diff --git a/.github/workflows/unit-tests.yml b/.github/workflows/unit-tests.yml
index fd5b2173e9..b7b8736ecf 100644
--- a/.github/workflows/unit-tests.yml
+++ b/.github/workflows/unit-tests.yml
@@ -234,8 +234,8 @@ jobs:
         # timings, uploaded by the last step of this job.
         #
         # #1958, the test tiers: tests/_tiers.py's module_tier() puts every test
-        # module in `ALL` or `LATEST`, by the `__tier__` constant a module states
-        # or by the rule where it states none, and `LATEST` runs on `latest`
+        # module in `ALL`, a curated list each stating `__tier__ = 'ALL'` (#1963),
+        # or else in `LATEST`, and `LATEST` runs on `latest`
         # alone, which is also the coverage leg, so coverage keeps its
         # selection. Every other leg -- `test-ktlo`'s too, as it runs these
         # steps -- passes each module latest_tier_paths() prints as an
diff --git a/docs/source/contributing/workflows.rst b/docs/source/contributing/workflows.rst
index 5323cd3d95..937da2705d 100644
--- a/docs/source/contributing/workflows.rst
+++ b/docs/source/contributing/workflows.rst
@@ -915,30 +915,57 @@ and fixture-dependent tiers, which decide what a module may read and so which jo
 collects it; a test tier decides on which of that job's legs it runs.
 
 ``module_tier()`` in :file:`tests/_tiers.py` decides, in one place, for every
-``tests/**/test_*.py``:
-
-* **The marker.** A module may state its tier outright with a module-level
-  ``__tier__ = 'ALL'`` or ``__tier__ = 'LATEST'``, read by AST, under a ``#:``
-  comment that gives the reason. It wins over the rule. It is a module constant
-  rather than a pytest marker, so ``unittest-ordering``'s plain ``unittest`` runs
-  read the same module. A value that is not one of the two string literals, or a
-  marker assigned twice or anywhere but at the module's top level, fails the tier
-  guard.
-* **The rule.** Otherwise a module is ``LATEST`` when its run does not vary by
-  interpreter, by the same AST test as the project pins below: it reads the
-  repository's text, a workflow or the docs, or drives a ``util/`` script, and
-  neither imports ``pcapkit`` nor compares the Python version. Everything that
-  exercises ``pcapkit`` is ``ALL``.
-
-The marker says what the AST cannot see. Each real sweep -- a round trip over every
-class or registry entry a table enumerates, every offset of a packet or every
-capture -- is marked ``LATEST``, as the owner ruled on #1732 and #1958; the
-hand-picked round trips, whatever their names, run on ``ALL``, and so do the #1202
-edge-case tables, whose cases are each written out by hand. A module that imports ``pcapkit`` only to read its text or
-structure, or that drives a docs hook or a generator other legs already run, is
-marked ``LATEST``; one that the rule would send to ``latest`` but that depends on
-the interpreter, such as the tier guard itself, which :file:`tests/conftest.py` runs at
-collection on every leg, is marked ``ALL``.
+``tests/**/test_*.py``. ``ALL`` is a curated, opt-in list (#1958, #1963): short enough
+to save time on every leg, the same on every leg so QA stays consistent, and enough
+on its own to validate a change on each interpreter, while ``latest`` runs the whole
+suite as the gate.
+
+* **The marker.** A module joins ``ALL`` with a module-level ``__tier__ = 'ALL'``,
+  read by AST, under a ``#: Test tier (#1958): <reason>`` comment. It is a module
+  constant rather than a pytest marker, so ``unittest-ordering``'s plain
+  ``unittest`` runs read the same module. A value that is not ``'ALL'`` or
+  ``'LATEST'``, or a marker assigned twice or anywhere but at the module's top
+  level, fails the tier guard.
+* **The default.** A module that states no tier is ``LATEST``, so a new module runs
+  on ``latest`` until it says why every leg needs it. ``__tier__ = 'LATEST'`` is
+  optional, and records why a module someone might expect on every leg is not; a
+  module that compares ``sys.version_info``, ``sys.implementation`` or
+  ``sys.maxsize``, reads
+  ``sys.flags``, calls ``sysconfig.get_config_var()`` or probes :mod:`sys` with
+  ``hasattr``/``getattr`` has to state its tier either way, so a version gate never
+  lands on ``latest`` alone by omission. The check is syntactic and best-effort: an
+  alias, a flag imported from ``pcapkit.utilities.compat`` or a gate in a helper is
+  not seen, and such a module states its tier by hand.
+
+A module opts in when it exercises behaviour that can differ between interpreters
+or versions:
+
+* compat shims and version or implementation gates;
+* C-accelerated versus pure paths, and code sensitive to garbage collection, weak
+  references or finalisation;
+* ``int``, ``bytes`` and ``struct`` edge cases, and ``sys.maxsize`` or recursion
+  limits;
+* pickling and copying, metaclass and ABC machinery, and enum mechanics;
+* stream and buffer semantics, threading and locking, and the warning and
+  exception hooks;
+* a third-party engine whose installable releases vary by interpreter, such as
+  ``pypcap`` and ``pypcapfile``, which install below 3.12 only, and Scapy, which the
+  ``integration`` and ``pypcap-parity`` legs install on every interpreter and a
+  ``test`` leg off ``latest`` skips;
+* the hand-picked round trips, whatever their names, and the #1202 edge-case
+  tables, whose cases are each written out by hand, as the owner ruled on #1958;
+  and
+* the smoke slice: a representative module per protocol family and per package,
+  so each leg still drives the main parse and build paths.
+
+Everything else is ``LATEST``: the per-protocol field-by-field tests, whose
+behaviour is interpreter-independent pure Python, each real sweep -- a round trip
+over every class or registry entry a table enumerates, every offset of a packet or
+every capture, as the owner ruled on #1732 and #1958 -- and the docs, project and
+structure tests. The vendor extra is installed on ``latest`` alone and the crawlers
+are pure Python, so the vendor modules are ``LATEST``, each saying so, but for the
+file-restore and environment-variable tests, which stay on every leg. The tier guard itself is ``ALL``, since :file:`tests/conftest.py`
+runs it at collection on every leg.
 
 Which legs run which tier:
 
diff --git a/tests/_tiers.py b/tests/_tiers.py
index 5bf864cd3d..4dca218957 100644
--- a/tests/_tiers.py
+++ b/tests/_tiers.py
@@ -47,10 +47,12 @@
   for real, and the complete pytest selection the fixture-dependent tier needs
   in order to still exercise them.
 * :func:`module_tier` -- which *test tier* does this module run in (#1958):
-  ``ALL``, on every leg, or ``LATEST``, on ``latest`` alone? A ``__tier__``
-  constant in the module (:func:`declared_tier`) answers first, and
-  :func:`rule_tier` otherwise. :func:`latest_tier_paths` lists the ``LATEST``
-  modules, which every leg but ``latest`` leaves out.
+  ``ALL``, on every leg, or ``LATEST``, on ``latest`` alone? ``ALL`` is opt-in:
+  a module is in it only when it says so with ``__tier__ = 'ALL'``
+  (:func:`declared_tier`), and :data:`DEFAULT_TIER`, ``LATEST``, otherwise.
+  :func:`all_tier_paths` and :func:`latest_tier_paths` list the two tiers; every
+  leg but ``latest`` leaves the second out. See "The curated ``ALL`` tier" below
+  for which modules opt in.
 * :func:`is_project_pin` and :func:`project_pin_paths` -- which
   :file:`tests/project` modules give one answer on every interpreter, and so run
   once per push, in the ``project-tests`` job, rather than on every leg (#1954)?
@@ -114,6 +116,47 @@
 capture reads go through :func:`~tests._support.sample_path`, and that is the door
 with the lock on it.
 
+The curated ``ALL`` tier
+------------------------
+
+``ALL`` is a tailored list rather than whatever a rule guesses (#1958, #1963): it
+is what every leg off ``latest`` runs, so it has to be short enough to save time,
+the same on every leg so QA stays consistent, and enough on its own to validate a
+change on each interpreter. ``latest`` runs the whole suite, and is the gate.
+
+A module opts in with ``__tier__ = 'ALL'`` when it exercises behaviour that can
+differ between interpreters or versions:
+
+* compat shims and ``sys.version_info`` or ``sys.implementation`` gates;
+* C-accelerated versus pure paths, and code sensitive to garbage collection,
+  weak references or finalisation;
+* ``int``, ``bytes`` and :mod:`struct` edge cases, and ``sys.maxsize`` or
+  recursion limits;
+* pickling and copying, metaclass and ABC machinery, and enum mechanics;
+* stream and buffer semantics, threading and locking, and the warning and
+  exception hooks;
+* a third-party engine whose installable releases vary by interpreter, which
+  ``latest`` alone could not cover;
+* the hand-picked round trips, the ``*_edge_roundtrip_unit`` tables among them,
+  which the owner ruled run on ``ALL`` as part of QA; and
+* the smoke slice: a representative module per protocol family and per
+  package, so each leg still drives the main parse and build paths.
+
+Everything else is ``LATEST`` by default: the per-protocol field-by-field tests,
+whose behaviour is interpreter-independent pure Python, the real sweeps, and the
+docs, project and structure tests. A module that branches on the interpreter
+itself (:func:`branches_on_interpreter`, a syntactic, best-effort check) has to
+state its tier either way, so a version gate never lands on ``latest`` alone by
+omission; a ``LATEST`` marker is otherwise optional, and records why a module
+someone might expect on every leg is not.
+
+Scapy is opted in like the other third-party engines: the ``integration`` and
+``pypcap-parity`` legs install it on every interpreter, and a ``test`` leg off
+``latest``, which does not, skips it. The vendor extra is installed on ``latest``
+alone, and the crawlers are pure Python over ``requests`` and ``bs4``, so the vendor
+modules are ``LATEST``, each saying so, but for the file-restore and
+environment-variable tests, which stay on every leg.
+
 Nothing here imports :mod:`tests._support` -- the dependency runs the other way,
 and adding the reverse edge would make it a cycle. Nothing here imports
 :mod:`pcapkit`, :mod:`pytest` or any optional engine either, so the guard is
@@ -141,7 +184,8 @@
     'ROOT', 'TESTS_ROOT', 'SAMPLE_ROOT', 'REGENERATE_SAMPLES_CMD', 'UNIT_TIER_SELECTION',
     'FIXTURE_TIER_SUFFIXES', 'FIXTURE_TIER_DIRS', 'FIXTURE_TIER_SELECTOR',
     'TIER_ALL', 'TIER_LATEST', 'TIERS', 'TIER_MARKER', 'LATEST_TIER_SELECTOR', 'FIXTURE_ALL_TIER_SELECTOR',
-    'InvalidTierError', 'suite_modules', 'declared_tier', 'rule_tier', 'module_tier', 'latest_tier_paths',
+    'InvalidTierError', 'DEFAULT_TIER', 'suite_modules', 'declared_tier', 'branches_on_interpreter', 'module_tier',
+    'all_tier_paths', 'latest_tier_paths',
     'PROJECT_PINS_DIR', 'PROJECT_PINS_SELECTOR', 'PROJECT_PIN_PACKAGE', 'LOADABLE_DIRS',
     'varies_by_interpreter', 'is_project_pin', 'project_pin_paths',
     'is_unit_tier', 'committed_captures', 'committed_capture_names',
@@ -195,6 +239,9 @@
 TIER_LATEST = 'LATEST'
 #: Every test tier, the values a :data:`TIER_MARKER` may take.
 TIERS = (TIER_ALL, TIER_LATEST)
+#: The tier of a module that states none: ``ALL`` is opt-in (#1958, #1963), so a
+#: new module runs on ``latest``, the gate, until it says why every leg needs it.
+DEFAULT_TIER = TIER_LATEST
 #: The module-level constant that states a module's test tier outright, e.g.
 #: ``__tier__ = 'LATEST'``, read by AST in :func:`declared_tier`. A constant rather
 #: than a pytest marker, so plain :mod:`unittest` runs see the same module.
@@ -282,7 +329,7 @@ class TierGuardWarning(UserWarning):
 class InvalidTierError(ValueError):
     """A test module's :data:`TIER_MARKER` cannot be read as one of :data:`TIERS`.
 
-    Raised rather than ignored: a marker that silently fell back to the rule would
+    Raised rather than ignored: a marker that silently fell back to the default would
     put the module on legs its author ruled it off, or the other way round.
 
     """
@@ -896,7 +943,7 @@ def declared_tier(path: 'pathlib.Path | str') -> 'Optional[str]':
 
     Read by AST, never by importing the module: one module-level statement,
     ``__tier__ = 'ALL'`` or ``__tier__ = 'LATEST'``, the value a string literal.
-    A module that does not parse states nothing here, and :func:`rule_tier`
+    A module that does not parse states nothing here, and :data:`DEFAULT_TIER`
     answers for it.
 
