diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 8e75ccd..e011b84 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -102,7 +102,7 @@ repos:
           - pytest
           - requests
           - gatelock @ git+https://github.com/kuhyx/utils@gatelock-v0.4.0#subdirectory=gatelock
-          - crdt-sync @ git+https://github.com/kuhyx/utils@crdt-sync-v0.5.1#subdirectory=crdt-sync
+          - crdt-sync @ git+https://github.com/kuhyx/utils@crdt-sync-v0.6.0#subdirectory=crdt-sync
           - mcp>=2,<3
           - pydantic
         # Test suites intentionally use patterns (protected-access, magic-value
diff --git a/CLAUDE.md b/CLAUDE.md
index a18d913..eb00270 100644
--- a/CLAUDE.md
+++ b/CLAUDE.md
@@ -178,9 +178,29 @@ Consequences worth knowing before touching `app/`:
   lives in that profile: changing either hides the entire local food log.
 - **The wrapper holds the GitHub token**, not the browser, and proxies
   `api.github.com` plus the CORS-less device flow (`lib/desktop/github_proxy.dart`).
-- **The desktop pushes as device id `desktop`**, the phone as `phone`, the
-  Python side as `pc`. Sharing an id means two devices overwrite each other's
-  pushed log every tick.
+- **Every device pushes under a persisted per-install uuid**, not the old
+  fixed `pc`/`phone`/`desktop` role constants. A role constant collides the
+  moment a second machine takes the same role, and a reinstall silently
+  inherits the previous install's CRDT identity. The id lives in
+  `~/.local/share/diet_guard/.device_id` on the PC (`diet_guard/_device.py`)
+  and under SharedPreferences `crdt.nodeId` in the app
+  (`lib/services/sync_device_id.dart`), the same key `todo` and
+  `home_inventory` use.
+  - The old role constant survives as the **legacy id**
+    (`SYNC_LEGACY_DEVICE_ID` / `legacySyncDeviceId`), passed to `sync_log` /
+    `syncLog` so `devices/<legacy>/` is skipped as *this device's own*. Drop
+    it to `None`/null only after that path has been deleted — until then,
+    removing it makes every tick re-download and re-merge this device's own
+    pre-migration log as though a peer wrote it.
+  - The app resolves its id **asynchronously at startup**
+    (`initSyncDeviceId()` in `main.dart`, before `LogStorageService.init()`).
+    HLC stamping is synchronous, so `currentSyncDeviceId` falls back to the
+    compile-time role constant until that completes: a caller that stamps too
+    early gets a valid-but-pre-migration id rather than a crash. Don't move
+    that call later.
+  - **Tests must redirect `_device.SYNC_DEVICE_ID_FILE`** (conftest already
+    does). Without it a test mints a uuid into the real state dir and this
+    machine's live sync identity is decided by whichever test ran first.
 - **Desktop reminders only exist while the window is open.** A browser has no
   WorkManager equivalent; the PC's real backstop is `diet-guard-gate.timer`,
   which locks the screen. Don't "fix" this with a background service without
diff --git a/app/lib/main.dart b/app/lib/main.dart
index 31095ed..95be649 100644
--- a/app/lib/main.dart
+++ b/app/lib/main.dart
@@ -2,8 +2,8 @@
 /// background due-slot check, then shows the primary meal-logging screen.
 library;
 
-import 'dart:async';
 
+import 'dart:async';
 import 'package:diet_guard_app/screens/log_meal_screen.dart';
 import 'package:diet_guard_app/services/app_settings_service.dart';
 import 'package:diet_guard_app/services/background_tasks.dart';
@@ -12,11 +12,15 @@ import 'package:diet_guard_app/services/foodbank_service.dart';
 import 'package:diet_guard_app/services/frame_stats.dart';
 import 'package:diet_guard_app/services/log_storage_service.dart';
 import 'package:diet_guard_app/services/notification_service.dart';
+import 'package:diet_guard_app/services/sync_device_id.dart';
 import 'package:diet_guard_app/ui/theme.dart';
 import 'package:flutter/material.dart';
 
