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 agentmods add skills/svngoku/coding-agents-skills/api-designnpx skills add svngoku/coding-agents-skills --skill api-designgit clone --depth 1 https://github.com/svngoku/coding-agents-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/svngoku/coding-agents-skills/api-design)<a href="https://agentmods.dev/skills/svngoku/coding-agents-skills/api-design"><img src="https://agentmods.dev/badge/skills/svngoku/coding-agents-skills/api-design.svg" alt="Measured on agentmods" 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 | $0.00144 | $0.04767 |
| Opus 5 | $0.00072 | $0.02384 |
| Sonnet 5 | $0.00029 | $0.00953 |
| Haiku 4.5 | $0.00014 | $0.00477 |
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 4d 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 — 301 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API Design
HTTP APIs are the contract between your service and every client that consumes it. A well-designed API is consistent and predictable: the URL says which resource you are touching, the HTTP method says what happens to it, and the status code plus body say whether it worked and why not. This skill covers designing and reviewing such APIs — resource modeling, HTTP semantics, status codes, error envelopes, pagination, versioning, spec-first OpenAPI 3.x, and authentication basics — REST first, with GraphQL and gRPC covered in passing.
Quick Reference
| Task | Reference |
|---|---|
| Error envelopes, RFC 7807, idempotency keys, retry policy | error-handling.md |
| Cursor vs offset pagination, filtering, sorting, field selection | pagination-filtering.md |
| Versioning strategies, backward compatibility, deprecation | versioning-evolution.md |
Core Workflow
Design an endpoint set in five steps:
- Model resources and map HTTP semantics — identify the nouns (users, orders, products) and their relationships; name them as plural collections; one method per operation: read → GET, create → POST, full replace → PUT, partial update → PATCH, delete → DELETE, domain action → POST to an action sub-resource.
- Design the responses — success shapes, the error envelope (problem+json), and the exact status code per failure mode.
- Paginate and filter collections — choose cursor or offset, set defaults, define filter/sort/field-selection syntax.
- Write the OpenAPI spec first — the spec is the contract; generate docs and clients from it.
- Lock down auth and limits — authentication scheme, least-privilege scopes, rate limits with standard headers.
Resource Modeling
Nouns, not verbs
Resources are nouns; HTTP methods are the verbs. URLs must never contain verbs.
| Anti-pattern | Good |
|---|---|
GET /getUser?id=1 |
GET /users/{id} |
POST /createOrder |
POST /orders |
POST /deleteOrder?id=1 |
DELETE /orders/{id} |
What ships with it
8 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
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.
- 4d ago First seen · 301 lines · 144 tokens per session scan A 59290f522958
api-design is a skill published in the GitHub repository svngoku/coding-agents-skills (9 stars, last pushed 22d ago), licensed MIT. It adds 144 tokens to every session and 4,767 once invoked, about $0.0007 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 2026-08-31.
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