What is OP_CAT?

General Wallet Use

15 min

Opcodes are like the building blocks of Bitcoin, forming the fundamental rules that govern how transactions are executed. Short for "operation code," opcodes contain instructions dictating the operations that can be performed within the scripting language of a blockchain network. 

However, implementing new opcodes on Bitcoin requires a Bitcoin Improvement Proposal (BIP) and network consensus, making it a complex process that requires community buy-in. OP_CAT, an opcode proposed in the early days of Bitcoin, was rejected because of concerns over its potential impact on the network's security. 

Today, OP_CAT is experiencing a resurgence, with developers and Bitcoin enthusiasts revisiting its potential benefits and considering its reintroduction into the protocol.

History of OP_CAT

OP_CAT is an abbreviation for operation code concatenate. It’s an opcode for the Bitcoin network that was included in the original Bitcoin stack by Satoshi Nakamoto — the creator of Bitcoin. Satoshi included OP_CAT in Bitcoin’s initial scripting system, but disabled it in 2010. Its purpose was to facilitate joining two data elements within a script. 

Imagine you have two pieces of a puzzle, and you want to put them together to see the whole picture. OP_CAT is like the glue that helps you join these pieces. In Bitcoin, it would allow you to combine different pieces of information within a script, making it easier to create more complex transactions. 

For example, you could use it to concatenate or link together different parts of a transaction to enable new features like smart contracts or more secure ways to move Bitcoin around. The goal of OP_CAT was to enrich the scripting language's capabilities and expand the capabilities of the network.

Satoshi viewed OP_CAT as a pivotal advancement for Bitcoin's scripting language. They believed that by incorporating OP_CAT, Bitcoin could transcend its role as a simple peer-to-peer electronic cash system. They wanted it to evolve into a platform capable of supporting more complex smart contract functionality and generalized logical spend conditions. Satoshi believed OP_CAT would empower developers to create innovative applications and use cases on the Bitcoin network, paving the way for broader adoption and utility.

The Initial Vote Against OP_CAT

Despite Satoshi's staunch support for OP_CAT, the proposal faced significant pushback from the broader Bitcoin community. One of the primary concerns was potential security risks associated with OP_CAT's implementation. Critics highlighted the possibility that OP_CAT could be exploited to launch denial-of-service (DoS) attacks or impose exponential memory requirements. These issues would pose serious threats to the network's stability and security. 

Because of the risks to security, the Bitcoin community felt growing apprehensions about implementing OP_CAT. And ultimately, enough votes were cast against the adoption of OP_CAT in its original proposal that consensus was never reached.

OP_CAT and Quantum Cats

Quantum Cats is an Ordinals collection launched with the goal of bringing back interest in OP_CAT. Created by Taproot Wizards, proponents of OP_CAT, Quantum Cats is meant to showcase the potential advantages of reintroducing OP_CAT into Bitcoin's scripting language. The Quantum Cats NFT collection is made up of 3,333 cat images and cost $66,000 to create, according to Taproot Wizards. 

Quantum Cats played a crucial role in amplifying the voices of those advocating for OP_CAT in the Bitcoin community. By providing a platform for discussions, presentations, and collaborative efforts, Quantum Cats signal boosted the arguments in favor of OP_CAT more effectively. Through research findings, use case demonstrations, and community engagement, Quantum Cats supporters worked to present a compelling case for OP_CAT’s reinstatement.

The popularity of Quantum Cats sparked a new wave of interest in OP_CAT among many who previously were not in the conversation. As discussions around Quantum Cats gained traction and attracted attention from developers, researchers, and enthusiasts, the relevance and potential benefits of OP_CAT became increasingly apparent. 

Quantum Cats served as a catalyst for reinvigorating the conversation about OP_CAT, reigniting debates, and prompting renewed evaluations of its merits and feasibility within the context of Bitcoin's evolving ecosystem.

What could OP_CAT do for Bitcoin?

In the last decade, Bitcoin's technology has improved significantly, addressing many of the security concerns leading to OP_CAT’s removal from Bitcoin. With advancements in network protocols, cryptographic techniques, and consensus mechanisms, several of the risks associated with OP_CAT have become less relevant and more manageable. 

These innovations and improvements to blockchain technology are what spurred proponents to argue for reinstating OP_CAT in October of 2024. They assert it could offer valuable functionalities to Bitcoin's scripting language without compromising network security.

In April of 2024, OP_CAT received a Bitcoin Improvement Proposal (BIP) number. Having a BIP number is required in the process of any new implementation on Bitcoin, and OP_CAT was granted BIP number 347.

Debating the Necessity

Opponents of OP_CAT question whether it’s even necessary, especially with new Bitcoin layer 2 solutions (L2s). Detractors argue that L2 technologies, such as the Lightning Network, already provide scalability, privacy, and smart contract functionalities without the need for additional opcodes in Bitcoin like OP_CAT. From their perspective, integrating OP_CAT into Bitcoin's protocol could introduce unnecessary complexity and unforeseen security vulnerabilities.

Today, the Bitcoin community is debating the pros and cons of bringing back OP_CAT, thanks to advocates and projects like Quantum Cats.

Pros of OP_CAT

  • Enhanced Smart Contract Functionality: OP_CAT could enable more sophisticated smart contracts on the Bitcoin network, expanding its utility beyond simple transactions.

  • Greater Flexibility: By reintroducing OP_CAT, developers would have access to a broader set of Bitcoin script capabilities, facilitating innovative use cases and dApps.

  • Improved Layer 2 Development: OP_CAT could streamline Layer 2 solutions by providing essential functionalities for constructing trustless bridges, decentralized exchanges, and other off-chain protocols.

Cons of OP_CAT

  • Security Risks: Despite advancements in Bitcoin's security infrastructure, reintroducing OP_CAT could enable new attack vectors or vulnerabilities like a service DoS attack.

  • Complexity Overhead: Integrating OP_CAT would add complexity to the network's codebase and maintenance processes, requiring thorough testing and validation.

While OP_CAT holds the promise of unlocking new possibilities for Bitcoin's scripting language, its implementation requires careful consideration of both its potential benefits and associated risks.


The 2023 Bitcoin building frenzy which started with Ordinals led the way to many refreshed conversations about the nature of Bitcoin and what it should or should not be used for. 

Implementations like OP_CAT provide avenues for greater innovation — but sometimes evolutions change the status quo. The Bitcoin community will likely continue to debate advancements like OP_CAT as the blockchain world becomes more complex.

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