 Future<void> main() async {
   WidgetsFlutterBinding.ensureInitialized();
+  // Before anything that stamps an Hlc or syncs: until this resolves, the
+  // device id falls back to the pre-migration role constant.
+  await initSyncDeviceId();
   await LogStorageService.init();
   await AppSettingsService.init();
   await BudgetHistoryService.init();
diff --git a/app/lib/services/sync_device_id.dart b/app/lib/services/sync_device_id.dart
new file mode 100644
index 0000000..eb6c04b
--- /dev/null
+++ b/app/lib/services/sync_device_id.dart
@@ -0,0 +1,55 @@
+/// This install's persisted sync device id.
+library;
+
+import 'package:diet_guard_app/services/github_client_factory.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:uuid/uuid.dart';
+
+/// SharedPreferences key holding this install's device id.
+///
+/// The same key the sibling apps (`todo`, `home_inventory`, `wake_alarm`)
+/// use for the same purpose.
+const kSyncDeviceIdKey = 'crdt.nodeId';
+
+String? _cached;
+
+/// This install's sync device id, for HLC stamping and the pushed path.
+///
+/// Synchronous because [Hlc] stamping is synchronous and threading a future
+/// through every merge adapter would be a large change for no benefit. Call
+/// [initSyncDeviceId] once during startup, before any sync or merge runs;
+/// until then this falls back to the compile-time role constant
+/// ([syncDeviceId]) so a caller that forgets still writes a *valid* id rather
+/// than crashing -- it just writes the pre-migration one.
+String get currentSyncDeviceId => _cached ?? syncDeviceId;
+
+/// The id this install pushed under before migrating to a persisted uuid.
+///
+/// Passed to `syncLog` as its legacy id so the log already sitting at
+/// `devices/<legacy>/` is treated as this device's own rather than a peer's;
+/// without it every tick re-downloads and re-merges this device's own
+/// pre-migration history. Set to null once that path has been reclaimed.
+const String legacySyncDeviceId = syncDeviceId;
+
+/// Loads (or mints and persists) this install's device id.
+///
+/// A per-install uuid rather than the fixed `phone`/`desktop` role constant:
+/// two installs sharing an id overwrite each other's pushed file on every
+/// tick, and a reinstall would inherit the previous install's CRDT identity.
+///
+/// Idempotent -- safe to call more than once.
+Future<String> initSyncDeviceId({SharedPreferences? prefs}) async {
+  final store = prefs ?? await SharedPreferences.getInstance();
+  final existing = store.getString(kSyncDeviceIdKey);
+  if (existing != null && existing.isNotEmpty) {
+    _cached = existing;
+    return existing;
+  }
+  final minted = const Uuid().v4();
+  await store.setString(kSyncDeviceIdKey, minted);
+  _cached = minted;
+  return minted;
+}
+
+/// Resets the cached id. Test-only.
+void resetSyncDeviceIdForTest() => _cached = null;
diff --git a/app/lib/services/sync_merge.dart b/app/lib/services/sync_merge.dart
index 224d4ef..69a1b74 100644
--- a/app/lib/services/sync_merge.dart
+++ b/app/lib/services/sync_merge.dart
@@ -29,15 +29,15 @@
 /// adapters field-for-field.
 library;
 
-import 'dart:convert';
 
+import 'dart:convert';
 import 'package:crdt_sync/crdt_sync.dart';
 import 'package:crypto/crypto.dart';
 import 'package:diet_guard_app/models/food_bank_record.dart';
 import 'package:diet_guard_app/models/food_entry.dart';
 import 'package:diet_guard_app/services/budget_schedule.dart';
-import 'package:diet_guard_app/services/github_client_factory.dart';
 import 'package:diet_guard_app/services/log_storage_service.dart';
+import 'package:diet_guard_app/services/sync_device_id.dart';
 
 /// Derives a deterministic [Hlc] for [entry] from its own `time` field.
 ///
@@ -50,7 +50,7 @@ import 'package:diet_guard_app/services/log_storage_service.dart';
 /// survives a merge.
 Hlc entryHlc(FoodEntry entry) {
   final wallTimeMs = DateTime.tryParse(entry.time)?.millisecondsSinceEpoch ?? 0;
-  return Hlc.newTick(syncDeviceId, wallTimeMsOverride: wallTimeMs);
+  return Hlc.newTick(currentSyncDeviceId, wallTimeMsOverride: wallTimeMs);
 }
 
 /// Deterministic id for a pre-`id` legacy entry, from `(time, desc)`.
@@ -206,7 +206,7 @@ Log foodBankToLog(Map<String, FoodBankRecord> bank) {
         'body': (
           mapEntry.value.toJson(),
           Hlc.newTick(
-            syncDeviceId,
+            currentSyncDeviceId,
             wallTimeMsOverride: mapEntry.value.count.toInt(),
           ),
         ),
@@ -270,7 +270,7 @@ Log manualBankToLog(Map<String, FoodBankRecord> bank) {
       fields: {
         'body': (
           body,
-          Hlc.newTick(syncDeviceId, wallTimeMsOverride: wallTimeMs),
+          Hlc.newTick(currentSyncDeviceId, wallTimeMsOverride: wallTimeMs),
         ),
       },
     );
@@ -337,7 +337,7 @@ Hlc budgetHlc(Map<String, dynamic> record) {
         record['t'] as String? ?? '',
       )?.toUtc().millisecondsSinceEpoch ??
       0;
-  return Hlc.newTick(syncDeviceId, wallTimeMsOverride: wallTimeMs);
+  return Hlc.newTick(currentSyncDeviceId, wallTimeMsOverride: wallTimeMs);
 }
 
 /// Field-name prefix for the effective-from budget history, one field per
@@ -355,7 +355,7 @@ const budgetHistoryFieldPrefix = 'hist:';
 Hlc historyHlc(BudgetEntry entry) {
   final wallTimeMs =
       DateTime.tryParse(entry.editedAt)?.toUtc().millisecondsSinceEpoch ?? 0;
-  return Hlc.newTick(syncDeviceId, wallTimeMsOverride: wallTimeMs);
+  return Hlc.newTick(currentSyncDeviceId, wallTimeMsOverride: wallTimeMs);
 }
 
