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.
git clone --depth 1 https://github.com/andr-ca/agentharnessWrote 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/rules/andr-ca/agentharness/api-design)<a href="https://agentmods.dev/rules/andr-ca/agentharness/api-design"><img src="https://agentmods.dev/badge/rules/andr-ca/agentharness/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/rules/andr-ca/agentharness/api-design"><img src="https://agentmods.dev/badge/rules/andr-ca/agentharness/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 | $0.00036 | $0.01494 |
| Opus 5.5 | $0.00014 | $0.00598 |
| Sonnet 5.5 | $0.00007 | $0.00299 |
| Haiku 4.5 | $0.00004 | $0.00149 |
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 — 166 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API Design
Design choices for HTTP APIs (REST-style and GraphQL). The goal is an API that is consistent, predictable, and easy to evolve without breaking callers.
Reference: Google API Design Guide, JSON:API, RFC 9457 (Problem Details for HTTP APIs).
Resource naming (REST)
Use nouns, not verbs. Collections are plural.
GET /users → list users
POST /users → create a user
GET /users/{id} → get one user
PATCH /users/{id} → partial update
PUT /users/{id} → full replace
DELETE /users/{id} → delete
GET /users/{id}/posts → posts belonging to a user
POST /users/{id}/posts → create a post for a user
Actions that don't map cleanly to CRUD go on a sub-resource with a verb:
POST /users/{id}/activate
POST /payments/{id}/refund
Naming rules:
kebab-casefor multi-word path segments (/user-profiles, not/userProfilesor/user_profiles)camelCasefor JSON field names (consistent with most JS/TS clients)- IDs in path, filters/pagination in query string
HTTP status codes — use them correctly
| Code | Meaning | Use for |
|---|---|---|
| 200 | OK | Successful GET, PATCH, PUT |
| 201 | Created | Successful POST that creates a resource |
| 204 | No Content | Successful DELETE or action with no response body |
| 400 | Bad Request | Client sent invalid data (validation failure) |
| 401 | Unauthorized | Not authenticated (no or invalid token) |
| 403 | Forbidden | Authenticated but not permitted for this resource |
| 404 | Not Found | Resource doesn't exist (or is hidden for auth reasons) |
| 409 | Conflict | Duplicate creation, optimistic lock failure |
| 422 | Unprocessable Entity | Syntactically valid but semantically invalid input |
| 429 | Too Many Requests | Rate limit exceeded |
| 500 | Internal Server Error | Unexpected server failure (never expose details) |
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 · 166 lines · 36 tokens per session scan A b8f408c067a0
api-design is a cursor rule published in the GitHub repository andr-ca/agentharness (1 stars, last pushed 5d ago), licensed MIT. It adds 36 tokens to every session and 1,494 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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