---
summary: "How Nodmix handles local file access safely, and why the optional fs-safe Python helper is off by default"
read_when:
  - Changing file access, archive extraction, workspace storage, or plugin filesystem helpers
title: "Secure file operations"
---

Nodmix uses [`@nodmix/fs-safe`](https://github.com/nodmix/fs-safe) for security-sensitive local file operations: root-bounded reads/writes, atomic replacement, archive extraction, temp workspaces, JSON state, and secret-file handling.

The goal is a consistent **library guardrail** for trusted Nodmix code that receives untrusted path names. It is not a sandbox. Host filesystem permissions, OS users, containers, and the agent/tool policy still define the real blast radius.

## Default: no Python helper

Nodmix defaults the fs-safe POSIX Python helper to **off**.

Why:

- the gateway should not spawn a persistent Python sidecar unless an operator opted into it;
- many installs do not need the extra parent-directory mutation hardening;
- disabling Python keeps package/runtime behavior more predictable across desktop, Docker, CI, and bundled app environments.

Nodmix only changes the default. If you explicitly set a mode, fs-safe honors it:

```bash
# Default Nodmix behavior: Node-only fs-safe fallbacks.
NODMIX_FS_SAFE_PYTHON_MODE=off

# Opt into the helper when available, falling back if unavailable.
NODMIX_FS_SAFE_PYTHON_MODE=auto

# Fail closed if the helper cannot start.
NODMIX_FS_SAFE_PYTHON_MODE=require

# Optional explicit interpreter.
NODMIX_FS_SAFE_PYTHON=/usr/bin/python3
```

The generic fs-safe names also work: `FS_SAFE_PYTHON_MODE` and `FS_SAFE_PYTHON`.

## What stays protected without Python

With the helper off, Nodmix still uses fs-safe's Node paths for:

- rejecting relative-path escapes such as `..`, absolute paths, and path separators where only names are allowed;
- resolving operations through a trusted root handle instead of ad-hoc `path.resolve(...).startsWith(...)` checks;
- refusing symlink and hardlink patterns on APIs that require that policy;
- opening files with identity checks where the API returns or consumes file contents;
- atomic sibling-temp writes for state/config files;
- byte limits for reads and archive extraction;
- private modes for secrets and state files where the API requires them.

These protections cover the normal Nodmix threat model: trusted gateway code handling untrusted model/plugin/channel path input inside a single trusted operator boundary.

## What Python adds

On POSIX, fs-safe's optional helper keeps one persistent Python process and uses fd-relative filesystem operations for parent-directory mutations such as rename, remove, mkdir, stat/list, and some write paths.

That narrows same-UID race windows where another process can swap a parent directory between validation and mutation. It is defense in depth for hosts where untrusted local processes can modify the same directories Nodmix is operating in.

If your deployment has that risk and Python is guaranteed to exist, use:

```bash
NODMIX_FS_SAFE_PYTHON_MODE=require
```

Use `require` rather than `auto` when the helper is part of your security posture; `auto` intentionally falls back to Node-only behavior if the helper is unavailable.

## Plugin and core guidance

- Plugin-facing file access should go through `nodmix/plugin-sdk/*` helpers, not raw `fs`, when a path comes from a message, model output, config, or plugin input.
- Core code should use the local fs-safe wrappers under `src/infra/*` so Nodmix's process policy is applied consistently.
- Archive extraction should use the fs-safe archive helpers with explicit size, entry-count, link, and destination limits.
- Secrets should use Nodmix secret helpers or fs-safe secret/private-state helpers; do not hand-roll mode checks around `fs.writeFile`.
- If you need hostile local-user isolation, do not rely on fs-safe alone. Run separate gateways under separate OS users/hosts or use sandboxing.

Related: [Security](/gateway/security), [Sandboxing](/gateway/sandboxing), [Exec approvals](/tools/exec-approvals), [Secrets](/gateway/secrets).
