	Ergonomics, productivity
4	(optional) "Design around ports & adapters for cross-boundary deps"	Testability, isolation

Each sub-agent produces:

Interface signature — types, methods, parameters
Usage example — how a real caller would use it
What it hides — complexity kept internal
Trade-offs — what you gain and what you give up

3. Present Designs

Show each design sequentially so the user can absorb one before seeing the next. Don't use comparison tables for the designs themselves — prose is better for understanding trade-offs.

4. Compare

After presenting all designs, compare them on these dimensions:

Interface simplicity: fewer methods and simpler params = easier to use correctly
Depth: small interface hiding significant complexity (good) vs large interface with thin implementation (bad)
Ease of correct use vs ease of misuse
Implementation efficiency: does the interface shape allow efficient internals?
Testability: can callers test against this interface without mocking internals?

Give your own recommendation — which design is strongest and why. If elements from different designs would combine well, propose a hybrid. Be opinionated.

5. Synthesize

Ask the user:

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

The final design often combines insights from multiple options.

Anti-Patterns

Don't let sub-agents produce similar designs — enforce radical difference via constraints
Don't skip the comparison step — the value is in the contrast
Don't implement — this is purely about interface shape
Don't evaluate based on implementation effort — that's a separate concern