My theory is Claude Sonnet 4.6.
> Claude Sonnet 4.6 was released Feb 17, 2026
> https://www.anthropic.com/news/claude-sonnet-4-6
For me, I switched from Sonnet 4.5 to Sonnet 4.6 as soon as it was available.
Within 24-48 hours, I noticed the quality of bug analysis and PRs generated by Copilot w/ Sonnet 4.6 went way up.
More specifically, I had a sense from using Sonnet 4.5 of how much AI slop I needed to review and clean up after each request and chat session. That amount of AI slop dropped dramatically for me after switching to Sonnet 4.6. It was noticeable right away.