OP_CAT Proposal to Bring Smart Contracts to Bitcoin Finally Gets a 'BIP Number'

A serious attempt to bring Ethereum-like smart contract functionality to Bitcoin called OP_CAT has finally been granted a “BIP number:” 347. This is the first step towards actually launching the long-proposed software upgrade.

“Getting a BIP number does not signal any sort of consensus on the part of the community," Ethan Heilman, one of the co-authors of the proposal alongside Armin Sabouri, said in an interview. "It just makes discussing and writing software around the proposal easier because the proposal now has a unique numeric identifier that everyone agrees on.”

In other words, getting assigned BIP 347 means the argument over the controversial proposal can finally begin in earnest.

On one side are those who want to reserve the Bitcoin network simply for monetary transactions; on the other are those who want to build new things on-chain, of which proponents of OP_CAT are just a sliver.

OP_CAT has a long history in Bitcoin circles. Initially included as one of the first op_codes (essentially programming shortcuts built into Bitcoin), Satoshi Nakamoto himself removed the functionality in 2010 after concerns were raised about excessive memory usage and the possibility of introducing vulnerabilities.

But in recent years, especially following the release of the Ordinals protocol that reinvigorated developers' desire to build on-chain, proponents have returned to OP_CAT as a possible way to increase the amount of things that can be built using Bitcoin. Other proposals include things like Bitcoin developer Jeremy Rubin’s CTV and feature-rich scaling solutions like Stacks and Ark.

See also: Bitcoin L2 Stacks Is Getting a Major Overhaul

Heilman and Sabouri began studying reintroducing OP_CAT in 2022, and first proposed launching it a year later on the Bitcoin Mailing List via a backward-compatible soft fork. The idea would be to redefine and expand upon an existing code called “OP_SUCCESS126,” without having to hard fork the chain.

If the proposal goes through, OP_CAT covenants could enable the creation of more sophisticated applications and multi-signature setups on Bitcoin. It works by introducing “covenants,” or rules that can be established to determine how a specific transaction will function, to Bitcoin.

“Bitcoin allows users to set rules on who and how their bitcoins can be spent. All CAT does is that it joins two values together. So if you have ‘abc’ and ‘def,’ CAT will join these two values together to make ‘abcdef,’” Heilman said, adding that such a basic maneuver isn’t possible today. “The CAT is just shorthand for conCATenate.”

“After the community is confident the software works as designed, we will put together a PR into bitcoin-core. This is where the real fun begins because the question changes from ‘is the software correct?’ to ‘does the Bitcoin community want OP_CAT?,’” Heilman said. “This could be a quick process or it could take years.”

Among the biggest proponents of OP_CAT have been the co-founders of popular Ordinals project Taproot Wizards, Eric Wall and Udi Wertheimer, who created the Quantum Cats inscriptions project as a sort of marketing campaign for Heilman and Sabouri’s proposal.

While Quantum Cats is one of the most popular inscription projects to date, OP_CAT itself is far from universally accepted. There is some speculation, for instance, that despite Heilman and Sabouri submitting their BIP proposal several months ago, it was being held off from approval by lone BIP editor and Bitcoin Core dev Luke Dashjr, who is not alone in his skepticism of recent on-chain experimentation.

On Monday, the Bitcoin community named five additional BIP editors. According to GitHub, OP_CAT's BIP number was assigned by an editor who goes by "Roasbeef."

See also: See also: Will Bitcoin's New BIP Editors Streamline Development?

Heilman said that now that OP_CAT has a BIP number, it’s up to the community to determine whether it should move forward.

“Speaking only for myself, at this point I plan to remove myself from the process and let the community debate if OP_CAT is something they want or not want," he said. "I don't plan to enter that debate except if needed to clarify technical questions.”