api-design

A guide for designing Next.js server actions and API route handlers. It covers checking inputs, authentication, permissions, response codes, errors, rate limits, and response formats.

In plain words
What is it for?
Use it when building form actions, webhooks, public APIs, REST endpoints, or file-upload and streaming handlers in Next.js.
Why use it?
It helps prevent invalid requests, unauthorized access, confusing errors, leaked database details, and uncontrolled public endpoints.

Skill for Claude CodeCodex

Install

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.

agentmods
npx agentmods add skills/m-binimran/dev-pack/api-design
Any agent
npx skills add m-binimran/dev-pack --skill api-design
Clone the repo
git clone --depth 1 https://github.com/m-binimran/dev-pack

Made for: Claude Code, Codex.

Per session 40 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 421 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

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.

ModelPer sessionOnce invoked
Fable 5 $0.00040 $0.00421
Opus 5 $0.00020 $0.00211
Sonnet 5 $0.00008 $0.00084
Haiku 4.5 $0.00004 $0.00042

Measured yesterday against content hash 57c4e0ef8433, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

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 yesterday.

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.

skills/api-design/SKILL.md · 34 lines

What it actually says

api-design

The server boundary between UI and database. Validate everything; never trust the caller.

Choose the right tool

  • Server Action — form submits / mutations from your own UI. Co-located, type-safe, no manual fetch.
  • Route Handler (app/api/.../route.ts) — webhooks, third-party callers, public/REST endpoints, file streams.

Every endpoint

  1. Validate input with Zod at the top. Reject early with 400 + a useful message. Re-validate even if the client already did.
  2. Authenticate + authorize: get the user (server), check they may do this. RLS is the backstop, not the only gate for sensitive actions.
  3. Correct status codes: 200/201 success, 400 bad input, 401 unauthenticated, 403 forbidden, 404 missing, 409 conflict, 429 rate-limited, 500 only for true server faults.
  4. Handle errors explicitly. Catch, log server-side, return a safe message — never leak stack traces or DB errors to the client.
  5. Rate-limit public/mutating endpoints (e.g. Upstash). Idempotency keys for anything chargeable.
  6. Shape responses consistently ({ data } / { error }); don't return raw DB rows with internal columns.

Output

  • The handler/action, the Zod schema, the auth check, and the status codes it returns.
  • Note what's validated, what's rate-limited, and what an attacker could try.

Guardrails

  • No unvalidated input reaching the DB or filesystem.
  • No secrets or internal error details in responses.
  • Mutations are POST/PUT/PATCH/DELETE, not GET.
Changes

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.

  1. yesterday First seen · 34 lines · 40 tokens per session scan A 57c4e0ef8433

Subscribe to this mod's changes

api-design is a skill published in the GitHub repository m-binimran/dev-pack (2 stars, last pushed 2mo ago), licensed MIT. It adds 40 tokens to every session and 421 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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