> ## 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:write

> 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:write`

<CardGroup cols={2}>
  <Card title="File" 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:write", args: { path: "./out.md", content: "...", create_dirs: true, overwrite: true } }
```

Write a file · `path:` names a FILE, never a directory · returns the path. A binary `content` value (an opaque bytes output from an upstream tool · e.g. MCP image content) is written as-is · no `output_format` declaration needed (the value carries its own type). A directory is never the target: it is made by writing its first file inside it with `create_dirs: true`, never by an empty write at the directory's path.

`overwrite:` defaults **true** · `create_dirs:` defaults **false** — and a false `create_dirs:` is ENFORCED, not ignored: a missing parent directory is refused loudly (a typo'd path surfaces rather than silently materializing a directory tree), so pass `create_dirs: true` to create the parent. **One carve-out, by decision (engine #433 option C): a declared `permits.fs.write` entry covering the target path IS the intent** — the boundary creates the parent tree before the builtin's gate, and the write lands without `create_dirs:` (measured on the reference engine: a literal grant `deep/dir/new.txt` and a glob grant `out/**` both materialize their tree · run green). The refusal keeps its teeth where no declared grant covers the path — and there the boundary itself speaks first: an uncovered write is `NIKA-SEC-004` before the parent gate can. A typo'd path under a *narrow* grant still surfaces (it falls outside the grant → SEC-004); the tree-scatter the enforcement exists to catch cannot hide inside a boundary the author drew. Throws · `NIKA-BUILTIN-WRITE-001` (IO failure, or a missing parent while `create_dirs: false` outside a covering grant · a directory-shaped `path:` — a trailing `/` is `write failed: path not found`, an existing directory is `filesystem I/O error: Is a directory` · a file in the way of `create_dirs:` — `create_dirs failed: path already exists`, the mark an earlier empty write at the directory's path leaves behind, and nothing deletes it; all three measured on the reference engine 0.118.7) · `-002` (`overwrite: false` and the path exists). Both `tool_error` · `transient: false`.

## 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#L153) · [Canonical registry](https://github.com/supernovae-st/nika-spec/blob/f8c5df2127c7867b2e51a2cd5712fa9d8461e73e/canon/builtins.yaml).

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

<Ecosystem />