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 Kin9Zeus/senior-engineer-skills --skill api-contractsgit clone --depth 1 https://github.com/Kin9Zeus/senior-engineer-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/kin9zeus/senior-engineer-skills/api-contracts)<a href="https://agentmods.dev/skills/kin9zeus/senior-engineer-skills/api-contracts"><img src="https://agentmods.dev/badge/skills/kin9zeus/senior-engineer-skills/api-contracts/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/kin9zeus/senior-engineer-skills/api-contracts"><img src="https://agentmods.dev/badge/skills/kin9zeus/senior-engineer-skills/api-contracts.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 | $0.00148 | $0.02679 |
| Opus 5.5 | $0.00059 | $0.01072 |
| Sonnet 5.5 | $0.00030 | $0.00536 |
| Haiku 4.5 | $0.00015 | $0.00268 |
Grade A, and why
api-contracts 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 — 279 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API Contracts
An API is a promise you cannot take back. Internal APIs can be refactored; anything a second party consumes becomes a compatibility obligation the moment the first integration ships.
Design for the consumer who is not in the room, does not read your changelog, and will retry on failure.
The seven properties of a good endpoint
Every endpoint should be:
- Predictable — its shape is inferable from the others.
- Authorised — per object, not merely authenticated.
- Validated — with a schema, at the boundary, rejecting unknown fields.
- Idempotent where it can be — safe to retry.
- Bounded — pagination capped, payload size capped, response time bounded.
- Observable — logged with a correlation id, latency and error rate measured.
- Documented — from the code, so it cannot drift.
An endpoint missing any of these is a finding, whatever it returns.
REST essentials
Resources are nouns; HTTP verbs are the operations.
GET /v1/invoices list
POST /v1/invoices create
GET /v1/invoices/{id} read
PATCH /v1/invoices/{id} partial update
DELETE /v1/invoices/{id} delete
POST /v1/invoices/{id}/void a state transition that is not CRUD
Actions that are genuinely not CRUD get a sub-resource with POST. Do not
contort them into PATCH with a magic status field, and do not invent
/getInvoices.
Status codes people actually need to distinguish
| Use for | |
|---|---|
200 / 201 / 204 |
Success; 201 with a Location on create; 204 for an empty response |
202 |
Accepted for async processing — return a status URL |
400 |
Malformed request |
401 |
Not authenticated |
403 |
Authenticated, not permitted — only when the resource's existence is not secret |
404 |
Not found, or found but not visible to you (prefer this over 403 for other users' data) |
409 |
Conflict — duplicate, or a state transition that is not valid now |
410 |
Gone — removed permanently; useful in deprecation |
422 |
Well-formed but semantically invalid (validation failures) |
429 |
Rate limited — always with Retry-After |
5xx |
Your fault. Never use a 4xx to hide a server error, and never a 200 with {"error": ...} |
What ships with it
2 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 · 279 lines · 148 tokens per session scan A 85f6233e33a9
api-contracts is a skill published in the GitHub repository Kin9Zeus/senior-engineer-skills (3 stars, last pushed 2d ago), licensed MIT. It adds 148 tokens to every session and 2,679 once invoked, about $0.0006 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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