Bring the agave, water, cilantro and jalapeño to a low simmer in a small saucepan.
Let it simmer for about five minutes, until agave is fully heated and the jalapeño, cilantro, flavors are infused into the liquid.
Place the syrup into a small airtight jar and store in the refrigerator for up to a month.
I like to use 1 teaspoon for 1 margarita.