From 23d7d6c85aa581af2d37c3c0d276e74ff75507df Mon Sep 17 00:00:00 2001
From: Tobias Raabe <raabe@posteo.de>
Date: Sat, 31 Jan 2026 19:52:57 +0100
Subject: [PATCH 1/9] Fix pickling errors by importing task modules in workers

---
 requirements-dev.lock           | 86 ---------------------------------
 requirements.lock               | 51 -------------------
 src/pytask_parallel/backends.py | 51 +++++++++++++++++--
 src/pytask_parallel/execute.py  |  9 +++-
 src/pytask_parallel/utils.py    | 30 ++++++++++++
 tests/test_execute.py           | 53 ++++++++++++++++++++
 6 files changed, 138 insertions(+), 142 deletions(-)
 delete mode 100644 requirements-dev.lock
 delete mode 100644 requirements.lock

diff --git a/requirements-dev.lock b/requirements-dev.lock
deleted file mode 100644
index 70e3e4a..0000000
--- a/requirements-dev.lock
+++ /dev/null
@@ -1,86 +0,0 @@
-# generated by rye
-# use `rye lock` or `rye sync` to update this lockfile
-#
-# last locked with the following flags:
-#   pre: false
-#   features: []
-#   all-features: false
-#   with-sources: false
-
--e file:.
-aiobotocore==2.12.3
-    # via s3fs
-aiohttp==3.9.4
-    # via aiobotocore
-    # via s3fs
-aioitertools==0.11.0
-    # via aiobotocore
-aiosignal==1.3.1
-    # via aiohttp
-attrs==23.2.0
-    # via aiohttp
-    # via pytask
-    # via pytask-parallel
-botocore==1.34.69
-    # via aiobotocore
-click==8.1.7
-    # via click-default-group
-    # via pytask
-    # via pytask-parallel
-click-default-group==1.2.4
-    # via pytask
-cloudpickle==3.0.0
-    # via loky
-    # via pytask-parallel
-frozenlist==1.4.1
-    # via aiohttp
-    # via aiosignal
-fsspec==2024.3.1
-    # via s3fs
-greenlet==3.0.3
-    # via sqlalchemy
-idna==3.7
-    # via yarl
-jmespath==1.0.1
-    # via botocore
-loky==3.4.1
-    # via pytask-parallel
-markdown-it-py==3.0.0
-    # via rich
-mdurl==0.1.2
-    # via markdown-it-py
-multidict==6.0.5
-    # via aiohttp
-    # via yarl
-networkx==3.3
-    # via pytask
-optree==0.11.0
-    # via pytask
-packaging==24.0
-    # via pytask
-pluggy==1.4.0
-    # via pytask
-    # via pytask-parallel
-pygments==2.17.2
-    # via rich
-pytask==0.4.7
-    # via pytask-parallel
-python-dateutil==2.9.0.post0
-    # via botocore
-rich==13.7.1
-    # via pytask
-    # via pytask-parallel
-s3fs==2024.3.1
-six==1.16.0
-    # via python-dateutil
-sqlalchemy==2.0.29
-    # via pytask
-typing-extensions==4.11.0
-    # via optree
-    # via sqlalchemy
-urllib3==2.2.1
-    # via botocore
-wrapt==1.16.0
-    # via aiobotocore
-yarl==1.9.4
-    # via aiohttp
diff --git a/requirements.lock b/requirements.lock
deleted file mode 100644
index 4979697..0000000
--- a/requirements.lock
+++ /dev/null
@@ -1,51 +0,0 @@
-# generated by rye
-# use `rye lock` or `rye sync` to update this lockfile
-#
-# last locked with the following flags:
-#   pre: false
-#   features: []
-#   all-features: false
-#   with-sources: false
-
--e file:.
-attrs==23.2.0
-    # via pytask
-    # via pytask-parallel
-click==8.1.7
-    # via click-default-group
-    # via pytask
-    # via pytask-parallel
-click-default-group==1.2.4
-    # via pytask
-cloudpickle==3.0.0
-    # via loky
-    # via pytask-parallel
-greenlet==3.0.3
-    # via sqlalchemy
-loky==3.4.1
-    # via pytask-parallel
-markdown-it-py==3.0.0
-    # via rich
-mdurl==0.1.2
-    # via markdown-it-py
-networkx==3.3
-    # via pytask
-optree==0.11.0
-    # via pytask
-packaging==24.0
-    # via pytask
-pluggy==1.4.0
-    # via pytask
-    # via pytask-parallel
-pygments==2.17.2
-    # via rich
-pytask==0.4.7
-    # via pytask-parallel
-rich==13.7.1
-    # via pytask
-    # via pytask-parallel
-sqlalchemy==2.0.29
-    # via pytask
-typing-extensions==4.11.0
-    # via optree
-    # via sqlalchemy
diff --git a/src/pytask_parallel/backends.py b/src/pytask_parallel/backends.py
index f5aa6ee..e064156 100644
--- a/src/pytask_parallel/backends.py
+++ b/src/pytask_parallel/backends.py
@@ -12,6 +12,8 @@
 from typing import Any
 from typing import ClassVar
 
