The first thing I can see from your explanation is that you mentioned bricks depend on another brick, testing, in their deps.edn. The most crucial rule in Polylith is that bricks do not depend on each other through deps.edn. They depend on interfaces, and which implementation will be included in the classpath will be decided at the project level. If you have a project named project_name, and if you want to use testing brick while testing that project, you can include the testing brick in the :test alias of the project_name project’s deps.edn. An example of this can be found in the Polylith repository. https://github.com/polyfy/polylith/blob/408ef27ecad00d3443120f2340fd0d95fa642535/projects/poly/deps.edn#L43, the poly project uses the test-helper brick while testing, and https://github.com/polyfy/polylith/blob/408ef27ecad00d3443120f2340fd0d95fa642535/components/creator/test/polylith/clj/core/creator/project_test.clj#L3 is an example of how the creator brick uses the test-helper. These two pages explain how testing works with Polylith: • https://cljdoc.org/d/polylith/clj-poly/0.2.19/doc/testing • https://cljdoc.org/d/polylith/clj-poly/0.2.19/doc/test-runners Without seeing the entire setup and configuration, it’s hard to tell where the problem is. Since poly doesn’t do any magic, it always comes down to what you have in the classpath.

"not being able to find the suitable driver and throws errors" -- this sounds a bit like the classloader isolation problem with tests that some people have gotten around by switching to the external test runner (which uses process isolation instead).

Problem solved! it all helped alot.