Cloud session fails at "setting up the git_repository source" — repo clones successfully, then container resume hangs indefinitely

What's Wrong?

Starting a Claude Code on the web session against a private repo fails during
environment setup. The repository clone step reports success, then an API error
appears and the container-resume panel never progresses.

Error text shown in the UI:

An unexpected error occurred while setting up the git_repository source. Try starting a new session. If the problem persists, the error details have been logged — contact support with your session ID.

After the error, the container-resume panel shows four steps, all of which
remain unstarted (empty circles) indefinitely:

1. Resuming cloud container
2. Updating 1 repository
3. Running setup script
4. Starting Claude Code

No command from the submitted prompt is ever executed.

(The UI was running in a Japanese locale; the step labels above are translated.)

What Should Happen?

The session should finish provisioning and run the submitted prompt.

Steps to Reproduce

1. Open claude.ai/code and start a new session
2. Select a private repository
3. Select a branch other than the default — in our case a feature branch that had

been created and pushed a few hours earlier

1. Submit any prompt
2. UI shows "Cloned 1 repository", then the API error above

Observed on 2026-07-27 (JST). Reproduced twice: once in the original session, and
once in a brand-new session as the error message itself suggests. Identical
failure at the same point both times. The first attempt was left for roughly an
hour with no change.

Notes

This is textually identical to #46771, which was not resolved: a bot flagged
it as a duplicate of #17888 / #27856 / #22277, the reporter replied that those
were not the same issue, and it was auto-closed anyway and later locked. Filing
fresh per the auto-lock message's instruction to reference the older issue.

Distinct from the following open issues, which fail at a later stage or from a
different cause:

• #78904 — session reaches "Starting Claude Code" and dies on the first turn
• #78382 — detached-HEAD over stale baked-in branch refs
• #76862 — deleted repository re-provisioning

Not verified

We did not isolate whether the repository's tracked .claude/settings.json
contributes. #78904 shows repo-tracked settings can affect cloud sessions, but our
failure occurs before the "Starting Claude Code" step that issue describes, so a
shared cause seems unlikely. Stated here rather than assumed away.

Session ID is not included in this report; it can be supplied on request.