+import os
+import sys
 import cloudpickle
 from attrs import define
 from loky import get_reusable_executor
@@ -19,7 +21,46 @@
 if TYPE_CHECKING:
     from collections.abc import Callable
 
-__all__ = ["ParallelBackend", "ParallelBackendRegistry", "WorkerType", "registry"]
+__all__ = [
+    "ParallelBackend",
+    "ParallelBackendRegistry",
+    "WorkerType",
+    "registry",
+    "set_worker_root",
+]
+
+_WORKER_ROOT: str | None = None
+
+
+def set_worker_root(path: os.PathLike[str] | str) -> None:
+    """Configure the root path for worker processes.
+
+    Spawned workers (notably on Windows) start with a clean interpreter and may not
+    inherit the parent's import path. We set both ``sys.path`` and ``PYTHONPATH`` so
+    task modules are importable by reference, which avoids pickling module globals.
+
+    """
+    global _WORKER_ROOT
+    root = os.fspath(path)
+    _WORKER_ROOT = root
+    if root not in sys.path:
+        sys.path.insert(0, root)
+    # Ensure custom process backends can import task modules by reference.
+    separator = os.pathsep
+    current = os.environ.get("PYTHONPATH", "")
+    parts = [p for p in current.split(separator) if p] if current else []
+    if root not in parts:
+        parts.insert(0, root)
+        os.environ["PYTHONPATH"] = separator.join(parts)
+
+
+def _configure_worker(root: str | None) -> None:
+    """Set cwd and sys.path for worker processes."""
+    if not root:
+        return
+    os.chdir(root)
+    if root not in sys.path:
+        sys.path.insert(0, root)
 
 
 def _deserialize_and_run_with_cloudpickle(fn: bytes, kwargs: bytes) -> Any:
@@ -75,12 +116,16 @@ def _get_dask_executor(n_workers: int) -> Executor:
 
 def _get_loky_executor(n_workers: int) -> Executor:
     """Get a loky executor."""
-    return get_reusable_executor(max_workers=n_workers)
+    return get_reusable_executor(
+        max_workers=n_workers, initializer=_configure_worker, initargs=(_WORKER_ROOT,)
+    )
 
 
 def _get_process_pool_executor(n_workers: int) -> Executor:
     """Get a process pool executor."""
-    return _CloudpickleProcessPoolExecutor(max_workers=n_workers)
+    return _CloudpickleProcessPoolExecutor(
+        max_workers=n_workers, initializer=_configure_worker, initargs=(_WORKER_ROOT,)
+    )
 
 
 def _get_thread_pool_executor(n_workers: int) -> Executor:
diff --git a/src/pytask_parallel/execute.py b/src/pytask_parallel/execute.py
index ef6529a..ec0bc1a 100644
--- a/src/pytask_parallel/execute.py
+++ b/src/pytask_parallel/execute.py
@@ -26,11 +26,13 @@
 
 from pytask_parallel.backends import WorkerType
 from pytask_parallel.backends import registry
+from pytask_parallel.backends import set_worker_root
 from pytask_parallel.typing import CarryOverPath
 from pytask_parallel.typing import is_coiled_function
 from pytask_parallel.utils import create_kwargs_for_task
 from pytask_parallel.utils import get_module
 from pytask_parallel.utils import parse_future_result
+from pytask_parallel.utils import should_pickle_module_by_value
 
 if TYPE_CHECKING:
     from concurrent.futures import Future
@@ -57,6 +59,7 @@ def pytask_execute_build(session: Session) -> bool | None:  # noqa: C901, PLR091
 
