One project builds a tool, every other project runs it, and each stays
in its sandbox. The tool's sources stay hidden.
Depend on a task, read its outputs
build: { dependsOn: ['packages/tools#build'], exec: { command: 'cat ../tools/out/bin/greet ../tools/src/greet.sh > greeting.txt', sandbox: { allow: { read: ['.'], write: ['greeting.txt'] } }, }, inputs: { files: ['src/**'] }, outputs: { files: ['greeting.txt'] }, cache: true, } The output reads fine. The source does not exist for this task:
┌─ packages/app#build > failed (exit 1) $ cat ../tools/out/bin/greet ../tools/src/greet.sh > greeting.txt ├─ OUTPUT ──────────────────────────────────────────────────── cat: ../tools/src/greet.sh: No such file or directory ├─ SANDBOX ─────────────────────────────────────────────────── vx: the sandbox hid paths outside the project that exist on this machine, which are not reported as violations: /work/demo/packages/tools/src/greet.sh. If the task reads one, grant it, e.g. `allow: { read: ['../tools/src/greet.sh'] }`. └─ packages/app#build ── (59ms) failed (exit 1) Demo workspace · Linux, bubblewrap · vx 18d0a91 · 2026-11-07
The outputs are safe to read. The task's key already moves with every
file that made them, so a cache hit never hides a changed tool.
hermetic: true closes off the machine too
A plain sandbox walls off the workspace. hermetic: true moves that
wall to /. The task reads only what you grant, plus the shell vx needs.
┌─ packages/app#build > failed (exit 127) $ cat ../tools/src/greet.sh > greeting.txt ├─ OUTPUT ──────────────────────────────────────────────────── /usr/bin/sh: 1: cat: not found [vx] exit 127 is the shell's "command not found": cat is /usr/bin/cat on the host, but no sandbox grant reads it, so inside the sandbox it is not there — add it (or its directory) to exec.sandbox.allow.read └─ packages/app#build ── (124ms) failed (exit 127) The task reached for the host's cat and found nothing. vx names the
host path and the grant that would fix it. A build cannot lean on a tool
that only your laptop has.
| Sandbox | Workspace | Rest of the machine |
|---|---|---|
| none | everything | everything |
sandbox: {...} | what you grant | readable |
hermetic: true | what you grant | what you grant |
How to grant reads: Sandboxing.
More at vznjs.github.io/vx.
Originally published on the vx blog. vx is open-source (MIT) task orchestration with a build cache: vznjs.github.io/vx · GitHub.
Written with AI assistance.
