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 sergei-aronsen/claude-code-toolkit --skill api-designgit clone --depth 1 https://github.com/sergei-aronsen/claude-code-toolkitWrote 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/sergei-aronsen/claude-code-toolkit/api-design)<a href="https://agentmods.dev/skills/sergei-aronsen/claude-code-toolkit/api-design"><img src="https://agentmods.dev/badge/skills/sergei-aronsen/claude-code-toolkit/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/sergei-aronsen/claude-code-toolkit/api-design"><img src="https://agentmods.dev/badge/skills/sergei-aronsen/claude-code-toolkit/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 | $0.00032 | $0.01721 |
| Opus 5.5 | $0.00013 | $0.00688 |
| Sonnet 5.5 | $0.00006 | $0.00344 |
| Haiku 4.5 | $0.00003 | $0.00172 |
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 — 356 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API Design Skill
Load this skill when designing REST APIs, creating endpoints, or working with OpenAPI.
Rule
DESIGN CONSISTENT, PREDICTABLE APIs!
- Use nouns for resources, HTTP verbs for actions
- Return appropriate status codes
- Provide clear error messages
REST Naming Conventions
Resources (nouns, plural)
GET /users # List users
POST /users # Create user
GET /users/{id} # Get user
PUT /users/{id} # Replace user
PATCH /users/{id} # Update user
DELETE /users/{id} # Delete user
Nested Resources
GET /users/{id}/posts # User's posts
POST /users/{id}/posts # Create post for user
GET /users/{id}/posts/{pid} # Specific post
Actions (when CRUD doesn't fit)
POST /users/{id}/activate # Custom action
POST /orders/{id}/cancel # Custom action
POST /reports/generate # Non-resource action
HTTP Status Codes
Success (2xx)
| Code | When to Use |
|---|---|
200 OK |
GET success, PUT/PATCH success |
201 Created |
POST created new resource |
204 No Content |
DELETE success, no body |
Client Errors (4xx)
| Code | When to Use |
|---|---|
400 Bad Request |
Invalid input/syntax |
401 Unauthorized |
Not authenticated |
403 Forbidden |
Authenticated but not allowed |
404 Not Found |
Resource doesn't exist |
409 Conflict |
Resource conflict (duplicate) |
422 Unprocessable Entity |
Validation failed |
429 Too Many Requests |
Rate limit exceeded |
Server Errors (5xx)
| Code | When to Use |
|---|---|
500 Internal Server Error |
Unexpected error |
502 Bad Gateway |
Upstream service failed |
503 Service Unavailable |
Temporarily unavailable |
Error Response Format (RFC 7807)
{
"type": "https://api.example.com/errors/validation",
"title": "Validation Error",
"status": 422,
"detail": "The request body contains invalid fields",
"instance": "/users/123",
"errors": [
{
"field": "email",
"message": "Invalid email format"
},
{
"field": "age",
"message": "Must be a positive number"
}
]
}
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 · 356 lines · 32 tokens per session scan A 712828b80d35
API Design is a skill published in the GitHub repository sergei-aronsen/claude-code-toolkit (5 stars, last pushed 2mo ago), licensed MIT. It adds 32 tokens to every session and 1,721 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.
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