Pour the olive oil into a Dutch oven or heavy stockpot over medium-high heat. When the shimmers, stir in the carrots, celery, onion, and garlic and cook, stirring occasionally, until the onion is translucent, about 10 minutes.
Stir in the curry powder and cook, stirring until fragrant, 1 to 2 minutes.
Add the lentils, 6 cups water, and salt. Add more water if needed to cover the lentils by 1 inch. Bring to a boil, then reduce the heat to low, cover, and cook until the lentils have softened but are not completely tender, about 10 minutes.
Stir in the lime zest, lime juice, and agave and continue cooking, uncovered, until the lentils are soft and slightly creamy but still hold their shape, about 5 minutes.
Remove from the heat and stir in the cilantro, harissa, and pepper. Taste and add more salt, harissa, and pepper if desired.
Serve hot.