All routes to claude.ai redirect to onboarding [BUG]

### Preflight Checklist

- [x] I have searched [existing issues](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20state%3Aopen%20label%3Abug) 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?

Attempting to log on to [existing, Pro] claude.ai account redirects to /onboarding page. This is consistently happening in the last week, through browser, or CLI, or using linux desktop app. I am then redirected to "Let's create your account", with yet more checkboxes to agree to the Consumer Terms and Acceptable Use Policy, and a "Create account" button.

### What Should Happen?

I should be able to log on to the website or app, see my account, and use Claude.

### Error Messages/Logs

```shell

```

### Steps to Reproduce

1. Attempt to log on through any browser or app, whether or not I am using a VPN.
2. Nothing more. Incorrect redirection is automatic at that point.

### Claude Model

None

### Is this a regression?

Yes, this worked in a previous version

### Last Working Version

Unknown

### Claude Code Version

2.1.263 (Claude Code)

### Platform

Anthropic API

### Operating System

Ubuntu/Debian Linux

### Terminal/Shell

Other

### Additional Information

_No response_

Start by reproducing the login flow for an existing Pro account through a browser and Claude Code on Ubuntu/Debian, as described in the issue. Compare the behavior with the Linux desktop app and check whether every route reaches /onboarding; done means the existing account opens normally without account-creation prompts.

索引モデルが issue の本文から書いたものです。