property-based-testing

Use a roundtrip property asserting that decode(encode(x)) equals the original input. In Python with Hypothesis, define a strategy for your message type with st.builds, then apply @given to generate inputs and assert the roundtrip holds.

Use Hypothesis for Python, fast-check for JavaScript/TypeScript, proptest for Rust, rapid for Go, and jqwik for Java. For Solidity smart contracts, use Echidna or Medusa fuzzers to test state invariants.

Avoid property-based testing for simple CRUD operations without transformation logic, UI code, integration tests with complex external setup, and code with side effects that cannot be isolated like network calls or database writes.

First verify the property against docstrings, type hints, and documented preconditions. Failures may indicate a genuine bug, an over-constrained strategy generating invalid inputs, or an ambiguous specification that needs clarification before reporting.

Use Echidna or Medusa fuzzers to test state invariants in EVM contracts. Write functions prefixed with echidna_ that return a boolean invariant, such as verifying balances remain non-negative, and the fuzzer generates transaction sequences to violate them.

Critical issues include tautological assertions comparing an expression to itself and vacuous tests where contradictory assume() calls filter out all inputs. Also watch for tests that reimplement the function logic or only assert no exception occurs.