In Chile, can I send a formal demand letter before suing my landlord for withheld deposit?

You must send a formal and verifiable letter demanding the return of your security deposit, specifying: the lease termination date, the return of the keys, the amount withheld, that the "repairs" are not properly documented, and a short and specific deadline for payment or justification with receipts/invoices. In urban rentals, if there are no proven damages or debts, the landlord must return the deposit; if they intend to deduct it, they must provide justification. If you do not receive a response, you should file a civil lawsuit.