 /// Converts a raw local budget record plus its history into a [Log].
diff --git a/app/lib/services/sync_service.dart b/app/lib/services/sync_service.dart
index 7243fbc..c4fe02f 100644
--- a/app/lib/services/sync_service.dart
+++ b/app/lib/services/sync_service.dart
@@ -17,8 +17,8 @@ import 'package:diet_guard_app/models/food_entry.dart';
 import 'package:diet_guard_app/services/app_settings_service.dart';
 import 'package:diet_guard_app/services/budget_history_service.dart';
 import 'package:diet_guard_app/services/foodbank_service.dart';
-import 'package:diet_guard_app/services/github_client_factory.dart';
 import 'package:diet_guard_app/services/log_storage_service.dart';
+import 'package:diet_guard_app/services/sync_device_id.dart';
 import 'package:diet_guard_app/services/sync_merge.dart';
 import 'package:diet_guard_app/services/sync_state_factory.dart';
 
@@ -35,7 +35,8 @@ Future<DayLog> runSync(RemoteStore client, {SyncStateStore? stateStore}) async {
 
   final mergedLog = await syncLog(
     client: client,
-    deviceId: syncDeviceId,
+    deviceId: currentSyncDeviceId,
+    legacyDeviceId: legacySyncDeviceId,
     pathPrefix: _devicesDir,
     localLog: dayLogToLog(local),
     encode: encodeLogForPush,
@@ -67,7 +68,8 @@ Future<DayLog> runSync(RemoteStore client, {SyncStateStore? stateStore}) async {
 Future<void> _syncFoodBank(RemoteStore client) async {
   final merged = await syncLog(
     client: client,
-    deviceId: syncDeviceId,
+    deviceId: currentSyncDeviceId,
+    legacyDeviceId: legacySyncDeviceId,
     pathPrefix: _devicesDir,
     localLog: foodBankToLog(await FoodBankService.instance.readBank()),
     encode: encodeFoodBankForPush,
@@ -88,7 +90,8 @@ Future<void> _syncManualBank(RemoteStore client) async {
   final local = await FoodBankService.instance.readManualBank();
   final merged = await syncLog(
     client: client,
-    deviceId: syncDeviceId,
+    deviceId: currentSyncDeviceId,
+    legacyDeviceId: legacySyncDeviceId,
     pathPrefix: _devicesDir,
     localLog: manualBankToLog(local),
     encode: encodeManualBankForPush,
@@ -120,7 +123,8 @@ Future<void> _syncBudget(RemoteStore client) async {
 
