Combine sugar, vinegar, water, garlic, chili flakes, and salt in a small saucepan over medium heat. Stir until the sugar dissolves, about 2 minutes.

Stir the cornstarch slurry, then whisk it into the pan. Bring to a gentle simmer and cook until the sauce turns glossy and thickens enough to coat a spoon, 2 to 3 minutes.

Remove from heat and let cool slightly. Pour into a jar and serve at room temperature, or refrigerate for later.