Whatever I used Sonnet 4.6 for, including Claude Code and Claude Chat, it made so many mistakes and totally awkward assumptions that I can’t fathom what it’s supposed to be good at. The mistakes were so blatant. Plan mode, several passes, couple grand in API costs… just disappointing at every task in every session over the past few weeks. Opus 4.6 has been good, still quite a few unexpected, silly mistakes, a few subtle but critical mistakes, but produced workable increments and code reviews, vastly subpar to GPT-5.x in chat mode (with and without identical customization).