Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add allemaar/open-skills --skill design-an-interfacegit clone --depth 1 https://github.com/allemaar/open-skillsWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/allemaar/open-skills/design-an-interface)<a href="https://agentmods.dev/skills/allemaar/open-skills/design-an-interface"><img src="https://agentmods.dev/badge/skills/allemaar/open-skills/design-an-interface/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/allemaar/open-skills/design-an-interface"><img src="https://agentmods.dev/badge/skills/allemaar/open-skills/design-an-interface.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00119 | $0.01515 |
| Opus 5 | $0.00060 | $0.00758 |
| Sonnet 5 | $0.00024 | $0.00303 |
| Haiku 4.5 | $0.00012 | $0.00152 |
Grade A, and why
design-an-interface scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 11d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 121 lines — stays where its author put it; the contents beside it link to each section on GitHub.
/design-an-interface
From "Design It Twice" (Ousterhout): your first idea is rarely the best. The value is in contrast — not in any single design.
Step 1 — Gather Requirements
Shallow intake produces shallow designs. Ask these specifically — not just "what does it do?":
- What is the one-sentence job of this module?
- Who are the callers? (other modules, end users, tests, CLI, external systems)
- Walk me through the happy path in code — what does the call site actually look like?
- What are the failure modes? What can go wrong, and who handles it?
- What must the interface hide? What should callers never need to know about?
- Any hard constraints? (performance, compatibility with existing patterns, language idioms)
- Is there a "most common case" that should be especially ergonomic?
Don't proceed until you have real answers. One vague sentence is not enough.
Step 2 — Generate Designs
If you have a Task tool: spawn all three designs simultaneously. Each agent gets the requirements plus its assigned lens and must produce a design that would surprise the others. Brief them to actively resist the obvious answer.
If you don't have Task: run them sequentially, but commit fully to each design before moving to the next. Don't let later designs learn from earlier ones — write each as if it were your only attempt.
Produce 3 radically different designs. Radically different means different information hiding boundaries — not just different method names or parameter orders. Three designs that all accept a config struct and return a result are not radically different, even if the method names differ.
Use these lenses to force divergence:
| Design | Constraint | Core question |
|---|---|---|
| Minimal | 1–3 operations max | What is the smallest surface that covers all cases? |
| Caller-empowering | Expose primitives; let caller compose | What if we trust the caller with more control? |
| Common-case-first | Optimize the happy path to one line | What if the 95% case has zero friction, even if edge cases are awkward? |
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 11d ago First seen · 121 lines · 119 tokens per session scan A 69167caf84b3
design-an-interface is a skill published in the GitHub repository allemaar/open-skills (14 stars, last pushed today), licensed Apache-2.0. It adds 119 tokens to every session and 1,515 once invoked, about $0.0006 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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