     Args:
@@ -912,19 +959,56 @@ def declared_tier(path: 'pathlib.Path | str') -> 'Optional[str]':
     return _declared_tier(_test_module(path))
 
 
-def rule_tier(path: 'pathlib.Path | str') -> 'str':
-    """The test tier the rule gives the module at ``path`` (#1958).
+#: Attributes of :mod:`sys` whose mere use asks which build is running, compared or not:
+#: ``sys.flags.context_aware_warnings``, ``sys._is_gil_enabled()``.
+_BUILD_FACTS = frozenset({'flags', '_is_gil_enabled'})
+#: Builtins that probe :mod:`sys` for an attribute, e.g. ``hasattr(sys, 'tracebacklimit')``.
+_PROBES = frozenset({'hasattr', 'getattr'})
+
+
+def _probes_interpreter(node: 'ast.AST') -> 'bool':
+    """Whether ``node`` asks the running interpreter something outside a comparison.
+
+    ``sys.flags`` or ``sys._is_gil_enabled``, a comparison with ``sys.maxsize``, a
+    ``sysconfig.get_config_var()`` call such as ``Py_GIL_DISABLED``'s, or a
+    ``hasattr``/``getattr`` probe of :mod:`sys`.
+
+    """
+    if isinstance(node, ast.Attribute):
+        return node.attr in _BUILD_FACTS and isinstance(node.value, ast.Name) and node.value.id == 'sys'
+    if isinstance(node, ast.Compare):
+        return any(isinstance(part, ast.Attribute) and part.attr == 'maxsize' and isinstance(part.value, ast.Name)
+                   and part.value.id == 'sys' for operand in [node.left] + list(node.comparators)
+                   for part in ast.walk(operand))
+    if not isinstance(node, ast.Call):
+        return False
+    func = node.func
+    if isinstance(func, ast.Attribute) and func.attr == 'get_config_var':
+        return isinstance(func.value, ast.Name) and func.value.id == 'sysconfig'
+    if isinstance(func, ast.Name) and func.id == 'get_config_var':
+        return True
+    return (isinstance(func, ast.Name) and func.id in _PROBES and bool(node.args)
+            and isinstance(node.args[0], ast.Name) and node.args[0].id == 'sys')
+
+
+def branches_on_interpreter(path: 'pathlib.Path | str') -> 'bool':
+    """Whether the test module at ``path`` itself asks which interpreter is running (#1958).
+
+    By its own AST alone, unlike :func:`varies_by_interpreter`, which follows what
+    the module runs: a comparison of ``sys.version_info``, ``sys.implementation``,
+    ``platform.python_version()`` or the like; a use of ``sys.flags`` or
+    ``sys._is_gil_enabled``; a comparison with ``sys.maxsize``; a
+    ``sysconfig.get_config_var()`` call, such as
+    ``Py_GIL_DISABLED``'s; or a ``hasattr``/``getattr`` probe of :mod:`sys`, anywhere
+    in it. Such a module has to state its tier with :data:`TIER_MARKER`, so a
+    version gate never runs on ``latest`` alone just because nobody opted it in to
+    ``ALL``. A module that does not parse branches on nothing here.
 
-    :data:`TIER_LATEST` when its run does not :func:`vary by interpreter
-    <varies_by_interpreter>` -- it reads the repository's text, a workflow or the
-    docs, or drives a ``util/`` script, and imports no ``pcapkit`` -- since one run
-    on ``latest`` is then every leg's answer. :data:`TIER_ALL` for everything
-    else, which is everything that exercises ``pcapkit``. What the AST cannot
-    see -- that a module is a sweep over every case of a table or every capture,
-    that one which imports ``pcapkit`` still answers a question about the
-    repository, or that one which imports nothing still depends on the
-    interpreter -- is stated with :data:`TIER_MARKER` instead; see
-    :func:`module_tier`.
+    The detection is syntactic and best-effort, not proof that a module is
+    interpreter-independent: it does not see :mod:`sys` under an alias, a probe of
+    any other module, a flag imported from :mod:`pcapkit.utilities.compat` or
+    another helper, or a gate in code the module imports. A module that relies on
+    any of those states its tier by hand.
 
     Args:
         path: Path to a test module, absolute or relative to :data:`ROOT`.
@@ -933,18 +1017,21 @@ def rule_tier(path: 'pathlib.Path | str') -> 'str':
         ValueError: ``path`` is not a test module under :file:`tests/`.
 
     """
-    return TIER_ALL if varies_by_interpreter(_test_module(path)) else TIER_LATEST
+    tree = _parse_module(_test_module(path))
+    return tree is not None and any(_compares_interpreter(node) or _probes_interpreter(node)
+                                    for node in ast.walk(tree))
 
 
 def module_tier(path: 'pathlib.Path | str') -> 'str':
     """The test tier of the module at ``path``: :data:`TIER_ALL` or :data:`TIER_LATEST` (#1958).
 
-    Its :func:`declared_tier` where it states one, its :func:`rule_tier`
-    otherwise, so every test module has exactly one. ``ALL`` modules run on every
-    leg; ``LATEST`` ones on ``latest`` alone, which is also the coverage leg, the
-    ``gate`` job's interpreter, and every ``unittest-ordering`` cell's. Within a
-    tier, the unit and fixture-dependent tiers still decide which job collects a
-    module.
+    Its :func:`declared_tier` where it states one, :data:`DEFAULT_TIER` otherwise,
+    so every test module has exactly one and ``ALL`` is opt-in: see "The curated
+    ``ALL`` tier" in this module's docstring for which modules opt in. ``ALL``
+    modules run on every leg; ``LATEST`` ones on ``latest`` alone, which is also
+    the coverage leg, the ``gate`` job's interpreter, and every
+    ``unittest-ordering`` cell's. Within a tier, the unit and fixture-dependent
+    tiers still decide which job collects a module.
 
     Args:
         path: Path to a test module, absolute or relative to :data:`ROOT`.
@@ -954,8 +1041,18 @@ def module_tier(path: 'pathlib.Path | str') -> 'str':
         InvalidTierError: Its marker is malformed; see :func:`declared_tier`.
 
     """
-    candidate = _test_module(path)
-    return _declared_tier(candidate) or rule_tier(candidate)
+    return _declared_tier(_test_module(path)) or DEFAULT_TIER
+
+
+@functools.lru_cache(maxsize=1)
+def all_tier_paths() -> 'tuple[str, ...]':
+    """Every :data:`TIER_ALL` test module, relative to :data:`ROOT`, sorted.
+
+    The curated tier every leg runs: the complement of :func:`latest_tier_paths`
+    within :func:`suite_modules`, and each of them marked ``__tier__ = 'ALL'``.
+
+    """
+    return tuple(path for path in suite_modules() if module_tier(path) == TIER_ALL)
 
 
 @functools.lru_cache(maxsize=1)
diff --git a/tests/cli/test_main.py b/tests/cli/test_main.py
index 166aa76468..526beedd36 100644
--- a/tests/cli/test_main.py
+++ b/tests/cli/test_main.py
@@ -48,6 +48,10 @@
 
 from tests._support import ISOLATED_PREFIXES, isolate_modules, snapshot_modules
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 ROOT = pathlib.Path(__file__).resolve().parents[2]
 
 #: The :data:`sys.modules` region this file stands things in for, and therefore
diff --git a/tests/const/test_const_apptype_lazy_1538_unit.py b/tests/const/test_const_apptype_lazy_1538_unit.py
index 0615b6509e..2b40365caf 100644
--- a/tests/const/test_const_apptype_lazy_1538_unit.py
+++ b/tests/const/test_const_apptype_lazy_1538_unit.py
@@ -39,6 +39,9 @@
     from types import ModuleType
     from typing import Any
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 ROOT = pathlib.Path(__file__).resolve().parents[2]
 
 #: The names ``dir(pcapkit.const.reg.apptype)`` listed on the eager package
diff --git a/tests/const/test_const_enum_builtin_parity.py b/tests/const/test_const_enum_builtin_parity.py
index 615d62fe5b..eb2cb244a9 100644
--- a/tests/const/test_const_enum_builtin_parity.py
+++ b/tests/const/test_const_enum_builtin_parity.py
@@ -66,6 +66,9 @@
 if TYPE_CHECKING:
     from typing import Optional
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 #: An integer no wire field in the library is wide enough to carry. Shared with
 #: :mod:`tests.const.test_const_enum_get`, which uses the same value for the
 #: same reason.
diff --git a/tests/const/test_const_enum_get.py b/tests/const/test_const_enum_get.py
index 8113f5fba1..7d7a3194b7 100644
--- a/tests/const/test_const_enum_get.py
+++ b/tests/const/test_const_enum_get.py
@@ -73,6 +73,10 @@
 
 from tests._support import ISOLATED_PREFIXES, purge_modules, restore_modules, snapshot_modules
 
+#: Test tier (#1958): exercises metaclass, ABC or enum machinery, which differs by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: An integer no wire field in the library is wide enough to carry, so every
 #: registry that bounds its own domain rejects it. Deliberately far outside the
 #: ``0..0xFF`` range the Mobility Header flag registries check, because a value
diff --git a/tests/const/test_const_enum_lookup.py b/tests/const/test_const_enum_lookup.py
index 510bc74984..47f868bb94 100644
--- a/tests/const/test_const_enum_lookup.py
+++ b/tests/const/test_const_enum_lookup.py
@@ -72,6 +72,10 @@
 from tests._support import (ISOLATED_PREFIXES, purge_modules, reimport_once_per_class,
                             restore_modules, snapshot_modules)
 
+#: Test tier (#1958): exercises metaclass, ABC or enum machinery, which differs by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: Fully qualified names of the :class:`~aenum.IntEnum` classes under
 #: :mod:`pcapkit.const` for which rejecting ``0`` is *correct*, because their
 #: registries simply do not define ``0`` (registries reserving ``0`` and
diff --git a/tests/const/test_const_register_loud_in_quiet_block_1916_unit.py b/tests/const/test_const_register_loud_in_quiet_block_1916_unit.py
index c272d97039..6818970993 100644
--- a/tests/const/test_const_register_loud_in_quiet_block_1916_unit.py
+++ b/tests/const/test_const_register_loud_in_quiet_block_1916_unit.py
@@ -30,6 +30,10 @@
 if TYPE_CHECKING:
     from typing import Callable
 
+#: Test tier (#1958): exercises threading, locking or warning and exception hooks, which
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 
 class RegisterLoudInQuietBlock1916Tests(unittest.TestCase):
     """``register`` on a generated registry logs its rejection inside a quiet block."""
diff --git a/tests/corekit/test_decorators.py b/tests/corekit/test_decorators.py
index f4819e19b2..1fb7552649 100644
--- a/tests/corekit/test_decorators.py
+++ b/tests/corekit/test_decorators.py
@@ -7,6 +7,10 @@
 from tests._support import (bootstrap_core_modules, install_fake_payload_protocols,
                             isolate_modules)
 
+#: Test tier (#1958): exercises metaclass, ABC or enum machinery, which differs by
+#: interpreter.
+__tier__ = 'ALL'
+
 
 class DecoratorTests(unittest.TestCase):
     def setUp(self) -> None:
diff --git a/tests/corekit/test_enum_get_exception_provenance_923_unit.py b/tests/corekit/test_enum_get_exception_provenance_923_unit.py
index 4e0d80db6d..06875ce259 100644
--- a/tests/corekit/test_enum_get_exception_provenance_923_unit.py
+++ b/tests/corekit/test_enum_get_exception_provenance_923_unit.py
@@ -73,6 +73,10 @@
 if TYPE_CHECKING:
     from typing import Any
 
+#: Test tier (#1958): exercises threading, locking or warning and exception hooks, which
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 __all__ = [
     'EnumKeyErrorClassTests', 'BaseGetProvenanceTests', 'BaseGetQuietnessTests',
     'MethodStillMintsTests', 'TransportProtocolCaseFoldingTests',
diff --git a/tests/corekit/test_enum_get_quiet_fallback_1845_unit.py b/tests/corekit/test_enum_get_quiet_fallback_1845_unit.py
index 67e15b789c..be684a6baf 100644
--- a/tests/corekit/test_enum_get_quiet_fallback_1845_unit.py
+++ b/tests/corekit/test_enum_get_quiet_fallback_1845_unit.py
@@ -47,6 +47,10 @@
 
     from tests.utilities._harness import Recorder
 
+#: Test tier (#1958): exercises threading, locking or warning and exception hooks, which
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 __all__ = ['QuietFallbackTests', 'QuietRaiseTests', 'LoudPathTests', 'MechanismTests']
 
 #: A value :class:`ToSPrecedence`'s ``_validate_value`` refuses: the field is 3 bits.
diff --git a/tests/corekit/test_enum_lookup_base_unit.py b/tests/corekit/test_enum_lookup_base_unit.py
index 6738556682..f9693f428c 100644
--- a/tests/corekit/test_enum_lookup_base_unit.py
+++ b/tests/corekit/test_enum_lookup_base_unit.py
@@ -64,6 +64,10 @@
 if TYPE_CHECKING:
     from typing import Any
 
+#: Test tier (#1958): exercises metaclass, ABC or enum machinery, which differs by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: The mutating half, which must stay on :class:`~pcapkit.corekit.enum.EnumRegistry`.
 MUTATING = ('register', 'register_alias', 'register_aliases', '_extend',
             '_unregistered_member')
diff --git a/tests/corekit/test_enum_lookup_reparent_877_unit.py b/tests/corekit/test_enum_lookup_reparent_877_unit.py
index 34f2057bf0..86d69dbc6e 100644
--- a/tests/corekit/test_enum_lookup_reparent_877_unit.py
+++ b/tests/corekit/test_enum_lookup_reparent_877_unit.py
@@ -60,6 +60,10 @@
 
 from pcapkit.corekit.enum import EnumLookup
 
+#: Test tier (#1958): exercises metaclass, ABC or enum machinery, which differs by
+#: interpreter.
+__tier__ = 'ALL'
+
 __all__ = [
     'ReparentedBasesTests', 'HttpV2FlagsHierarchyTests', 'PureReparentGetTests',
     'TransportProtocolGetTests', 'CriticalityGetTests', 'NoMintingTests',
diff --git a/tests/corekit/test_enum_lookup_reparent_930_unit.py b/tests/corekit/test_enum_lookup_reparent_930_unit.py
index 4c54d535da..6879729fdc 100644
--- a/tests/corekit/test_enum_lookup_reparent_930_unit.py
+++ b/tests/corekit/test_enum_lookup_reparent_930_unit.py
@@ -148,6 +148,10 @@
 from pcapkit.utilities.logging import logger
 from tests.utilities._harness import capture
 
