diff --git a/crdt-sync/crdt_sync/__init__.py b/crdt-sync/crdt_sync/__init__.py
index 83523e3..cda73bf 100644
--- a/crdt-sync/crdt_sync/__init__.py
+++ b/crdt-sync/crdt_sync/__init__.py
@@ -12,6 +12,7 @@ from crdt_sync._config import (
     firebase_client_for,
     mirror_client_for,
 )
+from crdt_sync._device import DeviceIdentity, load_device_identity
 from crdt_sync._firebase import (
     DatabaseNotFoundError,
     FirebaseSyncClient,
@@ -49,6 +50,7 @@ __all__ = [
     "ConfigError",
     "CredentialStore",
     "DatabaseNotFoundError",
+    "DeviceIdentity",
     "Field",
     "FileCredentialStore",
     "FileSyncStateStore",
@@ -76,6 +78,7 @@ __all__ = [
     "default_revs_path",
     "dump_log",
     "firebase_client_for",
+    "load_device_identity",
     "load_log",
     "merge_field",
     "merge_logs",
diff --git a/crdt-sync/crdt_sync/_device.py b/crdt-sync/crdt_sync/_device.py
new file mode 100644
index 0000000..091c511
--- /dev/null
+++ b/crdt-sync/crdt_sync/_device.py
@@ -0,0 +1,87 @@
+"""This device's stable sync identity.
+
+Every device that pushes into a namespace needs an id that is *its own*: it
+is the directory segment its log is pushed under
+(``<path_prefix>/<device_id>/<filename>``), the revision key, and the node
+component baked into every HLC stamp it writes (``<iso>-<counter>-<id>``).
+
+Two devices sharing an id overwrite each other's pushed file on every tick,
+so the id must be per-*install*, not per-role: a fixed ``"pc"``/``"phone"``
+constant silently collides the moment a second machine takes the same role,
+and a reinstall inherits the previous install's CRDT identity.
+
+A migration from a role constant to a persisted uuid cannot rewrite history.
+Stamps already written keep the old id, and the old path keeps the log that
+was pushed under it -- so a device that has just switched ids must still
+recognise its *former* id as itself. That is what [legacy_id] carries, and
+why [DeviceIdentity.own_ids] exists: skip-own-writes has to test membership
+of that set rather than equality with one id, or the device re-downloads and
+re-merges its own pre-migration history as though a peer had written it.
+"""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import TYPE_CHECKING
+import uuid
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+
+@dataclass(frozen=True)
+class DeviceIdentity:
+    """This device's current sync id, plus the id it used to push under.
+
+    Attributes:
+        device_id: The id to push, stamp and key revisions under from now on.
+        legacy_id: The id this device pushed under before the migration to
+            persisted uuids, or ``None`` once its old path has been reclaimed.
+    """
+
+    device_id: str
+    legacy_id: str | None = None
+
+    @property
+    def own_ids(self) -> frozenset[str]:
+        """Every id that means "this device", for skip-own-writes checks."""
+        if self.legacy_id is None:
+            return frozenset({self.device_id})
+        return frozenset({self.device_id, self.legacy_id})
+
+    def is_own(self, other_device_id: str) -> bool:
+        """Return whether [other_device_id] is one of this device's own ids."""
+        return other_device_id in self.own_ids
+
+
+def load_device_identity(path: Path, *, legacy_id: str | None = None) -> DeviceIdentity:
+    """Return the id persisted at [path], creating it on first call.
+
+    The file holds exactly one uuid4 and is written once; every later call
+    returns the same value, which is what makes the id stable across restarts
+    (and is why it must live in durable state, not a temp dir).
+
+    Args:
+        path: File holding this device's uuid. Parent dirs are created.
+        legacy_id: The role constant this device pushed under before
+            migrating, recorded on the returned identity so skip-own-writes
+            still recognises the old path as this device's own. Pass ``None``
+            once that path has been reclaimed.
+
+    Returns:
+        This device's [DeviceIdentity].
+    """
+    try:
+        existing = path.read_text(encoding="utf-8").strip()
+    except FileNotFoundError:
+        existing = ""
+    if existing:
+        return DeviceIdentity(device_id=existing, legacy_id=legacy_id)
+
+    # Deliberately not guarded: if the state directory cannot be created the
+    # id cannot be persisted, and returning an unpersisted one would mint a
+    # fresh uuid on every restart, stranding a device directory each time.
+    minted = str(uuid.uuid4())
+    path.parent.mkdir(parents=True, exist_ok=True)
+    path.write_text(f"{minted}\n", encoding="utf-8")
+    return DeviceIdentity(device_id=minted, legacy_id=legacy_id)
diff --git a/crdt-sync/crdt_sync/_sync.py b/crdt-sync/crdt_sync/_sync.py
index 947154a..0ae1307 100644
--- a/crdt-sync/crdt_sync/_sync.py
+++ b/crdt-sync/crdt_sync/_sync.py
@@ -204,7 +204,7 @@ def _remote_revs(client: RemoteStore, revs_path: str) -> dict[str, str]:
 
