diff --git a/conserver/links/tag/__init__.py b/conserver/links/tag/__init__.py
index 824cc6e..edce5e8 100644
--- a/conserver/links/tag/__init__.py
+++ b/conserver/links/tag/__init__.py
@@ -16,6 +16,12 @@ def run(
 
     vcon_redis = VconRedis()
     vCon = vcon_redis.get_vcon(vcon_uuid)
+    if vCon is None:
+        # get_vcon returns None when the vCon is missing from Redis and every
+        # storage backend (evicted/expired under chain latency). None is the
+        # documented "halt the chain" contract, so stop rather than crash.
+        logger.warning(f"tag: vCon {vcon_uuid} not found, halting chain")
+        return None
     for tag in opts.get("tags", []):
         vCon.add_tag(tag_name=tag, tag_value=tag)
     vcon_redis.store_vcon(vCon)
diff --git a/conserver/links/tag/test_tag.py b/conserver/links/tag/test_tag.py
index ffff22f..4eb6cee 100644
--- a/conserver/links/tag/test_tag.py
+++ b/conserver/links/tag/test_tag.py
@@ -33,6 +33,19 @@ def test_run_respects_custom_tags(mock_vcon_redis):
     mock_instance.store_vcon.assert_called_once_with(vcon)
 
 
+@patch("links.tag.VconRedis")
+def test_run_halts_chain_when_vcon_missing(mock_vcon_redis):
+    # Regression for CON-617: get_vcon returns None on a Redis/storage miss.
+    # The link must halt the chain, not crash with 'NoneType' has no add_tag.
+    mock_instance = mock_vcon_redis.return_value
+    mock_instance.get_vcon.return_value = None
+
+    result = run("missing-uuid", "tag")
+
+    assert result is None
+    mock_instance.store_vcon.assert_not_called()
+
+
 @patch("links.tag.VconRedis")
 def test_run_handles_empty_tag_list(mock_vcon_redis):
     vcon = Vcon.build_new()