# API Designer

> Use when a task needs API contract design, evolution planning, or compatibility review before implementation starts.

- Skill: `jshsakura/api-designer` (Agent Skill)
- Install (CLI): `npx skillmds@latest add jshsakura/api-designer`
- Raw SKILL.md: https://api.skillmd.com/api/skills/jshsakura/api-designer/raw
- Safety review: pending
- Works with: Claude Code, Claude.ai, OpenAI Codex
- Category: Integrations & APIs
- Author: jshsakura (https://skillmd.com/u/jshsakura)
- Updated: 2026-09-17
- Page: https://skillmd.com/skills/jshsakura/api-designer

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## Instructions

Design APIs as long-lived contracts between independently evolving producers and consumers.

Working mode:
1. Map actor flows, ownership boundaries, and current contract surface.
2. Propose the smallest contract that supports the required behavior.
3. Evaluate compatibility, migration, and operational consequences before coding.

Focus on:
- resource and endpoint modeling aligned to domain boundaries
- request and response schema clarity
- validation semantics and error model consistency
- auth, authorization, and tenant-scoping expectations in the contract
- pagination, filtering, sorting, and partial response strategy where relevant
- idempotency and retry behavior for mutating operations
- versioning and deprecation strategy
- observability-relevant contract signals (correlation keys, stable error codes)

Architecture checks:
- ensure contract behavior is explicit, not framework-default ambiguity
- isolate transport contract from internal storage schema where possible
- identify client-breaking changes and hidden coupling
- call out where "one endpoint" would blur ownership and increase long-term cost

Quality checks:
- provide one canonical success response and one canonical failure response per critical operation
- confirm field optionality/nullability reflects real behavior
- verify error taxonomy is actionable for clients
- describe migration path for changed fields or semantics

Return:
- proposed contract changes or new contract draft
- rationale tied to domain and client impact
- compatibility and migration notes
- unresolved product decisions that block safe implementation

Do not implement code unless explicitly asked by the parent agent.