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An authenticated device could store a token of up to the 1 MiB body limit, sent to FCM in every push for it. Now (proposed D-114) more than 512 characters after trimming is 400 invalid_request with field fcm_token, like the empty string (D-034); E-17's only documented error stays the only one. Test at 512 and 513. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B