   final mergedBudgetLog = await syncLog(
     client: client,
-    deviceId: syncDeviceId,
+    deviceId: currentSyncDeviceId,
+    legacyDeviceId: legacySyncDeviceId,
     pathPrefix: _devicesDir,
     localLog: budgetToLog(
       localRecord,
diff --git a/app/pubspec.lock b/app/pubspec.lock
index 6617784..386fc70 100644
--- a/app/pubspec.lock
+++ b/app/pubspec.lock
@@ -85,11 +85,11 @@ packages:
     dependency: "direct main"
     description:
       path: crdt_sync_dart
-      ref: "crdt_sync_dart-v0.6.0"
-      resolved-ref: fc99eff00599a4c3195e4c1573ba08f15909a10d
+      ref: "crdt_sync_dart-v0.7.0"
+      resolved-ref: "6eef99d66132e12eb5056ae8e1780b34237fa013"
       url: "https://github.com/kuhyx/utils"
     source: git
-    version: "0.6.0"
+    version: "0.7.0"
   crypto:
     dependency: "direct main"
     description:
diff --git a/app/pubspec.yaml b/app/pubspec.yaml
index 0697370..42d2d1a 100644
--- a/app/pubspec.yaml
+++ b/app/pubspec.yaml
@@ -11,7 +11,7 @@ dependencies:
   crdt_sync:
     git:
       url: https://github.com/kuhyx/utils
-      ref: crdt_sync_dart-v0.6.0
+      ref: crdt_sync_dart-v0.7.0
       path: crdt_sync_dart
   crypto: ^3.0.6
   flutter:
diff --git a/app/test/services/sync_device_id_test.dart b/app/test/services/sync_device_id_test.dart
new file mode 100644
index 0000000..3df55cf
--- /dev/null
+++ b/app/test/services/sync_device_id_test.dart
@@ -0,0 +1,73 @@
+import 'package:diet_guard_app/services/github_client_factory.dart';
+import 'package:diet_guard_app/services/sync_device_id.dart';
+import 'package:flutter_test/flutter_test.dart';
+import 'package:shared_preferences/shared_preferences.dart';
+import 'package:uuid/uuid.dart';
+
+void main() {
+  setUp(() {
+    SharedPreferences.setMockInitialValues({});
+    resetSyncDeviceIdForTest();
+  });
+
+  group('initSyncDeviceId', () {
+    test('mints and persists a uuid on first run', () async {
+      final minted = await initSyncDeviceId();
+
+      expect(Uuid.isValidUUID(fromString: minted), isTrue);
+      final prefs = await SharedPreferences.getInstance();
+      expect(prefs.getString(kSyncDeviceIdKey), minted);
+    });
+
+    test('returns the same id on every later run', () async {
+      final first = await initSyncDeviceId();
+      resetSyncDeviceIdForTest();
+
+      expect(await initSyncDeviceId(), first);
+    });
+
+    test('adopts an id already stored under the shared key', () async {
+      SharedPreferences.setMockInitialValues({
+        kSyncDeviceIdKey: 'pre-existing-id',
+      });
+
+      expect(await initSyncDeviceId(), 'pre-existing-id');
+    });
+
+    test('treats a blank stored id as absent', () async {
+      SharedPreferences.setMockInitialValues({kSyncDeviceIdKey: ''});
+
+      expect(await initSyncDeviceId(), isNotEmpty);
+    });
+
+    test('is idempotent', () async {
+      final first = await initSyncDeviceId();
+
+      expect(await initSyncDeviceId(), first);
+    });
+  });
+
+  group('currentSyncDeviceId', () {
+    test('falls back to the role constant before init runs', () {
+      // A caller that stamps an Hlc before startup finishes must still get a
+      // valid id -- the pre-migration one -- rather than crash.
+      expect(currentSyncDeviceId, syncDeviceId);
+    });
+
+    test('returns the persisted uuid once init has run', () async {
+      final minted = await initSyncDeviceId();
+
+      expect(currentSyncDeviceId, minted);
+      expect(currentSyncDeviceId, isNot(syncDeviceId));
+    });
+  });
+
+  group('legacySyncDeviceId', () {
+    test('is the role constant this build shipped with', () {
+      // Passed to syncLog so devices/<legacy>/ is skipped as this device's
+      // own; if it ever drifted from the pre-migration value, the old log
+      // would be re-merged as a peer's on every tick.
+      expect(legacySyncDeviceId, syncDeviceId);
+    });
+  });
+}
diff --git a/diet_guard/_constants.py b/diet_guard/_constants.py
index 625c057..9b5995c 100644
--- a/diet_guard/_constants.py
+++ b/diet_guard/_constants.py
@@ -88,7 +88,17 @@ GATE_LOCK_FILE: Path = DATA_DIR / ".gate.lock"
 # happens client-side (see _sync_merge.py), never via git.
 SYNC_REPO_OWNER: str = "kuhyx"
 SYNC_REPO_NAME: str = "syncs"
-SYNC_DEVICE_ID: str = "pc"
+# The id this machine pushed under before per-install uuids. The sync reader
+# still treats it as this device's own, so the log pushed under it is not
+# pulled back and re-merged as a peer's; drop to None once devices/pc/ has
+# been reclaimed. See SYNC_DEVICE_ID_FILE.
+SYNC_LEGACY_DEVICE_ID: str | None = "pc"
+# This device's sync id, minted once and persisted. A fixed "pc" collides the
+# moment a second machine takes the same role, and after a reinstall the new
+# install would inherit the old one's CRDT identity; a uuid makes both
+# impossible by construction. Lives beside the other state, not in ~/.config,
+# because losing it means being seen as a brand-new device.
+SYNC_DEVICE_ID_FILE: Path = DATA_DIR / ".device_id"
 # A fine-grained GitHub PAT, scoped to just SYNC_REPO_NAME's contents.  The
 # user creates this once via github.com (see CLAUDE.md) and saves it here,
 # mode 600.  Never committed -- this path is outside the repo entirely.
diff --git a/diet_guard/_device.py b/diet_guard/_device.py
new file mode 100644
index 0000000..17286c8
--- /dev/null
+++ b/diet_guard/_device.py
@@ -0,0 +1,28 @@
+"""This machine's persisted sync identity.
+
+Its own module rather than living in :mod:`diet_guard._sync`, because
+:mod:`diet_guard._sync_merge` needs it too for HLC stamping and importing
+``_sync`` from there would be a cycle.
+"""
+
+from __future__ import annotations
+
+from crdt_sync import DeviceIdentity, load_device_identity
+
+from diet_guard._constants import SYNC_DEVICE_ID_FILE, SYNC_LEGACY_DEVICE_ID
+
+
+def device_identity() -> DeviceIdentity:
+    """Return this machine's sync identity, minting it on first use.
+
+    Deliberately not cached at import time: the id file is redirected
+    per-test, and a module-level constant would freeze whichever value the
+    first import happened to see. Reading a small file once per sync tick is
+    cheaper than that surprise.
+    """
+    return load_device_identity(SYNC_DEVICE_ID_FILE, legacy_id=SYNC_LEGACY_DEVICE_ID)
+
+
+def device_id() -> str:
+    """Return just this machine's current sync id, for HLC stamping."""
+    return device_identity().device_id
diff --git a/diet_guard/_sync.py b/diet_guard/_sync.py
index ddf6979..1160215 100644
--- a/diet_guard/_sync.py
+++ b/diet_guard/_sync.py
@@ -22,6 +22,7 @@ import logging
 from crdt_sync import (
     CONFIG_FILE,
     ConfigError,
+    DeviceIdentity,
     FileSyncStateStore,
     FirebaseAuthError,
     GitHubSyncClient,
@@ -42,13 +43,13 @@ from diet_guard._budget_history import (
     write_raw_history,
 )
 from diet_guard._constants import (
-    SYNC_DEVICE_ID,
     SYNC_REPO_NAME,
     SYNC_REPO_OWNER,
     SYNC_STATE_FILE,
     SYNC_TIMEOUT_SECONDS,
     SYNC_TOKEN_FILE,
 )
+from diet_guard._device import device_identity
 from diet_guard._foodbank import read_food_bank, rebuild_food_bank, write_food_bank
 from diet_guard._foodbank_manual import read_manual_bank, write_manual_bank
 from diet_guard._state import DayLog, read_raw_log, resign_entry, write_raw_log
@@ -172,6 +173,7 @@ def _pull_remote_logs(
     remote_revs: dict[str, str],
     state: SyncState,
     seen_revs: dict[str, str],
+    identity: DeviceIdentity,
 ) -> list[Log]:
     """Return every other device's last-pushed log, skipping this one.
 
