Tauri Sidecar

Tauri v2 sidecar binaries and resources

Why sidecar

A sidecar is a helper binary bundled inside the Tauri app and launched as a child process at runtime. Use it when you need something the WebView can't do: a local HTTP server, a CLI tool, a language runtime (Python, Node), a native helper in Go/Rust. The Tauri shell does process management, arg-passing, and stdio piping; the sidecar stays a normal executable that you can also run standalone for debugging.

Don't sidecar for: simple FS access (use tauri-plugin-fs), shelling out to system binaries the user already has (use Command::new), or anything you could do as a #[tauri::command] in Rust (cheaper, no extra process).

Target-triple naming

Each binary in bundle.externalBin must exist on disk with a -<TARGET_TRIPLE> suffix. Tauri picks the right one at build time per target. There's no fallback — wrong name → "binary not found".

Get the host triple:

rustc --print host-tuple   # Rust ≥ 1.84
rustc -vV | grep '^host:'  # older Rust

Common triples:

So binaries/myproxy configured in externalBin requires src-tauri/binaries/myproxy-aarch64-apple-darwin (and one file per target you ship). Windows binaries keep their .exe extension after the triple: myproxy-x86_64-pc-windows-msvc.exe.

Config: tauri.conf.json

{
  "bundle": {
    "externalBin": ["binaries/myproxy"],
    "resources": {
      "../web-dist": "web-dist"
    }
  }
}

The externalBin path is relative to src-tauri/. Use bundle.resources for read-only static files (HTML, JSON, assets) that the sidecar or the WebView reads at runtime — see "Resources" below.

.. in resource paths becomes _up_ in the bundled output. Map to a clean name ("../web-dist": "web-dist") instead of leaving the escape visible.

Permissions: capabilities/default.json

The shell plugin gates sidecar execution. Without an shell:allow-execute entry naming the sidecar, spawn() returns "not allowed".

See templates/capabilities-sidecar.json for the full pattern. Minimal:

{
  "identifier": "shell:allow-execute",
  "allow": [
    {
      "name": "binaries/myproxy",
      "sidecar": true,
      "args": ["--port", { "validator": "^\\d{2,5}$" }]
    }
  ]
}

• sidecar: true — required; without it the entry matches a system binary of that name, not your bundled one.
• args — narrows what arg vector is allowed. Use true to allow any args (looser), or list literal strings and { "validator": "<regex>" } slots for positional values. Mismatched args at runtime → rejection.
• shell:allow-spawn — long-running spawn with the streaming rx channel (what you almost always want for a server sidecar). allow-execute covers the one-shot execute() form.
• shell:allow-kill — needed if you call child.kill() from JS. Rust's CommandChild::kill() doesn't go through the permission layer.

Run from JavaScript

import { Command } from '@tauri-apps/plugin-shell'

const sidecar = Command.sidecar('binaries/myproxy', ['--port', '4142'])
sidecar.stdout.on('data', (line) => console.log('[proxy]', line))
sidecar.stderr.on('data', (line) => console.error('[proxy]', line))
sidecar.on('close', ({ code }) => console.log('proxy exited', code))

const child = await sidecar.spawn()  // long-running; returns Child
// child.write('input\n'); await child.kill()

For a one-shot tool: const { stdout, code } = await sidecar.execute().

The first arg to Command.sidecar is the path string from externalBin — no triple suffix, Tauri resolves that.

See templates/spawn-sidecar.ts.