+#: Test tier (#1958): exercises threading, locking or warning and exception hooks, which
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 __all__ = [
     'ReparentedBasesTests', 'GetByNameAndValueTests', 'FailedLookupTests',
     'InheritedQuietnessTests', 'NoMintingTests', 'AllSevenInheritTheBareClassmethodTests',
diff --git a/tests/corekit/test_fields_byteorder_1519_unit.py b/tests/corekit/test_fields_byteorder_1519_unit.py
index 93748be3ef..c163ecf4e5 100644
--- a/tests/corekit/test_fields_byteorder_1519_unit.py
+++ b/tests/corekit/test_fields_byteorder_1519_unit.py
@@ -27,6 +27,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 SHARED = 'pcapkit.corekit.fields.byteorder'
 
 #: Where the helper lived before #1711; removed, not re-exported.
diff --git a/tests/corekit/test_fields_copy_dispatch_unit.py b/tests/corekit/test_fields_copy_dispatch_unit.py
index 658941c58d..364d791f47 100644
--- a/tests/corekit/test_fields_copy_dispatch_unit.py
+++ b/tests/corekit/test_fields_copy_dispatch_unit.py
@@ -41,6 +41,10 @@
 
     from pcapkit.corekit.fields.field import FieldBase
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 
 class FieldCallUsesCopyDunderDirectlyTests(unittest.TestCase):
     """Every ``__call__`` in :mod:`pcapkit.corekit.fields` calls ``self.__copy__()``.
diff --git a/tests/corekit/test_fields_field.py b/tests/corekit/test_fields_field.py
index 4258370c92..4b7db1c861 100644
--- a/tests/corekit/test_fields_field.py
+++ b/tests/corekit/test_fields_field.py
@@ -7,6 +7,10 @@
 
 from tests._support import reimport_once_per_class, time_limit
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 
 class FieldBaseUnpackBoundsTests(unittest.TestCase):
     """Bounds checking in :meth:`FieldBase.unpack <pcapkit.corekit.fields.field.FieldBase.unpack>`.
diff --git a/tests/corekit/test_fields_misc.py b/tests/corekit/test_fields_misc.py
index 973e9e6f93..2cbd94fa86 100644
--- a/tests/corekit/test_fields_misc.py
+++ b/tests/corekit/test_fields_misc.py
@@ -5,6 +5,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: Every address-typed branch of every address-typed
 #: :class:`~pcapkit.corekit.fields.misc.SwitchField` in the schema tree, as
 #: ``(module, class, attribute, label, packet)``. The ``packet`` is a selector
diff --git a/tests/corekit/test_fields_numbers.py b/tests/corekit/test_fields_numbers.py
index 185af50685..cfbec8a615 100644
--- a/tests/corekit/test_fields_numbers.py
+++ b/tests/corekit/test_fields_numbers.py
@@ -6,6 +6,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/corekit/test_fields_numbers_bit_length_1488_unit.py b/tests/corekit/test_fields_numbers_bit_length_1488_unit.py
index af26cc204b..b64d1d1a6f 100644
--- a/tests/corekit/test_fields_numbers_bit_length_1488_unit.py
+++ b/tests/corekit/test_fields_numbers_bit_length_1488_unit.py
@@ -20,6 +20,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 #: Octets of a two-octet field, as hex.
 WIRES = ('0000', '0001', '0fff', 'ffff', 'f000', '8000', '0800', 'f7ff', 'a5a5')
 
diff --git a/tests/corekit/test_fields_numbers_callable_length.py b/tests/corekit/test_fields_numbers_callable_length.py
index f974622f82..11358a16d4 100644
--- a/tests/corekit/test_fields_numbers_callable_length.py
+++ b/tests/corekit/test_fields_numbers_callable_length.py
@@ -24,6 +24,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/corekit/test_fields_numbers_port_enum_field_peek_1774_unit.py b/tests/corekit/test_fields_numbers_port_enum_field_peek_1774_unit.py
index 61459a253a..1d70ff5bdf 100644
--- a/tests/corekit/test_fields_numbers_port_enum_field_peek_1774_unit.py
+++ b/tests/corekit/test_fields_numbers_port_enum_field_peek_1774_unit.py
@@ -20,6 +20,10 @@
 if TYPE_CHECKING:
     from typing import Any
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: An unassigned port on all four transports, in none of ``_missing_``'s spans.
 PORT = 50000
 
diff --git a/tests/corekit/test_fields_numbers_port_option_no_mint_unit.py b/tests/corekit/test_fields_numbers_port_option_no_mint_unit.py
index dbdff0dd78..1ed8cf164f 100644
--- a/tests/corekit/test_fields_numbers_port_option_no_mint_unit.py
+++ b/tests/corekit/test_fields_numbers_port_option_no_mint_unit.py
@@ -52,6 +52,10 @@
 if TYPE_CHECKING:
     from typing import Any
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 
 class IntCode(aenum.IntEnum):
     """An :class:`int`-backed registry stand-in.
diff --git a/tests/corekit/test_fields_numbers_signed_decode_1274_unit.py b/tests/corekit/test_fields_numbers_signed_decode_1274_unit.py
index 2add91d246..bb79621f08 100644
--- a/tests/corekit/test_fields_numbers_signed_decode_1274_unit.py
+++ b/tests/corekit/test_fields_numbers_signed_decode_1274_unit.py
@@ -18,6 +18,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 
 class TestSignedNumberFieldDecode(unittest.TestCase):
     """Pin the decoded value and the rebuilt octets of signed fields."""
diff --git a/tests/corekit/test_fields_numbers_width_repair.py b/tests/corekit/test_fields_numbers_width_repair.py
index e363a6dd40..0982717132 100644
--- a/tests/corekit/test_fields_numbers_width_repair.py
+++ b/tests/corekit/test_fields_numbers_width_repair.py
@@ -42,6 +42,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/corekit/test_fields_strings.py b/tests/corekit/test_fields_strings.py
index 45d467ca68..c7ad145b4c 100644
--- a/tests/corekit/test_fields_strings.py
+++ b/tests/corekit/test_fields_strings.py
@@ -4,6 +4,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 
 class BitFieldTests(unittest.TestCase):
     """Packing and parsing of :class:`~pcapkit.corekit.fields.strings.BitField`."""
diff --git a/tests/corekit/test_info_child_finalisation_1490_unit.py b/tests/corekit/test_info_child_finalisation_1490_unit.py
index a4d07242d4..6ae1574d1b 100644
--- a/tests/corekit/test_info_child_finalisation_1490_unit.py
+++ b/tests/corekit/test_info_child_finalisation_1490_unit.py
@@ -23,6 +23,10 @@
 
 from tests.corekit._roundtrip import skip_without_runtime
 
+#: Test tier (#1958): depends on garbage collection, weak references or object
+#: finalisation.
+__tier__ = 'ALL'
+
 ROOT = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
 #: Parent and child, by ``module:qualname``; ``synthetic`` is declared in the script.
diff --git a/tests/corekit/test_infoclass.py b/tests/corekit/test_infoclass.py
index ae7807dd40..41e8a41b72 100644
--- a/tests/corekit/test_infoclass.py
+++ b/tests/corekit/test_infoclass.py
@@ -6,6 +6,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises metaclass, ABC or enum machinery, which differs by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/corekit/test_io.py b/tests/corekit/test_io.py
index 992a4c77f3..01af4a150b 100644
--- a/tests/corekit/test_io.py
+++ b/tests/corekit/test_io.py
@@ -9,6 +9,10 @@
 
 from tests._support import bootstrap_core_modules, load_module, reimport_once_per_class
 
+#: Test tier (#1958): exercises stream and buffer semantics, which differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 
 class SeekableReaderTests(unittest.TestCase):
     def _close_reader(self, reader) -> None:
diff --git a/tests/corekit/test_io_peekable_stream_copy_1524_unit.py b/tests/corekit/test_io_peekable_stream_copy_1524_unit.py
index f79e3ce41c..5a77d86d1b 100644
--- a/tests/corekit/test_io_peekable_stream_copy_1524_unit.py
+++ b/tests/corekit/test_io_peekable_stream_copy_1524_unit.py
@@ -23,6 +23,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: What :class:`_CopiesItself` returns from its own ``__deepcopy__``.
 _COPIED = 'the stream copied itself'
 
diff --git a/tests/corekit/test_io_seekable_reader_short_reads_1586_unit.py b/tests/corekit/test_io_seekable_reader_short_reads_1586_unit.py
index 5a09e97a1a..4b87bc6e29 100644
--- a/tests/corekit/test_io_seekable_reader_short_reads_1586_unit.py
+++ b/tests/corekit/test_io_seekable_reader_short_reads_1586_unit.py
@@ -24,6 +24,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises stream and buffer semantics, which differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: Octets the stream holds: more than a few buffers' worth.
 DATA = bytes(range(256)) * 4
 
diff --git a/tests/corekit/test_io_stream_proxies_1516_unit.py b/tests/corekit/test_io_stream_proxies_1516_unit.py
index 73e622d941..ca8b904f38 100644
--- a/tests/corekit/test_io_stream_proxies_1516_unit.py
+++ b/tests/corekit/test_io_stream_proxies_1516_unit.py
@@ -22,6 +22,10 @@
 
 from tests._support import reimport_once_per_class, sample_path
 
+#: Test tier (#1958): depends on garbage collection, weak references or object
+#: finalisation.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/corekit/test_ipv4interface_mask_unit.py b/tests/corekit/test_ipv4interface_mask_unit.py
index 75ff9800dd..d663cd7ef1 100644
--- a/tests/corekit/test_ipv4interface_mask_unit.py
+++ b/tests/corekit/test_ipv4interface_mask_unit.py
@@ -26,6 +26,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: Address ``10.0.0.1`` followed by four mask octets.
 MASKS = {
     'hostmask /24': '0a000001' '000000ff',
diff --git a/tests/corekit/test_module.py b/tests/corekit/test_module.py
index 6e03b691f3..57cafd499a 100644
--- a/tests/corekit/test_module.py
+++ b/tests/corekit/test_module.py
@@ -10,6 +10,10 @@
 
 from tests._support import load_module, reimport_once_per_class
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 
 class ModuleDescriptorTests(unittest.TestCase):
     def setUp(self) -> None:
diff --git a/tests/corekit/test_multidict.py b/tests/corekit/test_multidict.py
index 79fb37da84..5a7cd0b88e 100644
--- a/tests/corekit/test_multidict.py
+++ b/tests/corekit/test_multidict.py
@@ -5,6 +5,10 @@
 
 from tests._support import bootstrap_core_modules, reimport_once_per_class
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 
 class MultiDictTests(unittest.TestCase):
     def setUp(self) -> None:
diff --git a/tests/corekit/test_multiinfo_1484_unit.py b/tests/corekit/test_multiinfo_1484_unit.py
index ec8ff0ca01..736f775677 100644
--- a/tests/corekit/test_multiinfo_1484_unit.py
+++ b/tests/corekit/test_multiinfo_1484_unit.py
@@ -31,6 +31,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 PAIRS = [('a', 1), ('b', 2), ('a', 3)]
 
 
diff --git a/tests/corekit/test_ordered_multidict_copy_unit.py b/tests/corekit/test_ordered_multidict_copy_unit.py
index b411fd1a15..cfbf68970b 100644
--- a/tests/corekit/test_ordered_multidict_copy_unit.py
+++ b/tests/corekit/test_ordered_multidict_copy_unit.py
@@ -19,6 +19,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: Interleaved keys, so grouping by key changes the order.
 ITEMS = [('a', 1), ('b', 2), ('a', 3)]
 
diff --git a/tests/corekit/test_sentinel_py_abc_1960_unit.py b/tests/corekit/test_sentinel_py_abc_1960_unit.py
index 2732e2847f..5a10088226 100644
--- a/tests/corekit/test_sentinel_py_abc_1960_unit.py
+++ b/tests/corekit/test_sentinel_py_abc_1960_unit.py
@@ -33,6 +33,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises metaclass, ABC or enum machinery, which differs by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: Repository root, put first on the child interpreter's path so that it
 #: imports this tree rather than whatever :mod:`pcapkit` is installed.
 REPO_ROOT = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
diff --git a/tests/corekit/test_sentinel_singleton_1842_unit.py b/tests/corekit/test_sentinel_singleton_1842_unit.py
index fd7ceecc32..3103ec22a7 100644
--- a/tests/corekit/test_sentinel_singleton_1842_unit.py
+++ b/tests/corekit/test_sentinel_singleton_1842_unit.py
@@ -42,6 +42,10 @@
 if TYPE_CHECKING:
     from typing import Any, Callable
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: Where every sentinel is defined.
 CANONICAL_MODULE = 'pcapkit.corekit.sentinels'
 
diff --git a/tests/corekit/test_sentinels_housing_unit.py b/tests/corekit/test_sentinels_housing_unit.py
index 1a96d6f7bd..9e605b0ada 100644
--- a/tests/corekit/test_sentinels_housing_unit.py
+++ b/tests/corekit/test_sentinels_housing_unit.py
@@ -71,6 +71,10 @@
                                        NoDefaultType, NoValueType, NullType)
 from tests._support import purge_modules
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: Every sentinel, as ``(instance name, consumer module, instance, type)``. The
 #: consumer module is the one that compares against the object, and so still
 #: imports it from :mod:`pcapkit.corekit.sentinels` -- without re-exporting it.
diff --git a/tests/corekit/test_stringfield_lossless_unit.py b/tests/corekit/test_stringfield_lossless_unit.py
index 188ea64300..46705b5ca9 100644
--- a/tests/corekit/test_stringfield_lossless_unit.py
+++ b/tests/corekit/test_stringfield_lossless_unit.py
@@ -31,6 +31,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 #: Charsets whose codecs reject a ``'surrogateescape'`` error handler, so that a
 #: fallback through one would raise where ``'replace'`` does not.
 NO_SURROGATE_CHARSETS = ('utf-16', 'utf-16-le', 'utf-16-be', 'utf-32', 'utf-7')
diff --git a/tests/dumpkit/test_common_unit.py b/tests/dumpkit/test_common_unit.py
index c2d19b6624..aeebb79064 100644
--- a/tests/dumpkit/test_common_unit.py
+++ b/tests/dumpkit/test_common_unit.py
@@ -16,6 +16,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/dumpkit/test_fallback_cycle_unit.py b/tests/dumpkit/test_fallback_cycle_unit.py
index 3ec2a7790d..eb966d647c 100644
--- a/tests/dumpkit/test_fallback_cycle_unit.py
+++ b/tests/dumpkit/test_fallback_cycle_unit.py
@@ -31,6 +31,10 @@
 except ImportError:  # pragma: no cover - not POSIX
     resource = None  # type: ignore[assignment]
 
