Consider a race of humanoid, winged tree-dwellers. They can fly and generally spend most of their time either in the air or climbing around their tree-homes in the upper reaches of a forest. In order to climb, land, and perch effectively, they need claws capable of sinking easily into hardened tree bark and occasionally even stone. At the same time, in order to create and manipulate tools, they need humanlike hands with nimble fingers and opposable thumbs. They must be able to extend or retract their claws quickly, including in response to being startled, or as a weapon in a fight.
Given all this, what is the most likely digit structure for retractable claws in humanlike hands? Sitting at, and retracting into, the tips of the fingers? Or situated between the fingers and retracting into the palm?
What problems would a race with this kind of retractable claws have? Could a claw get "stuck"? How would the muscles support the weight in both configurations? How would the curvature of the claws affect the shape of the hand - the fingers for fingertip claws, or the palm for between-finger claws? What happens if a claw is broken? Are there any benefits to one claw structure over the other?