Which concept involves writing tests before actually coding in agile development?

Test-Driven Development, or TDD, is a core practice in agile development that emphasizes writing tests before the actual coding phase begins. The TDD process typically involves a cycle of writing a test for a new feature, running that test (which initially fails since the feature isn't implemented yet), implementing the code necessary to pass the test, and then refactoring the code while ensuring that all tests continue to pass.

This approach ensures that testing drives the development process, leading to better software design and more reliable code, as each piece of functionality is built with tests in mind. TDD helps in clarifying requirements, fostering continual improvement and ensuring that new features do not break existing functionality through automated tests. This practice aligns well with the agile principles of iterative development and customer feedback, making it a fundamental concept in agile environments.

The other concepts listed do not focus specifically on the practice of writing tests prior to coding. Documentation-Driven Development emphasizes documentation as a primary driver of development activities, Feature-Driven Development focuses on building features through a structured approach, and Code-First Development generally prioritizes coding before testing or other activities.