# What is OP\_RETURN?

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OP\_RETURN is a Bitcoin Script opcode that immediately terminates script execution. When a validation engine encounters OP\_RETURN, it stops processing and ends the script with the top value on the stack as the final result.

| Word       | Hex  | Input         | Output | Description                                               |
| ---------- | ---- | ------------- | ------ | --------------------------------------------------------- |
| OP\_RETURN | 0x6a | Any (ignored) | None   | ***Ends script with top value on stack as final result*** |

OP\_RETURN can also be used to create "False Return" outputs with a locking script consisting of OP\_FALSE OP\_RETURN followed by data. Such outputs are provably unspendable and should be given a value of zero Satoshis. These outputs can be pruned from storage in the UTXO set, reducing its size.

This might seem counterintuitive—why create an output that can't be spent? The answer lies in understanding Bitcoin's **UTXO (Unspent Transaction Output) set**. Before False Return outputs, developers storing data on the blockchain created fake payment scripts that looked like valid addresses but encoded data instead. These outputs permanently bloated the UTXO set—the database every node maintains of spendable coins.

False Return outputs and the usage of OP\_RETURN solved this elegantly. By making data outputs explicitly unspendable from creation, nodes can immediately recognize them and exclude them from the UTXO set. The data still gets recorded in the blockchain permanently, but doesn't create ongoing overhead for nodes.

**Key distinction:**

* **Blockchain storage**: Data is permanent but doesn't affect UTXO set
* **UTXO storage**: Creates ongoing costs for every node on the network

#### Data Capacity

The amount of data allowed in OP\_RETURN outputs has evolved significantly:

| Implementation | Period       | Limit               | Multiple Outputs |
| -------------- | ------------ | ------------------- | ---------------- |
| Bitcoin Core   | 2014-2015    | 40 bytes            | No               |
| Bitcoin Core   | 2015-present | 80 bytes            | No               |
| BSV blockchain | 2018-present | No artificial limit | Yes              |

BSV removed hardcoded restrictions on OP\_RETURN size, recognizing that arbitrary limits constrain legitimate use cases. Data size is now limited only by overall transaction size and practical cost considerations. This enables applications requiring substantial data storage while maintaining efficiency.


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