Hit a clean example of extended-thinking degeneration; the mechanics seemed worth discussing.

Setup: asked a reasoning model (Opus 4.8) whether truncating embedding vectors is the same as SVD. Its thinking fell into a verbatim repetition loop and couldn't exit until (presumably) a budget/watchdog cut it off - after which it produced a correct answer and handled a follow-up normally.

What stood out:

1. **Decoding failure, not knowledge failure.** The post-loop answer was correct. The model knew the material; the sampler was stuck.
2. **The trigger was a self-correction.** It noticed the loop and emitted "I'm repeating myself, let me be brief" - and that meta-comment got absorbed into the cycle, forming a 2-stroke limit cycle: \[content\] → \[I'm repeating\] → \[content\]. The self-monitoring text has no causal handle on decoding, so naming the loop doesn't break it.
3. **Precursor.** Before the verbatim loop it was already circling semantically (re-deriving the same point, grinding on diagram coordinates) - looks like the prodrome of the same attractor.
4. **A coupled summarizer** (the short thinking-summary line) also degenerated into English mid-stream ("could you provide the next chunk, I'll rewrite into 1-3 sentences") - consistent with a separate summarization model choking on degenerate input. (Inference.)

Open questions: how much is induction-head copying vs. general likelihood self-reinforcement (can't tell from a transcript)? Why are thinking channels more loop-prone than answer channels - weaker repetition penalties, longer budgets, both? Any clean defense for long reasoning, where legitimate repetition (recompute, rephrase) makes naive n-gram penalties lossy?

Screenshot of the loop attached. Curious if others have repro'd similar in long-reasoning modes.

Anatomy of a repetition loop in a reasoning model's extended thinking - the self-correction became part of the loop