diff --git a/examples/integration/mpi4py/__init__.py b/examples/integration/mpi4py/__init__.py
new file mode 100644
index 00000000..e69de29b
diff --git a/examples/integration/mpi4py/_child.py b/examples/integration/mpi4py/_child.py
new file mode 100644
index 00000000..e9d7186a
--- /dev/null
+++ b/examples/integration/mpi4py/_child.py
@@ -0,0 +1,5 @@
+import os
+
+
+async def child_fn() -> str:
+    return f"child OK  pid={os.getpid()}"
diff --git a/examples/integration/mpi4py/inherit_parent_main.py b/examples/integration/mpi4py/inherit_parent_main.py
new file mode 100644
index 00000000..60e30a95
--- /dev/null
+++ b/examples/integration/mpi4py/inherit_parent_main.py
@@ -0,0 +1,50 @@
+"""
+Integration test: spawning tractor actors from an MPI process.
+
+When a parent is launched via ``mpirun``, Open MPI sets ``OMPI_*`` env
+vars that bind ``MPI_Init`` to the ``orted`` daemon.  Tractor children
+inherit those env vars, so if ``inherit_parent_main=True`` (the default)
+the child re-executes ``__main__``, re-imports ``mpi4py``, and
+``MPI_Init_thread`` fails because the child was never spawned by
+``orted``::
+
+    getting local rank failed
+      --> Returned value No permission (-17) instead of ORTE_SUCCESS
+
+Passing ``inherit_parent_main=False`` and placing RPC functions in a
+separate importable module (``_child``) avoids the re-import entirely.
+
+Usage::
+
+    mpirun --allow-run-as-root -np 1 python -m \
+        examples.integration.mpi4py.inherit_parent_main
+"""
+
+from mpi4py import MPI
+
+import os
+import trio
+import tractor
+
+from ._child import child_fn
+
+
+async def main() -> None:
+    rank = MPI.COMM_WORLD.Get_rank()
+    print(f"[parent] rank={rank}  pid={os.getpid()}", flush=True)
+
+    async with tractor.open_nursery(start_method='trio') as an:
+        portal = await an.start_actor(
+            'mpi-child',
+            enable_modules=[child_fn.__module__],
+            # Without this the child replays __main__, which
+            # re-imports mpi4py and crashes on MPI_Init.
+            inherit_parent_main=False,
+        )
+        result = await portal.run(child_fn)
+        print(f"[parent] got: {result}", flush=True)
+        await portal.cancel_actor()
+
+
+if __name__ == "__main__":
+    trio.run(main)
diff --git a/tests/test_spawning.py b/tests/test_spawning.py
index 283d1785..7e230085 100644
--- a/tests/test_spawning.py
+++ b/tests/test_spawning.py
@@ -1,5 +1,12 @@
 """
-Spawning basics
+Spawning basics including audit of,
+
+- subproc bootstrap, such as subactor runtime-data/config inheritance,
+- basic (and mostly legacy) `ActorNursery` subactor starting and
+  cancel APIs.
+
+Simple (and generally legacy) examples from the original
+API design.
 
 """
 from functools import partial
@@ -98,7 +105,9 @@ async def movie_theatre_question():
 
 
 @tractor_test
-async def test_movie_theatre_convo(start_method):
+async def test_movie_theatre_convo(
+    start_method: str,
+):
     '''
     The main ``tractor`` routine.
 
@@ -151,13 +160,16 @@ async def test_most_beautiful_word(
                 name='some_linguist',
             )
 
-            print(await portal.result())
+            res: Any = await portal.wait_for_result()
+            assert res == return_value
     # The ``async with`` will unblock here since the 'some_linguist'
     # actor has completed its main task ``cellar_door``.
 
