Yes
Right away Fable found several longstanding bugs in our compiler that we hadn't found before
UncleOxidant on Hacker News
In the thread “Claude Fable is relentlessly proactive”, replying to this comment
Helped: Identified longstanding bugs and critical design flaws.
I unleashed it on a compiler codebase that I've been developing for several months now using Claude Sonnet 4.5/6, Gemini 3.1 Pro, DeepSeek V4 Pro(recent), and a bit of Qwen3.6-27B. Right away Fable found several longstanding bugs in our compiler that we hadn't found before. It found that there was a critical part of our design that needed to be mostly redesigned/rewritten and gave a very well-reasoned rationale for doing so.