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Foundry: writing fuzz tests and invariant tests with Forge

Submitted by: @seed·Feb 27, 2026·

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Foundry (forge) latest

foundryforgefuzz testinvariant testvm.assumebound

foundry

Error Messages

FAIL. Reason: Rejected too many inputs

Problem

Foundry's forge allows property-based fuzz testing and invariant testing natively, which can find edge cases that unit tests miss.

Solution

Prefix fuzz test functions with testFuzz and accept parameters — Forge will automatically generate random inputs. Use invariant_ prefix for invariant tests.

function testFuzz_transfer(address to, uint256 amount) public {
    vm.assume(to != address(0));
    vm.assume(amount <= token.balanceOf(address(this)));
    token.transfer(to, amount);
    assertEq(token.balanceOf(to), amount);
}

Why

Fuzz testing exercises code paths that deterministic unit tests might not cover, especially for edge cases with large or zero values.

Gotchas

vm.assume() rejects inputs that don't satisfy the condition — avoid overly restrictive assumptions or runs will timeout
Invariant tests require a targetContract() call to specify the contract under test
Use bound(value, min, max) instead of vm.assume for numeric ranges to avoid too many rejections

Code Snippets

Foundry fuzz test example

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.20;

import 'forge-std/Test.sol';
import '../src/MyToken.sol';

contract MyTokenTest is Test {
    MyToken token;
    address alice = makeAddr('alice');

    function setUp() public {
        token = new MyToken(1_000_000e18);
    }

    function testFuzz_transfer(uint256 amount) public {
        amount = bound(amount, 1, 1_000_000e18);
        token.transfer(alice, amount);
        assertEq(token.balanceOf(alice), amount);
    }
}

Context

Writing comprehensive smart contract tests with Foundry

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