Bring a large pot of generously salted water to a boil. Add spaghetti and cook until al dente according to package directions. Reserve 1 cup of pasta water before draining.

While pasta cooks, heat olive oil in a large skillet over medium heat. Add garlic and anchovies, cooking until garlic is golden and anchovies dissolve, about 2 minutes. Stir in red pepper flakes for 30 seconds.

Pour in crushed tomatoes, olives, and capers. Bring to a simmer and cook for 10–15 minutes, stirring occasionally, until the sauce thickens slightly. Season with a pinch of salt (careful, toppings are salty) and black pepper.

Add the drained spaghetti to the sauce and toss vigorously with tongs, adding a few splashes of pasta water as needed to create a glossy coat. Remove from heat and stir in most of the parsley.

Transfer to a warm serving bowl, scatter remaining parsley, and drizzle with olive oil. Serve immediately, with extra red pepper flakes on the side.