- Bitcoin Core’s new update removes the 80-byte limit on OP_RETURN, allowing more data and multiple OP_RETURN outputs per transaction—modernising the protocol to match real-world mining behavior.
- The change reignites discussion about OP_CAT, a disabled opcode that, if revived, could bring powerful smart contract capabilities like covenants, ZKPs, and decentralised bridges to Bitcoin.
- Critics argue the update breaks Bitcoin’s minimalist ethos and lacks community consensus, with figures like Luke Dashjr and Samson Mow warning it may bloat blocks and increase attack surfaces.
Bitcoin Core is shaking things up again, sparking intense debate by ditching its longstanding 80-byte OP_RETURN limit.
The upcoming update effectively kills the old ceiling, a move framed as aligning Bitcoin Core policy closer to real-world mining practices, but not everyone is thrilled.
The change was announced by developer Greg Sanders on GitHub, stating that it will allow transactions with OP_RETURN outputs exceeding 80 bytes as well as multiple such outputs per transaction. In plain terms: more data, fewer artificial restrictions, and a relay policy that finally mirrors on-chain reality.
The long-standing cap, originally a gentle signal that block space should be used sparingly for non-payment proof of publication data, has outlived its utility.
Greg Sanders Before we get into the details, let’s have a quick rundown on what this all means.
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OP_RETURN or OP_CAT?
OP_RETURN and OP_CAT are two Bitcoin script commands—one embedded, one exiled—that define the boundary between Bitcoin’s past limitations and its potential future.
Originally introduced as a way to embed small, unspendable pieces of data into the blockchain, OP_RETURN allows up to 80 bytes of user-defined data per transaction. The catch is that it burns the Bitcoin sent with it. That output is forever unspendable, a calculated tradeoff for preventing UTXO bloat.
Its use cases are narrow but persistent. It’s useful for things like proving a document existed at a certain time, creating basic tokens, or adding notes. And because it’s clearly unspendable, it doesn’t make the system slower or more complicated. But Bitcoin only lets one OP_RETURN note per transaction under normal conditions—so you can’t abuse it to spam the network.
OP_CAT, on the other hand, is the opcode that never really was. OP_CAT is a Bitcoin opcode that was disabled in 2010 because it could potentially crash nodes. All it does is combine two pieces of data from the stack into one. Sounds basic, right? But it’s actually very powerful.
If reactivated, OP_CAT would allow Bitcoin to support much more advanced smart contracts, opening the door to an entirely new class of smart contracts on the network —more composable, flexible, and modular. Think covenants, advanced multi-sigs, zero-knowledge proofs (ZKPs), or even decentralised bridges, etc.
Criticism and Controversy
The backlash was swift and sharp.
Luke Dashjr, leading the alternative Bitcoin Knots client with around 5% network presence, slammed the move as “utter insanity”, asserting it betrays Bitcoin’s minimalist principles.
Samson Mow called it an “undesirable change” on X, but stated there might be pragmatic reasons for it, adding that users can simply stick to version 29.0 or switch to alternative clients if they reject the new policy.
“There is no consensus at the moment on this OP_RETURN issue,” said Marty Bent, managing partner at Ten31 Fund.
Critics also took aim at the rollout process itself, saying the proposal lacked proper consensus.
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Bitcoin recently reclaimed US$97.5K (AU$151K), a change of over 20% in the last 30 days. The surge comes as the trade war tensions between China and the US seem to calm down, and US states like New Hampshire break ground by becoming the first to adopt a BTC reserve.