+#: Test tier (#1958): gates on GraalPy's sys.implementation, so it has to run on that
+#: leg.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/dumpkit/test_pcap_trace_truncated_record_1541_unit.py b/tests/dumpkit/test_pcap_trace_truncated_record_1541_unit.py
index 8caf566405..bc009b7b53 100644
--- a/tests/dumpkit/test_pcap_trace_truncated_record_1541_unit.py
+++ b/tests/dumpkit/test_pcap_trace_truncated_record_1541_unit.py
@@ -29,6 +29,10 @@
 from tests._support import close_extractor, reimport_once_per_class
 from tests.foundation._roundtrip import ethernet, ipv4, pcap, pcapng, tcp
 
+#: Test tier (#1958): a record whose claimed length runs towards the interpreter's size
+#: limit, which differs by implementation (sys.maxsize, #1963).
+__tier__ = 'ALL'
+
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None
                   for name in ('tbtrim', 'aenum', 'chardet', 'dictdumper'))
 
diff --git a/tests/dumpkit/test_plist_date_unit.py b/tests/dumpkit/test_plist_date_unit.py
index 1cd2233450..7883054e1a 100644
--- a/tests/dumpkit/test_plist_date_unit.py
+++ b/tests/dumpkit/test_plist_date_unit.py
@@ -24,6 +24,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises stream and buffer semantics, which differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/engines/test_engine_base.py b/tests/foundation/engines/test_engine_base.py
index 4310e4539c..78e798ce82 100644
--- a/tests/foundation/engines/test_engine_base.py
+++ b/tests/foundation/engines/test_engine_base.py
@@ -8,6 +8,9 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/engines/test_pcap_ct_engine.py b/tests/foundation/engines/test_pcap_ct_engine.py
index f3effa2f2e..0572306a91 100644
--- a/tests/foundation/engines/test_pcap_ct_engine.py
+++ b/tests/foundation/engines/test_pcap_ct_engine.py
@@ -28,6 +28,10 @@
 
 from tests._support import close_extractor, reimport_once_per_class, sample_path
 
+#: Test tier (#1958): drives a third-party engine whose installable releases vary by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/engines/test_pypcap_engine.py b/tests/foundation/engines/test_pypcap_engine.py
index a1bc946637..f1fbb9b146 100644
--- a/tests/foundation/engines/test_pypcap_engine.py
+++ b/tests/foundation/engines/test_pypcap_engine.py
@@ -20,6 +20,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): drives a third-party engine whose installable releases vary by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/engines/test_pypcapfile_engine.py b/tests/foundation/engines/test_pypcapfile_engine.py
index 1f36588e0d..941e131911 100644
--- a/tests/foundation/engines/test_pypcapfile_engine.py
+++ b/tests/foundation/engines/test_pypcapfile_engine.py
@@ -28,6 +28,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): drives a third-party engine whose installable releases vary by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/engines/test_pyshark_engine.py b/tests/foundation/engines/test_pyshark_engine.py
index f338e235c2..e15a5df368 100644
--- a/tests/foundation/engines/test_pyshark_engine.py
+++ b/tests/foundation/engines/test_pyshark_engine.py
@@ -48,6 +48,10 @@
 import unittest
 from unittest import mock
 
+#: Test tier (#1958): drives a third-party engine whose installable releases vary by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/engines/test_pyshark_non_packet_unit.py b/tests/foundation/engines/test_pyshark_non_packet_unit.py
index ebdd21f0b4..70d006a650 100644
--- a/tests/foundation/engines/test_pyshark_non_packet_unit.py
+++ b/tests/foundation/engines/test_pyshark_non_packet_unit.py
@@ -28,6 +28,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): drives a third-party engine whose installable releases vary by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/engines/test_scapy_engine.py b/tests/foundation/engines/test_scapy_engine.py
index 38353fd812..4d06ba61a3 100644
--- a/tests/foundation/engines/test_scapy_engine.py
+++ b/tests/foundation/engines/test_scapy_engine.py
@@ -46,6 +46,11 @@
 
 from tests._support import sample_path
 
+#: Test tier (#1958): drives Scapy, whose installable releases vary by interpreter; the
+#: integration and pypcap-parity legs install it on every interpreter, and a test leg
+#: off latest skips it.
+__tier__ = 'ALL'
+
 #: Repository root, which the child interpreter needs on :data:`sys.path` to
 #: import the :mod:`pcapkit` under test rather than an installed copy.
 ROOT = pathlib.Path(__file__).resolve().parents[3]
diff --git a/tests/foundation/engines/test_scapy_pcapng_sections_runtime.py b/tests/foundation/engines/test_scapy_pcapng_sections_runtime.py
index 40ddc31c0b..b2ef2bd83c 100644
--- a/tests/foundation/engines/test_scapy_pcapng_sections_runtime.py
+++ b/tests/foundation/engines/test_scapy_pcapng_sections_runtime.py
@@ -30,6 +30,11 @@
 
 from tests._support import close_extractor, reimport_once_per_class, sample_path
 
+#: Test tier (#1958): drives Scapy, whose installable releases vary by interpreter; the
+#: integration and pypcap-parity legs install it on every interpreter, and a test leg
+#: off latest skips it.
+__tier__ = 'ALL'
+
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None
                   for name in ('tbtrim', 'aenum', 'chardet', 'dictdumper'))
 HAS_SCAPY = importlib.util.find_spec('scapy') is not None
diff --git a/tests/foundation/engines/test_scapy_reader_hooks_unit.py b/tests/foundation/engines/test_scapy_reader_hooks_unit.py
index f0eb2b46f5..440fc8091c 100644
--- a/tests/foundation/engines/test_scapy_reader_hooks_unit.py
+++ b/tests/foundation/engines/test_scapy_reader_hooks_unit.py
@@ -26,6 +26,11 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): drives Scapy, whose installable releases vary by interpreter; the
+#: integration and pypcap-parity legs install it on every interpreter, and a test leg
+#: off latest skips it.
+__tier__ = 'ALL'
+
 HAS_SCAPY = importlib.util.find_spec('scapy') is not None
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None
                   for name in ('tbtrim', 'aenum', 'chardet', 'dictdumper'))
diff --git a/tests/foundation/engines/test_scapy_reader_interfaces_runtime.py b/tests/foundation/engines/test_scapy_reader_interfaces_runtime.py
index ded33cc6f0..02dbad5b9b 100644
--- a/tests/foundation/engines/test_scapy_reader_interfaces_runtime.py
+++ b/tests/foundation/engines/test_scapy_reader_interfaces_runtime.py
@@ -31,6 +31,11 @@
 
 from tests._support import close_extractor, reimport_once_per_class, sample_path
 
+#: Test tier (#1958): drives Scapy, whose installable releases vary by interpreter; the
+#: integration and pypcap-parity legs install it on every interpreter, and a test leg
+#: off latest skips it.
+__tier__ = 'ALL'
+
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None
                   for name in ('tbtrim', 'aenum', 'chardet', 'dictdumper'))
 HAS_SCAPY = importlib.util.find_spec('scapy') is not None
diff --git a/tests/foundation/engines/test_scapy_record_length_1717_runtime.py b/tests/foundation/engines/test_scapy_record_length_1717_runtime.py
index 5a5cd4e3dc..7a4a608883 100644
--- a/tests/foundation/engines/test_scapy_record_length_1717_runtime.py
+++ b/tests/foundation/engines/test_scapy_record_length_1717_runtime.py
@@ -19,6 +19,11 @@
 
 from tests._support import close_extractor, reimport_once_per_class, sample_path
 
+#: Test tier (#1958): drives Scapy, whose installable releases vary by interpreter; the
+#: integration and pypcap-parity legs install it on every interpreter, and a test leg
+#: off latest skips it.
+__tier__ = 'ALL'
+
 HAS_SCAPY = importlib.util.find_spec('scapy') is not None
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None
                   for name in ('tbtrim', 'aenum', 'chardet', 'dictdumper'))
diff --git a/tests/foundation/engines/test_scapy_record_length_1717_unit.py b/tests/foundation/engines/test_scapy_record_length_1717_unit.py
index 6df7769c95..4e9fcf3892 100644
--- a/tests/foundation/engines/test_scapy_record_length_1717_unit.py
+++ b/tests/foundation/engines/test_scapy_record_length_1717_unit.py
@@ -22,6 +22,11 @@
 
 from tests._support import close_extractor, reimport_once_per_class
 
+#: Test tier (#1958): drives Scapy, whose installable releases vary by interpreter; the
+#: integration and pypcap-parity legs install it on every interpreter, and a test leg
+#: off latest skips it.
+__tier__ = 'ALL'
+
 HAS_SCAPY = importlib.util.find_spec('scapy') is not None
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None
                   for name in ('tbtrim', 'aenum', 'chardet', 'dictdumper'))
diff --git a/tests/foundation/reassembly/test_ip.py b/tests/foundation/reassembly/test_ip.py
index 7bd1155d84..d4de9dfe6a 100644
--- a/tests/foundation/reassembly/test_ip.py
+++ b/tests/foundation/reassembly/test_ip.py
@@ -6,6 +6,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/reassembly/test_ip_max_length_edge_1739_unit.py b/tests/foundation/reassembly/test_ip_max_length_edge_1739_unit.py
index 6d7fd6b129..fcf0e6bbd5 100644
--- a/tests/foundation/reassembly/test_ip_max_length_edge_1739_unit.py
+++ b/tests/foundation/reassembly/test_ip_max_length_edge_1739_unit.py
@@ -19,6 +19,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 #: Datagram octets, with no zero octet in them, so a hole shows.
 DATA = (bytes(range(1, 256)) * 258)[:65536]
 
diff --git a/tests/foundation/reassembly/test_reassembly_base.py b/tests/foundation/reassembly/test_reassembly_base.py
index 5f96e9cd92..4d63c2197b 100644
--- a/tests/foundation/reassembly/test_reassembly_base.py
+++ b/tests/foundation/reassembly/test_reassembly_base.py
@@ -8,6 +8,9 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/reassembly/test_tcp.py b/tests/foundation/reassembly/test_tcp.py
index c93889cbc8..3468253eff 100644
--- a/tests/foundation/reassembly/test_tcp.py
+++ b/tests/foundation/reassembly/test_tcp.py
@@ -7,6 +7,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/registry/test_protocols.py b/tests/foundation/registry/test_protocols.py
index d0d91f6d52..b96a09744b 100644
--- a/tests/foundation/registry/test_protocols.py
+++ b/tests/foundation/registry/test_protocols.py
@@ -8,6 +8,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises metaclass, ABC or enum machinery, which differs by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/test_extraction.py b/tests/foundation/test_extraction.py
index 9e001284de..7da9238b73 100644
--- a/tests/foundation/test_extraction.py
+++ b/tests/foundation/test_extraction.py
@@ -15,6 +15,10 @@
 
 from tests._support import reimport_once_per_class, sample_path
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 #: Whether the optional DPKT engine can be selected. It is an extra
diff --git a/tests/foundation/test_extraction_no_eof.py b/tests/foundation/test_extraction_no_eof.py
index 9df6a21600..56b83110d3 100644
--- a/tests/foundation/test_extraction_no_eof.py
+++ b/tests/foundation/test_extraction_no_eof.py
@@ -108,6 +108,10 @@
 from tests._support import sample_path, scale_timeout, time_limit
 from tests._tiers import ROOT
 
+#: Test tier (#1958): exercises stream and buffer semantics, which differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/test_extraction_non_seekable_stream_1586_unit.py b/tests/foundation/test_extraction_non_seekable_stream_1586_unit.py
index 2581e81a80..41176f46c5 100644
--- a/tests/foundation/test_extraction_non_seekable_stream_1586_unit.py
+++ b/tests/foundation/test_extraction_non_seekable_stream_1586_unit.py
@@ -45,6 +45,10 @@
 from tests._support import reimport_once_per_class, scale_timeout
 from tests._tiers import ROOT
 
