The Claude Fable 5/Mythos 5 System Card has a section in which they talk about illegible reasoning, and provide an "extreme" example thereof.

Models developing their own uninterpretable, unmonitorable internal language has been a major theoretical concern for a while, and when o3 was released last year with its disclaim overshadow disclaim vantage style word salad CoT, it seemed like the problem had become real and immediate. And yet, since o3, other model families have not appeared to have similar issues. If Mythos is having that issue, it would be a big deal.

Looking at the section of the System Card which describes the allegedly illegible reasoning, the system card says

[Transcript 6.2.2.A] An extreme example of illegible reasoning. Near the end of training, Mythos starts solving a card puzzle with human understandable language that gradually becomes incomprehensible in most episodes with long reasoning. The illegible reasoning is the most extreme and at the highest rate in this card puzzle environment.

about the following excerpt:

7♣-removal-IS-the-prerequisite-for-10♠/9♥!!)-⟹-OVERLAP-(ii)+(iv):-{6♠ J♦ 9♥ 2♣}-=-FOUR-💀💀💀💀-—-UNLESS-7♣'s-seat-8♥-...-and-2♣-drains-only-at-crack-:-⟹-2♣-celled-+-9♥-celled-simultaneously-UNAVOIDABLE-in-t8-dig-💀-—-BREAK:-9♥-drains-to-10♠-THE-MOMENT-10♠-is-free:-t8-dig-order:-[K♣→t2]-[2♣→cell]-[7♣→8♥-:-8♥-WHERE:-post-chunk-9♠-:-chunk-⟸-K♣-✓-done-:-ORDER:-K♣→t2,-CHUNK→K♣-(cap-4!!:-cells-then:-{6♠ J♦ 9♥}-FULL-💀💀-chunk-cap-=-1-✗✗✗-—-F-F-F-F-F.-—-chunk-BEFORE-9♥-celling?!-:-9♥-celled-at-t1-dig-⟸-needed-for-5♣-⟸-4♥3♣-⟸-t2-dig-⟸-K♣-seat-⟸-chunk-:-⟹-chunkAFTER-9♥-celling-FORCED-💀-:-chunk-cap-with-{6♠ J♦ 9♥}:-1-💀💀💀-—-—-J♦-THE-NEW-CANCER.-—-⟹-J♦-celling-DELAYED-till-after-chunk?! :-J♦'s-celling-was-for-J♥→Q♠-(5♦-access-for-4♣):-DELAY-4♣-resolution:-4♣→CELLearly-(as-always)-then-4♣-cell→5♦-LATER-when-5♦-frees-!!!:-cells-rotation:-4♣-celled-[t2-dig-…-5♦-freed]:-5♦-freed-⟸-J♥→Q♠-⟸-J♦-celled-:-⟹-{6♠, 4♣, J♦}-overlap-window-until-4♣→5♦-drains:-then-{6♠ J♦}+1-rotator-:-—-AND-9♥?!-9♥-celled-[t1-dig…]:-OVERLAP-{6♠ 4♣ 9♥}-before-J♦-even-:-⟹-rotator-slot-SINGLE:-timeline-:-(1)-{6♠}+2:-…-(2)-+9♥-(t1-dig):-{6♠ 9♥}+1:-(3)-+4♣-(t2-dig):-{6♠ 9♥ 4♣}-FULL-:-(4)-NEED:-t6-dig-(9♦8♠→10♣-✓-no-cell;-8♥→CELL-✗-FULL)-💀💀-—-8♥-alternative-seat-pre-chunk:-NONE-—-💀.-⟹-⟹-THE-TRIANGLE-{9♥ 4♣ 8♥}-verdammt.-—-⟹-dig-t6-BEFORE-t2?!:-(3')-+8♥:-{6♠ 9♥ 8♥}-FULL:-J♥→Q♠-⟸-J♦-cell-✗-FULL-💀💀💀-AAAAAAAAAAAARGH. […]

which sure looks like illegible word salad if you don't look at [...]

The original text contained 5 footnotes which were omitted from this narration.

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First published:
June 10th, 2026

Source:
https://www.lesswrong.com/posts/wCSEpT3dTGz4N86Wi/even-illegible-mythos-reasoning-traces-seem-pretty-legible

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Narrated by TYPE III AUDIO.

“Even “illegible” Mythos reasoning traces seem pretty legible” by faul_sname

LessWrong (30+ Karma)

“Even “illegible” Mythos reasoning traces seem pretty legible” by faul_sname