Opus 4.8 output degenerates into unbounded single-token repetition ("court" loop), burning usage credits
Environment
- Claude Code version: 2.1.209 (Windows 11 Home 10.0.26200, desktop/CLI)
- Model: claude-opus-4-8 (switched from claude-fable-5 mid-session)
- Session ID: a9d99ed7-a5e6-45b7-a73d-6ee104aa2c70
Bug description
After switching the session model to Opus 4.8, assistant output repeatedly degenerated into an unbounded repetition of the single token "court" — thousands of consecutive repetitions with no other content, continuing until manually interrupted. This happened multiple times in the same session.
The same session running on Fable 5 (before and after the switch) never exhibited this.
Context / likely trigger
The session is a long-running orchestration workflow with an extremely repetitive turn structure: hundreds of near-identical cycles of (capture tmux pane output → request command approval → short status report). The degeneration started after this repetitive history had accumulated and persisted across turns; degenerate output already present in the history appeared to make recurrence more likely. /compact cleared the contaminated history.
Separately, in the same degraded state the model also failed to execute shell-helper commands that work reliably on Fable 5, suggesting general output-quality collapse rather than a one-off sampling glitch.
Impact
- Thousands of output tokens of pure single-token repetition were generated and billed as usage credits with zero utility.
- The work session was blocked (the loop had to be interrupted manually, repeatedly).
Expected behavior
- The runtime should detect and halt pathological single-token repetition loops (e.g. an n-gram/entropy guard on streaming output) instead of letting them run until user interruption.
- Usage caused by such degenerate output should not be billed; I have contacted support separately regarding the credit consumption.
Notes
Happy to provide the full transcript to Anthropic privately if useful — the session ID above should locate it.