Unit Testing Plugin

Click the _images/a-user-testing-icon.pngdouble check icon to get to the Solidity Unit Testing plugin.

If you haven’t used this plugin before and are not seeing double check icon, you have to activate it from Remix plugin manager.

Go to the plugin manager (by click the _images/a-plug.pngplug icon) and load up the unit testing plugin.


Now double check icon will appear on the left side icon bar. Clicking on icon will load the unit testing module in the side panel.

Alternatively, just select Solidity environment from remix IDE home page. This will activate Solidity Unit Testing plugin along with Solidity Compiler, Deploy & Run Transactions & Solidity Static Analysis plugins.



Select a solidity file which you want to test and click on the button Generate. It will generate a new sample solidity test file in the current folder suffixed with _test. This file contains the minimum you need for running unit testing.

Write Tests

Write tests to check the functionality of your contract. Remix injects a built-in assert library which can be used for testing. Visit the library documentation here.

Apart from this, Remix allows usage of some special functions to make testing more structural. They are:

  • beforeEach() - Runs before each test
  • beforeAll() - Runs before all tests
  • afterEach() - Runs after each test
  • afterAll() - Runs after all tests

To get started, see this simple example.


Once you are done with writing tests, select the _test.sol files in the list and click on the button Run to execute the tests in the selected files. The execution will run in a separate environment and the result will be displayed below.



If you have selected multiple files to run the tests and want to stop the execution, click on Stop button. It will stop execution after running the tests for current file.


Remix facilitates users with various types of customizations to test a contract properly.

1. Custom Compiler Context

Solidity Unit Testing refers Solidity Compiler plugin for compiler configurations. One can provide customized inputs for Compiler, EVM Version & Enable Optimization and these will be the configuration settings used for contract compilation before running unit tests.


2. Custom Transaction Context

For a contract method interaction, prime parameters of transaction are from address, value & gas. Usually, we need to test a method’s behaviour under different values of these parameters.

Remix provides the functionality of custom msg.sender & msg.value of transaction using method devdoc like:

/// #sender: account-0
/// #value: 10
function checkSenderIs0AndValueis10 () public payable {
    Assert.equal(msg.sender, TestsAccounts.getAccount(0), "wrong sender in checkSenderIs0AndValueis10");
    Assert.equal(msg.value, 10, "wrong value in checkSenderIs0AndValueis10");

Things to keep in mind while using custom transaction context:

  1. Parameters must be defined in devdoc of related method
  2. Each parameter key should be prefixed with a hash (#) and end with a colon following a space () like #sender: & #value:
  3. For now, customization is available for parameters sender & value only
  4. Sender is from address of a transaction which is accessed using msg.sender inside a contract method. It should be defined in a fixed format as ‘account-<account_index>’
  5. <account_index> varies from 0-2 before remix-ide release v0.10.0 and 0-9 afterwards
  6. remix_accounts.sol must be imported in your test file to use custom sender
  7. Value is value sent along with a transaction in wei which is accessed using msg.value inside a contract method. It should be a number.

Regarding gas, Remix estimates the required gas for each transaction internally. Still if a contract deployment fails with Out-of-Gas error, it tries to redeploy it by doubling the gas. Deployment failing with double gas will show error: contract deployment failed after trying twice: The contract code couldn't be stored, please check your gas limit

Various test examples can be seen in examples section.

Points to remember

  • A test contract cannot have a method with parameters. Having one such method will show error: Method 'methodname' can not have parameters inside a test contract
  • Number of test accounts are 3 before remix-ide release v0.10.0 and 10 afterwards
  • A test file which imports remix_accounts.sol might not compile successfully with Solidity Compiler plugin but it will work fine with Solidity Unit Testing plugin.


remix-tests is the module which works underneath of remix-ide Solidity Unit Testing plugin.

remix-tests is an NPM package. It can also be used as a CLI/CI solution, supporting node.js. Find more information about this type of usage in the remix-tests repository

For CI implementation example, see Su Squares contract and Travis build that uses remix-tests for continuous integration testing.