@@ -188,7 +190,7 @@ def _pull_remote_logs(
     """
     remote_logs: list[Log] = []
     for device_id in client.list_directory(_DEVICES_DIR):
-        if device_id == SYNC_DEVICE_ID:
+        if identity.is_own(device_id):
             continue
         remote_rev = remote_revs.get(device_id)
         if remote_rev is not None and remote_rev == state.peer_revs.get(device_id):
@@ -231,10 +233,11 @@ def _sync_food_bank(client: GitHubSyncClient) -> None:
     contains fixes that, and stays identical across devices because that
     rebuild comes from the *merged* log both devices share.
     """
+    identity = device_identity()
     local = read_food_bank()
     merged = food_bank_to_log(local)
     for device_id in client.list_directory(_DEVICES_DIR):
-        if device_id == SYNC_DEVICE_ID:
+        if identity.is_own(device_id):
             continue
         text = client.get_file_text(_device_food_bank_path(device_id))
         if text is None:
@@ -257,7 +260,7 @@ def _sync_food_bank(client: GitHubSyncClient) -> None:
     write_food_bank(resolved)
     merged = food_bank_to_log(resolved)
     client.put_file_text(
-        _device_food_bank_path(SYNC_DEVICE_ID),
+        _device_food_bank_path(identity.device_id),
         json.dumps(
             {record_id: record.to_dict() for record_id, record in merged.items()},
             indent=2,
@@ -274,9 +277,10 @@ def _sync_manual_bank(client: GitHubSyncClient) -> None:
     ``food_bank.json`` they need a real merge: last-writer-wins per food name
     by edit time, union across devices.
     """
+    identity = device_identity()
     merged = manual_bank_to_log(read_manual_bank())
     for device_id in client.list_directory(_DEVICES_DIR):
-        if device_id == SYNC_DEVICE_ID:
+        if identity.is_own(device_id):
             continue
         text = client.get_file_text(_device_manual_bank_path(device_id))
         if text is None:
@@ -295,7 +299,7 @@ def _sync_manual_bank(client: GitHubSyncClient) -> None:
         return
     write_manual_bank(log_to_manual_bank(merged))
     client.put_file_text(
-        _device_manual_bank_path(SYNC_DEVICE_ID),
+        _device_manual_bank_path(identity.device_id),
         json.dumps(
             {record_id: record.to_dict() for record_id, record in merged.items()},
             indent=2,
@@ -315,9 +319,10 @@ def _sync_budget(client: GitHubSyncClient) -> None:
     to the merge nor overwrites a real budget pulled from elsewhere, and if
     *no* device has ever set one, nothing is written or pushed.
     """
+    identity = device_identity()
     merged = budget_to_log(read_raw_record(), load_entries())
     for device_id in client.list_directory(_DEVICES_DIR):
-        if device_id == SYNC_DEVICE_ID:
+        if identity.is_own(device_id):
             continue
         text = client.get_file_text(_device_budget_path(device_id))
         if text is None:
@@ -347,7 +352,7 @@ def _sync_budget(client: GitHubSyncClient) -> None:
         indent=2,
     )
     client.put_file_text(
-        _device_budget_path(SYNC_DEVICE_ID),
+        _device_budget_path(identity.device_id),
         push_json,
         message="diet_guard sync",
     )
@@ -382,13 +387,16 @@ def run_sync() -> DayLog:
         )
     )
 