     # this should pull the cached final result already captured during
     # the nursery block exit.
-    print(await portal.result())
+    res: Any = await portal.wait_for_result()
+    assert res == return_value
+    print(res)
 
 
 async def check_loglevel(level):
@@ -168,16 +180,24 @@ async def check_loglevel(level):
     log.critical('yoyoyo')
 
 
+@pytest.mark.parametrize(
+    'level', [
+        'debug',
+        'cancel',
+        'critical'
+    ],
+    ids='loglevel={}'.format,
+)
 def test_loglevel_propagated_to_subactor(
-    start_method,
-    capfd,
-    reg_addr,
+    capfd: pytest.CaptureFixture,
+    start_method: str,
+    reg_addr: tuple,
+    level: str,
 ):
     if start_method == 'mp_forkserver':
         pytest.skip(
-            "a bug with `capfd` seems to make forkserver capture not work?")
-
-    level = 'critical'
+            "a bug with `capfd` seems to make forkserver capture not work?"
+        )
 
     async def main():
         async with tractor.open_nursery(
@@ -197,3 +217,121 @@ def test_loglevel_propagated_to_subactor(
     # ensure subactor spits log message on stderr
     captured = capfd.readouterr()
     assert 'yoyoyo' in captured.err
+
+
+async def check_parent_main_inheritance(
+    expect_inherited: bool,
+) -> bool:
+    '''
+    Assert that the child actor's ``_parent_main_data`` matches the
+    ``inherit_parent_main`` flag it was spawned with.
+
+    With the trio spawn backend the parent's ``__main__`` bootstrap
+    data is captured and forwarded to each child so it can replay
+    the parent's ``__main__`` as ``__mp_main__``, mirroring the
+    stdlib ``multiprocessing`` bootstrap:
+    https://docs.python.org/3/library/multiprocessing.html#the-spawn-and-forkserver-start-methods
+
+    When ``inherit_parent_main=False`` the data dict is empty
+    (``{}``) so no fixup ever runs and the child keeps its own
+    ``__main__`` untouched.
+
+    NOTE: under `pytest` the parent ``__main__`` is
+    ``pytest.__main__`` whose ``_fixup_main_from_name()`` is a no-op
+    (the name ends with ``.__main__``), so we cannot observe
+    a difference in ``sys.modules['__main__'].__name__`` between the
+    two modes.  Checking ``_parent_main_data`` directly is the most
+    reliable verification that the flag is threaded through
+    correctly; a ``RemoteActorError[AssertionError]`` propagates on
+    mismatch.
+
+    '''
+    import tractor
+    actor: tractor.Actor = tractor.current_actor()
+    has_data: bool = bool(actor._parent_main_data)
+    assert has_data == expect_inherited, (
+        f'Expected _parent_main_data to be '
+        f'{"non-empty" if expect_inherited else "empty"}, '
+        f'got: {actor._parent_main_data!r}'
+    )
+    return has_data
+
+
+def test_run_in_actor_can_skip_parent_main_inheritance(
+    start_method: str,  # <- only support on `trio` backend rn.
+):
+    '''
+    Verify ``inherit_parent_main=False`` on ``run_in_actor()``
+    prevents parent ``__main__`` data from reaching the child.
+
+    '''
+    if start_method != 'trio':
+        pytest.skip(
+            'parent main-inheritance opt-out only affects the trio backend'
+        )
+
+    async def main():
+        async with tractor.open_nursery(start_method='trio') as an:
+
+            # Default: child receives parent __main__ bootstrap data
+            replaying = await an.run_in_actor(
+                check_parent_main_inheritance,
+                name='replaying-parent-main',
+                expect_inherited=True,
+            )
+            await replaying.result()
+
+            # Opt-out: child gets no parent __main__ data
+            isolated = await an.run_in_actor(
+                check_parent_main_inheritance,
+                name='isolated-parent-main',
+                inherit_parent_main=False,
+                expect_inherited=False,
+            )
+            await isolated.result()
+
+    trio.run(main)
+
+
+def test_start_actor_can_skip_parent_main_inheritance(
+    start_method: str,  # <- only support on `trio` backend rn.
+):
+    '''
+    Verify ``inherit_parent_main=False`` on ``start_actor()``
+    prevents parent ``__main__`` data from reaching the child.
+
+    '''
+    if start_method != 'trio':
+        pytest.skip(
+            'parent main-inheritance opt-out only affects the trio backend'
+        )
+
+    async def main():
+        async with tractor.open_nursery(start_method='trio') as an:
+
+            # Default: child receives parent __main__ bootstrap data
+            replaying = await an.start_actor(
+                'replaying-parent-main',
+                enable_modules=[__name__],
+            )
+            result = await replaying.run(
+                check_parent_main_inheritance,
+                expect_inherited=True,
+            )
+            assert result is True
+            await replaying.cancel_actor()
+
+            # Opt-out: child gets no parent __main__ data
+            isolated = await an.start_actor(
+                'isolated-parent-main',
+                enable_modules=[__name__],
+                inherit_parent_main=False,
+            )
+            result = await isolated.run(
+                check_parent_main_inheritance,
+                expect_inherited=False,
+            )
+            assert result is False
+            await isolated.cancel_actor()
+
+    trio.run(main)
diff --git a/tractor/runtime/_runtime.py b/tractor/runtime/_runtime.py
index 0ffc6112..477d8f9b 100644
--- a/tractor/runtime/_runtime.py
+++ b/tractor/runtime/_runtime.py
@@ -119,6 +119,7 @@ from ..discovery._discovery import get_registry
 from ._portal import Portal
 from . import _state
 from ..spawn import _mp_fixup_main
+from ..spawn._mp_fixup_main import ParentMainData
 from . import _rpc
 
