Mock 404 Not Found Error Response

404 Not Found

A 404 response indicates that the requested resource does not exist. This is different from a 400 (bad input) or 403 (forbidden). The mock generates a clean error structure that includes the ID of the missing resource.

Response Structure

{
  "error": {
    "code": "NOT_FOUND",
    "message": "Resource with ID 'a1b2c3d4-...' was not found.",
    "details": []
  }
}

When to Return 404

• GET /api/users/:id where the ID does not exist
• PUT /api/users/:id attempting to update a non-existent record
• DELETE /api/users/:id when the resource has already been deleted
• Accessing a nested resource where the parent does not exist

404 vs 410 Gone

Use 404 Not Found when the resource may never have existed or you do not want to reveal that information. Use 410 Gone when you can confirm the resource existed previously but has been permanently deleted.

Frontend Handling

Your UI should display a meaningful empty state or redirect when encountering a 404:

• Show a "not found" illustration with a link back to the list view
• Log the error for debugging
• Avoid showing raw JSON to end users

Security Consideration

Be careful not to leak information in 404 messages. Saying "User with email john@example.com not found" reveals that the email is not registered, which can be exploited for account enumeration attacks.