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OP_CAT: The Missing Piece for BTC L2s?

This opcode has everyone talking; what does it do?
William M. Peaster William M. Peaster Jun 19, 20243 min read
OP_CAT: The Missing Piece for BTC L2s?

Bitcoin Layer 2s are the talk of the town, but not much of their promise has been realized yet.

What are Bitcoin's builders waiting for? One significant proposal could help mature the Bitcoin L2 landscape: the reintroduction of OP_CAT. This opcode, which was disabled early in Bitcoin's history, could unlock new capabilities and pave the way for more sophisticated and trustless Bitcoin L2 solutions.

Let's dig into why this opcode could be so transformative for BTC 👇

What is OP_CAT?

Okay, so why is this seemingly random opcode so controversial? OP_CAT, or "Operation Concatenate," is an opcode in Bitcoin's scripting language that allows for the concatenation of two data elements.

Initially proposed by Satoshi Nakamoto, OP_CAT was disabled in 2010 due to security concerns related to its original implementation, specifically potential denial-of-service (DoS) attacks caused by unbounded memory usage.

The introduction of the Taproot upgrade in 2021 has mitigated these original concerns. Taproot enforces a maximum stack element size of 520 bytes, addressing the memory usage issue that led to OP_CAT's deactivation. With this security improvement and resurgent Bitcoin development interest thanks to Ordinals and Runes, a movement to reintroduce OP_CAT to enhance Bitcoin's scripting capabilities is gaining steam, aiming to put more complex transaction capabilities in reach. 

Are Covenants and Smart Contracts Different?

One reason why OP_CAT is so compelling is because it can actualize covenants, i.e. spending conditions, on Bitcoin. Covenants and smart contracts are both mechanisms for controlling the flow of funds in a blockchain network, but they operate in distinct ways.


  • Purpose: Restrict how funds can be spent in future transactions.
  • Functionality: Implement specific spending constraints, such as limiting addresses or setting conditions on fund transfers.
  • Complexity: Generally involves simpler, deterministic logic without loops or complex computations.
  • Implementation: Can be implemented using Bitcoin Script with enhancements like OP_CAT.

Smart Contracts

  • Purpose: Automate a wide range of processes and transactions.
  • Functionality: Handle complex tasks such as decentralized exchanges, token issuance, and multi-party agreements.
  • Complexity: Support complex conditional logic, including loops and intricate computations.
  • Implementation: Commonly implemented on platforms like Ethereum using high-level programming languages.

Why Does OP_CAT Matter for Bitcoin L2s?

OP_CAT would prove crucial for advancing Bitcoin L2 solutions because it enhances the scripting capabilities needed for more complex and trustless Bitcoin infra. First, OP_CAT allows for covenants, enabling the construction of more complex scripts. This is essential for creating advanced transaction conditions and functionalities required for L2 solutions.

Additionally, with OP_CAT, users can unilaterally withdraw their funds from shared UTXOs, ensuring they can reclaim their assets without needing consent from other parties. This is also vital for trustless L2 protocols.

For example, CatVM, proposed by Taproot Wizards, would use OP_CAT to build a mechanism for locking and withdrawing funds in an L2 environment. It leverages OP_CAT's ability to concatenate data and verify Merkle tree branches, facilitating secure withdrawal processes.OP_CAT also enables the creation of fraud proofs a la optimistic rollups to prevent double-spending and ensure the integrity of offchain transactions, another hugely promising path for Bitcoin L2s. 

Will OP_CAT be Activated?

The reactivation of OP_CAT is currently under discussion within the Bitcoin community. While there is significant support for its potential benefits, the conservative approach to protocol changes in Bitcoin means that thorough testing and consensus-building are necessary steps before activation. However, if the proposal to re-enable OP_CAT is approved, it could be implemented within six months to a year, depending on the community's consensus and the results of the testing phase.

Additionally, if OP_CAT is activated, projects like StarkWare are on standby to develop atop it. StarkWare has plans to leverage OP_CAT for bringing zero-knowledge (ZK) scaling to Bitcoin to enable advanced financial instruments and apps on the network. StarkWare’s involvement here underscores the broader ecosystem's readiness to capitalize on the enhanced capabilities that OP_CAT would introduce.

As such, as the community continues to explore and test the possibilities around this proposal, the future of Bitcoin L2s looks promising and ripe with potential. At this point, reactivating OP_CAT seems like a matter of when not if

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William M. Peaster

Written by William M. Peaster

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William M. Peaster, Senior Writer, has been with Bankless since January 2021. Immersed in Ethereum since 2017, he writes the Metaversal newsletter on the onchain frontier, covering everything from AI projects to crypto games, as the team’s lead NFT analyst. With a background in creative writing, he writes fiction and publishes art on Ethereum in his free time. He lives in Washington.

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