diff --git a/src/main.zig b/src/main.zig
index 4567b26..074a301 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -59,6 +59,11 @@ pub const default_stack_size = 8 * 1024 * 1024;
 // ReleaseSafe GPF from fiber context-switch codegen bug, persistent ~10-15% coverage
 // degradation, zig team marks Evented as experimental. See docs/evented-attempt.md.
 // Can revisit when zig's Evented runtime matures.
+/// diagnostic build: zio prints loop-internal stats to stderr every 5s.
+/// the bug under investigation breaks the HTTP metrics path itself, so
+/// observation cannot depend on it.
+pub const zio_debug_loop_stats = true;
+
 const backend_config = @import("backend_config");
 const zio = @import("zio");