CoreThesis returns code quota_exceeded when any of three gates fire: your trial has ended, you've hit your monthly cap, or a single high-volume tool has hit a per-day sub-limit on the trial tier.
Check /billing — if it says "Trial ended" or a red pill, that's gate #1. Switch on pay-as-you-go to keep going.
If you're on a paid plan and seeing it mid-month, check /usage. Your cap rolls over at the start of the next calendar month, or you can keep going on pay-as-you-go (each extra call billed individually).
If you just upgraded and still see the error for 30 seconds, that's the usage cache warming up — the limiter re-reads your plan cap within a minute.