unchecked-arithmetic

Purpose

Identify arithmetic operations where the overflow/underflow invariant is provable from the surrounding code logic, then wrap those operations in unchecked {} to skip Solidity 0.8+'s automatic overflow guards (~30 gas per operation). Always require an explicit inline comment documenting the invariant. Never recommend unchecked without proof.

When to Use This Skill

Use when:

• Claude reviews a subtraction operation that is immediately preceded by a check proving the minuend is greater than or equal to the subtrahend
• Claude writes loop counter increments where the bound is proven by a for condition
• Claude reviews arithmetic on values proven bounded by type constraints or prior validation (e.g., a uint8 counter that cannot exceed 255)
• The user asks whether a specific arithmetic expression is safe to wrap in unchecked

When NOT to Use This Skill

Do NOT use for:

• Any arithmetic where the overflow/underflow condition is not explicitly proven in the same function scope — "it looks safe" is not proof; document the invariant in a comment or do not apply
• User-controlled inputs that have not been validated by a prior bounds check
• Business-critical arithmetic (token balances, prices, collateral ratios) unless fuzz test coverage specifically targeting boundary values is confirmed to exist — always document the fuzz test name in the finding
• Operations inside re-entrant paths where state can change between the check and the arithmetic (confirm reentrancy guard present or CEI pattern followed before applying)
• Solidity versions before 0.8.0 — those have no overflow checks to remove
• When uncertain about overflow safety: state "Cannot confirm bounds — do not apply unchecked without fuzz test proving safety" rather than guessing

Rationalizations to Reject