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 DevelopersGlobal/ai-agent-skills --skill api-designgit clone --depth 1 https://github.com/DevelopersGlobal/ai-agent-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/developersglobal/ai-agent-skills/api-design)<a href="https://agentmods.dev/skills/developersglobal/ai-agent-skills/api-design"><img src="https://agentmods.dev/badge/skills/developersglobal/ai-agent-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/developersglobal/ai-agent-skills/api-design"><img src="https://agentmods.dev/badge/skills/developersglobal/ai-agent-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 | $0.00034 | $0.00710 |
| Opus 5.5 | $0.00014 | $0.00284 |
| Sonnet 5.5 | $0.00007 | $0.00142 |
| Haiku 4.5 | $0.00003 | $0.00071 |
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 — 79 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Overview
APIs are contracts. Once published, every behavior — documented or not — becomes something users depend on (Hyrum's Law). This skill enforces the discipline of designing APIs that are stable, self-documenting, and difficult to misuse.
When to Use
- Creating any public API endpoint
- Designing function/library APIs
- Extending or versioning existing APIs
Process
Step 1: Design the Interface First
- Write the usage examples before writing the implementation.
- Ask: Is this easy to use correctly? Is it hard to use incorrectly?
- Apply the principle of least surprise — the API should do what it looks like it does.
- Design for the caller, not the implementer.
Verify: You can write 3 example usages without looking at the implementation.
Step 2: Apply Hyrum's Law
- Every observable behavior of your API will be depended upon by someone.
- Document what IS and IS NOT guaranteed:
- Stable: return type, error codes, semantic behavior
- Unstable: response time, field ordering, internal implementation
- Be conservative in what you expose — you can always add, never remove.
Verify: Every public field and behavior is either documented as stable or marked as internal.
Step 3: Versioning Strategy
- Version from day one:
/api/v1/,Content-Type: application/vnd.myapi.v1+json - Breaking changes require a new version.
- Maintain old versions for at least 6 months with deprecation notices.
- Additive changes (new optional fields) are non-breaking.
Verify: API version is in the URL or headers. Deprecation policy is documented.
Step 4: Self-Documentation
- Every endpoint: purpose, inputs, outputs, error codes — documented.
- Error messages tell the caller what went wrong AND how to fix it.
- Schema validation on all inputs with meaningful error messages.
- OpenAPI/Swagger spec generated (not hand-written).
Verify: A new developer can use the API from documentation alone, without reading source code.
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 · 79 lines · 34 tokens per session scan A b188642fe0e3
api-design is a skill published in the GitHub repository DevelopersGlobal/ai-agent-skills (177 stars, last pushed 5mo ago), licensed MIT. It adds 34 tokens to every session and 710 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 2026-08-30.
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