Both the spec and the plan asserted that Arc<Mutex<RotatingLogWriter>> satisfies tracing-subscriber's MakeWriter via impl MakeWriter for Arc<W>. That impl requires &'a W: io::Write, and &Mutex<W> does not implement io::Write. The error reached the implementer and cost a fix round before being caught; the shipped code uses a bare Mutex. Corrections are marked inline so the mistake stays visible rather than being silently erased. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01EGntTHCW3sEPy1VBRopNNp