 def _pull_remote_logs(
     client: RemoteStore,
-    device_id: str,
+    own_ids: frozenset[str],
     path_prefix: str,
     filename: str,
     decode: Callable[[str], Log],
@@ -219,12 +219,18 @@ def _pull_remote_logs(
     the remote is an external system boundary, and one bad device's file must
     not stall merging in every other device's.
 
+    ``own_ids`` is a *set* rather than a single id because a device that has
+    migrated from a role constant to a persisted uuid still owns the log it
+    pushed under the old id. Matching only the current id would pull that
+    file back and re-merge this device's own pre-migration history as though
+    a peer had written it -- idempotent, but pure wasted transfer.
+
     Records into ``seen_revs`` which peers are now merged in, so the next
     tick can skip them.
     """
     remote_logs: list[Log] = []
     for other_device_id in client.list_directory(path_prefix):
-        if other_device_id == device_id:
+        if other_device_id in own_ids:
             continue
         remote_rev = remote_revs.get(other_device_id)
         if remote_rev is not None and remote_rev == state.peer_revs.get(
@@ -265,6 +271,7 @@ def sync_log(
     commit_message: str = "crdt_sync: update log",
     state_store: SyncStateStore | None = None,
     revs_path: str | None = None,
+    legacy_device_id: str | None = None,
 ) -> Log:
     """Run one full sync tick: pull every other device's log, merge, push.
 
@@ -295,6 +302,11 @@ def sync_log(
             always did: fetch everything, push unconditionally.
         revs_path: Where revisions live; defaults to
             :func:`default_revs_path`.
+        legacy_device_id: The id this device pushed under before migrating to
+            a persisted uuid. Treated as this device's own for skip-own
+            purposes, so its pre-migration log is not pulled back and
+            re-merged as a peer's. Pass ``None`` once the old path has been
+            reclaimed.
 
     Returns:
         The merged log, as pushed.
@@ -311,11 +323,14 @@ def sync_log(
     state = state_store.load() if state_store is not None else SyncState()
     remote_revs = _remote_revs(client, revs) if state_store is not None else {}
 