 if TYPE_CHECKING:
@@ -218,7 +219,7 @@ class Actor:
         return self._ipc_server
 
     # Information about `__main__` from parent
-    _parent_main_data: dict[str, str]
+    _parent_main_data: ParentMainData
     _parent_chan_cs: CancelScope|None = None
     _spawn_spec: msgtypes.SpawnSpec|None = None
 
@@ -240,10 +241,11 @@ class Actor:
         name: str,
         uuid: str,
         *,
-        enable_modules: list[str] = [],
+        enable_modules: list[str] | None = None,
         loglevel: str|None = None,
         registry_addrs: list[Address]|None = None,
         spawn_method: str|None = None,
+        inherit_parent_main: bool = True,
 
         arbiter_addr: UnwrappedAddress|None = None,
 
@@ -265,12 +267,15 @@ class Actor:
         self._cancel_called_by: tuple[str, tuple]|None = None
         self._cancel_called: bool = False
 
-        # retreive and store parent `__main__` data which
+        # retrieve and store parent `__main__` data which
         # will be passed to children
-        self._parent_main_data = _mp_fixup_main._mp_figure_out_main()
+        self._parent_main_data: ParentMainData = _mp_fixup_main._mp_figure_out_main(
+            inherit_parent_main=inherit_parent_main,
+        )
 
         # TODO? only add this when `is_debug_mode() == True` no?
         # always include debugging tools module
+        enable_modules = list(enable_modules or [])
         if _state.is_root_process():
             enable_modules.append('tractor.devx.debug._tty_lock')
 
@@ -547,11 +552,15 @@ class Actor:
 
         '''
         try:
-            if self._spawn_method == 'trio':
-                parent_data = self._parent_main_data
+            if (
+                self._spawn_method == 'trio'
+                and
+                (parent_data := self._parent_main_data)
+            ):
                 if 'init_main_from_name' in parent_data:
                     _mp_fixup_main._fixup_main_from_name(
                         parent_data['init_main_from_name'])
+
                 elif 'init_main_from_path' in parent_data:
                     _mp_fixup_main._fixup_main_from_path(
                         parent_data['init_main_from_path'])
diff --git a/tractor/runtime/_supervise.py b/tractor/runtime/_supervise.py
index 3cd7d4c7..6d2d573f 100644
--- a/tractor/runtime/_supervise.py
+++ b/tractor/runtime/_supervise.py
@@ -194,18 +194,26 @@ class ActorNursery:
         loglevel: str|None = None,  # set log level per subactor
         debug_mode: bool|None = None,
         infect_asyncio: bool = False,
+        inherit_parent_main: bool = True,
 
         # TODO: ideally we can rm this once we no longer have
         # a `._ria_nursery` since the dependent APIs have been
         # removed!
         nursery: trio.Nursery|None = None,
-        proc_kwargs: dict[str, any] = {}
+        proc_kwargs: dict[str, typing.Any] | None = None,
 
     ) -> Portal:
         '''
         Start a (daemon) actor: an process that has no designated
         "main task" besides the runtime.
 
