@@ -95,6 +95,6 @@ def _missing_(cls, value: 'int') -> 'Group':
         if not (isinstance(value, int) and 0 <= value <= 255):
             raise ValueError('%r is not a valid %s' % (value, cls.__name__))
         if 12 <= value <= 255:
-            # Unassigned
+            #: Unassigned
             return cls._unregistered_member(value, 'Unassigned')
         return super()._missing_(value)
@@ -92,12 +92,12 @@ def _missing_(cls, value: 'int') -> 'Packet':
         if not (isinstance(value, int) and 0 <= value <= 127):
             raise ValueError('%r is not a valid %s' % (value, cls.__name__))
         if 5 <= value <= 15:
-            # Unassigned
+            #: Unassigned
             return cls._unregistered_member(value, 'Unassigned')
         if 21 <= value <= 31:
-            # Unassigned
+            #: Unassigned
             return cls._unregistered_member(value, 'Unassigned')
         if 33 <= value <= 127:
-            # Unassigned
+            #: Unassigned
             return cls._unregistered_member(value, 'Unassigned')
         return super()._missing_(value)
@@ -78,7 +78,7 @@ def process(self, data: 'list[str]') -> 'tuple[list[str], list[str]]':
                 start, stop = item[0].split('-')
 
                 miss.append(f'if {start} <= value <= {stop}:')
-                miss.append(f'    # {desc}')
+                miss.append(f'    #: {desc}')
                 miss.append(f"    return cls._unregistered_member(value, '{self.safe_name(name)}')")
         return enum, miss
 
 
@@ -79,7 +79,7 @@ def process(self, data: 'list[str]') -> 'tuple[list[str], list[str]]':
                 start, stop = item[0].split('-')
 
                 miss.append(f'if {start} <= value <= {stop}:')
-                miss.append(f'    # {desc}')
+                miss.append(f'    #: {desc}')
                 miss.append(f"    return cls._unregistered_member(value, '{self.safe_name(name)}')")
         return enum, miss
 
 
@@ -0,0 +1,267 @@
+# -*- coding: utf-8 -*-
+"""Every comment a crawler renders into ``_missing_`` is a ``#:`` one.
+
+GitHub issue #2331. :meth:`pcapkit.vendor.default.Vendor.process` renders a
+range-guarded :meth:`~enum.Enum._missing_` branch as three lines -- an ``if``
+header, a comment naming the span, then an indented ``return``::
+
+    miss.append(f'if {start} <= value <= {stop}:')
+    miss.append(f'    #: {desc}')
+    miss.append(f"    return cls._unregistered_member(value, '{self.safe_name(name)}')")
+
+``#:`` is Sphinx's attribute-doc marker, and every crawler rendered it bar
+:mod:`pcapkit.vendor.hip.group` and :mod:`pcapkit.vendor.hip.packet`, which
+rendered a plain ``#``. So their unassigned spans in
+:mod:`pcapkit.const.hip.group` and :mod:`pcapkit.const.hip.packet` read
+``# Unassigned`` where every other span across :mod:`pcapkit.const` reads
+``#: Unassigned``. The rendered documentation is not what the fix is about --
+autodoc reads a ``#:`` comment where an attribute is declared, not one inside a
+function body, as the issue itself notes -- it is that the generators emit one
+form rather than two.
+
+Nothing observable changed: a comment is a comment to the interpreter either
+way, so no behavioural suite could have caught it, and nothing checked the
+marker. Hence the two static sweeps below, one per layer:
+
+* :class:`VendorMissingCommentMarkerTests` reads the crawlers' own ASTs, so a
+  crawler that starts emitting ``#`` is caught before anyone regenerates.
+* :class:`ConstMissingCommentMarkerTests` reads the committed registries, so a
+  generated file that disagrees with the generator which will overwrite it is
+  caught too -- the same two-layer split
+  :file:`tests/vendor/test_vendor_missing_body_unit.py` makes for #866.
+
+Neither needs the network, a regeneration, or an importable
+:mod:`pcapkit.vendor`: both read the checked-out tree as text.
+
+"""
+from __future__ import annotations
+
+import ast
+import pathlib
+import re
+import unittest
+
+#: Test tier (#1958): reads the vendor templates and the committed constant modules
+#: as text, a question about the tree rather than about an interpreter.
+__tier__ = 'LATEST'
+
+#: Repository root, i.e. the grandparent of the directory holding this file.
+ROOT = pathlib.Path(__file__).resolve().parents[2]
+
+#: Root of the vendor crawler package, as a path rather than an import, so this
+#: module reads the same tree it is checked out in regardless of which
+#: ``pcapkit`` an editable install happens to resolve.
+VENDOR_ROOT = ROOT / 'pcapkit' / 'vendor'
+
+#: Root of the generated constant package, read the same way and for the same
+#: reason.
