> ## 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.

# nika:emit

> Usage, declared contract and source links for this Nika builtin.

export const Ecosystem = () => <CardGroup cols={3}>
    <Card title="Documentation" icon="book" href="/introduction">
      Guides, reference contracts and working examples.
    </Card>
    <Card title="Nika Lab" icon="diagram-project" href="https://github.com/supernovae-st/nika-lab">
      Internal workspace and graph projections of the spec, documentation and code.
    </Card>
    <Card title="Language spec" icon="file-contract" href="https://github.com/supernovae-st/nika-spec">
      the nine-key envelope, Apache-2.0. The contract these docs explain.
    </Card>
    <Card title="Engine source" icon="gem" href="https://github.com/supernovae-st/nika">
      Rust, AGPL-3.0-or-later. Every commit public.
    </Card>
    <Card title="TypeScript SDK" icon="code" href="/sdk/overview">
      Use one typed lifecycle locally or against authenticated nika serve HTTP.
    </Card>
    <Card title="Editor extension" icon="puzzle-piece" href="https://marketplace.visualstudio.com/items?itemName=supernovae.nika-lang">
      VS Code · Cursor · Windsurf: check-as-you-type, DAG view, trace replay.
    </Card>
    <Card title="Homebrew tap" icon="beer-mug-empty" href="https://github.com/supernovae-st/homebrew-tap">
      `brew install supernovae-st/tap/nika` for the latest tagged CLI release.
    </Card>
    <Card title="SuperNovae" icon="star" href="https://supernovae.studio">
      The Paris studio crafting Nika. 🦋
    </Card>
  </CardGroup>;

[Builtins catalog](/reference/builtins) / `nika:emit`

<CardGroup cols={2}>
  <Card title="Core" icon="layer-group">Tool family from the canonical registry.</Card>
  <Card title="invoke" icon="code" href="/reference/language/words/invoke">Call this builtin through the invoke verb.</Card>
</CardGroup>

## Read this contract

This reference preserves the specification at the revision linked below. Source examples are **fragments**: their surrounding tasks, inputs and permissions are not supplied here. Consult [engine status](/reference/status) and `nika catalog --tools --json` for the installed implementation; a registry declaration is not a runtime qualification.

## Usage and behavior

```yaml illustration theme={"system"}
invoke: { tool: "nika:emit", args: { event_type: custom.event, payload: { ... } } }
```

Emit a custom machine event (consumed by subscribers · journal).

\> **Why this is not `log` with a different word (normative rationale · 2026-08-11).**
\> The separation was stated as *"`log` = human diagnostic · `emit` = machine
\> event"* — a claim about the intended READER, and a reader is a property of the
\> consumer, not of the write. That reasoning would not have survived the
\> collapse sweep that merged `sleep`+`wait_until` and four introspection
\> builtins. **The load-bearing difference is the FAILURE MODE**, and it is
\> structural: `nika:log` has **no failure codes at all** — a log that cannot
\> land never fails its task — while `emit` **validates the event shape** and
\> refuses `NIKA-BUILTIN-EMIT-001`. Collapsing them forces one of two
\> regressions: either every diagnostic becomes failable (73 files in the
\> measured corpus rely on it not being), or machine events lose their shape
\> gate. Two builtins, two contracts, one event stream — **kept for what they
\> promise on failure, not for who reads them.**

`event_type:` matches `^[a-z][a-z0-9_.-]*$` · `payload:` any JSON value. Returns `null`. Delivery is engine-side (journal · subscribers) · best-effort once shape-valid. Throws · `NIKA-BUILTIN-EMIT-001` (invalid event shape · `validation_error`).

## Related concepts

<CardGroup cols={2}>
  <Card title="Arguments" icon="sliders" href="/reference/language/words/args">Pass the values required by the tool contract.</Card>
  <Card title="Permissions" icon="shield" href="/concepts/security">Understand authority before granting effects.</Card>
  <Card title="Errors and recovery" icon="rotate" href="/reference/error-codes">Read diagnostics and choose a recovery policy.</Card>
  <Card title="Workflow templates" icon="file-code" href="/guides/templates">Put the fragment inside a complete workflow.</Card>
</CardGroup>

## Contract provenance

[Read the pinned specification](https://github.com/supernovae-st/nika-spec/blob/f8c5df2127c7867b2e51a2cd5712fa9d8461e73e/stdlib/builtins-v0.1.md#L60) · [Canonical registry](https://github.com/supernovae-st/nika-spec/blob/f8c5df2127c7867b2e51a2cd5712fa9d8461e73e/canon/builtins.yaml).

Tool identity: `nika:emit`. Specification revision: `f8c5df2127c7`.
The Lab language identity is `language:tool:emit`; the registry view is `ecosystem:tool/nika:emit`. They refer to this contract without merging their graph identities.

<Ecosystem />