tl;dr: Starting June 9, 2026, Anthropic will require 30-day retention of prompts and outputs for its Mythos-class models (including Claude Mythos 5 and Fable 5), overriding zero-data-retention (ZDR) settings for API, Claude Code, and cloud marketplace customers. Anthropic says the retention is needed to detect multi-request attack patterns like best-of-N jailbreaks and state-sponsored misuse that single-prompt safeguards miss. Consumer plans are unaffected since they already retain data, and access to retained data is restricted to a small set of reviewers with tamper-proof audit logs.