+CONST_ROOT = ROOT / 'pcapkit' / 'const'
+
+#: The range guard :meth:`~pcapkit.vendor.default.Vendor.process` opens an
+#: unassigned-span branch with. Anchors :func:`_const_span_comments` to the
+#: *generated* shape, so the prose ``#`` comments that
+#: :mod:`pcapkit.const.ftp.command` and :mod:`pcapkit.const.http.method`
+#: hand-write above their own ``_unregistered_member`` calls are not swept:
+#: those are explanatory notes, not attribute docs, and ``#`` is right for them.
+SPAN_GUARD = re.compile(r'^\s*if .*<= value <=.*:$')
+
+#: The ``return`` such a branch closes on.
+SPAN_RETURN = re.compile(r'^\s*return cls\._unregistered_member\(')
+
+#: Crawlers the AST walk must reach, so a walk that silently matched nothing --
+#: an ``ast`` change, a rename of the ``miss`` local -- fails loudly instead of
+#: sweeping an empty set. Named rather than counted: a count of what the tree
+#: holds drifts as soon as the tree does, which :ref:`documentation`'s *state no
+#: count* rules against (:issue:`1103`), and the vendor cron regenerates this
+#: tree on its own schedule.
+REQUIRED_CRAWLERS = (
+    'pcapkit/vendor/default.py',      # the shared template every crawler inherits
+    'pcapkit/vendor/hip/group.py',    # #2331's pair
+    'pcapkit/vendor/hip/packet.py',
+    'pcapkit/vendor/hip/parameter.py',  # their already-correct HIP siblings
+    'pcapkit/vendor/hip/eddsa_curve.py',
+)
+
+#: Registries the text sweep must reach, for the same reason and named the same
+#: way.
+REQUIRED_REGISTRIES = (
+    'pcapkit/const/hip/group.py',
+    'pcapkit/const/hip/packet.py',
+)
+
+
+def _leading_text(node: 'ast.expr') -> 'str | None':
+    """Read the literal text a ``miss`` line starts with.
+
+    A crawler renders a line either as a plain :class:`str` constant or as an
+    f-string whose first segment is one -- ``f'    #: {desc}'``. Only that
+    leading segment decides whether the line is a comment, so an f-string's
+    interpolations are not inspected.
+
+    Args:
+        node: Expression appended to, or listed in, ``miss``.
+
+    Returns:
+        The leading literal text, or :obj:`None` when the line begins with
+        something other than literal text, e.g. ``f'{prefix}...'``.
+
+    """
+    if isinstance(node, ast.Constant) and isinstance(node.value, str):
+        return node.value
+    if isinstance(node, ast.JoinedStr) and node.values:
+        head = node.values[0]
+        if isinstance(head, ast.Constant) and isinstance(head.value, str):
+            return head.value
+    return None
+
+
+def _vendor_comment_lines() -> 'list[tuple[str, str]]':
+    """Collect every comment line the crawlers render into ``_missing_``.
+
+    Walks each crawler's AST for a line added to ``miss``, whether by
+    ``miss.append(...)`` or as an element of a ``miss = [...]`` literal, and
+    keeps the ones whose leading literal text is a comment.
+
+    Returns:
+        Pairs of repository-relative crawler path and comment line.
+
+    """
+    found = []  # type: list[tuple[str, str]]
+    for path in sorted(VENDOR_ROOT.rglob('*.py')):
+        tree = ast.parse(path.read_text(encoding='utf-8'))
+        lines = []  # type: list[ast.expr]
+        for node in ast.walk(tree):
+            if (isinstance(node, ast.Call)
+                    and isinstance(node.func, ast.Attribute) and node.func.attr == 'append'
+                    and isinstance(node.func.value, ast.Name) and node.func.value.id == 'miss'
+                    and node.args):
+                lines.append(node.args[0])
+            elif (isinstance(node, ast.Assign) and isinstance(node.value, ast.List)
+                    and any(isinstance(target, ast.Name) and target.id == 'miss'
+                            for target in node.targets)):
+                lines.extend(node.value.elts)
+        for line in lines:
+            text = _leading_text(line)
+            if text is not None and text.lstrip().startswith('#'):
+                found.append((str(path.relative_to(ROOT)), text))
+    return found
+
+
+def _const_span_comments() -> 'list[tuple[str, int, str]]':
+    """Collect every comment inside a generated unassigned-span branch.
+
+    A branch is recognised by its shape rather than by its text: a
+    :data:`SPAN_GUARD` header, one or more comment lines, then a
+    :data:`SPAN_RETURN`. A comment sitting anywhere else -- including one above
+    an ``_unregistered_member`` call that no range guard opens -- is not swept.
+
+    Returns:
+        Triples of repository-relative module path, 1-based line number and
+        comment line.