+    identity = device_identity()
     state_store = FileSyncStateStore(SYNC_STATE_FILE)
     state = state_store.load()
     remote_revs = _remote_revisions(client)
     seen_revs: dict[str, str] = {}
 
     merged = daylog_to_log(read_raw_log())
-    for remote_log in _pull_remote_logs(client, remote_revs, state, seen_revs):
+    for remote_log in _pull_remote_logs(
+        client, remote_revs, state, seen_revs, identity
+    ):
         merged = merge_logs(merged, remote_log)
 
     merged_daylog = log_to_daylog(merged)
@@ -410,14 +418,14 @@ def run_sync() -> DayLog:
     revision = revision_of(push_json)
     if revision != state.pushed_rev:
         client.put_file_text(
-            _device_log_path(SYNC_DEVICE_ID),
+            _device_log_path(identity.device_id),
             push_json,
             message="diet_guard sync",
         )
         # Published after the log, never before: a peer that cached "seen rev
         # X" against a log it never received would skip it forever.
         client.put_file_text(
-            f"{_REVS_DIR}/{SYNC_DEVICE_ID}",
+            f"{_REVS_DIR}/{identity.device_id}",
             revision,
             message="diet_guard sync: revision",
         )
diff --git a/diet_guard/_sync_merge.py b/diet_guard/_sync_merge.py
index e8ba294..e9d6d0c 100644
--- a/diet_guard/_sync_merge.py
+++ b/diet_guard/_sync_merge.py
@@ -42,7 +42,7 @@ from typing import TYPE_CHECKING
 from crdt_sync import Hlc, Record
 
 from diet_guard._budget_history import BudgetEntry
-from diet_guard._constants import SYNC_DEVICE_ID
+from diet_guard._device import device_id
 from diet_guard._foodbank_manual import record_edit_time
 
 if TYPE_CHECKING:
@@ -80,7 +80,7 @@ def _entry_hlc(entry: dict[str, object]) -> Hlc:
     except ValueError:
         dt = _EPOCH
     wall_time_ms = int(dt.timestamp() * 1000)
-    return Hlc.new_tick(SYNC_DEVICE_ID, wall_time_ms=wall_time_ms)
+    return Hlc.new_tick(device_id(), wall_time_ms=wall_time_ms)
 
 
 def _legacy_entry_id(entry: dict[str, object]) -> str:
@@ -229,7 +229,7 @@ def _budget_hlc(record: dict[str, object]) -> Hlc:
     except ValueError:
         dt = _EPOCH
     wall_time_ms = int(dt.timestamp() * 1000)
-    return Hlc.new_tick(SYNC_DEVICE_ID, wall_time_ms=wall_time_ms)
+    return Hlc.new_tick(device_id(), wall_time_ms=wall_time_ms)
 
 
 def _history_hlc(entry: BudgetEntry) -> Hlc:
@@ -245,7 +245,7 @@ def _history_hlc(entry: BudgetEntry) -> Hlc:
         moment = datetime.fromisoformat(entry.edited_at)
     except ValueError:
         moment = _EPOCH
-    return Hlc.new_tick(SYNC_DEVICE_ID, wall_time_ms=int(moment.timestamp() * 1000))
+    return Hlc.new_tick(device_id(), wall_time_ms=int(moment.timestamp() * 1000))
 
 
 def budget_to_log(
@@ -360,7 +360,7 @@ def food_bank_to_log(bank: Mapping[str, object]) -> Log:
             continue
         count = record.get("count")
         ticks = int(count) if isinstance(count, (int, float)) else 0
-        hlc = Hlc.new_tick(SYNC_DEVICE_ID, wall_time_ms=ticks)
+        hlc = Hlc.new_tick(device_id(), wall_time_ms=ticks)
         log[name] = Record(id=name, fields={"body": (dict(record), hlc)})
     return log
 
@@ -405,7 +405,7 @@ def manual_bank_to_log(bank: Mapping[str, object]) -> Log:
         if not isinstance(record, dict):
             continue
         hlc = Hlc.new_tick(
-            SYNC_DEVICE_ID,
+            device_id(),
             wall_time_ms=_wall_time_ms(record_edit_time(record)),
         )
         body = {k: v for k, v in record.items() if k != "t"}
diff --git a/diet_guard/tests/conftest.py b/diet_guard/tests/conftest.py
index 6d7c22b..6fb527a 100644
--- a/diet_guard/tests/conftest.py
+++ b/diet_guard/tests/conftest.py
@@ -120,6 +120,13 @@ def _isolate_state(tmp_path: Path) -> Iterator[None]:
             "diet_guard._sync.SYNC_STATE_FILE",
             tmp_path / "sync_state.json",
         ),
+        # Without this a test that syncs mints a uuid into the REAL
+        # ~/.local/share/diet_guard/.device_id, and this machine's live sync
+        # identity is decided by whichever test happened to run first.
+        patch(
+            "diet_guard._device.SYNC_DEVICE_ID_FILE",
+            tmp_path / ".device_id",
+        ),
         # `run_sync` reads this to decide whether to build a Firebase-primary
         # mirror. On a developer machine the real file exists, so without this
         # every sync test would sign in and push to the live database.
diff --git a/diet_guard/tests/test_sync.py b/diet_guard/tests/test_sync.py
index 3ac9f75..2c26da2 100644
--- a/diet_guard/tests/test_sync.py
+++ b/diet_guard/tests/test_sync.py
@@ -17,6 +17,7 @@ import pytest
 from diet_guard import _sync
 from diet_guard._budget import budget_weight, daily_budget, write_budget
 from diet_guard._budget_history import load_entries
+from diet_guard._device import device_id, device_identity
 from diet_guard._estimator import Nutrition
 from diet_guard._foodbank import lookup_food
 from diet_guard._state import load_log, log_meal
@@ -105,9 +106,9 @@ class TestRunSync:
         # device's revision -- published *after* the log, so a peer can never
         # cache "seen rev X" against a log it never received.
         assert pushed == [
-            "diet-guard-sync/devices/pc/food_bank.json",
-            "diet-guard-sync/devices/pc/food_log.json",
-            "diet-guard-sync/revs/pc",
+            f"diet-guard-sync/devices/{device_id()}/food_bank.json",
+            f"diet-guard-sync/devices/{device_id()}/food_log.json",
+            f"diet-guard-sync/revs/{device_id()}",
         ]
         log_call = next(
             call
@@ -252,7 +253,7 @@ class TestSyncBudget:
         with patch.object(_sync, "GitHubSyncClient", return_value=client):
             _sync.run_sync()
         pushed_paths = {call.args[0] for call in client.put_file_text.call_args_list}
-        assert "diet-guard-sync/devices/pc/budget.json" in pushed_paths
+        assert f"diet-guard-sync/devices/{device_id()}/budget.json" in pushed_paths
 