+#: Test tier (#1958): exercises stream and buffer semantics, which differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 HAS_DPKT = importlib.util.find_spec('dpkt') is not None
diff --git a/tests/foundation/test_extraction_ownership.py b/tests/foundation/test_extraction_ownership.py
index 9a0d0ca77d..e827559e88 100644
--- a/tests/foundation/test_extraction_ownership.py
+++ b/tests/foundation/test_extraction_ownership.py
@@ -56,6 +56,10 @@
 
 from tests._support import sample_path
 
+#: Test tier (#1958): depends on garbage collection, weak references or object
+#: finalisation.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/traceflow/test_tcp.py b/tests/foundation/traceflow/test_tcp.py
index 1f9448222e..6d07b76d82 100644
--- a/tests/foundation/traceflow/test_tcp.py
+++ b/tests/foundation/traceflow/test_tcp.py
@@ -8,6 +8,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/foundation/traceflow/test_traceflow_base.py b/tests/foundation/traceflow/test_traceflow_base.py
index cd114f78b0..05c03cea7b 100644
--- a/tests/foundation/traceflow/test_traceflow_base.py
+++ b/tests/foundation/traceflow/test_traceflow_base.py
@@ -9,6 +9,9 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/integration/test_engine_runtime.py b/tests/integration/test_engine_runtime.py
index e836cddbaa..e0ae06bee1 100644
--- a/tests/integration/test_engine_runtime.py
+++ b/tests/integration/test_engine_runtime.py
@@ -8,6 +8,10 @@
 
 from tests._support import close_extractor, reimport_once_per_class, sample_path
 
+#: Test tier (#1958): drives a third-party engine whose installable releases vary by
+#: interpreter.
+__tier__ = 'ALL'
+
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in ('tbtrim', 'aenum', 'chardet', 'dictdumper'))
 HAS_DPKT = importlib.util.find_spec('dpkt') is not None
 HAS_SCAPY = importlib.util.find_spec('scapy') is not None
diff --git a/tests/integration/test_frame_iteration.py b/tests/integration/test_frame_iteration.py
index 325ed764ac..41e06a8876 100644
--- a/tests/integration/test_frame_iteration.py
+++ b/tests/integration/test_frame_iteration.py
@@ -24,6 +24,10 @@
 if TYPE_CHECKING:
     from typing import Any
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 #: Frames of :file:`http6.cap` that carry an HTTP header block: the request and
 #: the response of each of the two connections.
 HTTP_FRAMES = (4, 6, 19, 21)
diff --git a/tests/interface/test_core.py b/tests/interface/test_core.py
index 2c7f39bece..2c01891541 100644
--- a/tests/interface/test_core.py
+++ b/tests/interface/test_core.py
@@ -10,6 +10,10 @@
 
 from tests._support import isolate_modules, load_module, reimport_once_per_class, sample_path
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 #: Whether the optional DPKT / Scapy engines can be selected. Both are extras
diff --git a/tests/project/test_curated_all_tier_1958.py b/tests/project/test_curated_all_tier_1958.py
new file mode 100644
index 0000000000..8fc5ede16b
--- /dev/null
+++ b/tests/project/test_curated_all_tier_1958.py
@@ -0,0 +1,129 @@
+# -*- coding: utf-8 -*-
+"""#1958, #1963: the ``ALL`` test tier is a curated, opt-in list.
+
+The owner's rulings, paraphrased: ``ALL`` is a tailored list of tests, to save
+time, to keep QA consistent across legs, and to be enough to validate a change on
+each of them; every implementation runs the same minimal ``ALL`` tier while
+``latest`` runs the whole suite as the gate; and the hand-picked round trips run
+on ``ALL`` as part of QA, with only the real sweeps on ``latest``.
+:func:`tests._tiers.module_tier` is the one source of truth, and
+:file:`tests/test_tier_guard.py` checks that every module lands in exactly one
+tier. This module pins the curation itself: a module is in ``ALL`` only when it
+says so, ``ALL`` stays a strict subset of the suite, and it keeps the ruling-B
+round trips.
+
+"""
+
+from __future__ import annotations
+
+import pathlib
+import tempfile
+import unittest
+import unittest.mock
+
+from tests import _tiers
+
+ROOT = pathlib.Path(__file__).resolve().parents[2]
+DOC = ROOT / 'docs' / 'source' / 'contributing' / 'workflows.rst'
+#: The #1202 edge-case tables, each case written out by hand (ruling B).
+EDGE_TABLES = tuple(sorted(path.relative_to(ROOT).as_posix()
+                           for path in (ROOT / 'tests' / 'protocols').glob('*/test_*_edge_roundtrip_unit.py')))
+#: The phrase every hand-picked round trip's marker reason carries.
+HAND_PICKED = 'hand-picked'
+
+
+def tier_of(source: 'str') -> 'str':
+    """The tier of a module holding ``source``, under a temporary :file:`tests/`."""
+    with tempfile.TemporaryDirectory(prefix='pcapkit-curated-tier-') as temp:
+        root = pathlib.Path(temp).resolve()
+        path = root / 'test_probe.py'
+        path.write_text(source, encoding='utf-8')
+        with unittest.mock.patch.object(_tiers, 'TESTS_ROOT', root):
+            return _tiers.module_tier(path)
+
+
+def reason(module: 'str') -> 'str':
+    """The ``#:`` comment above ``module``'s ``__tier__`` marker, joined into one line."""
+    lines = (ROOT / module).read_text(encoding='utf-8').splitlines()
+    index = next(number for number, line in enumerate(lines) if line.startswith(_tiers.TIER_MARKER))
+    comment = []  # type: list[str]
+    for line in reversed(lines[:index]):
+        if not line.startswith('#: '):
+            break
+        comment.insert(0, line[3:])
+    return ' '.join(comment)
+
+
+class TestAllIsOptIn(unittest.TestCase):
+    """Only ``__tier__ = 'ALL'`` puts a module in ``ALL``."""
+
+    def test_a_module_without_a_marker_is_latest(self) -> None:
+        for label, source in {
+                'imports pcapkit': 'import pcapkit\n',
+                'imports a pcapkit submodule': 'from pcapkit.protocols.link import Ethernet\n',
+                'reads text': 'TEXT = "pcapkit"\n',
+        }.items():
+            with self.subTest(case=label):
+                self.assertEqual(tier_of(source), _tiers.TIER_LATEST)
+
+    def test_the_marker_opts_in(self) -> None:
+        self.assertEqual(tier_of("import pcapkit\n__tier__ = 'ALL'\n"), _tiers.TIER_ALL)
+
+    def test_every_all_module_states_it(self) -> None:
+        for module in _tiers.suite_modules():
+            if _tiers.module_tier(module) == _tiers.TIER_ALL:
+                with self.subTest(module=module):
+                    self.assertEqual(_tiers.declared_tier(module), _tiers.TIER_ALL)
+
+
+class TestAllIsAStrictSubset(unittest.TestCase):
+    """``ALL`` is a curated part of the suite, never the suite, and keeps the ruling-B round trips."""
+
+    def test_it_is_a_strict_subset_smaller_than_latest(self) -> None:
+        modules = set(_tiers.suite_modules())
+        selected = {module for module in modules if _tiers.module_tier(module) == _tiers.TIER_ALL}
+        self.assertTrue(selected)
+        self.assertLess(selected, modules)
+        self.assertLess(len(selected), len(modules - selected))
+
+    def test_it_keeps_the_edge_case_tables(self) -> None:
+        self.assertTrue(EDGE_TABLES)
+        for module in EDGE_TABLES:
+            with self.subTest(module=module):
+                self.assertEqual(_tiers.module_tier(module), _tiers.TIER_ALL)
+
+    def test_it_keeps_every_hand_picked_round_trip(self) -> None:
+        picked = [module for module in _tiers.suite_modules()
+                  if _tiers.declared_tier(module) is not None and HAND_PICKED in reason(module)]
+        self.assertTrue(set(EDGE_TABLES) <= set(picked))
+        for module in picked:
+            with self.subTest(module=module):
+                self.assertEqual(_tiers.module_tier(module), _tiers.TIER_ALL)
+
+    def test_the_sweeps_stay_on_latest(self) -> None:
+        for module in ('tests/protocols/test_option_roundtrip_unit.py',
+                       'tests/protocols/test_short_read_roundtrip_unit.py',
+                       'tests/protocols/test_capture_roundtrip_runtime.py'):
+            with self.subTest(module=module):
+                self.assertEqual(_tiers.module_tier(module), _tiers.TIER_LATEST)
+
+
+class TestTheDocsSayIt(unittest.TestCase):
+
+    def test_the_workflows_page_states_the_criterion(self) -> None:
+        page = ' '.join(DOC.read_text(encoding='utf-8').split())
+        section = page[page.index('Test Tiers ----------'):]
+        for phrase in ("``__tier__ = 'ALL'``", 'opt-in', '#1963', 'smoke slice', 'hand-picked round trips',
+                       'A module that states no tier is ``LATEST``'):
+            with self.subTest(phrase=phrase):
+                self.assertIn(phrase, section)
+
+    def test_the_tier_module_states_the_criterion(self) -> None:
+        text = ' '.join(_tiers.__doc__.split())
+        for phrase in ('The curated ``ALL`` tier', 'opt-in', 'smoke slice', '``*_edge_roundtrip_unit``'):
+            with self.subTest(phrase=phrase):
+                self.assertIn(phrase, text)
+
+
+if __name__ == '__main__':
+    unittest.main()
diff --git a/tests/project/test_public_api.py b/tests/project/test_public_api.py
index f9021971b7..ebc388f15f 100644
--- a/tests/project/test_public_api.py
+++ b/tests/project/test_public_api.py
@@ -144,6 +144,10 @@ class exists at ``pcapkit/const/mh/handover_initiate_status.py`` and is used by
 import types
 import unittest
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 #: First path component under ``pcapkit.`` whose subtree the sweep skips. Empty:
 #: it held ``'vendor'`` until #1772, though ``requests`` and ``beautifulsoup4``
 #: were in the ``test`` extra before the exclusion was written, and
diff --git a/tests/project/test_sweeps_on_latest_only_1732.py b/tests/project/test_sweeps_on_latest_only_1732.py
index d9be81d7b5..515005a102 100644
--- a/tests/project/test_sweeps_on_latest_only_1732.py
+++ b/tests/project/test_sweeps_on_latest_only_1732.py
@@ -317,10 +317,20 @@ def flags() -> 'list[str]':
                 f'--junitxml={RUNNER_TEMP}/junit.xml']
 
     def test_the_all_selection_leaves_out_the_latest_modules(self) -> None:
-        latest = [module for module in _tiers.latest_tier_paths() if not _tiers.is_unit_tier(module)]
-        self.assertTrue(latest)
-        modules = [{entry.split('::', 1)[0] for entry in entries} for entries in (self.selection, self.all)]
-        self.assertEqual(sorted(modules[0] - modules[1]), latest)
+        """What the ``ALL`` part drops is exactly the ``LATEST`` modules the whole selection reaches."""
+        def reached(entries: 'list[str]') -> 'set[str]':
+            modules = set()  # type: set[str]
+            for entry in entries:
+                module = entry.split('::', 1)[0]
+                if (_tiers.ROOT / module).is_dir():
+                    modules.update(path for path in _tiers.suite_modules() if path.startswith(module.rstrip('/') + '/'))
+                else:
+                    modules.add(module)
+            return modules
+        everything, kept = reached(self.selection), reached(self.all)
+        dropped = everything - kept
+        self.assertTrue(dropped)
+        self.assertEqual(dropped, everything & set(_tiers.latest_tier_paths()))
 
     def test_only_latest_runs_them(self) -> None:
         """``latest`` passes the whole selection; every other leg its ``ALL`` part."""
diff --git a/tests/protocols/application/test_http_unit.py b/tests/protocols/application/test_http_unit.py
index 370baf6325..fb39439ba2 100644
--- a/tests/protocols/application/test_http_unit.py
+++ b/tests/protocols/application/test_http_unit.py
@@ -8,6 +8,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/application/test_httpv2_negative_length_unit.py b/tests/protocols/application/test_httpv2_negative_length_unit.py
index 55b1017af8..56fe4a6543 100644
--- a/tests/protocols/application/test_httpv2_negative_length_unit.py
+++ b/tests/protocols/application/test_httpv2_negative_length_unit.py
@@ -33,6 +33,10 @@
 import unittest
 import warnings
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/internet/test_ipv4_unit.py b/tests/protocols/internet/test_ipv4_unit.py
index a800ddac38..ea08cfe1a6 100644
--- a/tests/protocols/internet/test_ipv4_unit.py
+++ b/tests/protocols/internet/test_ipv4_unit.py
@@ -11,6 +11,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/internet/test_ipv6_calipso_make_validation_unit.py b/tests/protocols/internet/test_ipv6_calipso_make_validation_unit.py
index f7b7ecfbb1..162dbde506 100644
--- a/tests/protocols/internet/test_ipv6_calipso_make_validation_unit.py
+++ b/tests/protocols/internet/test_ipv6_calipso_make_validation_unit.py
@@ -22,6 +22,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 #: The protocol classes sharing the option code, by module and class name.
 PROTOCOLS = (
     ('pcapkit.protocols.internet.hopopt', 'HOPOPT'),
diff --git a/tests/protocols/internet/test_ipv6_ext_chain_limit_1604_unit.py b/tests/protocols/internet/test_ipv6_ext_chain_limit_1604_unit.py
index 0b900dfd00..7afb473cfa 100644
--- a/tests/protocols/internet/test_ipv6_ext_chain_limit_1604_unit.py
+++ b/tests/protocols/internet/test_ipv6_ext_chain_limit_1604_unit.py
@@ -27,6 +27,10 @@
 from tests._support import reimport_once_per_class
 from tests.foundation import _roundtrip as wire
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 #: The TCP segment every chain ends in.
 SEGMENT = wire.tcp(b'payload', seq=1)
 
diff --git a/tests/protocols/internet/test_ipv6_jumbogram_unit.py b/tests/protocols/internet/test_ipv6_jumbogram_unit.py
index ec4e3c043a..31d512be8f 100644
--- a/tests/protocols/internet/test_ipv6_jumbogram_unit.py
+++ b/tests/protocols/internet/test_ipv6_jumbogram_unit.py
@@ -32,6 +32,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 #: The two IPv6 addresses, ``::1`` and ``::2``.
 ADDRS = bytes(15) + b'\x01' + bytes(15) + b'\x02'
 #: A TCP header of 20 octets, no options.
diff --git a/tests/protocols/internet/test_ipv6_option_helpers_1519_unit.py b/tests/protocols/internet/test_ipv6_option_helpers_1519_unit.py
index 674b2b001a..d6f0ac10cf 100644
--- a/tests/protocols/internet/test_ipv6_option_helpers_1519_unit.py
+++ b/tests/protocols/internet/test_ipv6_option_helpers_1519_unit.py
@@ -23,6 +23,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 SHARED = 'pcapkit.protocols.schema.internet.ipv6_option'
 
