api-design

A guide to building APIs with REST, GraphQL, and gRPC. An API is an interface that lets different programs exchange data and request actions.

In plain words
What is it for?
Use it to design API endpoints and schemas, secure them, document them, test them, and plan operational work.
Why use it?
It helps address common API concerns such as authentication, documentation, testing, and running the service in production.

Skill for Claude CodeCodex

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.

agentmods
npx agentmods add skills/pluginagentmarketplace/custom-plugin-backend/api-design
Any agent
npx skills add pluginagentmarketplace/custom-plugin-backend --skill api-design
Clone the repo
git clone --depth 1 https://github.com/pluginagentmarketplace/custom-plugin-backend

Made for: Claude Code, Codex.

Per session 29 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 1,729 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 1 finding. Scan, not verified.
Origin unknown 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.

ModelPer sessionOnce invoked
Fable 5 $0.00029 $0.01729
Opus 5 $0.00015 $0.00864
Sonnet 5 $0.00006 $0.00346
Haiku 4.5 $0.00003 $0.00173

Measured yesterday against content hash 1bbaa4aa63f4, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

api-design scanned grade A with 1 finding 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 yesterday.

The scan reads SKILL.md. This mod also ships 2 executable files (scripts/generate_openapi.sh, scripts/helper.py), listed below but not scanned — reading those needs a real analyzer, not pattern matching.

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.

Makes network callslowCapability

Not a fault in itself. Listed so you know the mod talks to something, and to what.

2. Check authentication: `curl -H "Authorization: Bearer <token>"`
skills/api-design/SKILL.md · 288 lines

The source is not reproduced here

No licence file

A repository with no LICENSE is all rights reserved by default, so the body is not copied here. The metadata, the measurements and the link are.

Read it on GitHub

Files

What ships with it

9 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.

  1. yesterday First seen · 288 lines · 29 tokens per session scan A 1bbaa4aa63f4

Subscribe to this mod's changes

api-design is a skill published in the GitHub repository pluginagentmarketplace/custom-plugin-backend (4 stars, last pushed 7mo ago), with no licence file. It adds 29 tokens to every session and 1,729 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it A with 1 finding (makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-31.