[BUG] Usage credits consumed while plan allowance remains; enabling extra usage prevents the 5-hour window from starting (re-report of stale-closed #64949)

Bug Description

Claude Code is consuming usage credits (extra usage) while my plan's included allowance is largely untouched. Yesterday a session drained account credits even though the 5-hour plan window showed ~90% capacity remaining.

Worse: while extra usage is enabled, the 5-hour plan window does not appear to start at all — the session bills straight to credits from the first request, so plan capacity is never drawn down. Disabling extra usage restores normal plan metering.

The session model was Opus 4.8, not Fable 5, so this is not the (documented) Fable-5-bills-to-credits behavior — unless some component (subagent, background review, or silent routing) is running Fable 5 and billing its portion to credits without surfacing that anywhere. If that's the mechanism, it needs to be visible and opt-in.

Expected behavior

Included plan allowance is consumed first; usage credits are touched only after the plan limit is exhausted (this is how the extra-usage feature is described, and how claude.ai chat behaves).

Actual behavior

• Credits consumed while /usage shows the 5-hour window ~90% available.
• With extra usage on, the 5-hour window never starts / never accrues usage.

Environment Info

• Platform: darwin (macOS 26.5.2)
• Claude Code version: 2.1.218
• Plan: Max 20x
• Session model: Opus 4.8

Prior / related reports

• #64949 — same bug on Max20 (June 3), closed stale by the bot without triage. This is a re-report of that behavior, still occurring on 2.1.218.
• #64445 — 1M-context credits consumed without selecting 1M mode (possible related mechanism).
• The July 21–24 cluster of Fable-5/credit-gating issues (#80749, #80737, #80706, #80409, #80334) suggests the plan-vs-credits billing classification is actively misbehaving in recent releases; this looks like the same classification bug in the opposite direction (plan-eligible usage classified as credit-billed).
• Independent same-day reports from other users on r/ClaudeCode (Pro plan, same symptom): https://www.reddit.com/r/ClaudeCode/ "Bug: Anthropic ignores remaining plan usage capacity and consumes account credits first" (2026-07-24).

Repro

1. Max 20x (or Pro) plan with extra usage / usage credits enabled and a credit balance.
2. Start a Claude Code session on Opus 4.8 with most of the 5-hour window available.
3. Work normally; observe credit balance decreasing while /usage plan-window percentage stays flat.