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 proyecto26/system-design-skills --skill api-designgit clone --depth 1 https://github.com/proyecto26/system-design-skillsWrote 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/proyecto26/system-design-skills/api-design)<a href="https://agentmods.dev/skills/proyecto26/system-design-skills/api-design"><img src="https://agentmods.dev/badge/skills/proyecto26/system-design-skills/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/proyecto26/system-design-skills/api-design"><img src="https://agentmods.dev/badge/skills/proyecto26/system-design-skills/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 | $0.00129 | $0.03227 |
| Opus 5.5 | $0.00052 | $0.01291 |
| Sonnet 5.5 | $0.00026 | $0.00645 |
| Haiku 4.5 | $0.00013 | $0.00323 |
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 — 217 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API Design
Define the contract between clients and a service: the exact request and response shapes, how callers page through data, how retried writes stay safe, how errors are reported, and how the contract evolves. Get this vague and the rest of the diagram is guesswork (GUIDE #8) — a "NoSQL box" or "user service" solves nothing until you can write the request, the response, and the key it hits.
When to reach for this
The design has named services and a datastore, and you now need the interface: what a client sends, what it gets back, how it fetches the next page, how it retries a payment without double-charging, and how a v1 client survives a v2 deploy. Reach here the moment someone says "fetch the feed" or "store the post" without a shape.
When NOT to
Before requirements and scale are pinned — the protocol choice depends on
read/write ratio and latency target, not taste (→ requirements-scoping,
back-of-the-envelope). Don't reach for gRPC, GraphQL, or streaming because they
sound modern; a plain REST/JSON endpoint is the cheapest contract that meets most
constraints, and naming a fancier protocol you don't need is a YAGNI red flag.
Internal data-access keys and partition design live in data-storage; this skill
designs the external contract that mirrors them.
Clarify first
- Call shape — request/response (CRUD), bidirectional/real-time, or one request → many results (streaming)? This picks the protocol.
- Read/write ratio and result-set size — drives pagination and whether reads
need their own optimized path (→
back-of-the-envelope). - Retry safety — can a write be safely repeated? Which operations are naturally idempotent (PUT/DELETE) vs not (POST that creates/charges)?
- Client diversity & churn — public third parties (slow to upgrade, need strict versioning) vs your own apps (ship together)?
- Latency & payload budget — mobile/high-latency links favor compact binary and fewer round trips; browsers favor cacheable HTTP.
What ships with it
5 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.
- 1mo ago First seen · 217 lines · 129 tokens per session scan A e21704b1151d
api-design is a skill published in the GitHub repository proyecto26/system-design-skills (94 stars, last pushed 4mo ago), licensed MIT. It adds 129 tokens to every session and 3,227 once invoked, about $0.0005 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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