[Bug] DesignSync OAuth token scope acquisition fails with 400 error on valid subscription login

Bug Description
Title: DesignSync returns 400 "Could not add design scopes to the token" despite a valid subscription login

Summary: The DesignSync tool cannot acquire design scopes for the Claude Code OAuth token. Every method call (list_projects, get_project, list_files) fails identically before doing any work.

Environment: Claude Code, macOS (Darwin 25.5.0), model claude-opus-4-8[1m]. No ANTHROPIC_API_KEY set (confirmed), so auth is genuine subscription OAuth.

Steps to reproduce:

1. /login and select "Claude account with subscription" (login reports success).
2. Invoke DesignSync with any method, e.g. list_projects, or get_project on a project the account owns.

Expected: Design scopes are added to the token; the project is readable and writable.

Actual:
DesignSync needs a claude.ai login. Could not add design scopes to the token.
Run /login, select "Claude account with subscription", and retry. (Request failed with status code 400)

Already tried (all return the same 400): /login three times, a full /logout then /login, and repeated retries across a session resume.

Environment Info

• Platform: darwin
• Terminal: ghostty
• Version: 2.1.176
• Feedback ID: 78171ed2-ee7e-436e-b977-5c5dd727518d

Errors

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