 #: Each header's schema module, and the prefix its errors carry.
diff --git a/tests/protocols/internet/test_ipv6_unit.py b/tests/protocols/internet/test_ipv6_unit.py
index f227120cbb..eaa96fca89 100644
--- a/tests/protocols/internet/test_ipv6_unit.py
+++ b/tests/protocols/internet/test_ipv6_unit.py
@@ -7,6 +7,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/internet/test_mh_timestamp_info_1886_unit.py b/tests/protocols/internet/test_mh_timestamp_info_1886_unit.py
index 404ac34ab7..087244f36e 100644
--- a/tests/protocols/internet/test_mh_timestamp_info_1886_unit.py
+++ b/tests/protocols/internet/test_mh_timestamp_info_1886_unit.py
@@ -31,6 +31,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/link/test_link_unit.py b/tests/protocols/link/test_link_unit.py
index 3a7bb32d97..7527a9c147 100644
--- a/tests/protocols/link/test_link_unit.py
+++ b/tests/protocols/link/test_link_unit.py
@@ -8,6 +8,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/misc/pcap/test_frame_incl_len_overrun_1541_unit.py b/tests/protocols/misc/pcap/test_frame_incl_len_overrun_1541_unit.py
index 5e575a6a80..0a99ca0b40 100644
--- a/tests/protocols/misc/pcap/test_frame_incl_len_overrun_1541_unit.py
+++ b/tests/protocols/misc/pcap/test_frame_incl_len_overrun_1541_unit.py
@@ -21,6 +21,10 @@
 
 from tests._support import close_extractor, reimport_once_per_class
 
+#: Test tier (#1958): a record whose claimed length runs towards the interpreter's size
+#: limit, which differs by implementation (sys.maxsize, #1963).
+__tier__ = 'ALL'
+
 #: Little-endian global header: v2.4, Ethernet.
 GLOBAL = bytes.fromhex('d4c3b2a1' '0200' '0400' '00000000' '00000000' 'ffff0000' '01000000')
 #: Sixty octets of packet data, none of them zero.
diff --git a/tests/protocols/misc/pcap/test_header_frame_unit.py b/tests/protocols/misc/pcap/test_header_frame_unit.py
index 5fa536862e..b733134b7f 100644
--- a/tests/protocols/misc/pcap/test_header_frame_unit.py
+++ b/tests/protocols/misc/pcap/test_header_frame_unit.py
@@ -11,6 +11,10 @@
 
 from tests._support import reimport_once_per_class, sample_path
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/misc/test_pcapng_tzone_tsoffset_range_1340_unit.py b/tests/protocols/misc/test_pcapng_tzone_tsoffset_range_1340_unit.py
index 684ffe5ebc..a52e869760 100644
--- a/tests/protocols/misc/test_pcapng_tzone_tsoffset_range_1340_unit.py
+++ b/tests/protocols/misc/test_pcapng_tzone_tsoffset_range_1340_unit.py
@@ -32,6 +32,10 @@
 if TYPE_CHECKING:
     from typing import Any
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 #: ``datetime`` holds years 1 to 9999, so these are its first and last whole
 #: seconds since the UNIX epoch.
 DT_MIN = -62135596800
diff --git a/tests/protocols/misc/test_pcapng_unit.py b/tests/protocols/misc/test_pcapng_unit.py
index a2fa28f7bc..eea90ad237 100644
--- a/tests/protocols/misc/test_pcapng_unit.py
+++ b/tests/protocols/misc/test_pcapng_unit.py
@@ -23,6 +23,10 @@
 
 from tests._support import ROOT, reimport_once_per_class, sample_path, skip_if_stand_in
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/schema/test_enum_schema_registry_unit.py b/tests/protocols/schema/test_enum_schema_registry_unit.py
index d5905229fb..e8e943fe30 100644
--- a/tests/protocols/schema/test_enum_schema_registry_unit.py
+++ b/tests/protocols/schema/test_enum_schema_registry_unit.py
@@ -23,6 +23,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises metaclass, ABC or enum machinery, which differs by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/schema/test_schema_metaclass_abc_cache_unit.py b/tests/protocols/schema/test_schema_metaclass_abc_cache_unit.py
index bcb8ac5d6e..202356f3b6 100644
--- a/tests/protocols/schema/test_schema_metaclass_abc_cache_unit.py
+++ b/tests/protocols/schema/test_schema_metaclass_abc_cache_unit.py
@@ -8,6 +8,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises metaclass, ABC or enum machinery, which differs by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/schema/test_schema_unit.py b/tests/protocols/schema/test_schema_unit.py
index 8863c599e9..388338f0e1 100644
--- a/tests/protocols/schema/test_schema_unit.py
+++ b/tests/protocols/schema/test_schema_unit.py
@@ -10,6 +10,10 @@
 
 from tests._support import reimport_once_per_class, time_limit
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/test_construction_keyword_check_unit.py b/tests/protocols/test_construction_keyword_check_unit.py
index b4ef5e8b76..c85367a90c 100644
--- a/tests/protocols/test_construction_keyword_check_unit.py
+++ b/tests/protocols/test_construction_keyword_check_unit.py
@@ -82,6 +82,10 @@
 if TYPE_CHECKING:
     from typing import Any
 
+#: Test tier (#1958): exercises metaclass, ABC or enum machinery, which differs by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/test_frame_layer_limit_1610_unit.py b/tests/protocols/test_frame_layer_limit_1610_unit.py
index a1fea376a8..3263f57307 100644
--- a/tests/protocols/test_frame_layer_limit_1610_unit.py
+++ b/tests/protocols/test_frame_layer_limit_1610_unit.py
@@ -36,6 +36,10 @@
 if TYPE_CHECKING:
     from typing import Optional
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 #: Little-endian PCAP global header: v2.4, snaplen 65535, Ethernet.
 GLOBAL = bytes.fromhex('d4c3b2a1' '0200' '0400' '00000000' '00000000' 'ffff0000' '01000000')
 
diff --git a/tests/protocols/test_option_generator_deadline_scale_1428_unit.py b/tests/protocols/test_option_generator_deadline_scale_1428_unit.py
index 454a626a15..8cf5e7235d 100644
--- a/tests/protocols/test_option_generator_deadline_scale_1428_unit.py
+++ b/tests/protocols/test_option_generator_deadline_scale_1428_unit.py
@@ -47,6 +47,9 @@
 if TYPE_CHECKING:
     from typing import Any
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 #: The variable :mod:`tests._support` reads, spelled out so a generator reading a
 #: different name fails here rather than agreeing with itself.
 SCALE_ENV = 'PCAPKIT_TEST_TIMEOUT_SCALE'
diff --git a/tests/protocols/test_protocol_base_unit.py b/tests/protocols/test_protocol_base_unit.py
index e6126dee96..5db35cd3c1 100644
--- a/tests/protocols/test_protocol_base_unit.py
+++ b/tests/protocols/test_protocol_base_unit.py
@@ -10,6 +10,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/test_protocol_code_registration_unit.py b/tests/protocols/test_protocol_code_registration_unit.py
index 1b3530a283..23bb89836c 100644
--- a/tests/protocols/test_protocol_code_registration_unit.py
+++ b/tests/protocols/test_protocol_code_registration_unit.py
@@ -35,6 +35,9 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/protocols/test_protocol_make_unit.py b/tests/protocols/test_protocol_make_unit.py
index c60fb0fe33..9ac649e4e1 100644
--- a/tests/protocols/test_protocol_make_unit.py
+++ b/tests/protocols/test_protocol_make_unit.py
@@ -33,6 +33,9 @@
 if TYPE_CHECKING:
     from typing import Any
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 
 def _attr(path: 'str') -> 'Any':
     module, _, name = path.rpartition('.')
diff --git a/tests/protocols/test_to_dict_repeated_exthdr_1453_unit.py b/tests/protocols/test_to_dict_repeated_exthdr_1453_unit.py
index d78b69b9c7..6a1b89d082 100644
--- a/tests/protocols/test_to_dict_repeated_exthdr_1453_unit.py
+++ b/tests/protocols/test_to_dict_repeated_exthdr_1453_unit.py
@@ -28,6 +28,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 HOP, DST, ROUTE, UDP = 0, 60, 43, 17
 MAC = bytes.fromhex('0123456789ab' 'fedcba987654')
 SRC6 = bytes.fromhex('20010db8' + '00' * 11 + '01')
diff --git a/tests/protocols/test_trial_parse_warnings_1580_unit.py b/tests/protocols/test_trial_parse_warnings_1580_unit.py
index 61efc8e093..4b95a67e7c 100644
--- a/tests/protocols/test_trial_parse_warnings_1580_unit.py
+++ b/tests/protocols/test_trial_parse_warnings_1580_unit.py
@@ -40,6 +40,10 @@
 if TYPE_CHECKING:
     from typing import Any
 
+#: Test tier (#1958): exercises the warning machinery itself, which free-threaded builds
+#: make context-aware.
+__tier__ = 'ALL'
+
 #: Snaplen of the capture's interface, and so the octets each packet keeps.
 SNAPLEN = 10
 #: Original length of each packet, longer than the Ethernet header.
diff --git a/tests/protocols/transport/test_tcp_mptcp_length_arithmetic_unit.py b/tests/protocols/transport/test_tcp_mptcp_length_arithmetic_unit.py
index 75e6bf5530..d5c396367d 100644
--- a/tests/protocols/transport/test_tcp_mptcp_length_arithmetic_unit.py
+++ b/tests/protocols/transport/test_tcp_mptcp_length_arithmetic_unit.py
@@ -146,6 +146,10 @@
 
 import unittest
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 #: Header fields shared by the constructed TCP segments here, matching
 #: :data:`examples.generators.options.TCP_BASE` and
 #: :data:`tests.protocols.transport.test_tcp_mptcp_subtype_unit.TCP_BASE`.
diff --git a/tests/protocols/transport/test_tcp_udp_unit.py b/tests/protocols/transport/test_tcp_udp_unit.py
index a32ab1a089..b53851ba67 100644
--- a/tests/protocols/transport/test_tcp_udp_unit.py
+++ b/tests/protocols/transport/test_tcp_udp_unit.py
@@ -14,6 +14,10 @@
 if TYPE_CHECKING:
     from typing import Any
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/test_base_class_contract.py b/tests/test_base_class_contract.py
index ef8d5cddb2..934fe03c37 100644
--- a/tests/test_base_class_contract.py
+++ b/tests/test_base_class_contract.py
@@ -113,6 +113,10 @@
 if TYPE_CHECKING:
     from typing import Any, Iterator
 
+#: Test tier (#1958): exercises metaclass, ABC or enum machinery, which differs by
+#: interpreter.
+__tier__ = 'ALL'
+
 #: Every ``*Base`` name mapped to the public name it must never be aliased to.
 #: Keyed on the base rather than the public name because the base is what an
 #: import statement names; the alias is the thing being outlawed.
diff --git a/tests/test_final_enforcement.py b/tests/test_final_enforcement.py
index c59db390dc..ff48fb06fb 100644
--- a/tests/test_final_enforcement.py
+++ b/tests/test_final_enforcement.py
@@ -82,6 +82,9 @@
 import unittest
 import warnings
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/test_periodic_gc_1964_unit.py b/tests/test_periodic_gc_1964_unit.py
index 762437d9f7..e7235f2c9c 100644
--- a/tests/test_periodic_gc_1964_unit.py
+++ b/tests/test_periodic_gc_1964_unit.py
@@ -16,6 +16,10 @@
 
 from tests import conftest
 
+#: Test tier (#1958): depends on garbage collection, weak references or object
+#: finalisation.
+__tier__ = 'ALL'
+
 
 class TestPeriodicCollection(unittest.TestCase):
 
diff --git a/tests/test_support_helpers.py b/tests/test_support_helpers.py
index cbea1f5f21..a58f9a6cfb 100644
--- a/tests/test_support_helpers.py
+++ b/tests/test_support_helpers.py
@@ -81,6 +81,9 @@
 if TYPE_CHECKING:
     from typing import Iterator
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 #: Seconds a :class:`TimeLimitTests` body spins waiting for the deadline before it
 #: gives up and fails by name: ten times the one-second deadline those tests arm,
 #: so a deadline that fires at all, its repeat included, lands well inside it.
diff --git a/tests/test_tier_guard.py b/tests/test_tier_guard.py
index 3501b08457..52511d1999 100644
--- a/tests/test_tier_guard.py
+++ b/tests/test_tier_guard.py
@@ -520,14 +520,20 @@ def test_every_module_resolves_to_exactly_one_tier(self) -> None:
                 tiers.setdefault(tier, []).append(module)
         self.assertEqual(set(tiers), set(_tiers.TIERS))
         self.assertEqual(list(_tiers.latest_tier_paths()), tiers[_tiers.TIER_LATEST])
+        self.assertEqual(list(_tiers.all_tier_paths()), tiers[_tiers.TIER_ALL])
         self.assertEqual(sorted(tiers[_tiers.TIER_ALL] + tiers[_tiers.TIER_LATEST]), list(modules))
+        for module in tiers[_tiers.TIER_ALL]:
+            with self.subTest(module=module):
+                self.assertEqual(_tiers.declared_tier(module), _tiers.TIER_ALL)
         found = sorted(path.relative_to(_tiers.ROOT).as_posix() for path in _tiers.TESTS_ROOT.rglob('test_*.py'))
         self.assertEqual(list(modules), found)
 
