> For the complete documentation index, see [llms.txt](https://hub.bsvblockchain.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://hub.bsvblockchain.org/wiki/transactions-and-utxos/bitcoin-transactions/utxo.md). # UTXO UTXO is an acronym for unspent transaction output. Every Bitcoin transaction in every block contains at least one output. Outputs are then spent by inputs of later transactions and typically must be unlocked with a digital signature (most commonly ECDSA in Bitcoin). Until an output is used as an input in another transaction, this output is called a UTXO. All UTXOs at a given time can be collected into a set, called the UTXO set. The UTXO set represents the current state of ownership of all Bitcoin in existence. The inputs of transactions that are proposed for adding to the blockchain are checked against this set. If an input to a transaction is an output in the UTXO set, the input can be validated. The UTXO set is deterministic for every Bitcoin block, therefore, every node who has validated the history of the Bitcoin ledger up to a particular block should have an identical UTXO set. This is required in order for nodes to reach consensus on the validity of inputs in new transactions. Confusingly the term UTXO is also often used as shorthand to refer to the set. Generally, it is differentiated by the preceding word. When referring to the set, it is common to call it "the UTXO". When a node receives a new transaction, one of the checks on the transaction is if the input is spending a UTXO in the set. If this check is passed, the output that is being spent, is taken out of the UTXO set and temporarily stored in a buffer. If the transaction is accepted into a block, the corresponding UTXO is deleted from this temporary storage. On the other hand, if the transaction that spends the UTXO is not included in a block, the UTXO is returned to the UTXO set as the output is still unspent. As soon as an output is spent, it is no longer in the UTXO set. This process prevents an output from being double-spent. 1. Provably unspendable outputs There is a subset of outputs which are provably unspendable. These outputs, whilst ‘unspent’, are not included in the UTXO set, as they are provably unspendable and will never be needed to validate a future transaction. Prior to the Genesis upgrade in February 2020, one form of these outputs start with the OP\_RETURN opcode in the ScriptPubKey field of a transaction. Prior to this upgrade, The OP\_RETURN opcode ended the script execution and returned a failure. After the Genesis upgrade, this provably unspendable output will be unlocking scripts beginning with the opcodes OP\_FALSE OP\_RETURN. --- # Agent Instructions This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com. ## Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter: ``` GET https://hub.bsvblockchain.org/wiki/transactions-and-utxos/bitcoin-transactions/utxo.md?ask=&goal= ``` `ask` is the immediate question: it should be specific, self-contained, and written in natural language. `goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.