Core vs Knots: the 83-byte row that split the bitcoin community

In spring 2025, one of the biggest rows in years erupted in the bitcoin community. A technical proposal to lift the OP_RETURN data cap snowballed into a broad dispute between developers and users.

Bitcoin Core backed scrapping the 83-byte limit, while opponents rallied around the alternative client Bitcoin Knots. Together with the bitcoin mixer Mixer.Money we examine the technical issues and the potential impact on the future of the first cryptocurrency.

What is OP_RETURN

OP_RETURN is an opcode (a command) in bitcoin’s stack-based Script language that renders a transaction output unspendable while allowing up to 83 bytes of arbitrary data to be recorded. It is the standard way to add metadata to the blockchain without increasing the size of the UTXO set — the database of all unspent transaction outputs.

OP_RETURN was introduced in 2014 with Bitcoin Core 0.9.0. It was not intended to promote on-chain data storage; it was a compromise to prevent more harmful practices.

“This change is not an endorsement of data storage in the block chain. OP_RETURN creates a provably prunable output to avoid data storage schemes where arbitrary data such as images would be stored as permanently unspent transaction outputs, bloating the UTXO database. Storing arbitrary data on the block chain is still a bad idea; it is much cheaper and more efficient to store non-financial data elsewhere,” the Bitcoin Core documentation said.

According to Bitcoin Core developer Gregory Maxwell, before OP_RETURN users found ways to embed data by sending bitcoins to “fake” addresses. They created outputs that looked spendable but were not, causing permanent bloat in the UTXO set.

“While OP_RETURN outputs do add to bitcoin’s overall blockchain size, they are not included in the UTXO set, so nodes do not need to track them indefinitely. Nodes can ignore and prune these data after initial verification,” comment the team at Mixer.Money.

The initial cap was 40 bytes, raised to 80 in 2015 and to 83 in 2016. The limits were deliberate, nudging users towards storing hashes rather than raw data and underscoring that bitcoin’s blockchain should primarily serve financial transactions.

Importantly, the OP_RETURN cap is a standardness rule (mempool policy), not a consensus rule. By default, Bitcoin Core nodes do not relay transactions with OP_RETURN data above 83 bytes, but the bitcoin protocol does not forbid them.

Miners may include “non-standard” transactions in blocks, especially if fees make them profitable. If such a transaction lands in a valid block, all nodes will accept it regardless of their OP_RETURN policies.

Todd’s proposal

On April 27, 2025 Peter Todd filed Pull Request (PR) #32359 to the Bitcoin Core repository. The proposal would remove the cap on OP_RETURN output size entirely, formalising an initiative by Antoine Poinsot of Chaincode Labs.

The idea arose after Poinsot learned that the bitcoin ZK-rollup Citrea, constrained by OP_RETURN policy, chose to publish data on-chain using one OP_RETURN and two UTXOs. Those two unspent outputs would accumulate, even though limiting UTXO-set growth is a core Bitcoin Core aim.

“Theoretically, lifting the OP_RETURN cap would let projects like Citrea implement technical solutions without increasing the UTXO set. It could also affect miners that accept transactions via private mempools rather than organically through the node network,” comment the team at Mixer.Money.

Todd’s proposal required neither a soft fork nor a hard fork — it merely broadened the types of transactions that the network would relay.

Some welcomed the deletion of what they saw as redundant code. Others, while acknowledging the cap’s limited effectiveness, opposed removal, arguing it would encourage spam.

In particular, developer Luke Dashjr urged keeping the restriction and either staying on older Core versions or switching to his Bitcoin Knots client, which has a stricter 42-byte OP_RETURN limit.

On May 5, Bitcoin Core developer Greg Sanders said he intended to include the change in the next release, further inflaming debate. Many users publicly announced a move to Bitcoin Knots, and several bloggers (BTC Sessions, Southern Bitcoiner) posted how-to videos on YouTube.

