In a small bowl, stir together the cumin, coriander, cinnamon and 1 teaspoon each salt and pepper. In a large bowl, toss the chicken with 4 teaspoons of the spice mix; set aside. Reserve the remaining spice mix for cooking the tagine.
In a large Dutch oven over medium-high, heat the oil until shimmering. Add the onion and ½ teaspoon salt, then cook, stirring occasionally, until beginning to brown, 4 to 6 minutes.
Stir in the remaining spice mix, the carrots, chicken and 2½ cups water. Bring to a simmer, then reduce to medium-low and cook, uncovered and stirring occasionally, until a skewer inserted into the largest thigh meets no resistance, about 30 minutes.
Stir in the chickpeas and raisins. Bring to a simmer over medium-high, then reduce to medium and simmer, uncovered and stirring occasionally, until the sauce is thickened, about 15 minutes. Off heat, taste and season with salt and pepper.
