A guide to defining stable interfaces between pieces of software, such as REST or GraphQL APIs, including their inputs, outputs, versions, and errors.

Use it to design endpoints, module boundaries, component properties, type contracts, request and response shapes, versioning, and API error models.

It makes interfaces easier to use correctly and reduces accidental breakage when code, teams, or public APIs change.

api-design — Skill for Claude Code and Codex by int2t05

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 int2t05/engineering-skills --skill api-design

Clone the repo

git clone --depth 1 https://github.com/int2t05/engineering-skills

Made for: Claude Code, Codex.

Or install engineering-skills, the plugin that ships this one along with the rest of its 47 skills, 1 hook.

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/int2t05/engineering-skills/api-design/github.svg)](https://agentmods.dev/skills/int2t05/engineering-skills/api-design)

Your own site

<a href="https://agentmods.dev/skills/int2t05/engineering-skills/api-design"><img src="https://agentmods.dev/badge/skills/int2t05/engineering-skills/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/int2t05/engineering-skills/api-design"><img src="https://agentmods.dev/badge/skills/int2t05/engineering-skills/api-design.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 24 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 344 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	$0.00024	$0.00344
Opus 5.5	$0.00010	$0.00138
Sonnet 5.5	$0.00005	$0.00069
Haiku 4.5	$0.00002	$0.00034

Measured 22d ago against content hash 40a43f95823f, 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 22d 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/03-design/api-design/SKILL.md · 36 lines

What it actually says

API and Interface Design

When to use

Define a consumer-facing API contract with consistent behavior.

Not for: api-testing for contract verification.

Steps

Identify consumers, protocol, operations and trust boundaries; follow project contract tooling.
Specify requests, responses, authentication, authorization, validation and errors; distinguish malformed input from semantic invalidity.
Define pagination, concurrency, idempotency and evolution only where needed; include normal and failure examples.
Update the authoritative schema or approved contract document; validate schemas and consumer examples before human review.

Output: Suggested paths, subject to human audit: docs/API/*.md or the authoritative machine-readable contract.

Verify

Examples match the schema; authorization, errors and repeat-request behavior are testable.

References

engineering-principles.md — shared decision boundaries, scope and verification.
human-audit.md — document consent, readability and human review.
references/versioning-strategy.md — when breaking change is unavoidable: URI/header versioning, sunset/deprecation headers (RFC 8594), dual-version coexistence, sunset timeline
references/openapi-lifecycle.md — design-first vs code-first, Spectral/Redocly linting, openapi-generator SDK generation, spec versioning, CI pipeline

Files

What ships with it

3 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.

22d ago Changed · -107 lines · -39 tokens per session 40a43f95823f
29d ago Changed · +6 lines d751e819519b
2mo ago First seen · 137 lines · 63 tokens per session scan A 94582472ad64

Subscribe to this mod's changes

api-design is a skill published in the GitHub repository int2t05/engineering-skills (3 stars, last pushed 23d ago), licensed MIT. It adds 24 tokens to every session and 344 once invoked, about $0.0001 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 Claude Code Code review TDD agent-skills claude-code-plugin sdlc

api-design next to 6 skills from other repositories
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api-designint2t05/engineering-skills	Skill	3	23d ago	MIT	A	24	Claude Code, Codex
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