# Validation Error
URL: /reference/problems/validation

The API returns a `400 Bad Request` status with this problem type when the request payload is invalid.

## Response Format

```json
{
  "type": "https://docs.tambo.co/reference/problems/validation",
  "status": 400,
  "title": "Bad Request",
  "detail": "Initial message at index 0 must have content",
  "instance": "/threads"
}
```

## Fields

* **`type`** - Stable URI identifying this problem type
* **`status`** - HTTP status code (always `400`)
* **`title`** - Short summary of the problem (always `Bad Request`)
* **`detail`** - Human-readable description of what was invalid
* **`instance`** - The specific request path that triggered this error

## Handling

When you receive this error:

1. Read the `detail` field to see what failed validation
2. Update your request payload to match the expected schema
3. Retry the request

## See Also

* [REST API Reference](/reference/rest-api) - Complete API documentation
* [RFC 9457 Problem Details](https://www.rfc-editor.org/rfc/rfc9457.html) - Standard for error responses