Claude Cowork sandbox 403s every host despite "All domains" egress setting, user investigates
Easy-Flan-1698 · reddit · 2026-09-12
An Individuals-plan user documents a Claude Cowork sandbox networking failure:
- Settings don't apply: with outbound traffic allowed and domain list set to "All domains" (UI confirms it), every CONNECT request in the sandbox — including to their own site and google.com — is rejected with 403 by the gateway.
- The tell: the only reachable hosts are exactly the package-manager domains in noproxy (pypi, npm, crates, etc.), meaning the sandbox behaves as if in "Package managers only" mode; the account setting never reaches the sandbox.
- Ruled out: reproduced in fresh sessions; the local VM shows the same 403 (account-wide, not cloud-container); raw TCP connect works (gateway refusing, not routing); WebFetch works via a different path.
- Compounding bug: the Sept 8 Windows update (KB5124008/KB5124012) breaks the Plan9 share mount and kills devicebash entirely; uninstalling fixes that but not the 403. Combined, there is currently no path from a session to the user's own server.
The author references similar Enterprise egress-allowlist issues and asks whether anyone on Individuals/Pro has gotten "All domains" to actually take effect.
Related event: Claude Cowork Sandbox Network Whitelist Broken with 403s(2 posts)→
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