By May 2025 Bitcoin Knots’ share of nodes had neared 10%. In January 2023 its share was just 0.3%.

Bitcoin Core’s case

Proponents make several points:

• the cap is outdated and ineffective. This is Todd’s and Poinsot’s central claim. They argue it is “silly” to cling to parameters that do not work because users easily route around them. For instance, miner MARA built Slipstream to accept non-standard transactions directly.
• perverse incentives. Keeping the cap nudges users towards worse practices like encoding data into “fake” addresses, inflating the UTXO set.
• mempool policy calibration. Miners, driven by economics, will include profitable transactions regardless. “It’s very unlikely they will turn down this revenue source. Censoring these transactions just incentivizes private mempools, which harms small miners and makes fee estimation less reliable,” Todd noted.
• support for innovation. Removing the cap could improve support for second-layer designs such as Citrea.

The last point prompted a theory among opponents about Bitcoin Core’s financial motives. Developer Jameson Lopp, who backed Todd’s proposal, was accused of a conflict of interest over his investment in Citrea — the project whose constraints helped trigger the idea. Lopp said this was no secret and that he has long disclosed his investments (including Citrea) on his website.

Bitcoin Knots’ case

Opponents, dubbed “filterors”, see bitcoin primarily as a monetary network and do not want “to turn it into a general-purpose database”. Luke Dashjr called the proposal “complete madness”, while Jason Hughes of Ocean Mining thinks it could turn bitcoin into “a useless altcoin”.

Re: OP_RETURN, I have always been of the opinion that sidechain builders shouldn’t influence bitcoin core. Bitcoin on its base layer is money and should be only focused on money. Let all other functionalities be built around it.

Adding in increased OP_RETURN sizes is possible…

— Willem S ?⛓️ (@WillemSchroe) April 29, 2025

“Sidechain builders shouldn’t influence Bitcoin Core. Bitcoin on its base layer is money, and it should be focused only on money,” said Botanix Labs founder Willem Schroe.

Critics fear that unbounded OP_RETURN would open the floodgates to non-financial data, potentially overloading the network. While OP_RETURN data can be pruned from the UTXO set, it still consumes block space.

Opponents also argue existing limits work as a deterrent. The fact that “spammers” resort to costlier methods is, to them, evidence that limits raise the price of undesirable behaviour.

They point to the shift toward Bitcoin Knots as “node voting” — a decentralised protest in which node operators signal disagreement with Bitcoin Core’s direction by choosing alternative software.

Finally, “filterors” such as Bitcoin Mechanic accuse Bitcoin Core of pushing changes without broad consensus and of poor communication about decision-making.

Conclusions

On May 12, 2025 PR #32359 was closed, despite earlier statements of intent to include it in the next version (30.0), whose release is slated for early October.

The conflict laid bare key questions of governance in bitcoin’s ecosystem: who steers its development — developers, miners or the broader user community?

Many observers compare the situation to the 2015–17 blocksize wars, but there is an important difference: changing the OP_RETURN cap is client policy, not a consensus rule. A hard fork is therefore highly unlikely.

The rise of Bitcoin Knots shows users are willing to “vote with nodes”, opting for alternative implementations when they disagree with the direction set by Bitcoin Core.

“The row over OP_RETURN goes beyond a purely technical question. It is a fundamental clash of visions for what bitcoin should be: a pragmatically evolving system or a strictly defined monetary network,” comment representatives of Mixer.Money.

Experts add that the dispute does not touch privacy, and that preserving anonymity in the bitcoin network will remain a pressing issue for all participants regardless of their stance in this conflict.

The bitcoin mixer Mixer.Money has operated since 2016. The service mixes coins in three modes: “Mixer”, “Exact payment” and “Full anonymity”.

In the “Full anonymity” and “Exact payment” modes, Mixer.Money sends users bitcoins sourced directly from large exchanges with the corresponding status.