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Your Google API integration worked fine. Then, a few days later, every request started failing with: invalid_grant: "Token has been expired or revoked." You found the Stack Overflow thread, and the usual answer was “your app is in Testing mode.” Often that’s right. But what if you already switched to Production? What if your token came from the OAuth Playground? What if only one user is affected? This guide picks up where the forum answers stop. It covers every refresh-token expiry cause in Google’s own documentation, shows you how to tell them apart, and walks you to a fix that lasts.