diff --git a/ansible_mitogen/connection/mitogen.py b/ansible_mitogen/connection/mitogen.py
index 28e18c93..4e3c506c 100644
--- a/ansible_mitogen/connection/mitogen.py
+++ b/ansible_mitogen/connection/mitogen.py
@@ -30,11 +30,12 @@ import os
 
 import ansible.errors
 import ansible.plugins.connection
-import ansible_mitogen.helpers
+
 import mitogen.unix
+from mitogen.utils import cast
 
+import ansible_mitogen.helpers
 from ansible_mitogen.strategy.mitogen import ContextService
-from ansible_mitogen.utils import cast
 
 
 class Connection(ansible.plugins.connection.ConnectionBase):
diff --git a/ansible_mitogen/mixins.py b/ansible_mitogen/mixins.py
index 14c543eb..ed6ec42e 100644
--- a/ansible_mitogen/mixins.py
+++ b/ansible_mitogen/mixins.py
@@ -25,18 +25,37 @@
 # OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
+from __future__ import absolute_import
 import pwd
 import os
 import tempfile
 
 import ansible
+import ansible.plugins
 import ansible.plugins.action
 
+import mitogen.core
 import mitogen.master
+from mitogen.utils import cast
+
 import ansible_mitogen.helpers
 from ansible.module_utils._text import to_text
-from ansible_mitogen.utils import cast
-from ansible_mitogen.utils import get_command_module_name
+
+
+def get_command_module_name(module_name):
+    """
+    Given the name of an Ansible command module, return its canonical module
+    path within the ansible.
+
+    :param module_name:
+        "shell"
+    :return:
+        "ansible.modules.commands.shell"
+    """
+    path = ansible.plugins.module_loader.find_plugin(module_name, '')
+    relpath = os.path.relpath(path, os.path.dirname(ansible.__file__))
+    root, _ = os.path.splitext(relpath)
+    return 'ansible.' + root.replace('/', '.')
 
 
 class ActionModuleMixin(ansible.plugins.action.ActionBase):
diff --git a/ansible_mitogen/utils.py b/ansible_mitogen/utils.py
deleted file mode 100644
index 98193a0e..00000000
--- a/ansible_mitogen/utils.py
+++ /dev/null
@@ -1,75 +0,0 @@
-# Copyright 2017, David Wilson
-#
-# Redistribution and use in source and binary forms, with or without
-# modification, are permitted provided that the following conditions are met:
-#
-# 1. Redistributions of source code must retain the above copyright notice, this
-# list of conditions and the following disclaimer.
-#
-# 2. Redistributions in binary form must reproduce the above copyright notice,
-# this list of conditions and the following disclaimer in the documentation
-# and/or other materials provided with the distribution.
-#
-# 3. Neither the name of the copyright holder nor the names of its contributors
-# may be used to endorse or promote products derived from this software without
-# specific prior written permission.
-#
-# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
-# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
-# WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
-# DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
-# FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
-# DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
-# SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
-# CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
-# OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
-# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
-
-from __future__ import absolute_import
-import os
-
-import ansible
-import ansible.plugins
-import mitogen.core
-
-
-def cast(obj):
-    """
-    Ansible loves to decorate built-in types to implement useful functionality
-    like Vault, however cPickle loves to preserve those decorations during
-    serialization, resulting in CallError.
-
-    So here we recursively undecorate `obj`, ensuring that any instances of
-    subclasses of built-in types are downcast to the base type.
-    """
-    if isinstance(obj, dict):
-        return {cast(k): cast(v) for k, v in obj.iteritems()}
-    if isinstance(obj, (list, tuple)):
-        return [cast(v) for v in obj]
-    if obj is None or isinstance(obj, (int, float)):
-        return obj
-    if isinstance(obj, unicode):
-        return unicode(obj)
-    if isinstance(obj, str):
-        return str(obj)
-    if isinstance(obj, (mitogen.core.Context,
-                        mitogen.core.Dead,
-                        mitogen.core.CallError)):
-        return obj
-    raise TypeError("Cannot serialize: %r: %r" % (type(obj), obj))
-
-
-def get_command_module_name(module_name):
-    """
-    Given the name of an Ansible command module, return its canonical module
-    path within the ansible.
-
-    :param module_name:
-        "shell"
-    :return: 
-        "ansible.modules.commands.shell"
-    """
-    path = ansible.plugins.module_loader.find_plugin(module_name, '')
-    relpath = os.path.relpath(path, os.path.dirname(ansible.__file__))
-    root, _ = os.path.splitext(relpath)
-    return 'ansible.' + root.replace('/', '.')
diff --git a/docs/api.rst b/docs/api.rst
index 1b6b0b58..a3d5f6d1 100644
--- a/docs/api.rst
+++ b/docs/api.rst
@@ -971,6 +971,27 @@ Utility Functions
 
 A random assortment of utility functions useful on masters and children.
 
+.. currentmodule:: mitogen.utils
+.. function:: cast (obj)
+
+    Many tools love to subclass built-in types in order to implement useful
+    functionality, such as annotating the safety of a Unicode string, or adding
+    additional methods to a dict. However, cPickle loves to preserve those
+    subtypes during serialization, resulting in CallError during :py:meth:`call
+    <mitogen.master.Context.call>` in the target when it tries to deserialize
+    the data.
+
+    This function walks the object graph `obj`, producing a copy with any
+    custom sub-types removed. The functionality is not default since the
+    resulting walk may be computationally expensive given a large enough graph.
+
+    See :ref:`serialization-rules` for a list of supported types.
+
+    :param obj:
+        Object to undecorate.
+    :returns:
+        Undecorated object.
+
 .. currentmodule:: mitogen.utils
 .. function:: disable_site_packages
 
diff --git a/docs/getting_started.rst b/docs/getting_started.rst
index 56c486fd..234b556f 100644
--- a/docs/getting_started.rst
+++ b/docs/getting_started.rst
@@ -281,6 +281,9 @@ User-defined types may not be used, except for:
 * :py:class:`mitogen.core.Context`
 * :py:class:`mitogen.core._DEAD`
 
+Subclasses of built-in types must be undecorated using
+:py:func:`mitogen.utils.cast`.
+
 
 .. _troubleshooting:
 
diff --git a/mitogen/utils.py b/mitogen/utils.py
index f41a7dee..cd8a93d1 100644
--- a/mitogen/utils.py
+++ b/mitogen/utils.py
@@ -91,3 +91,22 @@ def with_router(func):
         return run_with_router(func, *args, **kwargs)
     wrapper.func_name = func.func_name
     return wrapper
+
+
+def cast(obj):
+    if isinstance(obj, dict):
+        return {cast(k): cast(v) for k, v in obj.iteritems()}
+    if isinstance(obj, (list, tuple)):
+        return [cast(v) for v in obj]
+    if obj is None or isinstance(obj, (int, float)):
+        return obj
+    if isinstance(obj, unicode):
+        return unicode(obj)
+    if isinstance(obj, str):
+        return str(obj)
+    if isinstance(obj, (mitogen.core.Context,
+                        mitogen.core.Dead,
+                        mitogen.core.CallError)):
+        return obj
+
+    raise TypeError("Cannot serialize: %r: %r" % (type(obj), obj))