A guide for designing HTTP APIs—the interfaces that let software systems communicate over the web. It covers common REST patterns, API descriptions, access control, and safe request handling.

Use it to design or review public or internal APIs, including pagination, error responses, versioning, rate limits, authentication, and repeat-safe write requests.

It helps prevent inconsistent endpoints, unclear errors, unsafe access, and changes that break existing clients.

api-design — Skill for Codex by san-npm/skills-ws

Install

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.

Any agent

npx skills add san-npm/skills-ws --skill api-design

Clone the repo

git clone --depth 1 https://github.com/san-npm/skills-ws

Made for: Codex.

Wrote 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.

README.md

[![agentmods](https://agentmods.dev/badge/skills/san-npm/skills-ws/api-design/github.svg)](https://agentmods.dev/skills/san-npm/skills-ws/api-design)

Your own site

<a href="https://agentmods.dev/skills/san-npm/skills-ws/api-design"><img src="https://agentmods.dev/badge/skills/san-npm/skills-ws/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.

Your own site · 80×15

<a href="https://agentmods.dev/skills/san-npm/skills-ws/api-design"><img src="https://agentmods.dev/badge/skills/san-npm/skills-ws/api-design.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 60 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 389 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

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	$0.00060	$0.00389
Opus 5.5	$0.00024	$0.00156
Sonnet 5.5	$0.00012	$0.00078
Haiku 4.5	$0.00006	$0.00039

Measured 2mo ago against content hash fb1e0235f9c6, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

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 2mo 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.

skills/api-design/SKILL.md · 22 lines

What it actually says

API Design

Reference guide

Read only the references needed for the current request:

REST Conventions That Actually Matter: references/rest-conventions-that-actually-matter.md
Pagination: Cursor vs Offset: references/pagination-cursor-vs-offset.md
Error Handling: RFC 9457 Problem Details: references/error-handling-rfc-9457-problem-details.md
API Versioning: references/api-versioning.md
Rate Limiting: references/rate-limiting.md
Authentication Patterns: references/authentication-patterns.md
Idempotency: references/idempotency.md
OpenAPI 3.1 Specification: references/openapi-3-1-specification.md
GraphQL vs REST: Decision Matrix: references/graphql-vs-rest-decision-matrix.md
Response Envelope: references/response-envelope.md
Checklist: Production-Ready API: references/checklist-production-ready-api.md

Files

What ships with it

12 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.

Changes

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.

2mo ago First seen · 22 lines · 60 tokens per session scan A fb1e0235f9c6

Subscribe to this mod's changes

api-design is a skill published in the GitHub repository san-npm/skills-ws (4 stars, last pushed 1mo ago), licensed MIT. It adds 60 tokens to every session and 389 once invoked, about $0.0002 per session on Opus 5.5. 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.

Backend and APIs

api-design next to 6 skills from other repositories
Mod	Type	Stars	Last push	Licence	Scan	Tokens	Works with
api-designsan-npm/skills-ws	Skill	4	1mo ago	MIT	A	60	Codex
mem0-oss-to-platformmem0ai/mem0	Skill	68k	2d ago	Apache-2.0	A	217	Claude Code, Codex
azure-mgmt-botservice-dotnetmicrosoft/skills	Skill	3.2k	yesterday	MIT	A	78	Claude Code
fastapi-router-pymicrosoft/skills	Skill	3.2k	yesterday	MIT	A	46	Claude Code
dev-runtimelangwatch/langwatch	Skill	5.0k	yesterday	Apache-2.0	A	172	Claude Code
api-designericrisco/rsc-harness	Skill	503	2d ago	MIT	A	105	Claude Code, Codex
migrate-segw-to-raparc-mcp/arc-1	Skill	252	yesterday	MIT	A	106	Claude Code