Bitcoin transforms limit size to 4MB

The Bitcoin Core development team announced on Tuesday that the upcoming Core 30 release in October will include a change to the default setting of OP_RETURN. The team said the update will increase the date carrier limit from 80 bytes to nearly 4MB.

The team also acknowledged that the decision marks a significant victory for the reformist faction led by Antoine Poinsot in their long-standing debate with conservative figures such as Luke Dashjr. Dashjr’s Bitcoin Knots popularity, which is an alternative implementation of Bitcoin node software, is already surging, as many network operators are concerned about the potential effects of the change.

Bitcoin transforms limit size to 4MB

Today is really sad day, Bitcoin Core is now fully malicious. There is no longer any filters on Bitcoin Core agains't shitcoins on the chain.

If you still run Core I very strongly urge you to switch to @BitcoinKnots or to not update Core.

Bitcoin is our only chance to escape… pic.twitter.com/SGgWZPIcgk

— Satoshi Vault 21 (@satoshivault21) June 9, 2025

Bitcoin, the largest cryptocurrency, is set to undergo a major transformation to the design of its most mainstream implementation. The Bitcoin Core team has gotten rid of the 80-byte limit size of OP_RETURN, an element of BTC transactions, to nearly 4MB. Starting from the Core 30 release in October, the first blockchain will mine transactions with bigger outputs and allow any number of these outputs.

Greg Sanders, core developer of Blockstream Bitcoin-centric studio, announced the update on his GitHub page. The author argued that the OP_RETURN size limit was designed to protect the network, but it is now failing its mission.

“The change re-affirms that Bitcoin is governed by transparent, minimal rules rather than editorial preference. By retiring a deterrent that no longer deters, Bitcoin Core keeps the policy surface lean and lets the fee market arbitrage competing demands.”

-Greg Sanders, Software Engineer at Blockstream.

Sanders also revealed that blocks remain limited to 4 million weighted units, dust outputs are rejected, and signature-operation and ancestor/descendant caps guard mempool growth. He acknowledged that the withdrawal of the 80-byte rule yields at least two tangible benefits, including a cleaner UTXO set and consistent default behavior.

According to Samson Mow, the former Blockstream CSO and current CEO of JAN3, many users find the update to be an undesirable change for a number of reasons. He acknowledged in May that users can refuse to upgrade and stay on 29.0 or run another implementation like Bitcoin Knots.

OP_RETURN limit size indicates how much data can be embedded into a cryptocurrency transaction. The limit is regular for standard ones, like payment transfers, though the existing limit censors more data-heavy use cases.

Bitcoin is set to process more data with the new upgrade, opening new opportunities for BTC DeFi and NFTs. The debate about the OP_RETURN size limit stems from 11 years ago. Sanders noted that the long-standing cap, originally a gentle signal that block space should be used sparingly for non-payment proof of publication data, had outlived its utility.

Sanders deems the current 80-byte ceiling counter-productive

Sander acknowledged that consensus rules decide whether a transaction can ever be included in a block. According to him, standardness rules implemented in Bitcoin Core’s relay code decide whether it is forwarded across the peer-to-peer network before it reaches a miner.

The tech enthusiast said standardness offers denial-of-service defense, where nodes decline transactions that waste CPU, RAM, or bandwidth disproportionate to their fee. Policies also offer incentive alignment, which gives policy nudges towards wallet authors for free-efficient yet UTXO-friendly constructions. 

Standardness also offers upgrade safety, where unknown opcodes or version bits remain non-standard until activated by a soft fork. According to Sanders, it prevents premature use that could hamper future consensus changes.

The software engineer said the standardized OP_RETURN outputs embody that philosophy. He noted that users already embed arbitrary data in spendable outputs, leaving toxic, unspendable entries in the UTXO set. Sanders also acknowledged that OP_RETURN gave users an unspendable output not added to the UTXO set; the 80-byte ceiling accompanied it as a soft deterrent, which was large enough for a hash or short commitment.

According to Sanders, the modern transaction landscape has rendered the legacy cap ineffective and, in several ways, damaging. He noted that several private mining accelerators simply do not enforce the limits, and other centralized services use alternative implementations to peer with these miners as well.

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