BitVM creator argues that Bitcoin upgrade OP_CAT is not all it’s cracked up to be

The Bitcoin developer community has spent months singing the praises of OP_CAT — a soft fork change that many believe is the next evolution of Bitcoin.

BitVM's creator, Robin Linus, has some disappointment, however, as he published an article on Thursday decrying common misconceptions about the widely heralded change.

Is OP_CAT the future of Bitcoin?

The developer's post – titled “Preventing a CATastrophe” – argued that OP_CAT could introduce risks to Bitcoin, which supporters tend to underestimate. “My goal is not to argue for or against op_cat, but to dispel these misconceptions and steer the discussion toward a more fact-based, rational analysis,” he wrote.

One includes the promotion of Bitcoin miners and the introduction of new programs that will significantly change the security of the entire network.

“The design space for op_cat-based DeFi applications on Bitcoin is largely unexplored and we cannot foresee all the innovations that may emerge and how they will affect MEV (Maximum Extractable Value),” Linus said.

OP_CAT is an old Bitcoin opcode that first creates – or “concatenates” – two pieces of data. It was removed by Satoshi Nakamoto in 2010 due to security concerns, but was extended to BIP 347 in 2023 to handle complex data structures such as Merkle trees and greatly enhance its functionality.

On the other hand, many see Bitcoin's simple code and resistance to change as a sign of safety and purity. As such, OP_CAT proponents point out how the change involves only 10 new lines of code and simply replaces old technology – thereby slowing down the attacker.

“Just because they don't use these 10 lines right away doesn't mean op_cat doesn't interfere with the wider system and incentive structure,” Linus countered. “Because it is impossible to predict the consequences of activating op_cat, we cannot confidently guarantee its security.”

Bets on Bitcoin?

One of the most anticipated benefits of OP_CAT is its ability to introduce pledges (conditional payments) to Bitcoin. Linus argued that OP_CAT-based commitments are “inefficient in terms of space and transaction fees”, and that other opcode proposals would be better suited for this purpose.

In a message to CryptoPotato, prominent OP_CAT supporter Bob Bodley – Bionic's founder and CEO – agreed with Linus's criticism of commitments. “There have been proposals for better collaterals (CTVs) that have been aggressively closed over the past few years, while CAT has the support of the broader Bitcoin ecosystem,” he added.

One of the most popular use cases for pledges is to enable trustless Bitcoin bridges, allowing Bitcoin to move to other blockchains and L2 networks in a truly decentralized manner. Instead, Linus and several colleagues recently published the “BitVM2” white paper, which describes a method for creating a trust-less Bitcoin bridge without any changes to Bitcoin Core.

According to Bodily, however, OP_CAT is a “low-level primitive” that provides more than just promises – including functions that help build Linus.

“Take the BitVM model (or BitVM2) and CAT makes it much better,” he wrote. “More efficient. Cheaper. More flexible. I don't know why Robin is clearly out on the CAT because he makes everything he does better.”