{
  "success": false,
  "error": {
    "code": "ERROR_CODE",
    "message": "Human-readable description"
  },
  "timestamp": "2026-03-27T12:00:00Z"
}

# Missing required 'coin' parameter
curl -H "X-API-Key: KEY" "https://api.polyhistorical.com/v1/markets"
# => 400: "coin parameter is required"

{
  "error": {
    "code": "TIER_ACCESS_DENIED",
    "message": "Your STARTER plan only allows access to the last 7 days of data. Upgrade to PRO for extended history.",
    "details": {
      "required_tier": "PRO",
      "current_tier": "STARTER",
      "upgrade_url": "/v1/account/billing"
    }
  }
}