+    own_ids = frozenset(
+        {device_id} if legacy_device_id is None else {device_id, legacy_device_id}
+    )
     merged = dict(local_log)
     seen_revs: dict[str, str] = {}
     for remote_log in _pull_remote_logs(
         client,
-        device_id,
+        own_ids,
         path_prefix,
         filename,
         decode,
diff --git a/crdt-sync/crdt_sync/tests/test_device.py b/crdt-sync/crdt_sync/tests/test_device.py
new file mode 100644
index 0000000..e0bc7e8
--- /dev/null
+++ b/crdt-sync/crdt_sync/tests/test_device.py
@@ -0,0 +1,113 @@
+"""Tests for this device's persisted sync identity."""
+
+from __future__ import annotations
+
+from typing import TYPE_CHECKING
+import uuid
+
+import pytest
+
+from crdt_sync import DeviceIdentity, load_device_identity
+
+if TYPE_CHECKING:
+    from pathlib import Path
+
+
+def test_mints_a_uuid_on_first_call(tmp_path: Path) -> None:
+    """A device with no id file gets a fresh uuid4, persisted."""
+    path = tmp_path / "device_id"
+
+    identity = load_device_identity(path)
+
+    assert uuid.UUID(identity.device_id).version == 4
+    assert path.read_text(encoding="utf-8").strip() == identity.device_id
+
+
+def test_returns_the_same_id_on_every_later_call(tmp_path: Path) -> None:
+    """The id is stable across restarts -- that is the whole point."""
+    path = tmp_path / "device_id"
+
+    first = load_device_identity(path)
+    second = load_device_identity(path)
+
+    assert first.device_id == second.device_id
+
+
+def test_creates_missing_parent_directories(tmp_path: Path) -> None:
+    """First run on a clean machine has no state dir yet."""
+    path = tmp_path / "nested" / "deeper" / "device_id"
+
+    identity = load_device_identity(path)
+
+    assert path.read_text(encoding="utf-8").strip() == identity.device_id
+
+
+def test_treats_a_blank_file_as_absent(tmp_path: Path) -> None:
+    """An empty/whitespace file (interrupted first write) re-mints."""
+    path = tmp_path / "device_id"
+    path.write_text("   \n", encoding="utf-8")
+
+    identity = load_device_identity(path)
+
+    assert uuid.UUID(identity.device_id).version == 4
+
+
+def test_reads_an_id_that_is_not_a_uuid(tmp_path: Path) -> None:
+    """Whatever is on disk wins; the file is the source of truth."""
+    path = tmp_path / "device_id"
+    path.write_text("hand-edited-id\n", encoding="utf-8")
+
+    assert load_device_identity(path).device_id == "hand-edited-id"
+
+
+def test_a_file_blocking_the_state_dir_surfaces_as_an_error(tmp_path: Path) -> None:
+    """A file where the state directory belongs is a real misconfiguration.
+
+    The read is tolerant (it reads as "no id yet"), but the subsequent mkdir
+    must not be: silently continuing would hand back an id that was never
+    persisted, so every restart would mint a new one and strand a device
+    directory in the namespace each time.
+    """
+    blocker = tmp_path / "blocker"
+    blocker.write_text("not a directory\n", encoding="utf-8")
+
+    with pytest.raises((NotADirectoryError, FileExistsError)):
+        load_device_identity(tmp_path / "blocker" / "device_id")
+
+
+def test_legacy_id_is_carried_onto_the_identity(tmp_path: Path) -> None:
+    """The pre-migration role constant travels with the new uuid."""
+    identity = load_device_identity(tmp_path / "device_id", legacy_id="pc")
+
+    assert identity.legacy_id == "pc"
+
+
+def test_legacy_id_is_carried_when_loading_an_existing_id(tmp_path: Path) -> None:
+    """The load branch carries it too, not just the mint branch."""
+    path = tmp_path / "device_id"
+    path.write_text("existing-uuid\n", encoding="utf-8")
+
+    identity = load_device_identity(path, legacy_id="pc")
+
+    assert (identity.device_id, identity.legacy_id) == ("existing-uuid", "pc")
+
+
+def test_own_ids_includes_both_ids_when_migrating() -> None:
+    """Skip-own must match the old path as well as the new one."""
+    identity = DeviceIdentity(device_id="new-uuid", legacy_id="pc")
+
+    assert identity.own_ids == frozenset({"new-uuid", "pc"})
+
+
+def test_own_ids_is_just_the_uuid_once_the_old_path_is_reclaimed() -> None:
+    """After GC there is no legacy id left to skip."""
+    assert DeviceIdentity(device_id="new-uuid").own_ids == frozenset({"new-uuid"})
+
+
+def test_is_own_matches_either_id() -> None:
+    """Both the current and the former id mean "this device"."""
+    identity = DeviceIdentity(device_id="new-uuid", legacy_id="pc")
+
+    assert identity.is_own("new-uuid")
+    assert identity.is_own("pc")
+    assert not identity.is_own("phone")
diff --git a/crdt-sync/crdt_sync/tests/test_sync.py b/crdt-sync/crdt_sync/tests/test_sync.py
index 45fdbb4..7f6a68c 100644
--- a/crdt-sync/crdt_sync/tests/test_sync.py
+++ b/crdt-sync/crdt_sync/tests/test_sync.py
@@ -79,6 +79,46 @@ class TestSyncLog:
 
         client.get_file_text.assert_called_once_with("devices/phone/log.json")
 
