diff --git a/src/agent_comms/thread_identity.py b/src/agent_comms/thread_identity.py
index f15551b46c..148c58acbc 100644
--- a/src/agent_comms/thread_identity.py
+++ b/src/agent_comms/thread_identity.py
@@ -14,6 +14,7 @@
 from functools import cached_property
 from typing import TYPE_CHECKING, Callable, ClassVar, Self, TypeVar, Annotated
 
+from metaclass_registry import AutoRegisterMeta, RegistryConfig
 
 from agent_comms.participant_identity import ParticipantIdentity
 from .agent_backend import AgentBackend, NewThreadBackend, RuntimeLocation
@@ -211,19 +212,16 @@ def commanding_identity(self) -> ThreadIncarnation:
         return self.authority
 
 
-class BirthlessIncarnation(ThreadIncarnation):
+class BirthlessIncarnation(ThreadIncarnation, metaclass=AutoRegisterMeta):
     """A member whose wire birth is a declared mark, not recorded evidence; never a lookup.
 
     Each member declares its mark in ``WIRE_BIRTH``; the record decodes a mark to its member."""
 
     WIRE_BIRTH: ClassVar[float]
-    members: ClassVar[dict[float, type[BirthlessIncarnation]]] = {}
-
-    def __init_subclass__(cls, **kwargs: object) -> None:
-        super().__init_subclass__(**kwargs)
-        if cls.WIRE_BIRTH in BirthlessIncarnation.members:
-            raise TypeError(f"Wire birth {cls.WIRE_BIRTH} is already declared")
-        BirthlessIncarnation.members[cls.WIRE_BIRTH] = cls
+    __registry_config__: ClassVar[RegistryConfig] = RegistryConfig(
+        registry_dict={}, key_attribute="WIRE_BIRTH", registry_name="birthless incarnation",
+    )
+    __registry__: ClassVar[dict[float, type[BirthlessIncarnation]]]
 
     @property
     def wire_birth(self) -> float:
@@ -357,8 +355,8 @@ class ThreadIncarnationRecord:
     created_at: float
 
     def incarnation(self) -> ThreadIncarnation:
-        if self.created_at in BirthlessIncarnation.members:
-            return BirthlessIncarnation.members[self.created_at].recorded_as(self.name)
+        if member := BirthlessIncarnation.__registry__.get(self.created_at):
+            return member.recorded_as(self.name)
         return RecordedThreadIncarnation(self.name, self.created_at)
 
     @classmethod
diff --git a/tests/test_birthless_declarations.py b/tests/test_birthless_declarations.py
new file mode 100644
index 0000000000..f7f1ef4438
--- /dev/null
+++ b/tests/test_birthless_declarations.py
@@ -0,0 +1,88 @@
+"""Only concrete birth-marker declarations decode; external identity spelling stays fixed."""
+
+import runpy
+from abc import abstractmethod
+from types import SimpleNamespace
+
+import pytest
+from metaclass_registry import DuplicateRegistryKeyError
+
+from agent_comms import thread_identity
+from agent_comms.thread_identity import (
+    CoreWatcherIncarnation,
+    DeletedThreadIncarnation,
+    RecordedThreadIncarnation,
+    ThreadIncarnation,
+    UnrecordedThreadIncarnation,
+)
+
+
+@pytest.mark.parametrize('birth,member,decoded_name', [
+    (-2.0, CoreWatcherIncarnation, 'core_watcher'),
+    (-1.0, DeletedThreadIncarnation, 'original'),
+    (0.0, UnrecordedThreadIncarnation, 'original'),
+    (1.0, RecordedThreadIncarnation, 'original'),
+])
+def test_external_birth_markers_survive_wire_and_cli_boundaries(birth, member, decoded_name):
+    wire = {'name': 'original', 'created_at': birth}
+    identity = ThreadIncarnation.from_wire(wire)
+    assert type(identity) is member
+    assert identity.to_wire() == {**wire, 'name': decoded_name}
+    assert ThreadIncarnation.from_arguments(identity.cli_arguments) == identity
+    renamed = identity.renamed('renamed')
+    assert type(renamed) is member
+    assert renamed.to_wire() == {**wire, 'name': 'renamed'}
+
+
+@pytest.mark.parametrize('birth', [-3.0, float('inf'), float('nan')])
+def test_unknown_nonrecorded_birth_cannot_create_a_live_identity(birth):
+    with pytest.raises(ValueError):
+        ThreadIncarnation.from_wire({'name': 'unknown', 'created_at': birth})
+
+
+@pytest.fixture
+def identity_family():
+    # Execute the actual class declarations without changing the canonical registry.
+    return SimpleNamespace(**runpy.run_path(thread_identity.__file__, run_name=thread_identity.__name__))
+
+
+def test_new_concrete_marker_drives_the_existing_decoder(identity_family):
+    class HistoricalMarker(identity_family.BirthlessIncarnation):
+        WIRE_BIRTH = -91.0
+
+        @property
+        def birth_description(self) -> str:
+            return 'historical marker'
+
+    wire = {'name': 'historical', 'created_at': HistoricalMarker.WIRE_BIRTH}
+    identity = identity_family.ThreadIncarnation.from_wire(wire)
+    assert type(identity) is HistoricalMarker
+    assert identity.to_wire() == wire
+    assert identity_family.ThreadIncarnation.from_arguments(identity.cli_arguments) == identity
+    with pytest.raises(ValueError, match='positive recorded birth'):
+        identity.require_recorded()
+
+
+def test_abstract_specialization_does_not_contest_an_existing_marker(identity_family):
+    class UnfinishedDeleted(identity_family.DeletedThreadIncarnation):
+        @abstractmethod
+        def unfinished(self): ...
+
+    with pytest.raises(TypeError, match='abstract'):
+        UnfinishedDeleted('name')
+    decoded = identity_family.ThreadIncarnation.from_wire({'name': 'name', 'created_at': -1.0})
+    assert type(decoded) is identity_family.DeletedThreadIncarnation
+
+
+def test_new_abstract_marker_is_not_a_decoder(identity_family):
+    class UnfinishedMarker(identity_family.BirthlessIncarnation):
+        WIRE_BIRTH = -92.0
+
+    with pytest.raises(ValueError, match='positive recorded birth'):
+        identity_family.ThreadIncarnation.from_wire({'name': 'name', 'created_at': UnfinishedMarker.WIRE_BIRTH})
+
+
+def test_two_concrete_owners_cannot_claim_one_marker(identity_family):
+    with pytest.raises(DuplicateRegistryKeyError):
+        class CompetingDeleted(identity_family.DeletedThreadIncarnation):
+            pass