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 fattain-naime/engineering-docs --skill api-design-documentgit clone --depth 1 https://github.com/fattain-naime/engineering-docsWrote 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/fattain-naime/engineering-docs/api-design-document)<a href="https://agentmods.dev/skills/fattain-naime/engineering-docs/api-design-document"><img src="https://agentmods.dev/badge/skills/fattain-naime/engineering-docs/api-design-document/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/fattain-naime/engineering-docs/api-design-document"><img src="https://agentmods.dev/badge/skills/fattain-naime/engineering-docs/api-design-document.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- NVIDIA SkillSpector pass
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 | $0.00062 | $0.03464 |
| Opus 5.5 | $0.00025 | $0.01386 |
| Sonnet 5.5 | $0.00012 | $0.00693 |
| Haiku 4.5 | $0.00006 | $0.00346 |
Grade A, and why
api-design-document 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 1mo 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 — 261 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Purpose
Produce an API design document that defines the complete interface contract for a new or revised API. This document is the authoritative specification that drives implementation, consumer integration, testing, and documentation simultaneously.
The cost of a bad API design is permanent. Once consumers depend on it, changes are breaking changes. Getting the design right before the first line of implementation code is written is the highest-ROI investment in an API's lifetime.
Input
Works best with: The name of the API or service being designed. Also valuable: The domain entities and operations, consumer use cases, authentication context, existing API conventions in the codebase, performance requirements.
Example invocation: Design the API for a payment links system. Merchants create payment links with a custom amount, expiry, and description. Customers use the link to pay. Merchants can list, deactivate, and delete their links. Needs HMAC webhook notification on payment. Auth via Bearer token.
Key Concepts
Richardson Maturity Model
Target Level 2 or Level 3 (HATEOAS for discovery-driven APIs):
- Level 0: Single endpoint, single verb - not REST
- Level 1: Resources with unique URIs
- Level 2: HTTP verbs and status codes used correctly - this is the minimum standard
- Level 3: Hypermedia controls (HATEOAS) - optional, for discovery-driven APIs
Resource Modeling
- Resources are nouns, not verbs:
/paymentsnot/createPayment - Resources map to domain entities:
/merchants/{id}/payment-links - Use plural nouns for collections:
/users, not/user - Nested resources imply ownership:
GET /merchants/{id}/transactions
HTTP Semantics
| Method | Idempotent? | Safe? | Use For |
|---|---|---|---|
| GET | Yes | Yes | Read resource(s) |
| POST | No | No | Create resource, non-idempotent action |
| PUT | Yes | No | Replace resource entirely |
| PATCH | No | No | Partial update |
| DELETE | Yes | No | Remove resource |
What ships with it
1 file 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.
- 1mo ago First seen · 261 lines · 62 tokens per session scan A 8f38cb88e803
api-design-document is a skill published in the GitHub repository fattain-naime/engineering-docs (6 stars, last pushed 23d ago), licensed MIT. It adds 62 tokens to every session and 3,464 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.
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