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 kumaran-is/claude-code-onboarding --skill api-design-principlesgit clone --depth 1 https://github.com/kumaran-is/claude-code-onboardingWrote 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/kumaran-is/claude-code-onboarding/api-design-principles)<a href="https://agentmods.dev/skills/kumaran-is/claude-code-onboarding/api-design-principles"><img src="https://agentmods.dev/badge/skills/kumaran-is/claude-code-onboarding/api-design-principles/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/kumaran-is/claude-code-onboarding/api-design-principles"><img src="https://agentmods.dev/badge/skills/kumaran-is/claude-code-onboarding/api-design-principles.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 | $0.00074 | $0.00960 |
| Opus 5 | $0.00037 | $0.00480 |
| Sonnet 5 | $0.00015 | $0.00192 |
| Haiku 4.5 | $0.00007 | $0.00096 |
Grade A, and why
api-design-principles 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 — 86 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Iron Law
NO API ENDPOINT DESIGN WITHOUT READING reference/rest-design-principles.md FIRST — HTTP semantics, status codes, idempotency, and caching strategies must be agreed before writing implementation code
API Design Principles
Stack-agnostic REST design principles for APIs built with Python FastAPI, NestJS 11.x, or Spring Boot WebFlux 3.5.x. Use this skill during the design phase — before writing controllers or route handlers.
When to Use
- Designing new REST endpoints from scratch
- Reviewing whether existing endpoints follow REST conventions
- Choosing a pagination strategy (offset vs cursor)
- Adding idempotency keys to mutation endpoints
- Designing caching strategy (ETags, Cache-Control)
- Designing bulk/batch endpoints with partial failure handling
- Running the pre-implementation API design checklist
How This Skill Relates to Others
| Skill | Scope |
|---|---|
| api-design-principles (this skill) | Design phase — REST semantics, patterns, checklist |
| openapi-spec-generation | Documentation phase — OpenAPI 3.1 spec, developer guide |
| java-spring-api | Implementation — Spring WebFlux controllers, services |
| nestjs-api | Implementation — NestJS modules, controllers, DTOs |
| python-dev | Implementation — FastAPI routes, Pydantic models |
Process
Step 1: Run the Pre-Implementation Checklist
Read assets/api-design-checklist.md before designing any endpoint. Focus on:
- Resource naming and URL structure
- HTTP method assignment
- Status codes per operation
- Pagination strategy choice
- Versioning strategy
Step 2: Apply REST Design Principles
Read reference/rest-design-principles.md for detailed patterns covering:
- URL structure and resource naming (plural nouns, shallow nesting)
- HTTP methods and correct status codes per operation type
- Pagination — offset-based vs cursor-based, with examples for all 3 stacks
- Versioning strategies (URL path recommended)
- Rate limiting headers (X-RateLimit-*)
- Authentication (Bearer token, 401 vs 403 distinction)
- Error response format (consistent structure across all 3 stacks)
- Caching — Cache-Control, ETags, conditional GET (304) — all 3 stacks
- Idempotency keys — mutation safety for payment and order endpoints — all 3 stacks
- Bulk operations — batch endpoints with 207 Multi-Status partial failure — all 3 stacks
- CORS configuration — all 3 stacks
- Health and monitoring endpoints
What ships with it
2 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.
- 6d ago First seen · 86 lines · 74 tokens per session scan A 92a0afe77396
api-design-principles is a skill published in the GitHub repository kumaran-is/claude-code-onboarding (35 stars, last pushed 2mo ago), licensed MIT. It adds 74 tokens to every session and 960 once invoked, about $0.0004 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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