A new overageInUse boolean is parsed from the anthropic-ratelimit-unified-overage-in-use: true response header. When this field is set on the rate limit state, registered overage listeners are notified. This enables the UI to react immediately when Fable 5 usage transitions into the credit-based overage regime.
Evidence
Header parsing and listener notification (search for "anthropic-ratelimit-unified-overage-in-use")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