     def test_nothing_pushed_when_no_budget_ever_set(self) -> None:
         """An uninitialized device contributes nothing -- no push, no crash."""
@@ -261,7 +262,7 @@ class TestSyncBudget:
         with patch.object(_sync, "GitHubSyncClient", return_value=client):
             _sync.run_sync()
         pushed_paths = {call.args[0] for call in client.put_file_text.call_args_list}
-        assert "diet-guard-sync/devices/pc/budget.json" not in pushed_paths
+        assert f"diet-guard-sync/devices/{device_id()}/budget.json" not in pushed_paths
 
     def test_remote_only_budget_is_adopted_locally(self) -> None:
         """Only the phone has ever set a budget -- the PC adopts it."""
@@ -448,7 +449,9 @@ class TestPullSkipsUnchangedPeers:
         state = SyncState(pushed_rev=None, peer_revs={"phone": "rev-1"})
         seen: dict[str, str] = {}
 
-        logs = _sync._pull_remote_logs(client, {"phone": "rev-1"}, state, seen)
+        logs = _sync._pull_remote_logs(
+            client, {"phone": "rev-1"}, state, seen, device_identity()
+        )
 
         assert logs == []
         assert seen == {"phone": "rev-1"}
@@ -463,7 +466,9 @@ class TestPullSkipsUnchangedPeers:
         state = SyncState(pushed_rev=None, peer_revs={"phone": "rev-1"})
         seen: dict[str, str] = {}
 
-        _sync._pull_remote_logs(client, {"phone": "rev-2"}, state, seen)
+        _sync._pull_remote_logs(
+            client, {"phone": "rev-2"}, state, seen, device_identity()
+        )
 
         client.get_file_text.assert_called_once()
         assert seen == {"phone": "rev-2"}
@@ -486,6 +491,8 @@ class TestNoOpPushSuppression:
             _sync.run_sync()
 
         pushed_second = [call.args[0] for call in client.put_file_text.call_args_list]
-        assert "diet-guard-sync/devices/pc/food_log.json" in pushed_first
-        assert "diet-guard-sync/devices/pc/food_log.json" not in pushed_second
-        assert "diet-guard-sync/revs/pc" not in pushed_second
+        assert f"diet-guard-sync/devices/{device_id()}/food_log.json" in pushed_first
+        assert (
+            f"diet-guard-sync/devices/{device_id()}/food_log.json" not in pushed_second
+        )
+        assert f"diet-guard-sync/revs/{device_id()}" not in pushed_second
diff --git a/diet_guard/tests/test_sync_banks.py b/diet_guard/tests/test_sync_banks.py
index 51771eb..319c74f 100644
--- a/diet_guard/tests/test_sync_banks.py
+++ b/diet_guard/tests/test_sync_banks.py
@@ -12,6 +12,7 @@ import json
 from unittest.mock import patch
 
 from diet_guard import _sync
+from diet_guard._device import device_id
 from diet_guard._estimator import Nutrition
 from diet_guard._foodbank import lookup_food, read_food_bank
 from diet_guard._foodbank_manual import add_manual_entry, read_manual_bank
@@ -33,7 +34,7 @@ class TestSyncFoodBank:
         with patch.object(_sync, "GitHubSyncClient", return_value=client):
             _sync.run_sync()
         pushed = [call.args[0] for call in client.put_file_text.call_args_list]
-        assert "diet-guard-sync/devices/pc/food_bank.json" in pushed
+        assert f"diet-guard-sync/devices/{device_id()}/food_bank.json" in pushed
 
     def test_a_food_the_phone_logged_reaches_the_gate(self) -> None:
         """The payoff: a food eaten on the phone autocompletes on the PC.
@@ -141,7 +142,7 @@ class TestSyncFoodBank:
         with patch.object(_sync, "GitHubSyncClient", return_value=client):
             _sync.run_sync()
         pushed = [call.args[0] for call in client.put_file_text.call_args_list]
-        assert "diet-guard-sync/devices/pc/food_bank.json" not in pushed
+        assert f"diet-guard-sync/devices/{device_id()}/food_bank.json" not in pushed
 
