diff --git a/tests/test_syncd_routing.py b/tests/test_syncd_routing.py
index 6d90c3b..fc6e866 100644
--- a/tests/test_syncd_routing.py
+++ b/tests/test_syncd_routing.py
@@ -126,6 +126,45 @@ class SyncdRoutingTest(unittest.TestCase):
             self.assertEqual(1, len(commands))
             self.assertEqual("sy_old", json.loads(state.read_text())["run_id"])
 
+    def test_daemon_skips_mtp_probe_while_canonical_sync_lock_is_busy(self):
+        with tempfile.TemporaryDirectory() as directory:
+            root = Path(directory)
+            transport = root / "transport"
+            transport.mkdir()
+            config = root / "config.json"
+            config.write_text(
+                json.dumps(
+                    {
+                        "format": "trainlog-sync-orchestrator-config",
+                        "version": 1,
+                        "enabled": True,
+                        "mode": "mtp",
+                        "expected_peer_id": "peer_11111111-1111-4111-8111-111111111111",
+                        "transport_root": str(transport),
+                        "owned_root": str(root / "owned"),
+                        "timeout_seconds": 30,
+                    }
+                )
+            )
+            args = argparse.Namespace(
+                mtp_adapter=root / "adapter",
+                state=root / "trainlog.db.sync-run.json",
+                trigger="android",
+                orchestrator=root / "sync_orchestrator.py",
+                database=root / "trainlog.db",
+            )
+
+            def flock(_fd, operation):
+                if operation == syncd.fcntl.LOCK_EX | syncd.fcntl.LOCK_NB:
+                    raise BlockingIOError
+                self.assertEqual(syncd.fcntl.LOCK_UN, operation)
+
+            with mock.patch.object(syncd.fcntl, "flock", side_effect=flock), mock.patch.object(
+                syncd.subprocess, "run"
+            ) as run:
+                self.assertEqual(3, syncd.run_full_generation(args, config))
+            run.assert_not_called()
+
     def test_android_production_publishes_full_generation_before_optional_legacy(self):
         source = (ROOT / "android/app/src/main/java/com/labfytools/trainlog/ui/SyncScreen.kt").read_text()
         production = source.split("internal suspend fun publishBundleAndRequest", 1)[1].split(
diff --git a/tools/trainlog_syncd.py b/tools/trainlog_syncd.py
index c35c2f8..116d8e6 100755
--- a/tools/trainlog_syncd.py
+++ b/tools/trainlog_syncd.py
@@ -4,6 +4,7 @@
 from __future__ import annotations
 
 import argparse
+import fcntl
 import json
 import os
 import re
@@ -168,12 +169,32 @@ def run_full_generation(args: argparse.Namespace, config_path: Path) -> int:
     root = Path(config["transport_root"])
     expected_peer = config["expected_peer_id"]
     adapter = args.mtp_adapter
-    pull = subprocess.run(
-        [str(adapter), "pull", expected_peer, str(root)],
-        capture_output=True,
-        text=True,
-        check=False,
-    )
+    # WHY: the daemon must probe MTP to discover Android-origin requests, but
+    # Web/TUI orchestration owns the same physical transport while it holds the
+    # canonical sync lock. Probing outside that lock lets libmtp race itself
+    # and fail with "device is busy". CONTRACT: idle discovery takes the same
+    # lock non-blockingly and simply skips this poll when another conversation
+    # is active. INVARIANT: the lock is released before launching the
+    # orchestrator, which remains the sole owner of business-sync locking.
+    lock_path = args.database.parent / "sync.lock"
+    lock_path.parent.mkdir(parents=True, exist_ok=True, mode=0o700)
+    lock_fd = os.open(lock_path, os.O_RDWR | os.O_CREAT, 0o600)
+    try:
+        try:
+            fcntl.flock(lock_fd, fcntl.LOCK_EX | fcntl.LOCK_NB)
+        except BlockingIOError:
+            return 3
+        pull = subprocess.run(
+            [str(adapter), "pull", expected_peer, str(root)],
+            capture_output=True,
+            text=True,
+            check=False,
+        )
+    finally:
+        try:
+            fcntl.flock(lock_fd, fcntl.LOCK_UN)
+        finally:
+            os.close(lock_fd)
     if pull.returncode != 0:
         return 3
     _, seen = load_request_state(args.state)