[BUG] Claude Desktop (Code tab): fast mode toggle shows "Fast" while the session cannot serve it (sdk_opt_in_required, then extra_usage_disabled)

Environment

• Windows 11, Claude Desktop 2.31226.0.0 (MSIX), bundled Claude Code 2.1.293
• Code tab session, model Opus 5.5
• Extra usage is disabled on the account

What happened

1. The model menu showed "Use fast mode" switched on and the composer showed "Opus 5.5 Fast".
2. Every response in the session log still had speed: standard. The session reported fast mode off with reason sdk_opt_in_required.
3. After switching the toggle off and on again, the reported reason changed to extra_usage_disabled. Responses stayed standard.
4. Shortly after, the fast mode entry disappeared from the model menu, while "fastMode": true remained in claude_desktop_config.json, with no UI left to turn it off.
5. Other sessions in the same app did not show the toggle at all, so the state was inconsistent between sessions.

Expected

• Do not show "Fast" or an enabled toggle when the session cannot serve fast mode.
• Show the reason instead (here: extra usage is required).
• Keep a way to switch the saved setting off even when fast mode is unavailable, so it cannot start applying later without the user noticing.

Notes

Possibly a regression of #82018.

🤖 Reported with Claude Code on the user's behalf