Bring a pot of lightly salted water to a boil, add couscous, and cook until tender, about 14 minutes. Remove from heat.
Reserve 1/4 cup cooking water, drain couscous thoroughly, and return to the pot.
Meanwhile, heat a nonstick pan over medium-high heat until hot. Add pine nuts and toast, stirring frequently, until golden brown and fragrant, 1 to 3 minutes. Remove pine nuts from skillet and set aside.
Melt butter in the same skillet. Add zucchini; sauté for 3 to 4 minutes. Add garlic and capers. Cook on medium-high, stirring constantly, until fragrant, 30 seconds to 1 minute. Stir in diced tomatoes; season with salt and pepper. Cook, stirring occasionally, until slightly reduced in volume, 3 to 5 minutes.
Stir in couscous and toasted pine nuts. If the sauce seems dry, gradually add reserved cooking water, 1 tablespoon at a time, until sauce is desired consistency. Serve immediately.
