OP_RETURN in Bitcoin Core 30: What You Need to Know

Bitcoin Core 30 introduces a change to how OP_RETURN transactions are handled, and it has sparked discussion in the community. This update is about increasing the amount of data that can be attached to a transaction using OP_RETURN. While the feature itself is not new, the size increase raises both opportunities and concerns.

What Is OP_RETURN?

OP_RETURN is a part of Bitcoin’s scripting system that lets users attach non-monetary data to a transaction. Because outputs with OP_RETURN are unspendable, they do not add weight to the UTXO set, which helps keep the system efficient. Developers and users have used OP_RETURN in the past for things like digital signatures, file timestamps, and proofs of ownership.

The Change in Bitcoin Core 30

Up until now, OP_RETURN outputs were capped at 80 bytes. This was enough for small pieces of metadata but limited for other uses. Bitcoin Core 30 changes the default policy, raising the OP_RETURN size allowance to 100,000 bytes. To put that in perspective:

92 bytes is enough for a very tiny, almost unreadable image.

Around 33,000 bytes can already hold a recognizable, high-quality image.

At 100,000 bytes, it becomes possible to embed even larger images or short video clips.

This adjustment gives much more room for data to be written into Bitcoin transactions, provided the sender is willing to pay transaction fees for the extra block space.

Potential Risks

The increase has raised concerns about how larger OP_RETURN transactions might be used. The key issue is that before they are confirmed into a block, transactions live in each node’s mempool, which is stored in RAM. If someone were to use OP_RETURN to embed harmful or illegal material, those unconfirmed transactions would be temporarily stored and relayed by any node running with the new default settings.

This has led to questions about the legal and ethical position of node operators. Since mempool data is not encrypted, operators may technically be relaying or storing sensitive or illegal content until the transaction either confirms or is dropped.

Bitcoin Knots’ Approach

Bitcoin Knots, a different implementation of Bitcoin software, uses stricter defaults for OP_RETURN. Knots nodes typically limit OP_RETURN data to 42 bytes, which is much smaller than Core’s upcoming default. This policy reduces the chances that large non-monetary files move through the network, including those that could be problematic or illegal.

At the time of writing, roughly a quarter of publicly visible Bitcoin nodes run Knots. Some operators prefer this setup because it gives them additional configuration options and, in this case, tighter controls on mempool policy.

Historical Example

This is not the first time OP_RETURN size has been raised. In 2019, the Bitcoin SV (BSV) network increased its OP_RETURN limit to 100,000 bytes. This opened the door to larger amounts of arbitrary data being broadcast, and it became a practical example of how larger OP_RETURN allowances can be used. The BSV case is often cited as a precedent when discussing Core’s upcoming change.

How Transactions Are Handled

To understand the issue, it helps to know how a Bitcoin node processes transactions:

Every node has a mempool, a temporary holding area for unconfirmed transactions.

When a transaction arrives, the node first checks whether it breaks consensus rules (for example, trying to spend the same coin twice). If it fails, the transaction is rejected.

Transactions that pass consensus rules are then tested against the node’s policy rules. These rules vary depending on the software and configuration. If the transaction breaks a policy rule, the node will not store or relay it.

The change in Bitcoin Core 30 is a shift in default policy rules. Large OP_RETURN transactions that would have been rejected in the past will now be accepted into the mempool and relayed across the network.

Closing Thoughts

The expansion of OP_RETURN in Bitcoin Core 30 highlights a broader tension in Bitcoin: balancing flexibility and innovation against efficiency and safety. Supporters argue that larger OP_RETURN space allows for cleaner, more standardized uses of data embedding, while critics worry about risks like network spam, resource strain, or exposure to harmful material.

Whether this change strengthens or complicates Bitcoin in the long run will depend on how people choose to use it and how node operators decide to configure their software.