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 agentmods add skills/mtvrkan/senior-dev-kit/api-designnpx skills add mtvrkan/senior-dev-kit --skill api-designgit clone --depth 1 https://github.com/mtvrkan/senior-dev-kitWrote 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/mtvrkan/senior-dev-kit/api-design)<a href="https://agentmods.dev/skills/mtvrkan/senior-dev-kit/api-design"><img src="https://agentmods.dev/badge/skills/mtvrkan/senior-dev-kit/api-design.svg" alt="Measured on agentmods" 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 | $0.00037 | $0.00495 |
| Opus 5 | $0.00018 | $0.00247 |
| Sonnet 5 | $0.00007 | $0.00099 |
| Haiku 4.5 | $0.00004 | $0.00049 |
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 3d 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.
What it actually says
api-design
Design the API contract. No implementation — wait for confirmation before coding.
- Read 1-2 existing endpoints. Extract URL pattern, response envelope, error shape, auth mechanism, status codes — match ALL in the new design.
- Changing an EXISTING endpoint → grep for callers first. Non-breaking → propose a backwards-compatible alternative. Breaking → version transition below.
Version transitions (breaking changes only)
Breaking = removed/renamed field, required field added, type/status-code/error-format/auth changed, endpoint removed. Non-breaking (no versioning): new optional field/param, new endpoint.
Plan: current /api/v[N] → new /api/v[N+1], both live in parallel — never remove old behavior on deploy. Old version gets Deprecation + Sunset + Link headers; sunset ≥6 months after v[N+1] ships. Write docs/api-migration-vN-to-vN+1.md, update OpenAPI spec, regenerate types.
Deep reference: agent_docs/api-versioning-guide.md — parallel routing code, deprecation headers, migration guide template, OpenAPI dual-version strategy.
Output
ENDPOINT: [METHOD] [/path/:param] | AUTH: [guard name | none | role:admin]
REQUEST: headers / params / body (with types) | RESPONSE 200/201: { field: type } ← match existing envelope
ERRORS: 400/401/403/404 — when each applies | IDEMPOTENCY: [required|not] | RATE LIMIT: [required|not]
BREAKING CHANGE RISK: [none|low|HIGH→mitigation] | OPEN QUESTIONS: [none|must-answer]
Every mutating endpoint needs auth. Sensitive endpoints must be rate-limited. Never implement — wait for "go".
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.
- 3d ago First seen · 32 lines · 37 tokens per session scan A 5fec7694c0a1
api-design is a skill published in the GitHub repository mtvrkan/senior-dev-kit (5 stars, last pushed 13d ago), licensed MIT. It adds 37 tokens to every session and 495 once invoked, about $0.0002 per session on Opus 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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