Grant De Swardt, an AI consultant from the UK, discovered abnormal token consumption on his Anthropic account with a Claude Max 20x subscription. Despite disabling all connected services and stopping use of the system, token usage continued to rise, increasing from 45% to 55% during a specific period.
Anthropic conducted an investigation and found that a compromised session key was used to create unauthorized OAuth tokens for Claude Code. The company suggested the account was likely used by an unauthorized third-party service to perform tasks for other users, though the exact method of access was not determined.
Anthropic partially refunded £44.49 ($60.51) of the $200 monthly subscription fee. The company suspended the subscription, canceled all sessions and tokens, and warned some users about potential malware infections from unofficial software or malicious ads, though De Swardt did not receive such a warning.
Other users on Reddit reported similar issues: one user's token consumption grew from 0% to 100% and their plan was automatically upgraded without consent, while another saw consumption jump from 0% to 49% in 12 minutes. A third user's token limit was exhausted for three consecutive days without any activity.
De Swardt has since switched to Cursor, stating that other models, including open-source alternatives, perform as well as Claude. He noted that Anthropic lacks tools for users to track detailed token expenditure, making it difficult to protect against such attacks.