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A smart contract is software deployed to a blockchain that runs when a user or another program calls it. On Ethereum, it consists of code and data stored at an address; a transaction can ask it to perform a defined action and record the resulting state change. The term “contract” describes the software, not a guarantee that it is legally binding.
What is a smart contract?
Ethereum.org defines a smart contract as “a program that runs on the Ethereum blockchain.” It is code plus data, often called its state, associated with an address on the network. Other blockchain platforms may use different execution environments, fee models, or terminology, so Ethereum’s details should not be treated as universal.
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The name can be confusing: a smart contract is not necessarily a legal contract. Whether an agreement is enforceable depends on the jurisdiction and circumstances. The technical label alone does not settle that question.
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How do smart contracts work?
- Deployment: A developer publishes the program to the blockchain through a transaction. The contract then has an address that users or applications can interact with.
- Invocation: A user or application submits a transaction calling a function and supplying inputs. On Ethereum, deployment and execution transactions incur network fees, commonly called gas.
- Execution and recording: The blockchain runs the function according to the contract’s code and the network’s rules. Any resulting state changes are recorded on-chain.
A vending machine is a limited analogy: valid inputs trigger a programmed result. A smart contract can handle more complex state and digital assets, and contracts can interact with one another through public interfaces. In every case, the outcome is bounded by the code, inputs, and blockchain environment.
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What are smart contracts used for?
They can automate actions involving data and digital assets. 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 software applications, not proof that a contract can observe every real-world event or that the application will behave as a user expects.
Can a smart contract use real-world information?
Not by itself. A blockchain contract cannot independently fetch facts such as the weather, a sports result, or whether a delivery arrived. Its execution needs deterministic inputs. An oracle or another external mechanism can supply off-chain information, but that creates a dependency: the contract relies on the mechanism and its data rather than making the outside fact native to the blockchain. Oracle systems differ in their data sources, architecture, and trust assumptions.
Can a deployed smart contract be changed?
Often, deployed code is difficult or impossible to change directly. Ethereum.org cautions that deployed code usually cannot be patched to fix security flaws, and assets stolen through vulnerabilities can be difficult to recover. Some systems use upgrade patterns that separate business logic into other contracts, allowing behavior to change. That flexibility shifts trust and control assumptions, so it matters who can authorize an upgrade and under what conditions.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallThere is no single rule that every smart contract is absolutely immutable or automatically upgradeable. The specific design and its controls determine what can change.
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How can users assess smart-contract safety?
- Check the intended action: Understand what a transaction or approval allows before signing. Public transaction data can be difficult to interpret, and blind signing—approving without understanding the details—carries risk.
- Distinguish source verification from safety: Source-code verification checks whether published high-level source corresponds to the bytecode deployed at an address. It does not prove that the code is correct or safe.
- Ask about specification and review: Formal verification examines whether behavior meets a defined specification. That is a different question from whether source code matches deployed bytecode, and neither label alone guarantees that an application is risk-free.
- Check upgrade authority: If the system can be upgraded, identify who controls that capability and what safeguards govern its use.
- Account for external dependencies: If an application relies on an oracle, consider the data source and trust model as part of its operation.
Ethereum.org’s security documentation describes the risks of unchangeable deployed code and difficult recovery after theft. Those risks make careful transaction review important; visibility on a public blockchain does not automatically make contract behavior easy to understand.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Is there a smart-contract size limit?
Ethereum.org states a maximum contract size of 24 KB. This is an Ethereum-specific technical detail, not a general blockchain limit, and protocol rules can evolve. Check current Ethereum documentation before relying on the figure for a deployment or design decision.
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