Thinly slice the cucumbers, carrots, and red onion. Place everything in a large mixing bowl.
In a small bowl or jar, whisk together the sesame oil, rice vinegar, garlic, agave syrup, soy sauce, sesame seeds, and chili flakes until well combined.
Pour the dressing over the vegetables and gently toss until evenly coated.
Let the salad rest for 10–15 minutes so the flavors can fully absorb.