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

# fallback

> Contract, placement, related fields and source examples for this Nika YAML field.

[Language reference](/reference/language/overview) / `fallback`

## Meaning and placement

### → run → access

What may replace the declared selection · none, also the meaning of an omitted fallback once access is declared: no other route, protocol, model or effort, before admission or after it — an unavailable route refuses before inference and a route failing mid-run fails its task, never re-routed. Declared alternatives are unspecified; a later value is additive.

| Property | Declaration |
| - | - |
| Requirement | Optional in this object |
| Schema location | `/properties/run/properties/access/properties/fallback` |
| enum | declaration below |

<details>
  <summary><code>enum</code> schema declaration (spec source, not a lesson)</summary>

  ```json theme={"system"}
  [
    "none"
  ]
  ```
</details>

**In the same object:** [protocol](/reference/language/words/protocol) · [via](/reference/language/words/via)

[Schema source (use the pointer above)](https://github.com/supernovae-st/nika-spec/blob/f3a952514b51ce884f42839c627bd88887fc941e/schemas/workflow.schema.json)

## In a source template

Excerpt from `templates/recover-optional-file.nika`, source lines 11–20. This is a fragment, not a runnable workflow.

```yaml illustration theme={"system"}
run:
  clock: system
const:
  fallback: '<SLOT: the explicit fallback content>'
permits:
  tools:
  - nika:assert
  - nika:jq
  - nika:read
  - nika:validate
```

### Templates containing this field

| Template | Source |
| - | - |
| `templates/recover-optional-file.nika` | [Line 14](https://github.com/supernovae-st/nika-spec/blob/f3a952514b51ce884f42839c627bd88887fc941e/templates/recover-optional-file.nika#L14) |

These are literal key occurrences in the source files, not an execution or conformance claim. [How to instantiate a template](/guides/templates).

## Contract and implementation

This page projects the named specification revision. Check [released engine status](/reference/status) and [the validation workflow](/guides/agent-authoring) before running a file. An optional field is not evidence that every engine supports every value.

Canonical identity: `language:word:fallback`. Spec revision: [`f3a952514b51`](https://github.com/supernovae-st/nika-spec/tree/f3a952514b51ce884f42839c627bd88887fc941e).

[How documentation stays connected](/reference/knowledge-system).


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