api-design

name	api-design
license	MIT
description	Guide API design for REST, GraphQL, gRPC, and CLI interfaces. Use this skill when designing new APIs, reviewing existing API contracts, or establishing API conventions for a project. Produces consistent, well-documented API specifications.

API Design

Design consistent, well-documented APIs across REST, GraphQL, gRPC, and CLI interfaces.

When to Use

• Designing new API endpoints or commands
• Reviewing existing API contracts for consistency
• Establishing API naming and versioning conventions
• Planning backward-compatible API changes
• Generating API documentation or OpenAPI specs

Principles

1. Consistency - Same patterns everywhere (naming, error format, pagination)
2. Discoverability - A developer should guess the right endpoint/flag without reading docs
3. Backward compatibility - Additions are safe; removals and renames require versioning
4. Minimal surface - Expose only what consumers need; internal details stay internal
5. Self-describing errors - Error responses should tell the caller what went wrong and how to fix it

REST API Checklist

Naming

• Use plural nouns for resources: /users, /tasks
• Use kebab-case for multi-word paths: /user-settings
• Nest for ownership: /users/{id}/tasks
• Use verbs only for actions that don't map to CRUD: /tasks/{id}/approve

HTTP Methods

Method	Purpose	Idempotent
GET	Read	Yes
POST	Create / action	No
PUT	Full replace	Yes
PATCH	Partial update	Yes
DELETE	Remove	Yes

Response Format

{
  "data": { ... },
  "meta": { "page": 1, "total": 42 }
}

Error format:

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Human-readable description",
    "details": [
      { "field": "email", "reason": "required" }
    ]
  }
}

Status Codes

Code	Use
200	Success (with body)
201	Created
204	Success (no body)
400	Client error (validation)
401	Not authenticated
403	Not authorized
404	Not found
409	Conflict (duplicate, state mismatch)
422	Unprocessable (semantically invalid)
500	Server error

Pagination

Use cursor-based for large datasets, offset-based for simple cases:

GET /tasks?cursor=abc123&limit=20
GET /tasks?page=2&per_page=20

Versioning

• Prefer URL prefix: /v1/tasks, /v2/tasks
• Use header-based only when URL versioning is impractical

CLI API Checklist

Command Structure

<tool> <noun> <verb> [args] [--flags]

Example: synapse send gemini "Review code" --wait

Flag Conventions

• Long form: --output, --verbose
• Short form for common flags: -o, -v
• Boolean flags: --force, --no-color
• Repeatable: --attach file1 --attach file2
• Mutually exclusive groups: --wait | --notify | --silent

Output

• Default: human-readable
• --json: machine-readable JSON
• --quiet / -q: minimal output
• Exit codes: 0 = success, 1 = error, 2 = usage error

Workflow

Step 1: Define Resources / Commands

List the entities and actions the API must support.

Step 2: Map to Endpoints / Subcommands

Apply naming conventions to produce the API surface.

Step 3: Define Request / Response Schemas

Specify required and optional fields, types, and validation rules.

Step 4: Document Error Cases

For each endpoint, list possible error responses and their meaning.

Step 5: Review for Consistency

Cross-check naming, error format, and pagination across all endpoints.