[BUG] Cloud sessions: GitHub gateway blocks github: archive tarballs for public repos even with Full network access (breaks Nix flakes)

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

In a Claude Code on the web session with the environment's network access set to Full, downloads of GitHub archive tarballs for public repos not attached to the session return 403. Anything that fetches https://github.com/<owner>/<repo>/archive/<rev>.tar.gz fails, which breaks nix develop / nix flake check for any flake that uses github: inputs. This looks like the same behavior as #86828 and #78330.

Observations

• google.com, cache.nixos.org, channels.nixos.org and api.github.com are reachable.
• git clone / git fetch of the same public repos works through the proxy.
• add_repo for those public repos replies "read access already available, nothing attached", but the tarball fetch still returns 403.
• Workaround: override each flake input (including transitive ones such as nix-systems/default) with git+https://github.com/<owner>/<repo>?rev=<locked rev>. This works, but is slower and has to be done per command.

What Should Happen?

With Full network access, public GitHub archives download normally, or the docs say plainly that GitHub is scoped to attached repos regardless of the network setting.

Error Messages/Logs

Steps to Reproduce

1. Set the environment's network access to Full.
2. Open a session on a repo whose flake.nix uses github:NixOS/nixpkgs, github:numtide/flake-utils and so on.
3. Run nix develop.

Actual result

error: unable to download 'https://github.com/numtide/flake-utils/archive/<rev>.tar.gz': HTTP error 403
{"message":"GitHub access to this repository is not enabled for this session. Use add_repo to request access. ..."}

Claude Model

None

Is this a regression?

No, this never worked

Last Working Version

_No response_

Claude Code Version

2.1.283

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

_No response_