Make the fajita seasoning: in a small bowl combine 2 teaspoons chili powder, 1 teaspoon ground cumin, 1 teaspoon smoked paprika, 1 teaspoon kosher salt, 1/2 teaspoon garlic powder, 1/2 teaspoon onion powder, and 1/8 teaspoon cayenne (optional).
Trim fat from the chicken and slice breasts into 1/2-inch-thick lengthwise strips; if using tenders, leave whole or slice as desired.
Toss the chicken with half of the fajita seasoning to coat evenly.
Pour the specified amount of chicken stock into the Instant Pot (1/2 cup for 3–6 qt models or 3/4 cup for 8-qt).
Add the seasoned chicken to the inner pot, then top with the sliced onion, sliced bell peppers, and minced garlic. Sprinkle the remaining fajita seasoning over the vegetables.
Seal the Instant Pot, set to High Pressure using the pressure cook/manual button, and cook for 4 minutes; after cooking, allow a natural pressure release for 10 minutes, then carefully quick-release any remaining pressure.
Use a slotted spoon to transfer the chicken, onions, and peppers to a serving bowl. Add the zest and juice of 2 medium limes and toss to combine.
Serve the fajita mixture on warmed tortillas or over rice with desired toppings.