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OP_CAT
BIP 347, CAT
A proposed Bitcoin opcode for tapscript that concatenates two stack values into one value.
OP_CAT is a proposed tapscript opcode described in BIP 347. It takes two values from the stack, joins them together in order, and pushes the combined value back onto the stack.
That small operation matters because Bitcoin script has many ways to hash, compare, and check signatures, but no active opcode for building larger values from smaller stack elements. Reintroducing OP_CAT in tapscript would make it easier to express some advanced contract patterns, including Merkle-style commitments, tree signatures, and covenant-like constructions.
OP_CAT existed in early Bitcoin versions and was disabled in 2010 along with several other opcodes. The modern proposal is narrower: it would only apply in tapscript and would keep tapscript's existing stack element size limit, so it is discussed as a soft-fork change rather than a return to the original scripting environment.