+    def test_skips_its_legacy_device_id_as_well_as_its_current_one(self) -> None:
+        """A migrated device must not re-merge its own pre-migration log.
+
+        Without this the old path looks like a peer, so the device pulls
+        back everything it pushed under its former id every single tick.
+        """
+        client = _mock_client(
+            devices=("pc", "new-uuid", "phone"),
+            files={"devices/phone/log.json": "{}"},
+        )
+
+        sync_log(
+            client=client,
+            device_id="new-uuid",
+            path_prefix="devices",
+            local_log={},
+            encode=_encode,
+            decode=_decode,
+            legacy_device_id="pc",
+        )
+
+        client.get_file_text.assert_called_once_with("devices/phone/log.json")
+
+    def test_pulls_the_old_path_when_no_legacy_id_is_declared(self) -> None:
+        """Sanity check the previous test: absent the legacy id, it *is* pulled."""
+        client = _mock_client(
+            devices=("pc", "new-uuid"), files={"devices/pc/log.json": "{}"}
+        )
+
+        sync_log(
+            client=client,
+            device_id="new-uuid",
+            path_prefix="devices",
+            local_log={},
+            encode=_encode,
+            decode=_decode,
+        )
+
+        client.get_file_text.assert_called_once_with("devices/pc/log.json")
+
     def test_skips_a_device_with_no_pushed_file_yet(self) -> None:
         client = _mock_client(devices=("phone",), files={})
 
diff --git a/crdt-sync/pyproject.toml b/crdt-sync/pyproject.toml
index 060ab75..a9184f7 100644
--- a/crdt-sync/pyproject.toml
+++ b/crdt-sync/pyproject.toml
@@ -1,6 +1,6 @@
 [project]
 name = "crdt-sync"
-version = "0.5.1"
+version = "0.6.0"
 description = "Shared CRDT merge scheme + GitHub-Contents-API sync transport for cross-device apps"
 requires-python = ">=3.10"
 dependencies = ["requests"]
diff --git a/crdt_sync_dart/lib/crdt_sync.dart b/crdt_sync_dart/lib/crdt_sync.dart
index b209e36..0c17979 100644
--- a/crdt_sync_dart/lib/crdt_sync.dart
+++ b/crdt_sync_dart/lib/crdt_sync.dart
@@ -1,6 +1,7 @@
 /// Shared CRDT merge scheme + pluggable remote sync transports.
 library;
 
