diff --git a/src/nostr/client.py b/src/nostr/client.py
index c04f89a..0155617 100644
--- a/src/nostr/client.py
+++ b/src/nostr/client.py
@@ -368,6 +368,7 @@ class NostrClient:
         start = time.perf_counter()
         if self.offline_mode or not self.relays:
             return Manifest(ver=1, algo="gzip", chunks=[]), ""
+        await self.ensure_manifest_is_current()
         await self._connect_async()
         manifest, chunks = prepare_snapshot(encrypted_bytes, limit)
         for meta, chunk in zip(manifest.chunks, chunks):
@@ -537,10 +538,39 @@ class NostrClient:
 
         return None
 
+    async def ensure_manifest_is_current(self) -> None:
+        """Verify the local manifest is up to date before publishing."""
+        if self.offline_mode or not self.relays:
+            return
+        await self._connect_async()
+        pubkey = self.keys.public_key()
+        ident = f"{MANIFEST_ID_PREFIX}{self.fingerprint}"
+        f = Filter().author(pubkey).kind(Kind(KIND_MANIFEST)).identifier(ident).limit(1)
+        timeout = timedelta(seconds=10)
+        try:
+            events = (await self.client.fetch_events(f, timeout)).to_vec()
+        except Exception:
+            return
+        if not events:
+            return
+        try:
+            data = json.loads(events[0].content())
+            remote = data.get("delta_since")
+            if remote is not None:
+                remote = int(remote)
+        except Exception:
+            return
+        with self._state_lock:
+            local = self.current_manifest.delta_since if self.current_manifest else None
+        if remote is not None and (local is None or remote > local):
+            self.last_error = "Manifest out of date"
+            raise RuntimeError("Manifest out of date")
+
     async def publish_delta(self, delta_bytes: bytes, manifest_id: str) -> str:
         """Publish a delta event referencing a manifest."""
         if self.offline_mode or not self.relays:
             return ""
+        await self.ensure_manifest_is_current()
         await self._connect_async()
 
         content = base64.b64encode(delta_bytes).decode("utf-8")
diff --git a/src/tests/test_nostr_dummy_client.py b/src/tests/test_nostr_dummy_client.py
index 6b24237..763e5b0 100644
--- a/src/tests/test_nostr_dummy_client.py
+++ b/src/tests/test_nostr_dummy_client.py
@@ -1,8 +1,9 @@
 import asyncio
 import gzip
 import math
+import pytest
 
-from helpers import create_vault, dummy_nostr_client
+from helpers import create_vault, dummy_nostr_client, TEST_SEED
 from seedpass.core.entry_management import EntryManager
 from seedpass.core.backup import BackupManager
 from seedpass.core.config_manager import ConfigManager
@@ -141,3 +142,44 @@ def test_fetch_snapshot_uses_event_ids(dummy_nostr_client):
         if getattr(f, "kind_val", None) == KIND_SNAPSHOT_CHUNK
     ]
     assert id_filters and all(id_filters)
+
+
+def test_publish_delta_aborts_if_outdated(tmp_path, monkeypatch, dummy_nostr_client):
+    client1, relay = dummy_nostr_client
+
+    from cryptography.fernet import Fernet
+    from nostr.client import NostrClient
+    from seedpass.core.encryption import EncryptionManager
+
+    enc_mgr = EncryptionManager(Fernet.generate_key(), tmp_path)
+
+    class DummyKeys:
+        def private_key_hex(self):
+            return "1" * 64
+
+        def public_key_hex(self):
+            return "2" * 64
+
+    class DummyKeyManager:
+        def __init__(self, *a, **k):
+            self.keys = DummyKeys()
+
+    with pytest.MonkeyPatch().context() as mp:
+        mp.setattr("nostr.client.KeyManager", DummyKeyManager)
+        mp.setattr(enc_mgr, "decrypt_parent_seed", lambda: TEST_SEED)
+        client2 = NostrClient(enc_mgr, "fp")
+
+    base = b"base"
+    manifest, _ = asyncio.run(client1.publish_snapshot(base))
+    with client1._state_lock:
+        client1.current_manifest.delta_since = 0
+    import copy
+
+    with client2._state_lock:
+        client2.current_manifest = copy.deepcopy(manifest)
+        client2.current_manifest_id = manifest_id = relay.manifests[-1].tags[0]
+
+    asyncio.run(client2.publish_delta(b"d1", manifest_id))
+
+    with pytest.raises(RuntimeError):
+        asyncio.run(client1.publish_delta(b"d2", manifest_id))