diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index cb756aa..62f201d 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -34,4 +34,4 @@ jobs:
           python-version-file: ".python-version"
 
       - name: Run tests
-        run: uv run pytest -n auto -v
+        run: uv run pytest -n 16 -v
diff --git a/pyproject.toml b/pyproject.toml
index 1f4b935..1d24b4c 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -56,6 +56,8 @@ ignore = [
 
 [tool.ruff.lint.per-file-ignores]
 "tests/*" = [
+    "D", # Test helpers don't need docstrings
+    "PLR2004", # Magic values are fine in test assertions
     "S", # It's tests, no need for security
     "PLC0415", # imports need to be late for pytest_in_docker to work.
 ]
diff --git a/pytest_in_docker/_container.py b/pytest_in_docker/_container.py
index 0f5342f..f92d1f8 100644
--- a/pytest_in_docker/_container.py
+++ b/pytest_in_docker/_container.py
@@ -5,8 +5,10 @@
 import sys
 import tarfile
 import time
+from types import FunctionType
 from typing import TYPE_CHECKING, Any
 
+import cloudpickle
 import rpyc
 
 from pytest_in_docker._types import ContainerPrepareError
@@ -173,76 +175,6 @@ def bootstrap_container(
     )
 
 
-def _make_picklable[T: Callable[..., Any]](func: T) -> T:
-    """Return a copy of *func* that cloudpickle will serialise by value.
-
-    Two things prevent a test function from being naively pickled into a
-    remote container:
-
-    1. cloudpickle pickles importable functions *by reference*
-       (module + qualname), but the test module does not exist in the
-       container.
-    2. pytest's assertion rewriter injects ``@pytest_ar`` into the
-       function's ``__globals__``, and that object drags in the test
-       module itself.
-
-    We fix both by creating a **new** function object whose
-    ``__module__`` is ``"__mp_main__"`` (forces pickle-by-value) and
-    whose ``__globals__`` are a *shared* clean dict stripped of the
-    assertion-rewriter helper.  All sibling callables (same module) are
-    cloned into the same ``clean_globals`` dict so transitive calls
-    between helpers resolve to the patched versions.
-    """
-    import types  # noqa: PLC0415
-
-    original_module = func.__module__
-
-    # First pass: build clean_globals with non-callable entries,
-    # collect names of same-module callables to patch.
-    clean_globals: dict[str, Any] = {}
-    to_patch: list[str] = []
-    for k, v in func.__globals__.items():
-        if k == "@pytest_ar":
-            continue
-        if (
-            isinstance(v, types.FunctionType)
-            and getattr(v, "__module__", None) == original_module
-        ):
-            to_patch.append(k)
-        else:
-            clean_globals[k] = v
-
-    # Second pass: clone callables so they all share clean_globals.
-    for k in to_patch:
-        orig = func.__globals__[k]
-        clone = types.FunctionType(
-            orig.__code__,
-            clean_globals,
-            orig.__name__,
-            orig.__defaults__,
-            orig.__closure__,
-        )
-        clone.__module__ = "__mp_main__"
-        clone.__qualname__ = orig.__qualname__
-        clone.__annotations__ = orig.__annotations__
-        clone.__kwdefaults__ = orig.__kwdefaults__
-        clean_globals[k] = clone
-
-    # Clone the test function itself into the same shared dict.
-    clone = types.FunctionType(
-        func.__code__,
-        clean_globals,
-        func.__name__,
-        func.__defaults__,
-        func.__closure__,
-    )
-    clone.__annotations__ = func.__annotations__
-    clone.__kwdefaults__ = func.__kwdefaults__
-    clone.__module__ = "__mp_main__"
-    clone.__qualname__ = func.__qualname__
-    return clone  # type: ignore[return-value]
-
-
 def run_pickled[T](
     conn: Any,  # noqa: ANN401
     func: Callable[..., T],
@@ -250,9 +182,30 @@ def run_pickled[T](
     **kwargs: Any,  # noqa: ANN401
 ) -> T:
     """Serialize *func* with cloudpickle, send to container, execute there."""
-    import cloudpickle  # noqa: PLC0415
+    module = sys.modules.get(func.__module__)
+    if module is not None:
+        cloudpickle.register_pickle_by_value(module)
+
+    # Pickle a shallow copy of the function, excluding pytestmark — it
+    # references host-only objects (e.g. container factories) that can't
+    # be unpickled in the container.  Copying avoids mutating the original.
+    func_copy = FunctionType(
+        func.__code__,
+        func.__globals__,
+        func.__name__,
+        func.__defaults__,
+        func.__closure__,
+    )
+    func_copy.__kwdefaults__ = func.__kwdefaults__
+    func_copy.__dict__.update(
+        {k: v for k, v in func.__dict__.items() if k != "pytestmark"}
+    )
+    try:
+        payload = cloudpickle.dumps(func_copy)
+    finally:
+        if module is not None:
+            cloudpickle.unregister_pickle_by_value(module)
 
-    payload = cloudpickle.dumps(_make_picklable(func))
     rpickle = conn.modules["pickle"]
     remote_func = rpickle.loads(payload)
     return remote_func(*args, **kwargs)
diff --git a/tests/test_class_support.py b/tests/test_class_support.py
new file mode 100644
index 0000000..250a3db
--- /dev/null
+++ b/tests/test_class_support.py
@@ -0,0 +1,88 @@
+"""Tests proving class serialization works with module-level classes."""
+
+from enum import Enum
+
+from pytest_in_docker import in_container
+
+
+class Greeter:
+    def __init__(self, name: str) -> None:
+        self.name = name
+
+    def greet(self) -> str:
+        return f"hello {self.name}"
+
+
+@in_container("python:alpine")
+def test_module_level_class() -> None:
+    """Test function instantiates a module-level class."""
+    g = Greeter("world")
+    assert g.greet() == "hello world"
+
+
+class Base:
+    def base_method(self) -> str:
+        return "base"
+
+
+class Child(Base):
+    def __init__(self, x: int) -> None:
+        self.x = x
+
+    @property
+    def value(self) -> int:
+        return self.x
+
+    @staticmethod
+    def static_thing() -> str:
+        return "static"
+
+    @classmethod
+    def from_string(cls, s: str) -> Child:
+        return cls(int(s))
+
+    def child_method(self) -> str:
+        return f"child {self.base_method()}"
+
+
+@in_container("python:alpine")
+def test_class_inheritance() -> None:
+    """Inherited classes serialize correctly."""
+    c = Child(42)
+    assert c.value == 42
+    assert c.static_thing() == "static"
+    assert c.child_method() == "child base"
+    c2 = Child.from_string("10")
+    assert c2.x == 10
+
+
+class Config:
+    def __init__(self, name: str) -> None:
+        self.name = name
+
+
+class Service:
+    def __init__(self) -> None:
+        self.config = Config("default")
+
+    def get_config_name(self) -> str:
+        return self.config.name
+
+
+@in_container("python:alpine")
+def test_class_referencing_another_class() -> None:
+    """A class whose methods instantiate another module-level class."""
+    s = Service()
+    assert s.get_config_name() == "default"
+
+
+class Color(Enum):
+    RED = 1
+    GREEN = 2
+
+
+@in_container("python:alpine")
+def test_module_level_enum() -> None:
+    """Enum classes serialize correctly."""
+    assert Color.RED.value == 1
+    assert Color.GREEN.name == "GREEN"