What Is OP_CAT? BTC Ordinals Promoters Want To Revive Defunct Opcode Scrapped By Satoshi Nakamoto

Written by

Joanna K


Published Mar 5, 2024 at 4:55 PM
Last updated Mar 6, 2024 at 10:00 AM

In the past month you may have read some of the hype about a revival of OP_CAT that was started by Bitcoin Ordinals project Taproot Wizards and their freshly launched collection of Quantum Cats NFTs on the Bitcoin blockchain.

But just what is OP_CAT

OP_Cat Bitcoin Ordinal NFT

Well, in short, OP_CAT is an old opcode, or operation code, that was part of the original Bitcoin scripting language. 

In computer-speak, OP_CAT also means ‘concatenate’ – or in other words, a method of linking items together in a chain or series.

In the past, OP_CAT was used to concatenate two sets of text or elements from a stack, and then add them back into the stack.

It was introduced by Bitcoin mastermind Satoshi Nakamoto in the first release but was later disabled in 2010 due to security fears such as denial-of-service (DoS) attacks if used in conjunction with other opcodes such as OP_DUP. 

Cat's out of the bag

Now Udi Wertheimer, a Taproot Wizard and Quantum Cats developer, believes the code could be quite useful if brought back into fashion. 

Even though the code fell away more than 10 years ago, according to Wertheimer, the OP_CAT Bitcoin Improvement Proposal (BIP) permits what are called 'covenants', allowing smart contracts and bridges to be assembled on Bitcoin.

"OP_CAT is a potential upgrade that would enable a type of smart contracts called 'covenants', and through that can enable Bitcoin L2s, bridges to other chains, and more," posted Wertheimer in January. 

Wertheimer has stated that reintroducing OP_CAT means making a small alteration to Bitcoin Core code using about 10 lines of code.

Famously, Bitcoin is tough to alter as it's controlled by a long-winded consensus process at the hands of a few devs. When software has been updated, it must then be implemented by thousands of full nodes and miners to begin taking full effect.

In May 2018, Bitcoin Cash, a fork of Bitcoin, brought back OP_CAT to increase the script capacity of Bitcoin Cash.

Quantum Cats Ordinals

According to the Taproot Wizards team, the Quantum Cats "are on a mission to revive Satoshi's beloved pet and scripting function, OP_CAT".

Launched in early 2024, Quantum Cats is a collection of 3,333 cat images minted using Ordinals Inscriptions at a cost of about $66k to create. They were designed with a system called 'evolving inscriptions' which are pre-programmed to reveal new changes over time.

Like NFTs based on Ethereum, Ordinals Inscriptions are digital art on the Bitcoin blockchain, with more than 50 million Ordinals being minted since the protocol launched in January 2023. These digital art assets are inscribed on a satoshi, the lowest unit of a Bitcoin (BTC).

The limited edition collection used 10MB of encrypted on-chain data, with the space of one single Bitcoin block being 1MB. 

At Sotheby’s on 22 January, a special edition Ordinal called 'Genesis Cat' (pictured below) by digital artist FAR that sold for 6.31 BTC – or $410k at today's market price.

FAR for Taproot Wizards. Source: Southeby's

Sadly, things did not go as smoothly for the rest of the collection. On 1 February, it was revealed that technical issues held up the much-anticipated sale of the other Quantum Cats for a third time following maximum hype for the project. 

Following the latest failed sale, Taproot Wizards posted on X: "There’s been an incredible demand for the cats today, and our servers simply couldn’t handle the amount of people who were trying to mint."

Bitcoin season 2?

Meanwhile, Taproot Wizards' Udi Wertheimer continues to tell the crypto community that "Bitcoin season 2 needs OP_CAT".

In a 29 February post on X (formerly Twitter), the dev explained: "When we launched @QuantumCatsXYZ and the OP_CAT campaign, people asked me 'but how will you convince miners to support the upgrade? how will you convince developers?'

"Some people are building basic multisig bridges for their L2s, the type that got hacked every other week last cycle and had people lose billions of dollars.

"Others are counting on BitVM, which is a really cool nerdy trick, but turns out to be impractical, and also still puts a permissioned council in control of bridged funds.

"OP_CAT solves these problems, and it does that effortlessly, with 10 lines of code, using a feature that was already part of Bitcoin 15 years ago."

He added: "We just have to turn it back on, and season 2 is LIVE. OP_CAT WILL happen, because all the biggest ecosystem players will NEED it."