> ## 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.

# tool

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

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

## Meaning and placement

### invoke

Tool reference · nika:\<path> (closed v0.1 builtin set) OR mcp:\<server>/\<tool> (requires the slash). The namespace set is CLOSED at v1 (spec/02-verbs.md) — an x-\<vendor>: prefix is RESERVED, not valid (engine-specific tools route through mcp: · spec/06-stdlib-contract.md §Namespace ownership).

| Property        | Declaration                     |
| --------------- | ------------------------------- |
| Requirement     | Optional in this object         |
| Schema location | `/$defs/invoke/properties/tool` |

**In the same object:** [args](/reference/language/words/args) · [workflow](/reference/language/words/workflow)

[Schema source (use the pointer above)](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/schemas/workflow.schema.json)

## In a source template

Excerpt from `templates/agent-loop.nika.yaml`, source lines 137–146. This is a fragment, not a runnable workflow.

```yaml illustration theme={"system"}
      # answer: that gap closes upstream, by binding the goal into the agent.
      has_findings: ${{ size(tasks.execute.output.findings) > 0 }}   # the check crosses as ONE boundary expression · SLOT: your law
    invoke:
      tool: "nika:assert"
      args:
        condition: "${{ with.has_findings }}"
        message: "Agent returned no findings, do not trust an empty run"   # SLOT

outputs:
  findings:
```

### Templates containing this field

| Template                                    | Source                                                                                                                                            |
| ------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------- |
| `templates/agent-loop.nika.yaml`            | [Line 140](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/agent-loop.nika.yaml#L140)          |
| `templates/api-upload-and-create.nika.yaml` | [Line 70](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/api-upload-and-create.nika.yaml#L70) |
| `templates/chain.nika.yaml`                 | [Line 48](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/chain.nika.yaml#L48)                 |
| `templates/classify-and-route.nika.yaml`    | [Line 96](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/classify-and-route.nika.yaml#L96)    |
| `templates/corpus-qa.nika.yaml`             | [Line 34](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/corpus-qa.nika.yaml#L34)             |
| `templates/docker-report.nika.yaml`         | [Line 92](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/docker-report.nika.yaml#L92)         |
| `templates/document-to-fields.nika.yaml`    | [Line 40](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/document-to-fields.nika.yaml#L40)    |
| `templates/etl-state.nika.yaml`             | [Line 72](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/etl-state.nika.yaml#L72)             |
| `templates/fanout.nika.yaml`                | [Line 88](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/fanout.nika.yaml#L88)                |
| `templates/gate-and-act.nika.yaml`          | [Line 53](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/gate-and-act.nika.yaml#L53)          |
| `templates/human-gated-ship.nika.yaml`      | [Line 95](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/human-gated-ship.nika.yaml#L95)      |
| `templates/media-asset-pack.nika.yaml`      | [Line 73](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/media-asset-pack.nika.yaml#L73)      |
| `templates/website-brief.nika.yaml`         | [Line 52](https://github.com/supernovae-st/nika-spec/blob/c5ebbb7b862b68bc6cf7235efba61173b844afc6/templates/website-brief.nika.yaml#L52)         |

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:tool`. Spec revision: [`c5ebbb7b862b`](https://github.com/supernovae-st/nika-spec/tree/c5ebbb7b862b68bc6cf7235efba61173b844afc6).

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