OP_CAT is a proposed Tapscript opcode that concatenates two byte strings when the resulting element does not exceed the 520-byte limit; it is not active on mainnet.
The specification redefines OP_SUCCESS126 in Tapscript. The operation removes the top two stack elements, concatenates them in stack order and pushes the result back onto the stack. [1]
The original OP_CAT was disabled in early Bitcoin together with other opcodes. The new proposal is limited to Tapscript and keeps the existing 520-byte limit for a single stack element. [2]
Concatenation lets a script construct and hash structured data. Proponents derive uses such as covenants, transaction trees, vaults and proofs about properties of a future transaction. [1]
The opcode by itself specifies neither one application nor a general-purpose virtual machine. The security of a particular protocol depends on how it combines OP_CAT with hashes, signatures and spending rules. [1]
The BIP has Complete status, which describes the maturity of the document rather than consensus activation. Mainnet nodes currently treat OP_SUCCESS126 as successful script termination under Tapscript rules. [3]
Activation would be a soft fork: the new rules would narrow the set of previously valid Tapscript spends. It would require a separate deployment process and broad technical review. [1]
OP_CAT is related to CTV, covenants and Simplicity, but is not synonymous with them. Each proposal offers different expressiveness, analysis costs and room for unintended constructions. [1]
Verification must separate the existing specification and experimental implementations from the actual mainnet rules. A wallet must not assume OP_CAT is available without activation. [4]
For the clearest picture, read this entry together with Bitcoin Script, Taproot, Covenants, OP_CHECKTEMPLATEVERIFY, Soft Fork, BIP (Bitcoin Improvement Proposal).
01Is OP_CAT active on Bitcoin mainnet today?+
No. BIP 347 is a completed proposal specification, but the change has not been activated as a mainnet consensus rule.
02Does OP_CAT mean general-purpose smart contracts?+
No. It adds one data-concatenation operation; capabilities and risks arise from the specific scripts and protocols that use it.