+
+    """
+    found = []  # type: list[tuple[str, int, str]]
+    for path in sorted(CONST_ROOT.rglob('*.py')):
+        lines = path.read_text(encoding='utf-8').splitlines()
+        for index, line in enumerate(lines):
+            if not SPAN_GUARD.match(line):
+                continue
+            cursor = index + 1
+            comments = []  # type: list[tuple[int, str]]
+            while cursor < len(lines) and lines[cursor].lstrip().startswith('#'):
+                comments.append((cursor + 1, lines[cursor]))
+                cursor += 1
+            if comments and cursor < len(lines) and SPAN_RETURN.match(lines[cursor]):
+                found.extend((str(path.relative_to(ROOT)), number, text)
+                             for number, text in comments)
+    return found
+
+
+class VendorMissingCommentMarkerTests(unittest.TestCase):
+    """The crawler layer: what the generators emit."""
+
+    @classmethod
+    def setUpClass(cls) -> 'None':
+        cls.comments = _vendor_comment_lines()  # type: ignore[attr-defined]
+
+    def test_the_sweep_reaches_the_crawlers_it_is_about(self) -> 'None':
+        """A walk that matched nothing would pass every check below vacuously."""
+        swept = {crawler for crawler, _ in self.comments}  # type: ignore[attr-defined]
+        for crawler in REQUIRED_CRAWLERS:
+            with self.subTest(crawler=crawler):
+                self.assertIn(crawler, swept)
+
+    def test_every_crawler_renders_a_sphinx_comment(self) -> 'None':
+        """One form of comment across the generators, which is ``#:``."""
+        for crawler, text in self.comments:  # type: ignore[attr-defined]
+            with self.subTest(crawler=crawler, line=text):
+                self.assertTrue(text.lstrip().startswith('#:'),
+                                f'{crawler} renders {text!r} into ``_missing_``, a plain '
+                                f'``#`` comment where every other crawler renders ``#:``; '
+                                f'see GitHub issue #2331')
+
+    def test_the_hip_group_and_packet_crawlers_render_sphinx_comments(self) -> 'None':
+        """The specific pair from #2331, pinned by name.
+
+        Kept alongside the general sweep because these are the crawlers the
+        issue was filed against, and a named test says which file to look at
+        without having to read a subTest label.
+
+        """
+        for crawler in ('pcapkit/vendor/hip/group.py', 'pcapkit/vendor/hip/packet.py'):
+            with self.subTest(crawler=crawler):
+                texts = [text for path, text in self.comments  # type: ignore[attr-defined]
+                         if path == crawler]
+                self.assertEqual(texts, ['    #: '])
+
+
+class ConstMissingCommentMarkerTests(unittest.TestCase):
+    """The generated layer: what the committed registries carry."""
+
+    @classmethod
+    def setUpClass(cls) -> 'None':
+        cls.comments = _const_span_comments()  # type: ignore[attr-defined]
+
+    def test_the_sweep_reaches_the_registries_it_is_about(self) -> 'None':
+        """A sweep that matched nothing would pass every check below vacuously."""
+        swept = {module for module, _, _ in self.comments}  # type: ignore[attr-defined]
+        for module in REQUIRED_REGISTRIES:
+            with self.subTest(module=module):
+                self.assertIn(module, swept)
+
+    def test_every_registry_carries_a_sphinx_comment(self) -> 'None':
+        """A generated file must not disagree with the generator."""
+        for module, number, text in self.comments:  # type: ignore[attr-defined]
+            with self.subTest(module=module, line=number):
+                self.assertTrue(text.lstrip().startswith('#:'),
+                                f'{module}:{number} reads {text.strip()!r} inside an '
+                                f'unassigned-span branch, a plain ``#`` comment where the '
+                                f'crawlers render ``#:``; see GitHub issue #2331')
+
+    def test_the_hip_group_and_packet_registries_carry_sphinx_comments(self) -> 'None':
+        """The spans #2331 names, pinned by module rather than by tally.
+
+        How many unassigned spans each registry has is IANA's business and
+        changes with each regeneration, so this checks what every one of them
+        reads instead of how many there are.
+
+        """
+        for module in REQUIRED_REGISTRIES:
+            with self.subTest(module=module):
+                texts = {text.strip() for path, _, text
+                         in self.comments if path == module}  # type: ignore[attr-defined]
+                self.assertEqual(texts, {'#: Unassigned'})
+
+    def test_the_hip_group_and_packet_lookups_still_resolve(self) -> 'None':
+        """Only the comment text changed, so the lookups behave as before."""
+        from pcapkit.const.hip.group import Group
+        from pcapkit.const.hip.packet import Packet
+
+        self.assertEqual(Group(12).name, 'Unassigned')
+        self.assertEqual(Packet(33).name, 'Unassigned')
+        self.assertNotIn('Unassigned', Group.__members__)
+        self.assertNotIn('Unassigned', Packet.__members__)
+
+
+if __name__ == '__main__':
+    unittest.main()