[Bug] Claude 3.5 Opus with effort "high" produces hallucinated code and incorrect claims
Status Open
Reported on v2.1.220
Maintainer reply None cached
Activity 0 comments · opened Jul 28, 2026
Bug Description
I entered into Opus 5 with high expectations. Asked if effort "high" claude-code confirmed. I struggled with hallucinated claims that later was pushed back. Today I read on Reddit the best performance is by using effort "medium". I can now confirm that the overly verbose, hallucnating Opus 5 is now back to normal. Opus 5 on effort "high" is unusable and should never be let loose on a tightly controlled setup with TDD, SCC and an appropriate skills set. Anthropic, you disappoint me.
Environment Info
- Platform: linux
- Terminal: xterm-256color
- Version: 2.1.220
- Feedback ID: 06a94ef6-9644-4bda-87b6-346ac336f351
Errors
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