+export 'src/device.dart';
 export 'src/firebase_auth_rest.dart';
 export 'src/firebase_client.dart';
 export 'src/firebase_config.dart';
diff --git a/crdt_sync_dart/lib/src/device.dart b/crdt_sync_dart/lib/src/device.dart
new file mode 100644
index 0000000..c1eedfe
--- /dev/null
+++ b/crdt_sync_dart/lib/src/device.dart
@@ -0,0 +1,51 @@
+/// This device's stable sync identity.
+library;
+
+/// This device's current sync id, plus the id it used to push under before.
+///
+/// Every device that pushes into a namespace needs an id that is *its own*:
+/// it is the directory segment its log is pushed under
+/// (`<pathPrefix>/<deviceId>/<filename>`), the revision key, and the node
+/// component baked into every HLC stamp it writes (`<iso>-<counter>-<id>`).
+/// Two devices sharing an id overwrite each other's pushed file on every
+/// tick, so the id must be per-*install*, not per-role.
+///
+/// A migration from a role constant (`phone`, `pc`, `desktop`) to a persisted
+/// uuid cannot rewrite history: stamps already written keep the old id, and
+/// the old path still holds the log pushed under it. So a device that has
+/// just switched ids must still recognise its *former* id as itself -- which
+/// is what [legacyId] carries, and why [ownIds] is a set. Skip-own-writes has
+/// to test membership of that set rather than equality with one id, or the
+/// device re-downloads and re-merges its own pre-migration history as though
+/// a peer had written it.
+class DeviceIdentity {
+  /// Creates an identity for [deviceId], optionally carrying a [legacyId].
+  const DeviceIdentity({required this.deviceId, this.legacyId});
+
+  /// The id to push, stamp and key revisions under from now on.
+  final String deviceId;
+
+  /// The id this device pushed under before migrating to a persisted uuid.
+  ///
+  /// Null once the old path has been reclaimed.
+  final String? legacyId;
+
+  /// Every id that means "this device", for skip-own-writes checks.
+  Set<String> get ownIds => {deviceId, ?legacyId};
+
+  /// Whether [otherDeviceId] is one of this device's own ids.
+  bool isOwn(String otherDeviceId) => ownIds.contains(otherDeviceId);
+
+  @override
+  bool operator ==(Object other) =>
+      other is DeviceIdentity &&
+      other.deviceId == deviceId &&
+      other.legacyId == legacyId;
+
+  @override
+  int get hashCode => Object.hash(deviceId, legacyId);
+
+  @override
+  String toString() =>
+      'DeviceIdentity(deviceId: $deviceId, legacyId: $legacyId)';
+}
diff --git a/crdt_sync_dart/lib/src/sync.dart b/crdt_sync_dart/lib/src/sync.dart
index bd64b79..1862ab5 100644
--- a/crdt_sync_dart/lib/src/sync.dart
+++ b/crdt_sync_dart/lib/src/sync.dart
@@ -155,6 +155,12 @@ String revisionOf(String encodedLog) =>
 /// after which those peers would look permanently unchanged and never be
 /// fetched again. Per-device keys make that failure impossible rather than
 /// merely avoided.
+///
+/// Pass [legacyDeviceId] on a device that has migrated from a fixed role id
+/// (`phone`, `pc`, `desktop`) to a persisted uuid: the old path is then
+/// treated as this device's own rather than as a peer's, so its
+/// pre-migration log is not pulled back and re-merged every tick. Pass null
+/// once that path has been reclaimed.
 Future<Log> syncLog({
   required RemoteStore client,
   required String deviceId,
@@ -166,6 +172,7 @@ Future<Log> syncLog({
   String commitMessage = 'crdt_sync: update log',
   SyncStateStore? stateStore,
   String? revsPath,
+  String? legacyDeviceId,
 }) async {
   final revs = revsPath ?? defaultRevsPath(pathPrefix);
   final state = await stateStore?.load() ?? const SyncState();
@@ -173,10 +180,15 @@ Future<Log> syncLog({
       ? const <String, String>{}
       : await _remoteRevs(client, revs);
 
+  // A set rather than one id: a device that has migrated from a role
+  // constant to a persisted uuid still owns the log it pushed under the old
+  // id. Matching only the current id would pull that file back and re-merge
+  // this device's own pre-migration history as though a peer wrote it.
+  final ownIds = <String>{deviceId, ?legacyDeviceId};
   var mergedLog = Map<String, Record>.from(localLog);
   final seenRevs = <String, String>{};
   for (final otherDeviceId in await client.listDirectory(pathPrefix)) {
-    if (otherDeviceId == deviceId) continue;
+    if (ownIds.contains(otherDeviceId)) continue;
     final remoteRev = remoteRevs[otherDeviceId];
     if (remoteRev != null && remoteRev == state.peerRevs[otherDeviceId]) {
       // Unchanged since we last merged it, and that merge is already part of
diff --git a/crdt_sync_dart/pubspec.yaml b/crdt_sync_dart/pubspec.yaml
index af7a17a..c7225f0 100644
--- a/crdt_sync_dart/pubspec.yaml
+++ b/crdt_sync_dart/pubspec.yaml
@@ -2,7 +2,7 @@ name: crdt_sync
 description: >-
   Shared CRDT merge scheme + GitHub-Contents-API sync transport for
   cross-device apps. Pure Dart, no Flutter dependency.
-version: 0.6.0
+version: 0.7.0
 publish_to: 'none'
 
