API Forge

--- name: api-forge description: Design REST/GraphQL APIs with OpenAPI 3.1, error handling, pagination, rate limiting, webhooks, and idempotency. Use when user asks to design an API, create endpoints, define REST/GraphQL schema, or generate OpenAPI spec. Do NOT use for database schema design, frontend API integration, or non-HTTP protocols (gRPC, WebSocket, MQTT). triggers: - "design an API" - "create endpoints" - "REST API" - "GraphQL schema" - "OpenAPI spec" - "rate limiting" - "webhooks" - "idempotency" - "API pagination" - "API versioning" - "API security" negatives: - "database schema" - "frontend integration" - "gRPC" - "WebSocket" - "MQTT" - "database design" license: MIT compatibility: opencode metadata: workflow: backend audience: developers version: "4.0.0" author: shokunin

API Forge

Design APIs that developers love. Based on patterns from Stripe, GitHub, Twilio, Slack, and the OpenAPI 3.1 specification.

Sub-Commands

Workflow

Step 1: Determine API type

Step 2: Define resources and naming

Step 3: Map HTTP methods

Step 4: Design response format

Stripe-style standard envelope:

{
  "data": {},
  "meta": {
    "page": 1,
    "per_page": 25,
    "total": 100
  },
  "error": null,
  "request_id": "req_abc123"
}

If using JSON:API or GraphQL, use their standard envelopes instead.

Step 5: Implement pagination

Cursor-based for production. Page-based only for admin/internal tools.

GET /items?cursor=abc123&limit=25
{
  "data": [...],
  "meta": {
    "next_cursor": "def456",
    "has_more": true
  }
}

Cursor must be opaque (base64-encoded compound key). Never expose internal IDs. Maximum limit: 100. Default: 25.

Error Handling

Every error response includes:

• code: machine-readable error code (VALIDATION_ERROR, NOT_FOUND, RATE_LIMITED)
• message: human-readable summary, max 150 chars
• details: array of field-level errors for validation
• request_id: UUIDv4 for debugging correlation
• docs_url: link to error documentation (optional, strongly recommended)

{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Email is required",
    "details": [
      { "field": "email", "code": "required", "message": "Email is required" }
    ],
    "request_id": "req_a1b2c3d4e5f6",
    "docs_url": "https://docs.example.com/errors/validation"
  }
}

Status codes — exact mapping

Rate Limiting

Algorithm decision

Default: Token Bucket.

Headers (every response)

X-RateLimit-Limit: 100
X-RateLimit-Remaining: 42
X-RateLimit-Reset: 1700000000

Return 429 Too Many Requests with Retry-After header when exceeded.

Rate limit tiers (exact values)

Critical: login (5/15min per IP+username), password reset (3/60min per email), MFA (3/15min per user).

Versioning

Prefer URL path for public APIs. Deprecate with sunset headers. 6-month migration window minimum.

Deprecation: true
Sunset: Sat, 12 May 2027 00:00:00 GMT

Webhooks

Delivery format (Stripe pattern)

{
  "id": "wh_abc123",
  "type": "order.created",
  "created": 1700000000,
  "data": {
    "id": "order_456",
    "status": "paid",
    "total": 2999
  }
}

Delivery protocol

• Retry: exponential backoff (1s, 2s, 4s, 8s, 16s, 32s…)
• Max retries: 3. Max TTL: 24 hours.
• Expect 200 response within 5 seconds.
• Signature: HMAC-SHA256.

Signature verification (exact implementation)

X-Webhook-Signature: t=1700000000,v1=abc123def456...

function verifyWebhook(payload: string, signature: string, secret: string): boolean {
  const [timestampStr, signatures] = signature.split(',').map(s => s.trim())
  const timestamp = timestampStr.split('=')[1]

  const expected = crypto
    .createHmac('sha256', secret)
    .update(`${timestamp}.${payload}`)
    .digest('hex')

  return crypto.timingSafeEqual(
    Buffer.from(expected),
    Buffer.from(signatures.split('=')[1])
  )
}

Idempotency

Return cached response (same status, same body) if same key seen within TTL. Return 409 Conflict if different request body arrives with same key.

API Security Checklist

• HTTPS enforced (HTTP → 301 redirect)
• TLS 1.2+ only (no TLS 1.0/1.1)
• CORS whitelist per environment, never * with credentials
• Input validation at boundary (Zod, Joi, Pydantic)
• Parameterized queries (no SQL injection). Never string interpolation.
• No secrets in responses, logs, or error messages
• Rate limiting on auth + password endpoints
• Request size limit: 1MB default, configurable per endpoint
• Body parsing limits (depth, field count, string length)
• Security headers: X-Content-Type-Options: nosniff, X-Frame-Options: DENY, Content-Security-Policy

OpenAPI 3.1 Generation

Every endpoint needs:

• summary: one sentence. Verb + resource.
• parameters: name, in, required, schema, description, example
• responses: every possible status code
• requestBody (for POST/PUT/PATCH): content, schema, required

GraphQL

Schema design

• Queries for read, Mutations for write, Subscriptions for real-time
• Max 3 nesting levels per query
• @deprecated(reason: "Use fieldX instead") for removals
• DataLoader for N+1 prevention
• Complexity limits: max depth 5, max cost 1000

Error handling

{
  "errors": [
    {
      "message": "Validation error",
      "extensions": {
        "code": "VALIDATION_ERROR",
        "field": "email",
        "request_id": "req_abc123"
      }
    }
  ]
}

Anti-Patterns

Production Checklist

• All endpoints documented with OpenAPI 3.1
• Envelope: { data, meta, error, request_id } on every response
• Cursor-based pagination for public endpoints
• Rate limit headers on every response
• Rate limiting on auth endpoints (5/15min login, 3/60min reset)
• Idempotency-Key support on POST/PATCH
• Webhook signature verification (HMAC-SHA256)
• Security headers on every response
• CORS whitelist (never * with credentials)
• Input validation at boundary
• Parameterized queries everywhere
• Error responses include request_id and code
• Versioning strategy defined (URL path preferred)
• Deprecation + Sunset headers on deprecated endpoints

Sources

• Stripe API Reference — idempotency, pagination, webhooks, error format
• GitHub REST API — resource naming, versioning
• Twilio API — webhook signature verification
• OpenAPI 3.1 Specification (openapis.org)
• JSON:API Specification (jsonapi.org)
• GraphQL Relay Connection Specification
• IETF RFC 7231 — HTTP semantics
• IETF RFC 6585 — Additional HTTP status codes
• Slack API — rate limiting headers

Checklist

• Skill loads without errors in the AI agent
• YAML frontmatter is valid (description, compatibility, audience)
• Workflow section provides clear step-by-step instructions
• Error handling section covers common failure modes
• All referenced files (references/, scripts/, assets/) exist
• Skill triggers correctly for intended use cases
• No broken links or missing resources