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/d-padmanabhan/agent-engineering-handbookWrote 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/d-padmanabhan/agent-engineering-handbook/320-api-design)<a href="https://agentmods.dev/rules/d-padmanabhan/agent-engineering-handbook/320-api-design"><img src="https://agentmods.dev/badge/rules/d-padmanabhan/agent-engineering-handbook/320-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/d-padmanabhan/agent-engineering-handbook/320-api-design"><img src="https://agentmods.dev/badge/rules/d-padmanabhan/agent-engineering-handbook/320-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 | $0.00000 | $0.01208 |
| Opus 5.5 | $0.00000 | $0.00483 |
| Sonnet 5.5 | $0.00000 | $0.00242 |
| Haiku 4.5 | $0.00000 | $0.00121 |
Grade A, and why
320-api-design scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 112 lines — stays where its author put it; the contents beside it link to each section on GitHub.
API Design Gates
This rule owns mandatory API policy only. Use the API design skill
(${HANDBOOK_ROOT}/skills/api-design/SKILL.md) for workflow, examples,
trade-offs, and protocol-specific guidance.
Semantics and Contracts
- Design around consumers, business invariants, trust boundaries, failure behavior, and compatibility commitments before defining endpoints.
- In REST/HTTP, let paths identify resources and methods express operations. Never place state-changing behavior behind GET, HEAD, or a query action.
- Follow HTTP safety, idempotency, cache, conditional-request, and status-code semantics. Use RFC 9457 Problem Details for new public HTTP error contracts unless compatibility requires a documented alternative.
- Define bounded schemas for every request and response, including path, query, header, cookie, body, message, error, pagination, and asynchronous state.
- Keep naming, timestamps, identifiers, money, units, nullability, enums, filtering, sorting, pagination, and envelope semantics consistent.
- Protect concurrent mutations with preconditions when lost updates matter.
A stale
If-Matchproduces412 Precondition Failed. - A
202 Acceptedresponse requires a durable operation lifecycle, authorized status resource, terminal result or problem, and retry or polling contract.
Identity, Validation, and Data
- Authenticate and enforce server-side authorization for every protected object, tenant, relationship, collection, field, action, batch item, stream, and asynchronous status resource.
- Never derive owner, tenant, role, price, approval, or resource authority from untrusted client input.
- Parse untrusted input into constrained schemas, reject or intentionally handle unknown fields, and bound sizes, nesting, counts, ranges, execution, concurrency, and response volume.
- Serialize allowlisted output schemas. Never expose persistence objects, raw exceptions, stack traces, secrets, policy internals, or unnecessary personal data.
- JWT validation includes trusted issuer, audience, allowed algorithms, key source, signature, expiry, not-before, token type where applicable, and subject. API keys are high-entropy, hashed, scoped, expiring, rotatable, revocable, rate-attributed, and never logged or transported in query strings.
- User-influenced outbound URLs require scheme and destination allowlists, redirect and resolved-address revalidation, private-address denial, timeouts, size limits, and restricted egress.
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 Changed · -1,733 lines · -20 tokens per session cf5afc003969
- 1mo ago First seen · 1,845 lines · 20 tokens per session scan A 08c1ac96a381
320-api-design is a cursor rule published in the GitHub repository d-padmanabhan/agent-engineering-handbook (19 stars, last pushed 2d ago), licensed MIT. It costs nothing until one of its globs matches a file; then it loads 1,208 tokens. 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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