BIP-347, titled “OP_CAT in Tapscript,” is listed in the official Bitcoin BIPs repository with an assigned date of December 11, 2023, authors Ethan Heilman and Armin Sabouri, status “Complete,” and type “Specification.” The proposal defines OP_CAT as a Tapscript opcode for concatenating two stack values, but its publication does not mean the change has community consensus or is close to mainnet adoption.
The specification explicitly says OP_CAT would be activated through a soft fork by redefining OP_SUCCESS126, decimal 126 and hexadecimal 0x7e, as OP_CAT. That OP_SUCCESS126 routing is part of the BIP text itself, not an external interpretation, and the document also states that OP_CAT would fail if the concatenated value exceeded Bitcoin’s 520-byte maximum script element size.
OP_SUCCESS126 and the 520-Byte Limit Anchor the Design
The 520-byte cap is presented as a key safety boundary in the proposal. BIP-347 says earlier OP_CAT concerns centered on scripts that could expand stack elements dramatically, while Tapscript’s existing 520-byte maximum script element size changes that risk profile. The proposal does not claim this removes every review concern around OP_CAT, but it frames the limit as central to making the opcode viable in Tapscript.
The document also explains why OP_SUCCESS126 was chosen. BIP-347 says OP_SUCCESS126 uses the same opcode value that OP_CAT had before being disabled, reducing confusion that could arise from assigning the restored functionality to a different opcode value.
Signet Work Is Experimental, Not Mainnet Activation
Testing around OP_CAT has been tied to Bitcoin Inquisition and signet environments rather than Bitcoin mainnet. Bitcoin Optech reported on March 13, 2024, that an Armin Sabouri PR reintroduced OP_CAT only on Bitcoin Inquisition signet and only for Tapscript, while a later Optech note said Bitcoin Inquisition 27.0 added signet enforcement of OP_CAT as specified in BIN24-1 and BIP-347.
Additional experimentation has appeared in separate developer efforts. The Bitcoin-Wildlife-Sanctuary Catnet repository describes Catnet as a custom Bitcoin signet with OP_CAT enabled, used to test a Bitcoin Circle STARK Verifier, and says it is built on Bitcoin Inquisition v27.0. Bitcoin Optech’s recap of a STARK verification proof of concept also described CAT-based covenants on signet splitting a STARK verification script across 10 Bitcoin transactions.
StarkWare’s November 28, 2024 post, written as a guest post by Mihael Šinkec, separately described a proof-of-concept bridge covenant built by sCrypt on OP_CAT-enabled Bitcoin using the sCrypt embedded DSL. That work should be attributed to StarkWare’s publication and sCrypt’s implementation, not treated as formal validation of BIP-347 for production deployment.
The activation path remains unresolved. BIP-347 defines proposed behavior, but it does not provide mainnet activation parameters, miner signaling thresholds, deployment timelines or an activation client. Under the BIPs repository’s own guidance, publication means a proposal has met formal repository criteria, not that it has consensus or is about to be adopted.