Claude Cowork Sandbox Not Working? Troubleshooting Guide (Startup, Permissions, Windows)

Why the sandbox matters

Claude Cowork runs every autonomous task inside an isolated virtual machine (Apple's VZVirtualMachine on macOS) — not directly on your system. That VM is the "sandbox": Claude gets controlled access to your files, network, and apps, and anything it does is contained. When the sandbox misbehaves, every task fails at once, which is why "sandbox not working" feels like the whole app is down.

Most sandbox problems fall into one of five buckets. Walk through them in order.

1. Sandbox stuck on "initializing" or spinning forever

Symptom: You start a task and it hangs on "Starting sandbox…" / "Initializing…" for minutes.

Try, in order:

Wait 30–60 seconds. First launch after an update is slower — the sandbox rebuilds its disk image.
Restart Claude Cowork completely (Cmd/Ctrl+Q, not just closing the window), then retry. A stuck VM process is the #1 cause.
Restart your computer. Virtualization extensions (Hypervisor.framework on macOS, WSL2/Hyper-V on Windows) can be left in a bad state after sleep/wake cycles, and only a full reboot reliably clears them.
Check for a leftover VM process: on macOS open Activity Monitor and look for Claude* or VZ processes; on Windows check Task Manager for orphaned Claude*/vmcompute entries. Force-quit anything stuck, then relaunch.

If it still hangs, skip to Resetting the sandbox below.

2. "Sandbox is down" / tasks fail instantly

Symptom: Every task fails within seconds with a sandbox-related error, or the app shows a "sandbox unavailable" banner.

Check your disk space. The sandbox reserves several GB of disk for its image. If your drive is nearly full, the sandbox can't allocate space and dies instantly. Free up space and retry.
Check macOS Sequoia+ / Windows storage settings — if your system is set to "Optimize Storage" aggressively, the sandbox image may have been purged. Relaunching Cowork should rebuild it.
Update Claude Cowork. Sandbox regressions are usually fixed in point releases. Check for updates under the app menu, and also check your macOS/Windows system updates (a stale virtualization stack breaks the sandbox).
Security software conflicts. On Windows especially, third-party antivirus/EDR tools sometimes block the sandbox's VM process. Add Claude Cowork to your AV's allowlist, or temporarily disable the AV to test.

3. Sandbox can't access your files / permissions errors

Symptom: Claude can see the sandbox but can't read or write your folders, or you get "permission denied" on files.

This is almost always a grant problem, not a sandbox problem:

macOS: Cowork requests file access through the system permission prompts (the same TCC prompts you'd see for any app). Open System Settings → Privacy & Security → Files and Folders (and Full Disk Access if you've granted it) and confirm Claude Cowork is listed and enabled. If it's missing, the sandbox has no permission to map those folders.
Windows: Check the app's permissions in Settings → Privacy & security → File system and Documents/Pictures/Downloads toggles.
Folders on external/network drives: The sandbox maps your home folders by default. Files on external volumes or network shares often need an explicit grant — look for a permission prompt the first time you ask Claude to touch them, and if you dismissed it, re-trigger it by asking Claude to access the folder again.

After changing permissions, restart Claude Cowork so the sandbox re-reads the grants.

4. Network issues inside the sandbox

Symptom: Claude can't reach the web — research tasks return nothing, connectors time out.

Check if the task actually needs network. Some sandbox profiles are network-restricted by design; if a workflow previously worked and now doesn't, it's not the profile.
Corporate VPNs / firewalls. The sandbox uses a NAT network by default. Strict VPNs or firewall policies can block the sandbox's traffic even when your browser works. Try disconnecting the VPN briefly to confirm.
DNS issues. Flush your system DNS (macOS: dscacheutil -flushcache; Windows: ipconfig /flushdns), then retry.
Check proxy settings. If you use a system proxy, make sure it's not set to a localhost-only proxy that the sandbox can't reach.

5. Sandbox timeout / "task killed" mid-run

Symptom: Long tasks get killed at the same point every time, or you see a timeout error.

Very long-running tasks can hit the session limit — that's expected behavior, not a bug. Split the task into smaller steps or use Scheduled Tasks for recurring work.
Memory pressure: the sandbox borrows system RAM. If your machine is low on memory (check Activity Monitor / Task Manager while a task runs), close heavy apps and retry.
Thermal throttling on laptops: sustained heavy workloads can throttle the VM. Let the machine cool down and retry.

6. Windows-specific sandbox issues (the ones users search for most)

Windows support shipped in February 2026, and Windows sandbox issues have their own common causes:

WSL2 / Virtual Machine Platform must be enabled. The sandbox on Windows depends on the Windows Hypervisor Platform. Verify in Windows Features that "Virtual Machine Platform" and "Windows Hypervisor Platform" are turned on. If you just enabled them, reboot — the hypervisor loads at boot.
Conflicts with other hypervisors. Running Docker Desktop (WSL2 mode), VirtualBox, or VMware simultaneously can conflict with the Windows Hypervisor Platform. Stop those apps while using Cowork, or switch Docker to a non-WSL backend.
Windows Sandbox / Hyper-V is disabled by your IT policy. On managed work laptops, group policy may disable virtualization. Check systeminfo — if "Hyper-V Requirements" shows "A hypervisor has been detected" as No, virtualization is off at the BIOS/OS level.
Antivirus interference (see #2 above) is disproportionately common on Windows — Defender and third-party AVs have flagged the VM process before. Allowlist Claude Cowork and retry.

7. When it's NOT a bug

Network-restricted tasks: some connectors or commands legitimately run with limited access. The UI usually says so.
First-run slowness: the first task after install/update builds the sandbox image — allow a couple of minutes.
Plan limits: if you're on a free or basic plan, the sandbox may have stricter session limits. Check Pricing.

8. Resetting the sandbox

The nuclear option that fixes most stubborn cases:

Quit Claude Cowork completely.
Delete the sandbox state folder:
- macOS: ~/Library/Containers/com.anthropic.claude-cowork/Data/Library/Application Support/Sandbox (exact path may vary by version — look for anything with "sandbox" or "vm" under the app's container).
- Windows: %LOCALAPPDATA%\Anthropic\Claude Cowork\Sandbox.
Relaunch Cowork — it rebuilds a fresh sandbox image on the next task.

⚠️ Resetting wipes any files you saved inside the sandbox (not your normal folders). Back up anything in the sandbox you need first.

If a fresh sandbox still fails, the problem is environmental (disk, virtualization, AV, or OS-level) rather than the sandbox itself — work through sections 1–6 again.

Still stuck?

Check the official status page / help center — "sandbox down" is sometimes genuinely server-side during an Anthropic incident.
Ask the community: the r/ClaudeCowork and Anthropic Discord communities catalog known issues per version.
File a report with your macOS/Windows version, Cowork version, and the exact error text — that's what makes fixes land in the next release.

This guide is community-maintained and based on publicly reported issues. Always defer to Anthropic's official documentation and support channels.