+        Pass ``inherit_parent_main=False`` to keep this child on its
+        own bootstrap module for the trio spawn backend instead of
+        applying the parent ``__main__`` re-exec fixup during startup.
+        This does not affect ``multiprocessing`` ``spawn`` or
+        ``forkserver`` which reconstruct the parent's ``__main__`` as
+        part of their normal stdlib bootstrap.
+
         '''
         __runtimeframe__: int = 1  # noqa
         loglevel: str = (
@@ -224,7 +232,8 @@ class ActorNursery:
             _rtv['_debug_mode'] = debug_mode
             self._at_least_one_child_in_debug = True
 
-        enable_modules = enable_modules or []
+        enable_modules = list(enable_modules or [])
+        proc_kwargs = dict(proc_kwargs or {})
 
         if rpc_module_paths:
             warnings.warn(
@@ -242,6 +251,7 @@ class ActorNursery:
             # modules allowed to invoked funcs from
             enable_modules=enable_modules,
             loglevel=loglevel,
+            inherit_parent_main=inherit_parent_main,
 
             # verbatim relay this actor's registrar addresses
             registry_addrs=current_actor().registry_addrs,
@@ -289,7 +299,8 @@ class ActorNursery:
         enable_modules: list[str] | None = None,
         loglevel: str | None = None,  # set log level per subactor
         infect_asyncio: bool = False,
-        proc_kwargs: dict[str, any] = {},
+        inherit_parent_main: bool = True,
+        proc_kwargs: dict[str, typing.Any] | None = None,
 
         **kwargs,  # explicit args to ``fn``
 
@@ -310,6 +321,7 @@ class ActorNursery:
             # use the explicit function name if not provided
             name = fn.__name__
 
+        proc_kwargs = dict(proc_kwargs or {})
         portal: Portal = await self.start_actor(
             name,
             enable_modules=[mod_path] + (
@@ -320,6 +332,7 @@ class ActorNursery:
             # use the run_in_actor nursery
             nursery=self._ria_nursery,
             infect_asyncio=infect_asyncio,
+            inherit_parent_main=inherit_parent_main,
             proc_kwargs=proc_kwargs
         )
 
diff --git a/tractor/spawn/_mp_fixup_main.py b/tractor/spawn/_mp_fixup_main.py
index 11d5f1c6..2bd19d1b 100644
--- a/tractor/spawn/_mp_fixup_main.py
+++ b/tractor/spawn/_mp_fixup_main.py
@@ -14,103 +14,72 @@
 # You should have received a copy of the GNU Affero General Public License
 # along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
-"""
-Helpers pulled mostly verbatim from ``multiprocessing.spawn``
+'''
+(Originally) Helpers pulled verbatim from ``multiprocessing.spawn``
 to aid with "fixing up" the ``__main__`` module in subprocesses.
 
-These helpers are needed for any spawing backend that doesn't already
-handle this. For example when using ``trio_run_in_process`` it is needed
-but obviously not when we're already using ``multiprocessing``.
+Now just delegates directly to appropriate `mp.spawn` fns.
 
-"""
-import os
-import sys
-import platform
-import types
-import runpy
+Note
+----
+These helpers are needed for any spawning backend that doesn't already
+handle this. For example it's needed when using our
+`start_method='trio'` backend but not when we're already using
+a ``multiprocessing`` backend such as 'mp_spawn', 'mp_forkserver'.
+
+?TODO?
+- what will be required for an eventual subint backend?
+
+The helpers imported from `mp.spawn` provide the stdlib's
+spawn/forkserver bootstrap that rebuilds the parent's `__main__` in
+a fresh child interpreter. In particular, we capture enough info to
+later replay the parent's main module as `__mp_main__` (or by path)
+in the child process.
+
+See:
+https://docs.python.org/3/library/multiprocessing.html#the-spawn-and-forkserver-start-methods
+
+'''
+import multiprocessing as mp
+from multiprocessing.spawn import (
+    _fixup_main_from_name as _fixup_main_from_name,
+    _fixup_main_from_path as _fixup_main_from_path,
+    get_preparation_data,
+)
+from typing import NotRequired
+from typing import TypedDict
 
 
-ORIGINAL_DIR = os.path.abspath(os.getcwd())
+class ParentMainData(TypedDict):
+    init_main_from_name: NotRequired[str]
+    init_main_from_path: NotRequired[str]
 
 
-def _mp_figure_out_main() -> dict[str, str]:
-    """Taken from ``multiprocessing.spawn.get_preparation_data()``.
+def _mp_figure_out_main(
+    inherit_parent_main: bool = True,
+) -> ParentMainData:
+    '''
+    Delegate to `multiprocessing.spawn.get_preparation_data()`
+    when `inherit_parent_main=True`.
 
