> ## Documentation Index
> Fetch the complete documentation index at: https://openrouter.ai/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# MessagesFallbackParam

Fallback model to try when the primary model fails or refuses. Only the `model` field is supported; per-attempt overrides are rejected.

## Fields

| Field | Type | Required | Description | Example |
| - | - | - | - | - |
| `Model` | `string` | :heavy\_check\_mark: | N/A | |
| `AdditionalProperties` | map\[string]`any` | :heavy\_minus\_sign: | N/A | \{<br />"model": "claude-opus-4-8"<br />} |