Session silently falls back from Fable 5 to Sonnet 5 mid-conversation; manual switch back blocked by out-of-credits message despite ~12% weekly limit used

On a Max (5x) plan that includes Fable 5 access.

1. settings.json has "model": "claude-fable-5[1m]" explicitly pinned.
2. At the start of a session, the status line correctly showed Fable 5.
3. After resuming/continuing that same conversation, the status line and /model switched to showing Sonnet 5, with no user-initiated model change.
4. Attempting to manually switch back to Fable 5 via /model fails with a message saying I'm out of usage credits.
5. /model reports Kept model as Sonnet 5 after the failed switch attempt.
6. This is inconsistent with the account's actual usage: only ~12% of the weekly limit has been used.
7. Restarted Claude Code twice; behavior persists across restarts.

Expected: Model should stay on Fable 5 as configured, or if a genuine quota limit was hit, usage reporting (12% used) should be consistent with that block.

Actual: Silent mid-session fallback to Sonnet 5, and the credit-block message appears to contradict the account's own usage percentage.

Environment: Windows 11 Pro, Claude Code CLI.