OP_Return is a command in Bitcoin’s scripting language. When a transaction uses OP_Return, the output cannot be spent. As a result, it does not add a new unspent output to the usual set, and nodes can remove or ignore the data while still keeping the transaction record. This lets developers add metadata to the blockchain without making every node track extra spendable outputs.
The opcode became a formal way to add small pieces of data to Bitcoin transactions around 2013, and later updates like version 0.9.0 made it more official. It was seen as a compromise to let data be added without increasing the number of unspent outputs that full nodes have to keep.
A transaction output with OP_Return stores any data instead of a spendable script. When nodes see this, they mark the output as unspendable, so the coins in that output cannot be used again. There are also limits on how much data you can include—most systems allow up to 80 bytes, while older versions allowed less, such as 40 bytes. These limits make OP_Return useful for short hashes, IDs, or small messages.
Developers and services use OP_Return when they need an unchangeable timestamp or proof that something existed. Common uses include saving a document’s fingerprint (a hash) so the blockchain can later show the document existed at a certain time, adding metadata for a protocol, or linking to content stored elsewhere. Some newer token and inscription systems also use OP_Return to store protocol data.
Adding data to a public blockchain has pros and cons. The data stays there forever and anyone can see it. Even though OP_Return outputs cannot be spent and nodes can remove them, large amounts of data can still use up bandwidth and storage for services that track them. This has led to debates about how big OP_Return data should be and how to balance usefulness with network costs.
OP_Return is similar to other Bitcoin features that use transaction fields for extra information or signals. Related topics include the unspent transaction output model, prunable outputs, and higher-level protocols built on Bitcoin. Some protocols use OP_Return directly, while others just store a small reference on the blockchain and keep the main content elsewhere.
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