     # The executor can only be created after the collection to give users the
     # possibility to inject their own executors.
+    set_worker_root(session.config["root"])
     session.config["_parallel_executor"] = registry.get_parallel_backend(
         session.config["parallel_backend"], n_workers=session.config["n_workers"]
     )
@@ -208,7 +211,8 @@ def pytask_execute_task(session: Session, task: PTask) -> Future[WrapperResult]:
         # cloudpickle will pickle it with the function. See cloudpickle#417, pytask#373
         # and pytask#374.
         task_module = get_module(task.function, getattr(task, "path", None))
-        cloudpickle.register_pickle_by_value(task_module)
+        if should_pickle_module_by_value(task_module):
+            cloudpickle.register_pickle_by_value(task_module)
 
         return cast("Any", wrapper_func).submit(
             task=task,
@@ -230,7 +234,8 @@ def pytask_execute_task(session: Session, task: PTask) -> Future[WrapperResult]:
         # cloudpickle will pickle it with the function. See cloudpickle#417, pytask#373
         # and pytask#374.
         task_module = get_module(task.function, getattr(task, "path", None))
-        cloudpickle.register_pickle_by_value(task_module)
+        if should_pickle_module_by_value(task_module):
+            cloudpickle.register_pickle_by_value(task_module)
 
         return session.config["_parallel_executor"].submit(
             wrap_task_in_process,
diff --git a/src/pytask_parallel/utils.py b/src/pytask_parallel/utils.py
index 269c72d..4f033de 100644
--- a/src/pytask_parallel/utils.py
+++ b/src/pytask_parallel/utils.py
@@ -2,7 +2,9 @@
 
 from __future__ import annotations
 
+import importlib.util
 import inspect
+from pathlib import Path
 from functools import partial
 from typing import TYPE_CHECKING
 from typing import Any
@@ -39,6 +41,7 @@ class CoiledFunction: ...
     "create_kwargs_for_task",
     "get_module",
     "parse_future_result",
+    "should_pickle_module_by_value",
 ]
 
 
@@ -150,3 +153,30 @@ def get_module(func: Callable[..., Any], path: Path | None) -> ModuleType:
     if path:
         return inspect.getmodule(func, path.as_posix())  # type: ignore[return-value]
     return inspect.getmodule(func)  # type: ignore[return-value]
+
+
+def should_pickle_module_by_value(module: ModuleType) -> bool:
+    """Return whether a module should be pickled by value.
+
+    We only pickle by value when the module is not importable by name in the worker.
+    This avoids serializing all module globals, which can fail for non-picklable
+    objects (e.g., closed file handles or locks stored at module scope).
+
+    """
+    module_name = getattr(module, "__name__", None)
+    module_file = getattr(module, "__file__", None)
+    if not module_name or module_name == "__main__" or module_file is None:
+        return True
+
+    try:
+        spec = importlib.util.find_spec(module_name)
+    except (ImportError, ValueError, AttributeError):
+        return True
+
+    if spec is None or spec.origin is None:
+        return True
+
+    try:
+        return Path(spec.origin).resolve() != Path(module_file).resolve()
+    except OSError:
+        return True
diff --git a/tests/test_execute.py b/tests/test_execute.py
index c06919a..79c04d4 100644
--- a/tests/test_execute.py
+++ b/tests/test_execute.py
@@ -364,3 +364,56 @@ def task_create_file(
     )
     assert result.exit_code == ExitCode.OK
     assert tmp_path.joinpath("file.txt").read_text() == "This is the text."
+
+
+@pytest.mark.parametrize(
+    "parallel_backend",
+    [
+        ParallelBackend.PROCESSES,
+        pytest.param(ParallelBackend.LOKY, marks=skip_if_deadlock),
+    ],
+)
+def test_parallel_execution_with_mark_import(runner, tmp_path, parallel_backend):
+    source = """
+    from pytask import mark, task
+
+    @task
+    def task_assert_math():
+        assert 2 + 2 == 4
+    """
+    tmp_path.joinpath("task_mark.py").write_text(textwrap.dedent(source))
+    result = runner.invoke(
+        cli, [tmp_path.as_posix(), "-n", "2", "--parallel-backend", parallel_backend]
+    )
+    assert result.exit_code == ExitCode.OK
+
+
+@pytest.mark.parametrize(
+    "parallel_backend",
+    [
+        ParallelBackend.PROCESSES,
+        pytest.param(ParallelBackend.LOKY, marks=skip_if_deadlock),
+    ],
+)
+def test_parallel_execution_with_closed_file_handle(
+    runner, tmp_path, parallel_backend
+):
+    source = """
+    from pathlib import Path
+    from pytask import task
+
+    data_path = Path(__file__).parent / "data.txt"
+    data_path.write_text("hello", encoding="utf-8")
+
+    with data_path.open(encoding="utf-8") as f:
+        content = f.read()
+
+    @task
+    def task_assert_math():
+        assert content == "hello"
+    """
+    tmp_path.joinpath("task_file.py").write_text(textwrap.dedent(source))
+    result = runner.invoke(
+        cli, [tmp_path.as_posix(), "-n", "2", "--parallel-backend", parallel_backend]
+    )
+    assert result.exit_code == ExitCode.OK

