Second on-device failure in one session: after the wordlist fix, dev3
blocked cloud turns AGAIN mid-session as 13-10's history grew — splitting
on every non-alphabetic character let words from unrelated JSON fields
chain into one run. Both failures took the whole feature down rather than
protecting anything, which is the worse failure for a screen to have.
Shape is the wrong signal. A real mnemonic's last word encodes a checksum
over the rest, so an accidental run of English words parses as a mnemonic
only about one time in sixteen. Candidate runs are now validated with the
same bip39 crate the wallet uses:
- tokenize on whitespace (a seed phrase is space-separated); a token's
leading alphabetic segment counts, and alphanumerics after it end the
phrase, so a seed glued to a closing quote is still caught
- block only if a 12/15/18/21/24 window parses as a real mnemonic
- IMPLAUSIBLE_MEMBER_RUN (20) backstops checksum-invalid material such as
a typo'd 24-word seed, which prose cannot plausibly produce
Documented trade-off: a checksum-invalid run under 20 words no longer
blocks. The rule that did block it also blocked every legitimate turn,
twice, on a live node. 15/15 egress tests green, including the real
system prompt, scattered-JSON prose, and a genuine mnemonic in JSON.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
