I am testing the Foundry Fuzzing feature. Therefore I created this simple contract:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.0;

contract Vault {
bool public s_locked;
uint private s_password;

constructor(uint password) {
    s_locked = true;
    s_password = password;

function unlock(uint password) external {
    if (s_password == password) {
        s_locked = false;

And here its test:

// SPDX-License-Identifier: MIT
pragma solidity ^0.8.18;

import "forge-std/Test.sol";
import "forge-std/console.sol";
import "../src/Vault.sol";

contract VaultFuzzTest is Test {

Vault public vault;

function setUp() public {
    vault = new Vault(5);

function testFuzz_password(uint x) public {
  assertEq(vault.s_locked(), true);

When I run it, the test is passing successfully, however it shouldn't when the password is equal to 5. See output:

Test result: ok. 1 passed; 0 failed; 0 skipped; finished in 10.94ms
  Ran 1 test suites: 1 tests passed, 0 failed, 0 skipped (1 total tests)

That is why I was wondering, is Foundry just trying random 256 values (I assume all 256 values are big numbers) and if it doesn't find 5 among them, it just returns success. If so, is there a way I can fuzz my contract with more than 256 values, so eventually I get my test fail?

What else I have tried: adding vm.assume(x<10);

function testFuzz_password(uint x) public {
  assertEq(vault.s_locked(), true);

Now when I run my test it fails:

Failing tests:
Encountered 1 failing test in test/Vault.t.sol:VaultFuzzTest
[FAIL. Reason: Assertion failed. Counterexamplecalldata=0x0ce2d6010000000000000000000000000000000000000000000000000000000000000005, args=[5]] testFuzz_password(uint256) (runs: 6, μ: 13283, ~: 13283)

  Encountered a total of 1 failing tests, 0 tests succeeded

When I put a constraint where the password is assumed to be <10, then the fuzzing works as expected and throws an error.

Therefore I wanted to ask someone with more experience with Foundry to explain me how it works and should I increase the runs in order to achieve the same behaviour but without using vm.assume(x<10);

1 Answer 1


Ok, I found how to increase fuzz runs: Inside the foundry.toml Add this

runs = 1000000

But I don't know whether that is correct solution.

  • This is. The fuzzer tries semi-random variables as input, sort of like a brute-force attempting to crack the code. So the more runs, the better. Jul 29, 2023 at 19:50

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