diff --git a/internal/observability/duration_test.go b/internal/observability/duration_test.go
new file mode 100644
index 00000000..83163f6f
--- /dev/null
+++ b/internal/observability/duration_test.go
@@ -0,0 +1,23 @@
+package observability
+
+import (
+	"testing"
+)
+
+func TestFormatDurationMillisFormatsFloat(t *testing.T) {
+	if got := formatDurationMillis(123.456); got != "123.456ms" {
+		t.Fatalf("got %q, want %q", got, "123.456ms")
+	}
+}
+
+func TestFormatDurationMillisFormatsInt(t *testing.T) {
+	if got := formatDurationMillis(42); got != "42ms" {
+		t.Fatalf("got %q, want %q", got, "42ms")
+	}
+}
+
+func TestFormatDurationMillisReturnsStringAsIs(t *testing.T) {
+	if got := formatDurationMillis("fast"); got != "fast" {
+		t.Fatalf("got %q, want %q", got, "fast")
+	}
+}