diff --git a/pcapkit/corekit/fields/collections.py b/pcapkit/corekit/fields/collections.py
index 20b5a0ad95..3c00960b03 100644
--- a/pcapkit/corekit/fields/collections.py
+++ b/pcapkit/corekit/fields/collections.py
@@ -122,7 +122,7 @@ def __call__(self, packet: 'dict[str, Any]') -> 'Self':
 
         """
         new_self = self.__copy__()
-        new_self._callback(self, packet)
+        new_self._callback(new_self, packet)
         if new_self._length_callback is not None:
             new_self._length = new_self._length_callback(packet)
             new_self._template = f'{new_self._length}s'
diff --git a/pcapkit/corekit/fields/misc.py b/pcapkit/corekit/fields/misc.py
index b92a5c0e31..2c8e2c4a16 100644
--- a/pcapkit/corekit/fields/misc.py
+++ b/pcapkit/corekit/fields/misc.py
@@ -695,7 +695,7 @@ def __call__(self, packet: 'dict[str, Any]') -> 'Self':
         new_self._callback(new_self, packet)
         if new_self._length_callback is not None:
             new_self._length = new_self._length_callback(packet)
-            new_self._template = f'{new_self._length}s' if self._length >= 0 else '1024s'  # use a reasonable default
+            new_self._template = f'{new_self._length}s' if new_self._length >= 0 else '1024s'  # use a reasonable default
         return new_self
 
     def pack(self, value: 'Optional[_TS | bytes]', packet: 'dict[str, Any]') -> 'bytes':
diff --git a/tests/corekit/test_fields_call_new_self_2173_unit.py b/tests/corekit/test_fields_call_new_self_2173_unit.py
new file mode 100644
index 0000000000..7364146c20
--- /dev/null
+++ b/tests/corekit/test_fields_call_new_self_2173_unit.py
@@ -0,0 +1,75 @@
+# -*- coding: utf-8 -*-
+"""``ListField`` and ``SchemaField`` calls work on the copy, not the template.
+
+GitHub issue #2173. Calling a field with a packet returns a fresh copy, which is
+what the schema then reads with. :meth:`ListField.__call__
+<pcapkit.corekit.fields.collections.ListField.__call__>` handed its ``callback``
+the template instead of that copy, so a callback adjusting its field mutated the
+template shared by every later packet and missed the copy actually in use.
+:meth:`SchemaField.__call__ <pcapkit.corekit.fields.misc.SchemaField.__call__>`
+sized the copy's ``template`` from the template's ``-1`` length rather than the
+copy's resolved one, so it stayed ``'1024s'``. Both read the copy.
+"""
+
+import unittest
+from typing import TYPE_CHECKING
+
+from pcapkit.corekit.fields.collections import ListField
+from pcapkit.corekit.fields.misc import SchemaField
+from pcapkit.corekit.fields.numbers import UInt8Field, UInt16Field
+from pcapkit.protocols.schema.schema import Schema
+
+if TYPE_CHECKING:
+    from typing import Any
+
+
+class Pair(Schema):
+    """Four-octet schema for :class:`SchemaField`."""
+
+    a: 'int' = UInt16Field()
+    b: 'int' = UInt16Field()
+
+
+def tag_field(field: 'Any', packet: 'dict[str, Any]') -> 'None':
+    """Callback that adjusts the field it is given, as a byte-order one would."""
+    field.tag = packet['n']
+
+
+class ListFieldCallbackTests(unittest.TestCase):
+    """``ListField.__call__`` passes its callback the copy."""
+
+    def test_callback_receives_the_copy(self) -> None:
+        seen = []  # type: list[ListField]
+        template = ListField(length=lambda pkt: pkt['n'], item_type=UInt8Field(),
+                             callback=lambda field, _: seen.append(field))
+        copy = template({'n': 2})
+        self.assertEqual(len(seen), 1)
+        self.assertIs(seen[0], copy)
+        self.assertIsNot(seen[0], template)
+
+    def test_template_is_untouched_across_calls(self) -> None:
+        template = ListField(length=lambda pkt: pkt['n'], item_type=UInt8Field(), callback=tag_field)
+        first = template({'n': 2})
+        second = template({'n': 3})
+        self.assertFalse(hasattr(template, 'tag'))
+        self.assertEqual((first.tag, second.tag), (2, 3))  # type: ignore[attr-defined]
+        self.assertEqual((first.length, second.length), (2, 3))
+
+
+class SchemaFieldTemplateTests(unittest.TestCase):
+    """``SchemaField.__call__`` sizes the copy's template from the copy's length."""
+
+    def test_callable_length_sizes_the_template(self) -> None:
+        template = SchemaField(length=lambda pkt: pkt['n'], schema=Pair)
+        copy = template({'n': 24})
+        self.assertEqual(copy.length, 24)
+        self.assertEqual(copy.template, '24s')
+        self.assertEqual(template.template, '1024s')
+
+    def test_negative_resolved_length_keeps_the_default(self) -> None:
+        copy = SchemaField(length=lambda pkt: pkt['n'], schema=Pair)({'n': -1})
+        self.assertEqual(copy.template, '1024s')
+
+
+if __name__ == '__main__':
+    unittest.main()