-    def test_a_marker_wins_over_the_rule(self) -> None:
+    def test_all_is_opt_in(self) -> None:
+        """#1963: ``ALL`` is a curated list, so only a marker puts a module in it."""
         cases = {
-            'the rule, exercising pcapkit': ('import pcapkit\n', _tiers.TIER_ALL),
-            'the rule, reading text': ('TEXT = "pcapkit"\n', _tiers.TIER_LATEST),
+            'no marker, exercising pcapkit': ('import pcapkit\n', _tiers.TIER_LATEST),
+            'no marker, reading text': ('TEXT = "pcapkit"\n', _tiers.TIER_LATEST),
+            'opted in, exercising pcapkit': ("import pcapkit\n__tier__ = 'ALL'\n", _tiers.TIER_ALL),
             'a sweep, marked': ("import pcapkit\n__tier__ = 'LATEST'\n", _tiers.TIER_LATEST),
             'an interpreter-bound guard, marked': ("import ast\n__tier__ = 'ALL'\n", _tiers.TIER_ALL),
             'annotated': ("import pcapkit\n__tier__: str = 'LATEST'\n", _tiers.TIER_LATEST),
@@ -554,6 +560,42 @@ def test_an_invalid_marker_fails(self) -> None:
                 self.assertIn('test_probe.py', str(caught.exception))
                 self.assertIn(_tiers.TIER_MARKER, str(caught.exception))
 
+    def test_a_module_that_branches_on_the_interpreter_states_its_tier(self) -> None:
+        """A version gate is never ``LATEST`` by omission: such a module says which tier it is in.
+
+        Detection is syntactic and best-effort: :mod:`sys` under an alias, a flag imported
+        from :mod:`pcapkit.utilities.compat`, or a gate in an imported helper is not seen.
+
+        """
+        cases = {
+            'a version gate': ('import sys\nNEW = sys.version_info >= (3, 12)\n', True),
+            'an implementation gate': ('import sys\nGRAAL = sys.implementation.name == "graalpy"\n', True),
+            'a bare import': ('from sys import version_info\nOLD = version_info < (3, 9)\n', True),
+            'pcapkit alone': ('import pcapkit\n', False),
+            'a version only read': ('import sys\nprint(sys.version_info)\n', False),
+            'a build flag': ('import sys\nAWARE = sys.flags.context_aware_warnings\n', True),
+            'the GIL probe': ('import sys\nGIL = sys._is_gil_enabled()\n', True),
+            'a config var': ('import sysconfig\nFT = sysconfig.get_config_var("Py_GIL_DISABLED")\n', True),
+            'hasattr on sys': ('import sys\nSET = hasattr(sys, "tracebacklimit")\n', True),
+            'getattr on sys': ('import sys\nLIMIT = getattr(sys, "tracebacklimit", None)\n', True),
+            'getattr on another object': ('import os\nSEP = getattr(os, "sep")\n', False),
+            'a size limit': ('import sys\nBIG = 1 << 40 > sys.maxsize\n', True),
+            'a size limit only read': ('import sys\nprint(sys.maxsize)\n', False),
+        }
+        for label, (source, expected) in cases.items():
+            with self.subTest(case=label), tempfile.TemporaryDirectory(prefix='pcapkit-test-tier-') as temp:
+                path = write_module(pathlib.Path(temp), 'test_probe.py', source)
+                with unittest.mock.patch.object(_tiers, 'TESTS_ROOT', pathlib.Path(temp).resolve()):
+                    self.assertIs(_tiers.branches_on_interpreter(path), expected)
+        for module in _tiers.suite_modules():
+            if _tiers.branches_on_interpreter(module):
+                with self.subTest(module=module):
+                    self.assertIsNotNone(_tiers.declared_tier(module),
+                                         f'{module} branches on the interpreter; state its tier with '
+                                         f"{_tiers.TIER_MARKER} = {_tiers.TIER_ALL!r} or {_tiers.TIER_LATEST!r} "
+                                         '(the check is syntactic and best-effort: aliases, probes of other '
+                                         'modules and flags imported from compat are not seen)')
+
     def test_only_a_test_module_has_a_tier(self) -> None:
         for path in ('tests/_support.py', 'tests/_tiers.py', 'util/ci_timing.py', 'tests/project',
                      'tests/project/test_no_such_module.py'):
@@ -562,7 +604,7 @@ def test_only_a_test_module_has_a_tier(self) -> None:
                     _tiers.module_tier(path)
 
     def test_every_marker_states_a_reason(self) -> None:
-        """A ``__tier__`` the rule would also give says why it is there, as one that overrules it does."""
+        """Every ``__tier__`` says why it is there, under a ``#: Test tier (#1958):`` comment."""
         for module in _tiers.suite_modules():
             if _tiers.declared_tier(module) is None:
                 continue
@@ -620,13 +662,17 @@ def test_a_directory_holding_a_latest_module_is_split(self) -> None:
         """pytest ignores no path it was handed, so the ``ALL`` selection names a mixed directory's modules."""
         integration = f'tests/{next(iter(_tiers.FIXTURE_TIER_DIRS))}'
         members = [path for path in _tiers.suite_modules() if path.startswith(integration + '/')]
-        self.assertIn(integration, _tiers.fixture_tier_paths(_tiers.TIER_ALL))
-        marked = members[0]
         _tiers.fixture_tier_paths.cache_clear()
         self.addCleanup(_tiers.fixture_tier_paths.cache_clear)
         real = _tiers.module_tier
         with unittest.mock.patch.object(_tiers, 'module_tier',
-                                        lambda path: _tiers.TIER_LATEST if path == marked else real(path)):
+                                        lambda path: _tiers.TIER_ALL if path in members else real(path)):
+            self.assertIn(integration, _tiers.fixture_tier_paths(_tiers.TIER_ALL))
+        _tiers.fixture_tier_paths.cache_clear()
+        marked = members[0]
+        with unittest.mock.patch.object(_tiers, 'module_tier',
+                                        lambda path: _tiers.TIER_LATEST if path == marked
+                                        else _tiers.TIER_ALL if path in members else real(path)):
             selected = _tiers.fixture_tier_paths(_tiers.TIER_ALL)
         self.assertNotIn(integration, selected)
         self.assertNotIn(marked, selected)
diff --git a/tests/toolkit/test_dpkt_timestamp_copy_1647_unit.py b/tests/toolkit/test_dpkt_timestamp_copy_1647_unit.py
index 7de7a3f450..6f01fda56a 100644
--- a/tests/toolkit/test_dpkt_timestamp_copy_1647_unit.py
+++ b/tests/toolkit/test_dpkt_timestamp_copy_1647_unit.py
@@ -36,6 +36,10 @@
 
     from pcapkit.toolkit.dpkt import RecordTimestamp
 
+#: Test tier (#1958): exercises pickling and copying, whose protocols differ by
+#: interpreter.
+__tier__ = 'ALL'
+
 ROOT = os.path.dirname(os.path.dirname(os.path.dirname(os.path.abspath(__file__))))
 
 #: Rerun :func:`clones` over :func:`stamps` with :mod:`_decimal` unimportable, as
diff --git a/tests/toolkit/test_dpkt_unit.py b/tests/toolkit/test_dpkt_unit.py
index a6c1d335c1..63deb45bfd 100644
--- a/tests/toolkit/test_dpkt_unit.py
+++ b/tests/toolkit/test_dpkt_unit.py
@@ -12,6 +12,10 @@
 
 from tests._support import close_extractor, reimport_once_per_class, sample_path
 
+#: Test tier (#1958): the representative smoke slice for its package's main parse and
+#: build paths.
+__tier__ = 'ALL'
+
 HAS_DPKT = importlib.util.find_spec('dpkt') is not None
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
diff --git a/tests/toolkit/test_engine_pcap_trace_runtime.py b/tests/toolkit/test_engine_pcap_trace_runtime.py
index 2f921f5f2e..a35a1816dd 100644
--- a/tests/toolkit/test_engine_pcap_trace_runtime.py
+++ b/tests/toolkit/test_engine_pcap_trace_runtime.py
@@ -38,6 +38,10 @@
 if TYPE_CHECKING:
     from typing import Any
 
+#: Test tier (#1958): drives a third-party engine whose installable releases vary by
+#: interpreter.
+__tier__ = 'ALL'
+
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None
                   for name in ('tbtrim', 'aenum', 'chardet', 'dictdumper'))
 HAS_DPKT = importlib.util.find_spec('dpkt') is not None
diff --git a/tests/toolkit/test_pcap_ct_unit.py b/tests/toolkit/test_pcap_ct_unit.py
index cc8226e569..62e429da21 100644
--- a/tests/toolkit/test_pcap_ct_unit.py
+++ b/tests/toolkit/test_pcap_ct_unit.py
@@ -20,6 +20,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): drives a third-party engine whose installable releases vary by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/toolkit/test_pypcap_unit.py b/tests/toolkit/test_pypcap_unit.py
index 67b953e7ef..040d7c24bc 100644
--- a/tests/toolkit/test_pypcap_unit.py
+++ b/tests/toolkit/test_pypcap_unit.py
@@ -15,6 +15,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): drives a third-party engine whose installable releases vary by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/toolkit/test_pypcapfile_unit.py b/tests/toolkit/test_pypcapfile_unit.py
index ec0021c000..9565aaf411 100644
--- a/tests/toolkit/test_pypcapfile_unit.py
+++ b/tests/toolkit/test_pypcapfile_unit.py
@@ -24,6 +24,10 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): drives a third-party engine whose installable releases vary by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/toolkit/test_pyshark_unit.py b/tests/toolkit/test_pyshark_unit.py
index dc28ee05cb..28d29fb822 100644
--- a/tests/toolkit/test_pyshark_unit.py
+++ b/tests/toolkit/test_pyshark_unit.py
@@ -15,6 +15,10 @@
 if TYPE_CHECKING:
     from typing import Any, Callable
 
+#: Test tier (#1958): drives a third-party engine whose installable releases vary by
+#: interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/toolkit/test_scapy_linktype_unit.py b/tests/toolkit/test_scapy_linktype_unit.py
index b2ce69bba0..3b9602a3d1 100644
--- a/tests/toolkit/test_scapy_linktype_unit.py
+++ b/tests/toolkit/test_scapy_linktype_unit.py
@@ -17,6 +17,11 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): drives Scapy, whose installable releases vary by interpreter; the
+#: integration and pypcap-parity legs install it on every interpreter, and a test leg
+#: off latest skips it.
+__tier__ = 'ALL'
+
 HAS_SCAPY = importlib.util.find_spec('scapy') is not None
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None
                   for name in ('tbtrim', 'aenum', 'chardet', 'dictdumper'))
diff --git a/tests/toolkit/test_scapy_packet2dict_unit.py b/tests/toolkit/test_scapy_packet2dict_unit.py
index e116f636f5..de02d8deb4 100644
--- a/tests/toolkit/test_scapy_packet2dict_unit.py
+++ b/tests/toolkit/test_scapy_packet2dict_unit.py
@@ -7,6 +7,11 @@
 
 from tests._support import reimport_once_per_class
 
+#: Test tier (#1958): drives Scapy, whose installable releases vary by interpreter; the
+#: integration and pypcap-parity legs install it on every interpreter, and a test leg
+#: off latest skips it.
+__tier__ = 'ALL'
+
 HAS_SCAPY = importlib.util.find_spec('scapy') is not None
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
diff --git a/tests/toolkit/test_scapy_unit.py b/tests/toolkit/test_scapy_unit.py
index 27b949e453..a81cd7410b 100644
--- a/tests/toolkit/test_scapy_unit.py
+++ b/tests/toolkit/test_scapy_unit.py
@@ -13,6 +13,11 @@
 
 from tests._support import close_extractor, purge_modules, reimport_once_per_class
 