From b036df9d392444c31d7b9d9999f098a523c306d4 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Sat, 31 Jan 2026 18:52:01 +0000
Subject: [PATCH 2/9] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 src/pytask_parallel/backends.py | 4 ++--
 src/pytask_parallel/utils.py    | 3 +--
 tests/test_execute.py           | 4 +---
 3 files changed, 4 insertions(+), 7 deletions(-)

diff --git a/src/pytask_parallel/backends.py b/src/pytask_parallel/backends.py
index e064156..45aa6b7 100644
--- a/src/pytask_parallel/backends.py
+++ b/src/pytask_parallel/backends.py
@@ -2,6 +2,8 @@
 
 from __future__ import annotations
 
+import os
+import sys
 import warnings
 from concurrent.futures import Executor
 from concurrent.futures import Future
@@ -12,8 +14,6 @@
 from typing import Any
 from typing import ClassVar
 
-import os
-import sys
 import cloudpickle
 from attrs import define
 from loky import get_reusable_executor
diff --git a/src/pytask_parallel/utils.py b/src/pytask_parallel/utils.py
index 4f033de..c0be884 100644
--- a/src/pytask_parallel/utils.py
+++ b/src/pytask_parallel/utils.py
@@ -4,8 +4,8 @@
 
 import importlib.util
 import inspect
-from pathlib import Path
 from functools import partial
+from pathlib import Path
 from typing import TYPE_CHECKING
 from typing import Any
 
@@ -22,7 +22,6 @@
 if TYPE_CHECKING:
     from collections.abc import Callable
     from concurrent.futures import Future
-    from pathlib import Path
     from types import ModuleType
     from types import TracebackType
 
diff --git a/tests/test_execute.py b/tests/test_execute.py
index 79c04d4..b1a83f9 100644
--- a/tests/test_execute.py
+++ b/tests/test_execute.py
@@ -395,9 +395,7 @@ def task_assert_math():
         pytest.param(ParallelBackend.LOKY, marks=skip_if_deadlock),
     ],
 )
-def test_parallel_execution_with_closed_file_handle(
-    runner, tmp_path, parallel_backend
-):
+def test_parallel_execution_with_closed_file_handle(runner, tmp_path, parallel_backend):
     source = """
     from pathlib import Path
     from pytask import task

From 725634fd298e9cb40bacf62d2ef6f43badd131a3 Mon Sep 17 00:00:00 2001
From: Tobias Raabe <raabe@posteo.de>
Date: Sat, 31 Jan 2026 19:54:04 +0100
Subject: [PATCH 3/9] Document fix for pickling errors

---
 docs/source/changes.md | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/docs/source/changes.md b/docs/source/changes.md
index f7a82dd..85c2e52 100644
--- a/docs/source/changes.md
+++ b/docs/source/changes.md
@@ -11,6 +11,8 @@ releases are available on [PyPI](https://pypi.org/project/pytask-parallel) and
 - {pull}`130` switches type checking to ty.
 - {pull}`131` updates pre-commit hooks.
 - {pull}`132` removes the tox configuration in favor of uv and just.
+- {pull}`137` fixes pickling errors in parallel workers when task modules contain
+  non-picklable globals. Fixes {issue}`136`.
 
