Does the Rust engine run each Python test in a separate process?

Two mechanisms. Plugs run as persistent processes, one per plug instance, addressed over a local TCP socket, so instrument objects and their state stay alive across phases. Test phases run on a pool of worker processes, which is what isolates a crashing phase from the rest of the run. Both are standard Python subprocesses, not an embedded interpreter, and the environment and dependencies are managed automatically with uv.