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A smart contract is software deployed on a blockchain that runs when a user or another program invokes it. On Ethereum, it has an address and stores code and data, called its state. It can automatically carry out defined actions, but it only works with the logic and inputs available to it; the word “contract” does not, by itself, mean the software is a legally enforceable agreement.
What is a smart contract?
Ethereum.org puts it simply: “A ‘smart contract’ is simply a program that runs on the Ethereum blockchain.” The program is deployed at an Ethereum address, where its code and data reside. Users and applications interact with it by sending transactions that call its functions. Ethereum.org’s smart-contract overview describes the concept and its components.
The name can be confusing. A smart contract is software that can carry out programmed actions; the technical term does not establish that a legal agreement exists or is enforceable. Legal treatment depends on the jurisdiction and circumstances.
How do smart contracts work?
- A developer deploys the code. Deployment submits the contract to the blockchain, where it is associated with an address.
- A user or application calls it. The caller submits a transaction with a function and its inputs.
- The blockchain executes the defined logic. The network processes the transaction according to the contract code and blockchain rules.
- The resulting state is recorded. If execution changes the contract’s data or other blockchain state, that change is recorded on-chain.
On Ethereum, deployment and execution are transactions and can incur network fees, commonly called gas. The amount depends on the transaction and network conditions; there is no single fee implied by the definition. See Ethereum.org’s smart-contract documentation and Ethereum.org’s explanation of gas.
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A vending machine is a useful but limited analogy: valid inputs trigger a programmed result. A smart contract can handle more complex data and digital assets, and contracts can interact with other contracts through their interfaces. Unlike a machine that can sense its surroundings, however, a blockchain contract cannot independently know what happened in the physical world.
What are smart contracts used for?
Smart contracts can automate actions involving digital assets and data. Ethereum.org gives examples such as stablecoins, unique digital assets, exchanges, games, and insurance policies designed to pay when encoded conditions are met. These are possible applications, not a guarantee that a contract can observe or correctly evaluate every real-world condition. Ethereum.org’s overview describes these uses.
What can a smart contract know about the real world?
A contract can process information available on its blockchain, but it cannot independently retrieve a weather report, sports result, or delivery confirmation from an off-chain source. To use such information, a system needs an external mechanism—often called an oracle—to deliver it to the blockchain. That creates a dependency on the oracle’s sources and design; it does not make the external fact native to the chain. Oracle systems vary in how they obtain and relay data. Ethereum.org’s oracle documentation explains the role of these services.
Can smart contracts be changed after deployment?
Do not assume that every contract can be edited like an ordinary application or that every contract is permanently unchangeable. Ethereum.org warns that deployed code usually cannot be changed to patch flaws. Some systems use upgrade patterns that route behavior through other contracts, allowing changes but introducing a control question: who can authorize an upgrade, and under what conditions? The answer depends on the particular design. Ethereum.org’s security guidance discusses the risks of deployed code, while its upgrade documentation describes upgrade approaches.
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How can you check a smart contract, and what does verification prove?
Source-code verification and formal verification answer different questions. Source-code verification checks whether published high-level source code compiles to the bytecode deployed at a particular address. It helps readers inspect what was deployed, but does not prove that the code behaves safely or matches a user’s expectations. Formal verification examines whether behavior satisfies a specification; it is a separate assurance technique, not a consequence of publishing source code. Ethereum.org’s verification documentation explains source verification.
Even when code and transaction data are public, they may be difficult for a nontechnical user to interpret. A request to approve or sign a transaction can have effects that are not obvious from a wallet prompt; Ethereum.org describes this risk as blind signing. Before approving an interaction, check which contract and action are involved and whether the request makes sense for the task. Ethereum.org’s smart-contract security guidance covers this risk.
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What risks should users understand?
- Bugs can have lasting effects. If deployed code cannot be patched, a flaw may remain exploitable. Ethereum.org warns that assets stolen through smart-contract vulnerabilities can be very difficult to recover.
- Execution follows code, not intent. A contract carries out its programmed rules; it does not automatically correct a mistaken input or an incomplete design.
- External data adds dependencies. If an action relies on off-chain facts, the oracle or other delivery mechanism becomes part of the system’s trust and security assumptions.
- Upgrade controls matter. Where upgrades are possible, the parties or mechanisms controlling them can change how the system behaves.
- Public does not always mean understandable. Raw code and transaction data can be hard to interpret, and signing without understanding a transaction can expose assets.
These risks are not unique in form to Ethereum, but the specifics of execution, fees, verification, and upgrades vary by blockchain platform. Ethereum.org’s technical guidance is specific to Ethereum and should not be treated as a ranked comparison of chains.
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Not necessarily. “Smart contract” names a software mechanism, not a legal conclusion. Whether a particular arrangement is legally binding depends on its terms, facts, and applicable jurisdiction. A technical explanation cannot determine that status for a specific transaction; consult a qualified lawyer for jurisdiction-specific advice.
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