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 kid-sid/codex-spellbook --skill api-designgit clone --depth 1 https://github.com/kid-sid/codex-spellbookWrote 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/kid-sid/codex-spellbook/api-design)<a href="https://agentmods.dev/skills/kid-sid/codex-spellbook/api-design"><img src="https://agentmods.dev/badge/skills/kid-sid/codex-spellbook/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.
<a href="https://agentmods.dev/skills/kid-sid/codex-spellbook/api-design"><img src="https://agentmods.dev/badge/skills/kid-sid/codex-spellbook/api-design.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 | $0.00045 | $0.01212 |
| Opus 5.5 | $0.00018 | $0.00485 |
| Sonnet 5.5 | $0.00009 | $0.00242 |
| Haiku 4.5 | $0.00005 | $0.00121 |
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 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 — 158 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API Design
Design resource-oriented HTTP APIs with stable contracts, predictable errors, and explicit pagination and versioning choices.
When to Activate
- Design a new HTTP or REST endpoint
- Review existing route naming or handler contracts
- Choose between offset and cursor pagination
- Define status codes and error payloads
- Introduce a breaking API change
- Add rate limiting or idempotency behavior
- Generate or review an OpenAPI spec
Resource Design
| Concern | Preferred | Avoid |
|---|---|---|
| Collections | /users |
/getUsers |
| Single resource | /users/{user_id} |
/user?id=123 |
| Nested resource | /users/{user_id}/tokens |
/userTokens/{user_id} |
| Actions | /jobs/{job_id}:cancel when no resource exists |
/cancelJob/{job_id} |
Rules:
- Use plural nouns for collections.
- Use kebab-case for multi-word path segments.
- Put identifiers in the path and filters in the query string.
- Model actions as resources first; use action suffixes only when CRUD does not fit.
BAD
GET /getUserOrders?userId=42&limit=5000
GOOD
GET /users/42/orders?limit=100&cursor=eyJpZCI6Im9yZF8xIn0=
HTTP Semantics
| Method | Use When | Idempotent | Response |
|---|---|---|---|
GET |
Read resources | Yes | 200, 206, 304 |
POST |
Create resource or submit command | No | 201, 202 |
PUT |
Replace full resource | Yes | 200, 204 |
PATCH |
Partial update | Usually | 200, 204 |
DELETE |
Remove resource | Yes | 204 |
| Situation | Code | Notes |
|---|---|---|
| Created synchronously | 201 Created |
Return Location header |
| Accepted async work | 202 Accepted |
Return job resource |
| Validation failure | 400 Bad Request or 422 Unprocessable Entity |
Match validator semantics |
| Authentication missing | 401 Unauthorized |
Include auth challenge when applicable |
| Permission denied | 403 Forbidden |
Caller is authenticated |
| Missing resource | 404 Not Found |
Do not leak existence across tenants |
| Version conflict | 409 Conflict |
ETag or business conflict |
| Rate limited | 429 Too Many Requests |
Include reset headers |
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 · 158 lines · 45 tokens per session scan A 0fc2f2b41a8e
api-design is a skill published in the GitHub repository kid-sid/codex-spellbook (22 stars, last pushed 5mo ago), licensed MIT. It adds 45 tokens to every session and 1,212 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 2026-08-30.
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