Load the crockpot: Add chicken, marinara, cream of chicken soup, diced tomatoes, chicken broth, onion, garlic, Italian seasoning, smoked paprika, red pepper flakes, salt, and pepper. Stir to coat the chicken evenly.
Cook the chicken: Cover and cook on Low for 4–5 hours or High for 2.5–3.5 hours, until the chicken shreds easily with two forks.
Shred and enrich: Remove the chicken to a plate and shred. Add cream cheese cubes to the slow cooker and whisk until mostly smooth. Return shredded chicken and stir. Taste and adjust salt and pepper.
Cook the pasta: Boil spaghetti in salted water until just shy of al dente (1–2 minutes under package time). Reserve 1/2 cup pasta water, then drain.
Finish in the crockpot: Add cooked spaghetti to the creamy sauce along with cheddar and butter. Toss until the cheese melts and the pasta drinks up the sauce. Use splashes of pasta water to loosen as needed.
Serve: Top with parsley and Parmesan. Plate it up while hot and extra creamy.
Don’t overcook the chicken: Shred as soon as it’s tender; this keeps it juicy.
Warm your cream cheese: Room-temp cream cheese blends faster and smoother.
Salt your pasta water: It seasons the spaghetti from the inside out.
Hold back some sauce: If you prefer extra saucy bowls, reserve a cup before adding pasta, then spoon it on top when serving.
