When should I run Bitcoin Core functional tests?

Functional testing is part of the testing framework used in Bitcoin Core development.

When developers work on proposed changes to their code, they regularly run functional tests and other tests to validate those changes. It’s up to the developer to decide which tests are helpful. Often, significant changes also require changes or additions to the testing framework.

In fact, if a change is proposed, every Testing on various platforms is performed automatically by the Continuous Integration (CI) infrastructure. Maintainers do not merge changes that do not pass all tests.