 ## 0.5.1 - 2025-03-09
 

From 0ddd1c325f3494445d247aa18bfd17f8fe8dbcf6 Mon Sep 17 00:00:00 2001
From: Tobias Raabe <raabe@posteo.de>
Date: Sat, 31 Jan 2026 20:02:19 +0100
Subject: [PATCH 4/9] Fix typing, lint, and docs checks

---
 pyproject.toml                  |  2 +-
 src/pytask_parallel/backends.py | 13 ++++++++-----
 src/pytask_parallel/wrappers.py |  2 +-
 3 files changed, 10 insertions(+), 7 deletions(-)

diff --git a/pyproject.toml b/pyproject.toml
index 8520904..d010638 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -33,7 +33,7 @@ docs = [
     "matplotlib",
     "myst-parser",
     "nbsphinx",
-    "sphinx",
+    "sphinx<9",
     "sphinx-autobuild",
     "sphinx-click",
     "sphinx-copybutton",
diff --git a/src/pytask_parallel/backends.py b/src/pytask_parallel/backends.py
index 45aa6b7..5e4abe4 100644
--- a/src/pytask_parallel/backends.py
+++ b/src/pytask_parallel/backends.py
@@ -29,7 +29,7 @@
     "set_worker_root",
 ]
 
-_WORKER_ROOT: str | None = None
+_WORKER_STATE: dict[str, str | None] = {"root": None}
 
 
 def set_worker_root(path: os.PathLike[str] | str) -> None:
@@ -40,9 +40,8 @@ def set_worker_root(path: os.PathLike[str] | str) -> None:
     task modules are importable by reference, which avoids pickling module globals.
 
     """
-    global _WORKER_ROOT
     root = os.fspath(path)
-    _WORKER_ROOT = root
+    _WORKER_STATE["root"] = root
     if root not in sys.path:
         sys.path.insert(0, root)
     # Ensure custom process backends can import task modules by reference.
@@ -117,14 +116,18 @@ def _get_dask_executor(n_workers: int) -> Executor:
 def _get_loky_executor(n_workers: int) -> Executor:
     """Get a loky executor."""
     return get_reusable_executor(
-        max_workers=n_workers, initializer=_configure_worker, initargs=(_WORKER_ROOT,)
+        max_workers=n_workers,
+        initializer=_configure_worker,
+        initargs=(_WORKER_STATE["root"],),
     )
 
 
 def _get_process_pool_executor(n_workers: int) -> Executor:
     """Get a process pool executor."""
     return _CloudpickleProcessPoolExecutor(
-        max_workers=n_workers, initializer=_configure_worker, initargs=(_WORKER_ROOT,)
+        max_workers=n_workers,
+        initializer=_configure_worker,
+        initargs=(_WORKER_STATE["root"],),
     )
 
 
diff --git a/src/pytask_parallel/wrappers.py b/src/pytask_parallel/wrappers.py
index e663e8a..e26deba 100644
--- a/src/pytask_parallel/wrappers.py
+++ b/src/pytask_parallel/wrappers.py
@@ -217,7 +217,7 @@ def _render_traceback_to_string(
     traceback = Traceback(exc_info, show_locals=show_locals)
     segments = console.render(cast("Any", traceback), options=console_options)
     text = "".join(segment.text for segment in segments)
-    return (*exc_info[:2], text)  # ty: ignore[invalid-return-type]
+    return (*exc_info[:2], text)
 
 
 def _handle_function_products(

From 81c84e0cdbc1b4b8a0418803106f7a156efd14c4 Mon Sep 17 00:00:00 2001
From: Tobias Raabe <raabe@posteo.de>
Date: Sat, 31 Jan 2026 20:02:56 +0100
Subject: [PATCH 5/9] Allow global worker root with ruff ignore

---
 src/pytask_parallel/backends.py | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/src/pytask_parallel/backends.py b/src/pytask_parallel/backends.py
index 5e4abe4..2021782 100644
--- a/src/pytask_parallel/backends.py
+++ b/src/pytask_parallel/backends.py
@@ -29,7 +29,7 @@
     "set_worker_root",
 ]
 