+#: Test tier (#1958): drives Scapy, whose installable releases vary by interpreter; the
+#: integration and pypcap-parity legs install it on every interpreter, and a test leg
+#: off latest skips it.
+__tier__ = 'ALL'
+
 HAS_SCAPY = importlib.util.find_spec('scapy') is not None
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
diff --git a/tests/utilities/test_compat.py b/tests/utilities/test_compat.py
index 55e505b935..c6bae26df8 100644
--- a/tests/utilities/test_compat.py
+++ b/tests/utilities/test_compat.py
@@ -67,6 +67,9 @@
 from tests._support import load_module, purge_modules, reimport_once_per_class
 from tests._tiers import ROOT
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 # NOTE: :mod:`aenum` is deliberately *not* imported at module scope, however
 # convenient that would be for the assertions below. A module-level import would
 # warm the cache issue #687 is about as a side effect of this file being
diff --git a/tests/utilities/test_config_unit.py b/tests/utilities/test_config_unit.py
index 7f47b7ad83..78d0b833ea 100644
--- a/tests/utilities/test_config_unit.py
+++ b/tests/utilities/test_config_unit.py
@@ -34,6 +34,10 @@
 if TYPE_CHECKING:
     from typing import Any, Iterator
 
+#: Test tier (#1958): exercises integer, bytes and struct edge cases, whose limits
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 #: Root of the :mod:`pcapkit` package under test.
 PACKAGE = pathlib.Path(__file__).resolve().parents[2] / 'pcapkit'
 
diff --git a/tests/utilities/test_exceptions_excepthook.py b/tests/utilities/test_exceptions_excepthook.py
index 0c1ee6c292..d6eb25b599 100644
--- a/tests/utilities/test_exceptions_excepthook.py
+++ b/tests/utilities/test_exceptions_excepthook.py
@@ -47,6 +47,10 @@
 from tests._support import purge_modules
 from tests.utilities._harness import bootstrap, capture
 
+#: Test tier (#1958): exercises threading, locking or warning and exception hooks, which
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 #: Repository root -- three directories up from this file
 #: (``tests/utilities/test_exceptions_excepthook.py``). Passed to the child
 #: interpreter as ``PYTHONPATH`` explicitly, rather than relied on via the
diff --git a/tests/utilities/test_exceptions_warnings.py b/tests/utilities/test_exceptions_warnings.py
index 9bcc5af685..c87da7ba73 100644
--- a/tests/utilities/test_exceptions_warnings.py
+++ b/tests/utilities/test_exceptions_warnings.py
@@ -7,6 +7,10 @@
 
 from tests._support import bootstrap_core_modules, reimport_once_per_class
 
+#: Test tier (#1958): exercises threading, locking or warning and exception hooks, which
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 
 class ExceptionsWarningsTests(unittest.TestCase):
     def setUp(self) -> None:
diff --git a/tests/utilities/test_logging.py b/tests/utilities/test_logging.py
index e63be66845..9b2005fedd 100644
--- a/tests/utilities/test_logging.py
+++ b/tests/utilities/test_logging.py
@@ -9,6 +9,10 @@
 
 from tests._support import load_module, reimport_once_per_class
 
+#: Test tier (#1958): exercises threading, locking or warning and exception hooks, which
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 RUNTIME_DEPS = ('tbtrim', 'aenum', 'chardet', 'dictdumper')
 HAS_RUNTIME = all(importlib.util.find_spec(name) is not None for name in RUNTIME_DEPS)
 
diff --git a/tests/utilities/test_quiet_exceptions.py b/tests/utilities/test_quiet_exceptions.py
index 4ed1a19490..f655aeec26 100644
--- a/tests/utilities/test_quiet_exceptions.py
+++ b/tests/utilities/test_quiet_exceptions.py
@@ -26,6 +26,10 @@
 from tests._support import purge_modules
 from tests.utilities._harness import bootstrap, capture
 
+#: Test tier (#1958): exercises threading, locking or warning and exception hooks, which
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 
 def _unrelated_failure() -> 'int':
     """Format an unrelated exception and count the traceback's lines.
diff --git a/tests/utilities/test_stacklevel.py b/tests/utilities/test_stacklevel.py
index d7fca2a91e..8ded5a628b 100644
--- a/tests/utilities/test_stacklevel.py
+++ b/tests/utilities/test_stacklevel.py
@@ -58,6 +58,10 @@
 
     _T = TypeVar('_T')
 
+#: Test tier (#1958): exercises threading, locking or warning and exception hooks, which
+#: differ by interpreter.
+__tier__ = 'ALL'
+
 #: The path fragment :func:`~pcapkit.utilities.exceptions.stacklevel` treats as
 #: "inside :mod:`pcapkit`".
 MARKER = f'{os.path.sep}pcapkit{os.path.sep}'
diff --git a/tests/utilities/test_warning_emission.py b/tests/utilities/test_warning_emission.py
index 1dc38c8b97..e6b7969c0d 100644
--- a/tests/utilities/test_warning_emission.py
+++ b/tests/utilities/test_warning_emission.py
@@ -27,6 +27,10 @@
 from tests._support import purge_modules
 from tests.utilities._harness import bootstrap, capture
 
+#: Test tier (#1958): exercises the warning machinery itself, which free-threaded builds
+#: make context-aware.
+__tier__ = 'ALL'
+
 
 class WarningEmissionTests(unittest.TestCase):
     def setUp(self) -> None:
diff --git a/tests/utilities/test_warning_filters.py b/tests/utilities/test_warning_filters.py
index f697e4c3e8..049d30d69a 100644
--- a/tests/utilities/test_warning_filters.py
+++ b/tests/utilities/test_warning_filters.py
@@ -29,6 +29,10 @@
 from tests.utilities import _unrelated_warning
 from tests.utilities._harness import bootstrap, capture
 
+#: Test tier (#1958): exercises the warning machinery itself, which free-threaded builds
+#: make context-aware.
+__tier__ = 'ALL'
+
 #: Every warning category the module exports, by name.
 CATEGORY_NAMES = [
     'BaseWarning',
diff --git a/tests/vendor/test_ftp_return_code_unit.py b/tests/vendor/test_ftp_return_code_unit.py
index 6445018c7f..1669b95686 100644
--- a/tests/vendor/test_ftp_return_code_unit.py
+++ b/tests/vendor/test_ftp_return_code_unit.py
@@ -44,9 +44,12 @@
 if TYPE_CHECKING:
     from typing import Any
 
-#: Test tier (#1958): exercises pcapkit.vendor, which users run on any supported
-#: interpreter.
-__tier__ = 'ALL'
+#: Test tier (#1958): the vendor extra is installed on latest alone, where the whole
+#: suite runs, and the crawlers are pure Python over requests and bs4; the vendor
+#: package's interpreter-sensitive parts, file restore and environment-variable parsing,
+#: run on every leg in tests/vendor/test_vendor_snapshot_restore_unit.py and
+#: tests/vendor/test_vendor_variables_1726_unit.py.
+__tier__ = 'LATEST'
 
 #: Repository root, i.e. the grandparent of the directory holding this file.
 ROOT = pathlib.Path(__file__).resolve().parents[2]
diff --git a/tests/vendor/test_re_sub_positional_flag_unit.py b/tests/vendor/test_re_sub_positional_flag_unit.py
index 380ad26cb3..1866757c1c 100644
--- a/tests/vendor/test_re_sub_positional_flag_unit.py
+++ b/tests/vendor/test_re_sub_positional_flag_unit.py
@@ -86,6 +86,9 @@
 import unittest
 import warnings
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 #: Repository root, i.e. the grandparent of the directory holding this file.
 ROOT = pathlib.Path(__file__).resolve().parents[2]
 
diff --git a/tests/vendor/test_request_prompt_unit.py b/tests/vendor/test_request_prompt_unit.py
index 039ba91ec9..98f4a54185 100644
--- a/tests/vendor/test_request_prompt_unit.py
+++ b/tests/vendor/test_request_prompt_unit.py
@@ -88,9 +88,12 @@
 if TYPE_CHECKING:
     from typing import Any, Iterator
 
-#: Test tier (#1958): exercises pcapkit.vendor, which users run on any supported
-#: interpreter.
-__tier__ = 'ALL'
+#: Test tier (#1958): the vendor extra is installed on latest alone, where the whole
+#: suite runs, and the crawlers are pure Python over requests and bs4; the vendor
+#: package's interpreter-sensitive parts, file restore and environment-variable parsing,
+#: run on every leg in tests/vendor/test_vendor_snapshot_restore_unit.py and
+#: tests/vendor/test_vendor_variables_1726_unit.py.
+__tier__ = 'LATEST'
 
 #: Repository root, i.e. the grandparent of the directory holding this file.
 ROOT = pathlib.Path(__file__).resolve().parents[2]
diff --git a/tests/vendor/test_user_agent_unit.py b/tests/vendor/test_user_agent_unit.py
index 9fa7286825..2cf613b881 100644
--- a/tests/vendor/test_user_agent_unit.py
+++ b/tests/vendor/test_user_agent_unit.py
@@ -50,9 +50,12 @@
 if TYPE_CHECKING:
     from typing import Any, Iterator
 
-#: Test tier (#1958): exercises pcapkit.vendor, which users run on any supported
-#: interpreter.
-__tier__ = 'ALL'
+#: Test tier (#1958): the vendor extra is installed on latest alone, where the whole
+#: suite runs, and the crawlers are pure Python over requests and bs4; the vendor
+#: package's interpreter-sensitive parts, file restore and environment-variable parsing,
+#: run on every leg in tests/vendor/test_vendor_snapshot_restore_unit.py and
+#: tests/vendor/test_vendor_variables_1726_unit.py.
+__tier__ = 'LATEST'
 
 #: Repository root, i.e. the grandparent of the directory holding this file.
 ROOT = pathlib.Path(__file__).resolve().parents[2]
diff --git a/tests/vendor/test_vendor_dest_path_unit.py b/tests/vendor/test_vendor_dest_path_unit.py
index 493e666da3..4e8cb0b7f5 100644
--- a/tests/vendor/test_vendor_dest_path_unit.py
+++ b/tests/vendor/test_vendor_dest_path_unit.py
@@ -123,9 +123,12 @@
 if TYPE_CHECKING:
     from typing import Any
 
-#: Test tier (#1958): exercises pcapkit.vendor, which users run on any supported
-#: interpreter.
-__tier__ = 'ALL'
+#: Test tier (#1958): the vendor extra is installed on latest alone, where the whole
+#: suite runs, and the crawlers are pure Python over requests and bs4; the vendor
+#: package's interpreter-sensitive parts, file restore and environment-variable parsing,
+#: run on every leg in tests/vendor/test_vendor_snapshot_restore_unit.py and
+#: tests/vendor/test_vendor_variables_1726_unit.py.
+__tier__ = 'LATEST'
 
 #: Repository root, i.e. the grandparent of the directory holding this file.
 ROOT = pathlib.Path(__file__).resolve().parents[2]
diff --git a/tests/vendor/test_vendor_exit_code_unit.py b/tests/vendor/test_vendor_exit_code_unit.py
index 50c596253e..ef0a15f245 100644
--- a/tests/vendor/test_vendor_exit_code_unit.py
+++ b/tests/vendor/test_vendor_exit_code_unit.py
@@ -63,9 +63,12 @@
 import warnings
 from unittest import mock
 
-#: Test tier (#1958): exercises pcapkit.vendor, which users run on any supported
-#: interpreter.
-__tier__ = 'ALL'
+#: Test tier (#1958): the vendor extra is installed on latest alone, where the whole
+#: suite runs, and the crawlers are pure Python over requests and bs4; the vendor
+#: package's interpreter-sensitive parts, file restore and environment-variable parsing,
+#: run on every leg in tests/vendor/test_vendor_snapshot_restore_unit.py and
+#: tests/vendor/test_vendor_variables_1726_unit.py.
+__tier__ = 'LATEST'
 
 #: Same gate :file:`tests/vendor/test_crawler_reachability_unit.py` and
 #: :file:`tests/vendor/test_vendor_dest_path_unit.py` use. Importing
diff --git a/tests/vendor/test_vendor_missing_extra_1850_unit.py b/tests/vendor/test_vendor_missing_extra_1850_unit.py
index 9afffc7ba6..1af475a412 100644
--- a/tests/vendor/test_vendor_missing_extra_1850_unit.py
+++ b/tests/vendor/test_vendor_missing_extra_1850_unit.py
@@ -53,6 +53,9 @@
 if TYPE_CHECKING:
     from typing import Sequence
 
+#: Test tier (#1958): branches on the running interpreter's version or implementation.
+__tier__ = 'ALL'
+
 #: The package under test, found from this file rather than by importing it, so
 #: that a child interpreter can be pointed at the same tree.
 PACKAGE = pathlib.Path(__file__).resolve().parents[2] / 'pcapkit'
diff --git a/tests/vendor/test_vendor_ngap_unit.py b/tests/vendor/test_vendor_ngap_unit.py
index fc01df9840..4e9b6ef14d 100644
--- a/tests/vendor/test_vendor_ngap_unit.py
+++ b/tests/vendor/test_vendor_ngap_unit.py
@@ -31,9 +31,12 @@
 import unittest
 from unittest import mock
 
-#: Test tier (#1958): exercises pcapkit.vendor, which users run on any supported
-#: interpreter.
-__tier__ = 'ALL'
+#: Test tier (#1958): the vendor extra is installed on latest alone, where the whole
+#: suite runs, and the crawlers are pure Python over requests and bs4; the vendor
+#: package's interpreter-sensitive parts, file restore and environment-variable parsing,
+#: run on every leg in tests/vendor/test_vendor_snapshot_restore_unit.py and
+#: tests/vendor/test_vendor_variables_1726_unit.py.
+__tier__ = 'LATEST'
 
 HAS_PYCRATE = importlib.util.find_spec('pycrate_asn1dir') is not None