A Bitcoin smart contract is a set of conditions that Bitcoin checks before allowing a particular transaction output to be spent. The value is locked in an unspent transaction output (UTXO); a later transaction can spend it only by providing data that satisfies the output’s script. The Bitcoin Developer Guide describes contracts as “transactions which use the decentralized Bitcoin system to enforce financial agreements.”
How a Bitcoin smart contract works
Bitcoin does not treat each coin as an account balance that an application continuously edits. Instead, transactions create outputs, and each output specifies how its value may be spent. Bitcoin nodes validate a proposed spend against those conditions.
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In technical terms, the core mechanism is Bitcoin Script and transaction validation. “Smart contract” is a useful broad term for arrangements built from those mechanisms, but it should not be confused with a continuously running application that can freely update arbitrary internal state.
What conditions can a Bitcoin contract enforce?
Bitcoin Script can combine conditions such as signatures, hash locks, and time locks. A signature condition can require authorization from one or more key holders; a hash lock can make spending depend on revealing data whose hash matches a specified value; and a time lock can delay a spend until a specified time or block-height condition is met.
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Miniscript is a structured way to represent and analyze a subset of Bitcoin Script. BIP 379 describes how this approach can help assess whether spending paths are permitted and analyze properties such as correctness, security, malleability, and resource use. BIP 379 is labeled Draft Informational, not an activated consensus upgrade.
Example: multisignature escrow
The Bitcoin Developer Guide illustrates escrow using a 2-of-3 multisignature arrangement. The customer, merchant, and arbitrator each contribute a public key, and two signatures are required to spend the funds. The script can be constructed to support different settlement paths:
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- The customer and merchant can agree and settle together.
- The customer and arbitrator can authorize a refund path.
- The merchant and arbitrator can authorize another agreed resolution.
The script enforces the chosen signature conditions; it does not determine whether the goods arrived as promised. Resolving a dispute about delivery or quality depends on the parties’ agreement and any arbitration process, not on the script independently interpreting real-world events.
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Taproot, specified by BIP 341, allows a Taproot output to be spent through a key path or a script path. A key-path spend provides a signature for the output key. A script-path spend reveals the selected committed script and the Merkle proof needed to show that it belongs to the output. If a spend uses one path, unused script alternatives need not be revealed.
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BIP 342 specifies Tapscript, the execution rules for Taproot script-path spends. It updates script execution and signature-opcode behavior and defines resource rules. These changes do not make Bitcoin an unrestricted virtual machine or turn its script language into a general-purpose application platform.
What a Bitcoin smart contract does not do
A Bitcoin contract is best understood as a rule for spending one or more outputs, not as an always-running program with arbitrary persistent state. Bitcoin Script has constrained execution and resource rules. Features that require unrestricted computation or application state carried forward and freely updated between transactions need additional mechanisms or a different execution environment.
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The Ethereum whitepaper offers a historical comparison, describing Bitcoin Script as limited in areas such as looping, persistent internal state, and access to blockchain data. That is the whitepaper’s comparative framing; the current mechanics of Bitcoin spending should be understood through Bitcoin’s protocol documentation and specifications.
What determines a contract’s security?
A contract is only as reliable as its specific script, transaction construction, signatures, and assumptions about participants or outside information. A script can ensure that a spend follows encoded conditions, but it cannot make an ambiguous agreement clear or verify an off-chain event on its own. In arrangements involving delivery, service, or disputes, the parties must decide how relevant facts will be established and who can authorize each outcome.
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