-    Retrieve parent actor `__main__` module data.
-    """
-    d = {}
-    # Figure out whether to initialise main in the subprocess as a module
-    # or through direct execution (or to leave it alone entirely)
-    main_module = sys.modules['__main__']
-    main_mod_name = getattr(main_module.__spec__, "name", None)
-    if main_mod_name is not None:
-        d['init_main_from_name'] = main_mod_name
-    # elif sys.platform != 'win32' or (not WINEXE and not WINSERVICE):
-    elif platform.system() != 'Windows':
-        main_path = getattr(main_module, '__file__', None)
-        if main_path is not None:
-            if (
-                not os.path.isabs(main_path) and (
-                    ORIGINAL_DIR is not None)
-            ):
-                # process.ORIGINAL_DIR is not None):
-                # main_path = os.path.join(process.ORIGINAL_DIR, main_path)
-                main_path = os.path.join(ORIGINAL_DIR, main_path)
-            d['init_main_from_path'] = os.path.normpath(main_path)
+    Retrieve parent (actor) proc's  `__main__` module data.
+
+    '''
+    if not inherit_parent_main:
+        return {}
+
+    proc: mp.Process = mp.current_process()
+    prep_data: dict = get_preparation_data(
+        name=proc.name,
+    )
+    # XXX, unserializable (and unneeded by us) by default
+    # see `mp.spawn.get_preparation_data()` impl details.
+    prep_data.pop('authkey', None)
+
+    d: ParentMainData = {}
+    if 'init_main_from_name' in prep_data:
+        d['init_main_from_name'] = prep_data['init_main_from_name']
+    if 'init_main_from_path' in prep_data:
+        d['init_main_from_path'] = prep_data['init_main_from_path']
 
     return d
-
-
-# Multiprocessing module helpers to fix up the main module in
-# spawned subprocesses
-def _fixup_main_from_name(mod_name: str) -> None:
-    # __main__.py files for packages, directories, zip archives, etc, run
-    # their "main only" code unconditionally, so we don't even try to
-    # populate anything in __main__, nor do we make any changes to
-    # __main__ attributes
-    current_main = sys.modules['__main__']
-    if mod_name == "__main__" or mod_name.endswith(".__main__"):
-        return
-
-    # If this process was forked, __main__ may already be populated
-    if getattr(current_main.__spec__, "name", None) == mod_name:
-        return
-
-    # Otherwise, __main__ may contain some non-main code where we need to
-    # support unpickling it properly. We rerun it as __mp_main__ and make
-    # the normal __main__ an alias to that
-    # old_main_modules.append(current_main)
-    main_module = types.ModuleType("__mp_main__")
-    main_content = runpy.run_module(mod_name,
-                                    run_name="__mp_main__",
-                                    alter_sys=True)  # type: ignore
-    main_module.__dict__.update(main_content)
-    sys.modules['__main__'] = sys.modules['__mp_main__'] = main_module
-
-
-def _fixup_main_from_path(main_path: str) -> None:
-    # If this process was forked, __main__ may already be populated
-    current_main = sys.modules['__main__']
-
-    # Unfortunately, the main ipython launch script historically had no
-    # "if __name__ == '__main__'" guard, so we work around that
-    # by treating it like a __main__.py file
-    # See https://github.com/ipython/ipython/issues/4698
-    main_name = os.path.splitext(os.path.basename(main_path))[0]
-    if main_name == 'ipython':
-        return
-
-    # Otherwise, if __file__ already has the setting we expect,
-    # there's nothing more to do
-    if getattr(current_main, '__file__', None) == main_path:
-        return
-
-    # If the parent process has sent a path through rather than a module
-    # name we assume it is an executable script that may contain
-    # non-main code that needs to be executed
-    # old_main_modules.append(current_main)
-    main_module = types.ModuleType("__mp_main__")
-    main_content = runpy.run_path(main_path,
-                                  run_name="__mp_main__")  # type: ignore
-    main_module.__dict__.update(main_content)
-    sys.modules['__main__'] = sys.modules['__mp_main__'] = main_module