initialize [%here] must be called at the top level of any files that have inline or expect tests that use Rpc_parallel. Further, these calls must come before the definitions of the tests, but after the definitions of any workers used in the tests.

For example:

  let () = Rpc_parallel.For_testing.initialize [%here]

  let%expect_test "" =
    run_code_with_rpc_parallel ();
    [%expect {| output |}]
  ;;