OP_RETURN

OP_RETURN is a Bitcoin Script opcode that immediately marks the script it appears in as invalid, making any output that uses it provably unspendable. Because the output can never be redeemed, it does not need to live in the UTXO set forever — full nodes can recognise and prune it. This makes OP_RETURN the sanctioned way to commit a small amount of arbitrary data to the timechain (for timestamps, asset issuance proofs, or commitment hashes) without bloating the set of spendable coins.

How it works

A typical data-carrying output sets a value of zero satoshis and a scriptPubKey of OP_RETURN <data>. When a node evaluates the script, OP_RETURN halts execution and fails the output, so the coins are burned by design. The data after the opcode is simply stored, never executed.

The relay-policy limit

For years, default node relay policy (a standardness rule, not a consensus rule) capped a single OP_RETURN payload at 80 bytes via the datacarriersize setting. Bitcoin Core v30 (October 2025) relaxed this default, raising the cap and admitting multiple OP_RETURN outputs — a change that drew vigorous community debate. Miners and node operators running alternative policies (for example, Knots) can still set tighter limits. Consensus rules were untouched: OP_RETURN behaviour at the validation layer is unchanged.

OP_RETURN is one way data interacts with Bitcoin's UTXO model; unlike a normal spend, it never creates a coin that a future block must track. In our experience explaining this to miners, the key intuition is that the network charges you for the data via the fee while sparing every node the cost of storing it as live state.