A developer should define a contract’s permitted behavior and trust assumptions first, then write Solidity code that makes state changes explicit, test expected and failure cases, and obtain an appropriately qualified independent review before considering deployment. The escrow example discussed below is illustrative only: code cannot determine whether an off-chain event, such as delivery of a physical item, actually occurred, and the example is not a production recommendation.

Define the contract’s scope and trust assumptions

Before coding, describe the contract’s intended behavior in plain language. Identify the information it must retain, the actions it should permit, and the outcomes callers should be able to observe. Then specify the conditions for each state-changing operation: who may request it, what must be true beforehand, and how the contract’s state should change afterward. This small specification gives you something concrete to compare against the code and tests.

A state is the set of values the program uses to represent its current condition. A state transition is a change from one defined condition to another after an operation. For example, an escrow workflow might distinguish funds being held from funds being released. Describing those conditions first helps reveal missing paths, such as what should happen if a caller attempts an operation at the wrong stage.

Separate on-chain rules from off-chain assumptions. A contract applies programmed behavior to its inputs and state; it does not automatically establish every fact about the physical or business world. Solidity’s safe remote purchase example holds buyer and seller deposits until the buyer confirms receipt, then releases funds according to its state transitions. It cannot independently determine whether the physical item arrived. The workflow therefore depends on a participant’s confirmation, rather than on the code verifying delivery. That limitation belongs in the design description and review checklist. source

Also identify dependencies before implementation. A contract may call or reuse other on-chain components through public interfaces. This composability can connect programs, but every dependency adds behavior, upgrade, permission, availability, and economic assumptions that the developer must account for. A specification should name such dependencies and explain what the contract expects from them, without treating an interface as proof that the component is suitable. source

Keep the example’s scope narrow. It can illustrate how to reason about permissions, states, and transitions, but it does not decide the right business policy or resolve off-chain disputes. Do not copy an illustrative flow into a real application without separately specifying its requirements and reviewing its complete behavior.

Read the Solidity code for smart contracts as distinct components

A smart contract is a reusable program deployed into blockchain state and invoked through transaction requests. On Ethereum, a user account can invoke a contract function by submitting a transaction to the deployed contract. This is the protocol interaction: an application or wallet may help prepare and present a request, while the contract’s programmed behavior governs what happens when the request is processed. source source

When reading Solidity source, identify the parts and their responsibilities:

  • The contract declaration groups the code that defines the contract.
  • State variables represent information the contract retains between operations.
  • Functions define operations that callers can request. Some operations read information; others change state.
  • Visibility choices express how a function or variable is exposed for use. Review who can reach each operation and whether that access matches the specification.
  • Mutability choices describe whether a function is intended to change state or instead operate without a state change. Check the exact meaning and constraints against the documentation for the compiler version selected for the project.

Do not infer access control from a function’s name or from the interface presented by an application. For each state-changing function, follow its path from entry to exit: what inputs it receives, what conditions it checks, which stored values it updates, and what outcomes a caller can observe. Ask whether an unauthorized caller can reach the same path and whether an operation can be repeated or called in an unexpected order. Those questions tie the source back to the behavior specification.

A useful review technique is to write a short transition table before relying on code. List the initial condition, the requested action, the caller and preconditions, and the expected resulting condition. Then compare each row to a function and its checks. If a behavior has no corresponding specified transition, do not assume it is intended. If a transition has no obvious code path, investigate whether the implementation is incomplete or the specification is unclear.

Compiler compatibility is another review item, not a detail to guess from an example. The language features accepted by a project must be checked against the actual compiler version and configuration used. This guide does not prescribe a version, present a deployable code listing, or claim that compilation establishes correctness. A compiler accepting source under a configuration is only one technical step; the intended behavior and assumptions still need separate review.

Test expected behavior and failure cases

Translate each requirement into a test case. For a permitted call, check that the operation succeeds under the stated conditions, that the resulting state matches the specification, and that any relevant return value or observable outcome is correct. Keep the test’s purpose explicit so a passing result shows which behavior was exercised.

Test prohibited behavior as deliberately as expected behavior. Include calls by unauthorized users, invalid inputs, and attempts to act when the contract is in the wrong state. A failure test should check that the operation is rejected and that it does not create a state change the specification forbids. If the implementation returns or signals a particular error, verify that behavior only where it is part of the project’s stated requirements.

Then examine boundaries and sequences. Test the smallest and largest permitted values, repeated calls, and calls made in a different order from the ordinary workflow. For a bounded escrow-style state machine, check that each allowed transition works and that a caller cannot skip or repeat a transition if the specification disallows it. Consider both successful completion and failure at intermediate steps. These cases help expose gaps that a single “happy path” test would miss.

Review the test setup as part of the evidence. Confirm which initial state each case assumes, which caller is acting, and which conditions the test changes. When a test fails, first check whether the specification, test, or implementation is inconsistent; do not simply weaken the test to make the suite pass. Keep a record of behaviors that are not covered or depend on off-chain confirmation.

Passing tests show that the tested cases behaved as expected under the test conditions. They do not establish that every possible input, ordering, dependency, or failure mode is safe. In particular, a test cannot turn an off-chain claim into a fact the contract independently verified. Treat test results as one input to review, not as a blanket security conclusion.

Review security and deployment limits

Before considering deployment, compare the implementation against the specification and examine permission boundaries, state changes, failure behavior, and dependencies. Ask whether each function has the intended callers, whether conditions are checked before changing state, and whether unusual sequences leave the contract in a state the design did not anticipate. If a function interacts with another component, include that component’s behavior and assumptions in the review rather than examining the caller in isolation.

Use current authoritative documentation for Solidity syntax and compiler behavior, and for any development or testing tools selected. Tool commands, supported behavior, and compiler details should be verified for the actual project setup. This guide does not name a tool or claim that a particular workflow, build result, or test command establishes production readiness.

Seek independent review by a suitably qualified security professional appropriate to the contract’s scope and risk. Give the reviewer the specification, source, tests, assumptions, and dependency information, including any steps that rely on a person or external system. The review should be able to challenge both the code and the assumptions behind it. A tutorial, compilation, automated test results, and independent review are distinct forms of evidence; none should be misrepresented as a guarantee that the contract is secure.

Keep conclusions about code separate from legal, regulatory, tax, and compliance conclusions. This guide does not determine what obligations apply in a jurisdiction, and a Solidity implementation does not itself settle those questions. Any jurisdiction-specific statement requires current authoritative evidence and appropriate expertise; do not infer it from a function, a successful build, or a test result.

Similarly, compiling or testing does not establish that deployment is permitted, that an application or service will be available, or that fees, recovery, or compliance conditions have been met. Those are separate matters requiring their own evidence. Before moving beyond an illustrative exercise, document the intended behavior, authorized callers, state transitions, off-chain assumptions, test coverage, dependencies, and limits of review. This workflow helps developers understand what the code does and what remains unverified; it does not certify a contract for production use.

Sources and references