-_WORKER_STATE: dict[str, str | None] = {"root": None}
+_WORKER_ROOT: str | None = None
 
 
 def set_worker_root(path: os.PathLike[str] | str) -> None:
@@ -41,7 +41,8 @@ def set_worker_root(path: os.PathLike[str] | str) -> None:
 
     """
     root = os.fspath(path)
-    _WORKER_STATE["root"] = root
+    global _WORKER_ROOT  # noqa: PLW0603
+    _WORKER_ROOT = root
     if root not in sys.path:
         sys.path.insert(0, root)
     # Ensure custom process backends can import task modules by reference.
@@ -118,7 +119,7 @@ def _get_loky_executor(n_workers: int) -> Executor:
     return get_reusable_executor(
         max_workers=n_workers,
         initializer=_configure_worker,
-        initargs=(_WORKER_STATE["root"],),
+        initargs=(_WORKER_ROOT,),
     )
 
 
@@ -127,7 +128,7 @@ def _get_process_pool_executor(n_workers: int) -> Executor:
     return _CloudpickleProcessPoolExecutor(
         max_workers=n_workers,
         initializer=_configure_worker,
-        initargs=(_WORKER_STATE["root"],),
+        initargs=(_WORKER_ROOT,),
     )
 
 

From c2166c35f866609eedbae3debcafac5d2034c96a Mon Sep 17 00:00:00 2001
From: Tobias Raabe <raabe@posteo.de>
Date: Mon, 2 Feb 2026 22:37:30 +0100
Subject: [PATCH 6/9] Add regression test for mark import loop

---
 tests/test_execute.py | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/tests/test_execute.py b/tests/test_execute.py
index b1a83f9..6df9ef5 100644
--- a/tests/test_execute.py
+++ b/tests/test_execute.py
@@ -388,6 +388,32 @@ def task_assert_math():
     assert result.exit_code == ExitCode.OK
 
 
+@pytest.mark.parametrize(
+    "parallel_backend",
+    [
+        ParallelBackend.PROCESSES,
+        pytest.param(ParallelBackend.LOKY, marks=skip_if_deadlock),
+    ],
+)
+def test_parallel_execution_with_mark_import_in_loop(
+    runner, tmp_path, parallel_backend
+):
+    source = """
+    from pytask import mark, task
+
+    for data_name in ("a", "b", "c"):
+
+        @task(id=data_name)
+        def task_assert_math_loop():
+            assert 2 + 2 == 4
+    """
+    tmp_path.joinpath("task_mark_loop.py").write_text(textwrap.dedent(source))
+    result = runner.invoke(
+        cli, [tmp_path.as_posix(), "-n", "2", "--parallel-backend", parallel_backend]
+    )
+    assert result.exit_code == ExitCode.OK
+
+
 @pytest.mark.parametrize(
     "parallel_backend",
     [

From 5ba097a727ca7172f74a6f703c18f0dd6c3e1831 Mon Sep 17 00:00:00 2001
From: Tobias Raabe <raabe@posteo.de>
Date: Wed, 4 Feb 2026 21:20:28 +0100
Subject: [PATCH 7/9] Strip local from task.

---
 src/pytask_parallel/execute.py |  3 +++
 src/pytask_parallel/utils.py   | 14 ++++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/src/pytask_parallel/execute.py b/src/pytask_parallel/execute.py
index ec0bc1a..2d7c843 100644
--- a/src/pytask_parallel/execute.py
+++ b/src/pytask_parallel/execute.py
@@ -32,6 +32,7 @@
 from pytask_parallel.utils import create_kwargs_for_task
 from pytask_parallel.utils import get_module
 from pytask_parallel.utils import parse_future_result
+from pytask_parallel.utils import strip_annotation_locals
 from pytask_parallel.utils import should_pickle_module_by_value
 
 if TYPE_CHECKING:
@@ -198,6 +199,7 @@ def pytask_execute_task(session: Session, task: PTask) -> Future[WrapperResult]:
     kwargs = create_kwargs_for_task(task, remote=remote)
 
