In a small saucepan, combine water, vinegar, sugar, garlic, chili flakes, and salt over medium heat.
Stir and bring to a gentle simmer until the sugar fully dissolves.
Whisk cornstarch with cold water in a separate bowl to make a smooth slurry.
Slowly pour the slurry into the saucepan while stirring constantly.
Simmer for 1–2 minutes until the sauce thickens, then remove from heat.
Let it cool completely, then transfer it to a clean glass jar. Store in the fridge for up to 3 weeks.