 
 class TestSyncManualBank:
@@ -162,7 +163,7 @@ class TestSyncManualBank:
         with patch.object(_sync, "GitHubSyncClient", return_value=client):
             _sync.run_sync()
         pushed = [call.args[0] for call in client.put_file_text.call_args_list]
-        assert "diet-guard-sync/devices/pc/food_bank_manual.json" in pushed
+        assert f"diet-guard-sync/devices/{device_id()}/food_bank_manual.json" in pushed
 
     def test_a_remote_curated_entry_is_adopted_locally(self) -> None:
         """The point of the whole feature: a food added on the phone is usable
diff --git a/diet_guard/tests/test_sync_device_id.py b/diet_guard/tests/test_sync_device_id.py
new file mode 100644
index 0000000..c5aec3a
--- /dev/null
+++ b/diet_guard/tests/test_sync_device_id.py
@@ -0,0 +1,64 @@
+"""Tests for this machine's persisted sync device id.
+
+Split out of ``test_sync.py`` to keep it under the repo's 500-line cap.
+Covers the migration from the fixed ``"pc"`` role constant to a per-install
+uuid -- specifically that the old path is still recognised as this device's
+own, so its pre-migration log is not re-merged as a peer's.
+"""
+
+from __future__ import annotations
+
+from unittest.mock import MagicMock
+
+from crdt_sync import DeviceIdentity, SyncState
+
+from diet_guard import _sync
+from diet_guard._device import device_id, device_identity
+
+
+def _mock_client(*, devices: tuple[str, ...] = ()) -> MagicMock:
+    """Build a mock sync client that lists ``devices`` and holds no files."""
+    client = MagicMock()
+    client.list_directory.return_value = list(devices)
+    client.get_file_text.side_effect = lambda _path: None
+    del client.get_string_map
+    return client
+
+
+class TestPullSkipsOwnLegacyPath:
+    """The uuid migration leaves this device's old 'pc' log behind."""
+
+    def test_skips_the_devices_own_legacy_id(self) -> None:
+        """devices/pc/ is this machine's own history, not a peer's.
+
+        Without the legacy id in the skip set, every tick would download and
+        re-merge the log this machine pushed before it migrated -- idempotent
+        under LWW, but hundreds of KB of pure waste, 96 times a day.
+        """
+        client = _mock_client(devices=("pc", "phone"))
+        state = SyncState(pushed_rev=None, peer_revs={})
+        seen: dict[str, str] = {}
+
+        _sync._pull_remote_logs(client, {}, state, seen, device_identity())
+
+        fetched = [call.args[0] for call in client.get_file_text.call_args_list]
+        assert "diet-guard-sync/devices/pc/food_log.json" not in fetched
+        assert "diet-guard-sync/devices/phone/food_log.json" in fetched
+
+    def test_pulls_the_legacy_path_once_it_is_disowned(self) -> None:
+        """Negative control: with no legacy id, devices/pc/ IS a peer.
+
+        Proves the skip above is the legacy id's doing, not an artifact of
+        the fixture -- and documents what happens after the old path is
+        reclaimed and SYNC_LEGACY_DEVICE_ID drops to None.
+        """
+        client = _mock_client(devices=("pc",))
+        state = SyncState(pushed_rev=None, peer_revs={})
+        seen: dict[str, str] = {}
+
+        _sync._pull_remote_logs(
+            client, {}, state, seen, DeviceIdentity(device_id=device_id())
+        )
+
+        fetched = [call.args[0] for call in client.get_file_text.call_args_list]
+        assert "diet-guard-sync/devices/pc/food_log.json" in fetched
diff --git a/pyproject.toml b/pyproject.toml
index 3b41ec9..ff5ce26 100644
--- a/pyproject.toml
+++ b/pyproject.toml
@@ -5,7 +5,7 @@ description = "Tkinter/systemd log-to-unlock meal gate with calorie/macro budget
 requires-python = ">=3.10"
 dependencies = [
     "gatelock @ git+https://github.com/kuhyx/utils@gatelock-v0.4.0#subdirectory=gatelock",
-    "crdt-sync @ git+https://github.com/kuhyx/utils@crdt-sync-v0.5.1#subdirectory=crdt-sync",
+    "crdt-sync @ git+https://github.com/kuhyx/utils@crdt-sync-v0.6.0#subdirectory=crdt-sync",
     "requests>=2.31.0",
 ]
 
diff --git a/requirements.txt b/requirements.txt
index 30324bf..05d4340 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -3,7 +3,7 @@
 bandit>=1.7.0
 codespell>=2.2.0
 coverage>=7.4.0
-crdt-sync @ git+https://github.com/kuhyx/utils@crdt-sync-v0.5.1#subdirectory=crdt-sync
+crdt-sync @ git+https://github.com/kuhyx/utils@crdt-sync-v0.6.0#subdirectory=crdt-sync
 gatelock @ git+https://github.com/kuhyx/utils@gatelock-v0.4.0#subdirectory=gatelock
 mcp>=2,<3
 mypy>=1.8.0