     if is_coiled_function(task):
+        strip_annotation_locals(task)
         # Prevent circular import for coiled backend.
         from pytask_parallel.wrappers import (  # noqa: PLC0415
             rewrap_task_with_coiled_function,
@@ -225,6 +227,7 @@ def pytask_execute_task(session: Session, task: PTask) -> Future[WrapperResult]:
         )
 
     if worker_type == WorkerType.PROCESSES:
+        strip_annotation_locals(task)
         # Prevent circular import for loky backend.
         from pytask_parallel.wrappers import wrap_task_in_process  # noqa: PLC0415
 
diff --git a/src/pytask_parallel/utils.py b/src/pytask_parallel/utils.py
index c0be884..b5472d2 100644
--- a/src/pytask_parallel/utils.py
+++ b/src/pytask_parallel/utils.py
@@ -40,6 +40,7 @@ class CoiledFunction: ...
     "create_kwargs_for_task",
     "get_module",
     "parse_future_result",
+    "strip_annotation_locals",
     "should_pickle_module_by_value",
 ]
 
@@ -154,6 +155,19 @@ def get_module(func: Callable[..., Any], path: Path | None) -> ModuleType:
     return inspect.getmodule(func)  # type: ignore[return-value]
 
 
+def strip_annotation_locals(task: PTask) -> None:
+    """Remove annotation locals from task functions before pickling.
+
+    The locals snapshot is only needed during collection to evaluate annotations.
+    Keeping it around for execution can break pickling when it contains non-serializable
+    objects (for example, when importing ``pytask.mark`` in loop-generated tasks).
+
+    """
+    meta = getattr(task.function, "pytask_meta", None)
+    if meta is not None and getattr(meta, "annotation_locals", None) is not None:
+        meta.annotation_locals = None
+
+
 def should_pickle_module_by_value(module: ModuleType) -> bool:
     """Return whether a module should be pickled by value.
 

From c8528dc58fdf4486c480280af19c8a440ca6d997 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Wed, 4 Feb 2026 20:20:45 +0000
Subject: [PATCH 8/9] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 src/pytask_parallel/execute.py | 2 +-
 src/pytask_parallel/utils.py   | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/pytask_parallel/execute.py b/src/pytask_parallel/execute.py
index 2d7c843..aa0f3a0 100644
--- a/src/pytask_parallel/execute.py
+++ b/src/pytask_parallel/execute.py
@@ -32,8 +32,8 @@
 from pytask_parallel.utils import create_kwargs_for_task
 from pytask_parallel.utils import get_module
 from pytask_parallel.utils import parse_future_result
-from pytask_parallel.utils import strip_annotation_locals
 from pytask_parallel.utils import should_pickle_module_by_value
+from pytask_parallel.utils import strip_annotation_locals
 
 if TYPE_CHECKING:
     from concurrent.futures import Future
diff --git a/src/pytask_parallel/utils.py b/src/pytask_parallel/utils.py
index b5472d2..fa1137c 100644
--- a/src/pytask_parallel/utils.py
+++ b/src/pytask_parallel/utils.py
@@ -40,8 +40,8 @@ class CoiledFunction: ...
     "create_kwargs_for_task",
     "get_module",
     "parse_future_result",
-    "strip_annotation_locals",
     "should_pickle_module_by_value",
+    "strip_annotation_locals",
 ]
 
 

From df4878c8f09549203da363f77126cd7224f747f8 Mon Sep 17 00:00:00 2001
From: Tobias Raabe <tobias.raabe@quantilope.com>
Date: Thu, 5 Feb 2026 18:08:02 +0100
Subject: [PATCH 9/9] Release 0.5.2

---
 docs/source/changes.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/docs/source/changes.md b/docs/source/changes.md
index 85c2e52..239e57c 100644
--- a/docs/source/changes.md
+++ b/docs/source/changes.md
@@ -5,7 +5,7 @@ chronological order. Releases follow [semantic versioning](https://semver.org/)
 releases are available on [PyPI](https://pypi.org/project/pytask-parallel) and
 [Anaconda.org](https://anaconda.org/conda-forge/pytask-parallel).
 
-## Unreleased
+## 0.5.2 - 2026-02-05
 
 - {pull}`129` drops support for Python 3.8 and 3.9 and adds support for Python 3.14.
 - {pull}`130` switches type checking to ty.