3524e04 added the counter to the args tuple but never added its line to the format string, leaving 15 args against 14 placeholders. Extra args are a hard compile error in 0.16, so the production build did not build at all -- and the metric the incident follow-up asks operators to watch was not emitted. The default suite could not catch it: build.zig defaults to -Dbackend=threaded and the whole block is behind `if (comptime backend_config.use_zio)`, so it is never semantically analyzed. `zig build test` stayed green while ReleaseSafe-zio, which is what production runs, could not compile. CLAUDE.md now requires `zig build -Dbackend=zio` before pushing. Verified: zig fmt --check, zig build test, zig build test -Dbackend=zio, and zig build -Dbackend=zio -Dtarget=x86_64-linux-gnu -Doptimize=ReleaseSafe. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>