OP_RETURN

OP_RETURN is a command in Bitcoin’s scripting language. An output that begins with it can never be spent. Data can be stored after it, for example a timestamp proof for a document. Because such outputs are provably unspendable, full nodes don’t have to keep them in their list of spendable coins (the UTXOGlossaryUTXO (Unspent Transaction Output)An output of an earlier transaction that hasn’t been spent yet – like a single banknote in your wallet. Bitcoin has no account balances: your balance is the sum of all the UTXOs you can spend with your keys.On the learning path: Stage 5 · Step 4 – First withdrawal →In the glossary →× set).^[1]

How much data nodes relay

From version 0.12.0, Bitcoin CoreGlossaryBitcoin CoreThe most widely used software for Bitcoin nodes, a direct descendant of Satoshi Nakamoto’s first program. It is open source, verifies all blocks and transactions itself and includes a wallet.In the glossary →× relayed up to 83 bytes in a single OP_RETURN output per transaction by default.^[1] Since version 30.0 (10 October 2025), the default has been 100,000 bytes – effectively no limit – and multiple OP_RETURN outputs per transaction are allowed. Setting -datacarriersize=83 restores the old value (as of September 2026).^[2]

This only affects relay (policy), not the consensus rules: miners could already include larger OP_RETURN data in a block before.^[1]

The dispute

• For the new default: Nodes shouldn’t filter what miners include in blocks anyway. Filters merely push such transactions onto routes directly to miners.^[3]
• Against (including users of Bitcoin Knots): Bitcoin is money, not data storage. Data competes with payments for block space and makes running a node more expensive.^[4]

In August 2026, a small minority split off over this with the BIP-110 proposal; hardly any miners followed it.^[5] Forks & controversies explains both sides. For normal payments, you don’t need to do anything.