Most engineers assume refresh-token failures mean 'token expired, need to refresh.' But Google will delete the entire OAuth client after roughly 30 days of inactivity, which simultaneously revokes every refresh token tied to it. From the calling code's perspective this looks identical to a routine expiry — a refresh error — but no refresh will ever work again. You need a brand-new client (new client ID + secret) plus a fresh consent flow, even if the old tokens look valid.
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