Fast mode now has a three-state model: "on", "off", and "cooldown" (after a rate limit). Network errors show a temporary unavailability message rather than fully disabling fast mode. The fast_mode_state field is now included in API response schemas.
Evidence
Fast mode state function (search for "Fast mode is temporarily unavailable due to a network error")
Strings lifted out of the shipped bundle, so the claim above can be checked against them.