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 OKHP3/skillz --skill api-designgit clone --depth 1 https://github.com/OKHP3/skillzWrote 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/okhp3/skillz/api-design)<a href="https://agentmods.dev/skills/okhp3/skillz/api-design"><img src="https://agentmods.dev/badge/skills/okhp3/skillz/api-design/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/okhp3/skillz/api-design"><img src="https://agentmods.dev/badge/skills/okhp3/skillz/api-design.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 | $0.00096 | $0.01037 |
| Opus 5 | $0.00048 | $0.00518 |
| Sonnet 5 | $0.00019 | $0.00207 |
| Haiku 4.5 | $0.00010 | $0.00104 |
Grade A, and why
api-design 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 6d 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 — 98 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API design
An API is a promise. Implementation is cheap to change; a published interface is not, because every caller has to move with you. Design it as though you cannot change it, because roughly speaking you cannot.
Design from the caller's side. Write the code that calls your API before you write the API. If that code is awkward, the design is wrong, and you have found out in two minutes instead of after three teams have integrated.
1. Model the domain, not the storage
The most common failure is exposing your database schema through HTTP. It couples every consumer to your internal structure, so a routine table split becomes everyone's migration.
Name resources and operations in the language your users use, not your tables. If a caller must understand your persistence model to use the API, the abstraction has already failed.
Done when: you could change the storage layer without touching the interface.
2. Make the wrong call impossible
Design so misuse does not compile, or fails immediately and clearly:
- Required things are required. Not optional-with-a-runtime-check.
- Enums over free strings for anything with a fixed set of values.
- Distinct types over booleans.
create(user, true, false)is unreadable at the call site and easy to get backwards. - No parameter whose valid values the caller has to guess.
- Reject unknown fields rather than silently ignoring them — a typo'd field name that is quietly dropped is a bug that surfaces days later.
Done when: a caller who ignores the docs still cannot fail silently.
3. Design the errors as carefully as the successes
Errors are half the interface and usually get a tenth of the thought.
- A stable machine-readable code, separate from the human message. Callers branch on the code; the message is free to change.
- Say what to do, not only what went wrong. "Order not found — it may belong to another account" beats "404".
- Distinguish retryable from terminal. A caller cannot back off correctly without this.
- Never leak internals: stack traces, SQL, internal hostnames.
- Be consistent. One error shape across the whole surface.
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.
- 6d ago First seen · 98 lines · 96 tokens per session scan A d04116d3be9f
api-design is a skill published in the GitHub repository OKHP3/skillz (3 stars, last pushed today), licensed MIT. It adds 96 tokens to every session and 1,037 once invoked, about $0.0005 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 2026-09-03.
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