 environment:
diff --git a/crdt_sync_dart/test/device_test.dart b/crdt_sync_dart/test/device_test.dart
new file mode 100644
index 0000000..2925275
--- /dev/null
+++ b/crdt_sync_dart/test/device_test.dart
@@ -0,0 +1,46 @@
+import 'package:crdt_sync/crdt_sync.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('DeviceIdentity', () {
+    test('ownIds includes both ids while migrating', () {
+      const identity = DeviceIdentity(deviceId: 'new-uuid', legacyId: 'phone');
+
+      expect(identity.ownIds, {'new-uuid', 'phone'});
+    });
+
+    test('ownIds is just the uuid once the old path is reclaimed', () {
+      const identity = DeviceIdentity(deviceId: 'new-uuid');
+
+      expect(identity.ownIds, {'new-uuid'});
+    });
+
+    test('isOwn matches either id, and nothing else', () {
+      const identity = DeviceIdentity(deviceId: 'new-uuid', legacyId: 'phone');
+
+      expect(identity.isOwn('new-uuid'), isTrue);
+      expect(identity.isOwn('phone'), isTrue);
+      expect(identity.isOwn('pc'), isFalse);
+    });
+
+    test('value equality covers both fields', () {
+      const a = DeviceIdentity(deviceId: 'u', legacyId: 'phone');
+      const b = DeviceIdentity(deviceId: 'u', legacyId: 'phone');
+      const differentLegacy = DeviceIdentity(deviceId: 'u', legacyId: 'pc');
+      const noLegacy = DeviceIdentity(deviceId: 'u');
+
+      expect(a, equals(b));
+      expect(a.hashCode, equals(b.hashCode));
+      expect(a, isNot(equals(differentLegacy)));
+      expect(a, isNot(equals(noLegacy)));
+      expect(a, isNot(equals(Object())));
+    });
+
+    test('toString names both ids', () {
+      const identity = DeviceIdentity(deviceId: 'u', legacyId: 'phone');
+
+      expect(identity.toString(), contains('u'));
+      expect(identity.toString(), contains('phone'));
+    });
+  });
+}
diff --git a/crdt_sync_dart/test/sync_test.dart b/crdt_sync_dart/test/sync_test.dart
index 84204d1..ec9dcf2 100644
--- a/crdt_sync_dart/test/sync_test.dart
+++ b/crdt_sync_dart/test/sync_test.dart
@@ -105,6 +105,58 @@ void main() {
       expect(putCalls, hasLength(1));
     });
 
+    test('skips its legacy device id as well as its current one', () async {
+      // The old path holds a record this device pushed before migrating.
+      // Pulling it back would re-merge its own history as a peer's.
+      final legacyLog = _encode({
+        'stale': Record(id: 'stale', fields: {'text': ('old', _make(1))}),
+      });
+      final (:client, :putCalls) = _client(
+        contentResponses: {
+          'devices': _directoryOf(['pc', 'new-uuid']),
+          'devices/pc/log.json': _fileContaining(legacyLog),
+        },
+      );
+
+      final merged = await syncLog(
+        client: client,
+        deviceId: 'new-uuid',
+        pathPrefix: 'devices',
+        localLog: {},
+        encode: _encode,
+        decode: _decode,
+        legacyDeviceId: 'pc',
+      );
+
+      expect(merged, isEmpty);
+      expect(putCalls.single.path, 'devices/new-uuid/log.json');
+    });
+
+    test('pulls the old path when no legacy id is declared', () async {
+      // Negative control for the test above: absent legacyDeviceId, the old
+      // path really is treated as a peer.
+      final legacyLog = _encode({
+        'stale': Record(id: 'stale', fields: {'text': ('old', _make(1))}),
+      });
+      final (:client, :putCalls) = _client(
+        contentResponses: {
+          'devices': _directoryOf(['pc', 'new-uuid']),
+          'devices/pc/log.json': _fileContaining(legacyLog),
+        },
+      );
+
+      final merged = await syncLog(
+        client: client,
+        deviceId: 'new-uuid',
+        pathPrefix: 'devices',
+        localLog: {},
+        encode: _encode,
+        decode: _decode,
+      );
+
+      expect(merged.keys, contains('stale'));
+    });
+
     test('skips a device with no pushed file yet', () async {
       final (:client, :putCalls) = _client(
         contentResponses: {