Is AI code rotting open source? 450 repos, 12 months

An analysis of 450 top GitHub repositories found that the average repo had 11.5% of its merged pull requests AI-signed in September 2026, up from 2.3% in October 2025, though the median repo was at just 1.5% and 264 of the 450 repos have committed agent instruction files, making every AI figure a floor. Among 100 TypeScript and C# repos, complexity per 1,000 lines rose a median 4.8% in AI-heavy repos versus 3.0% in others, with the increase concentrated in fast-growing codebases while mature Microsoft projects held flat. AI PRs were typically 2.4× larger than other PRs and fixed within 30 days only slightly more often.