diff --git a/src/modules/util.c b/src/modules/util.c
index 507ba69..f06c71a 100644
--- a/src/modules/util.c
+++ b/src/modules/util.c
@@ -181,7 +181,9 @@ static ant_value_t util_format_impl(ant_t *js, ant_value_t *args, int nargs, int
       util_sb_append_json(js, &sb, v);
     } else if (spec == 'o' || spec == 'O') {
       util_sb_append_jsval(js, &sb, v);
-    } else if (spec == 'c') // style placeholder: consume arg, emit nothing
+    } else if (spec == 'c') {
+      // style placeholder: consume arg, emit nothing.
+    }
 
     i++;
   }
diff --git a/tests/repro_obug_format.mjs b/tests/repro_obug_format.mjs
new file mode 100644
index 0000000..8250def
--- /dev/null
+++ b/tests/repro_obug_format.mjs
@@ -0,0 +1,32 @@
+import { formatWithOptions } from 'node:util';
+import process from 'node:process';
+
+function humanize(value) {
+  if (value >= 1000) return `${(value / 1000).toFixed(1)}s`;
+  return `${value}ms`;
+}
+
+function writeDebugLine(namespace, args, diff) {
+  const prefix = `  \u001B[32;1m${namespace} \u001B[0m`;
+  args[0] = prefix + args[0].split('\n').join(`\n${prefix}`);
+  args.push(`\u001B[32m+${humanize(diff)}\u001B[0m`);
+  return formatWithOptions({}, ...args);
+}
+
+const raw = formatWithOptions({}, '%s %s : %s', 'fn', '/', '/foo.png');
+const debugLine = writeDebugLine('repro:test', ['%s %s : %s', 'fn', '/', '/foo.png'], 0);
+
+console.log('raw:');
+console.log(raw);
+console.log('debug:');
+console.log(debugLine);
+
+if (raw !== 'fn / : /foo.png') {
+  throw new Error(`util.formatWithOptions mismatch: ${JSON.stringify(raw)}`);
+}
+
+if (!debugLine.includes('fn / : /foo.png')) {
+  throw new Error(`obug-style line mismatch: ${JSON.stringify(debugLine)}`);
+}
+
+process.exit(0);