Run from Rust

use tauri_plugin_shell::{ShellExt, process::CommandEvent};

let (mut rx, child) = app
    .shell()
    .sidecar("myproxy")?
    .args(["--port", "4142"])
    .env("RUST_LOG", "info")
    .spawn()?;

// Stash child in managed state so you can kill it later.
app.state::<Sidecar>().set(child);

tauri::async_runtime::spawn(async move {
    while let Some(event) = rx.recv().await {
        match event {
            CommandEvent::Stdout(line) => { /* parse */ }
            CommandEvent::Stderr(line) => { /* log */ }
            CommandEvent::Terminated(payload) => {
                eprintln!("sidecar exited: {payload:?}");
                break;
            }
            _ => {}
        }
    }
});

sidecar("myproxy") takes just the basename — no binaries/ prefix, no triple. The rx channel yields CommandEvent::{Stdout, Stderr, Error, Terminated} until the process ends; iterate it on a Tokio task so the main thread isn't blocked.

child.write(b"...\n")? pipes to the sidecar's stdin.

See templates/spawn-sidecar.rs for a complete pattern including managed-state storage and kill-on-exit.

Lifecycle: kill on app exit

The single biggest sidecar bug on macOS is leaking the child process when the user quits. Tauri does not kill sidecars automatically. The fix:

1. Store the CommandChild in managed state (app.manage(Sidecar::new())).
2. In app.run(|handle, event| { ... }), match RunEvent::ExitRequested and call child.kill() exactly once.

app.run(|handle, event| {
    if let RunEvent::ExitRequested { .. } = event {
        if let Some(child) = handle.state::<Sidecar>().take() {
            let _ = child.kill();   // SIGTERM on POSIX, TerminateProcess on Win
        }
    }
});

kill() consumes the handle — wrap it in Mutex<Option<CommandChild>> and use Option::take() so a second exit signal is a harmless no-op.

If your sidecar holds resources (sockets, file locks), make sure it handles SIGTERM cleanly. Bun-compiled and Node binaries do by default; a naive Rust binary may need a tokio::signal::ctrl_c() handler.

Resources: read-only bundled files

For static assets the sidecar (or the WebView) needs to read — built frontend dist, JSON config templates, vendored libs — use bundle.resources instead of externalBin. They land in a platform-specific resource directory you resolve at runtime.

"bundle": {
  "resources": {
    "../web-dist": "web-dist",
    "templates/": "templates/"
  }
}

From Rust:

let dir = app.path().resource_dir()?;
let html = std::fs::read_to_string(dir.join("web-dist/index.html"))?;

From JS:

import { resolveResource } from '@tauri-apps/api/path'
const p = await resolveResource('web-dist/index.html')

Grant fs access for the WebView path with fs:scope on $RESOURCE/**/* (needed only if the JS side reads them directly; sidecar-side reads bypass the plugin entirely).

Dev caveat: tauri dev does NOT materialise the resources mapping — that's a bundler step. If your sidecar reads bundled resources, either ship them inside the sidecar binary (Bun import attributes, include_bytes!, Go embed) or detect dev and fall back to a path relative to CWD. See shell/src-tauri/src/lib.rs::locate_dev_settings_dist in this repo for a working example.

Node.js sidecars

Node can't ship as node script.js — the user won't have Node installed. Compile to a single binary first, then sidecar it like any other.

Options (all produce one file you can drop in src-tauri/binaries/):

Bun example (the pattern this repo uses for its own proxy sidecar — see scripts/build-sidecar.ts for a polished version with mtime-based skip, git-sha stamping, and dev/CI force flags):

bun build --compile \
  --target=bun-darwin-arm64 \
  src/main.ts \
  --outfile=src-tauri/binaries/myproxy-aarch64-apple-darwin

Repeat per target. Wire it into your build pipeline before tauri build.

Verification checklist

Before declaring sidecar wiring done:

1. ls src-tauri/binaries/ shows one binary per target triple you ship.
2. bundle.externalBin lists the basename (no suffix).
3. capabilities/*.json has a shell:allow-execute entry with sidecar: true matching the basename.
4. bun run tauri dev (or equivalent) — sidecar spawns, you see its stdout in the Tauri console.
5. Quit the app. ps aux | grep myproxy shows no orphan.
6. bun run tauri build produces a bundle; install it, launch, sidecar still runs (this catches resource-dir misconfigurations that only bite in packaged builds).