diff --git a/op/op.go b/op/op.go
index 09e2a86..9486f49 100644
--- a/op/op.go
+++ b/op/op.go
@@ -166,12 +166,19 @@ func Assert(cond bool, msgs ...any) {
 	if !cond {
 		msg := "assertion failed"
 		if len(msgs) > 0 {
-			msg += ": " + fmt.Sprint(msgs)
+			msg = fmt.Sprint(msgs)
 		}
 		panic(msg)
 	}
 }
 
+// Assertf panics with a formatted message if cond is false.
+func Assertf(cond bool, format string, args ...any) {
+	if !cond {
+		panic(fmt.Sprintf(format, args...))
+	}
+}
+
 // ReverseCompare returns a comparison function that reverses the order of the original comparison function.
 func ReverseCompare[T any](cmp func(T, T) int) func(T, T) int {
 	return func(x, y T) int {