diff --git a/gns3server/compute/marker/marker_listener.py b/gns3server/compute/marker/marker_listener.py
index 40cdd97b6..a30769c1e 100644
--- a/gns3server/compute/marker/marker_listener.py
+++ b/gns3server/compute/marker/marker_listener.py
@@ -114,13 +114,24 @@ class MarkerListener(asyncio.DatagramProtocol):
         # signals that carry no `link=`.
         signal_link = link if link and link != "-" else None
 
+        # Normalize the tag to int so the event matches the REST schema
+        # (MarkerCreate.tag is Optional[int]): the signal merely echoes the
+        # decimal we installed via `mark <bpf> tag <id>`, so parsing cannot
+        # fail for well-formed signals; a malformed value keeps the registered
+        # int, and the tag is None only when neither side carries one.
+        event_tag = registered_tag
+        if tag and tag != "-":
+            try:
+                event_tag = int(tag)
+            except ValueError:
+                pass  # malformed signal tag: keep the registered value
+
         event = {
             "project_id": project_id,
             "node_id": node_id,
             "link_id": signal_link or link_id,
             "filter": filter_name,
-            # Prefer the value carried in the signal; fall back to the one we registered.
-            "tag": tag if tag and tag != "-" else registered_tag,
+            "tag": event_tag,
             "ts": ts,
             "len": int(length) if length and length.isdigit() else 0,
             # Travel direction relative to the capture node (node_id above);
diff --git a/tests/compute/marker/test_marker_manager.py b/tests/compute/marker/test_marker_manager.py
index da495ae92..5917870d7 100644
--- a/tests/compute/marker/test_marker_manager.py
+++ b/tests/compute/marker/test_marker_manager.py
@@ -121,7 +121,11 @@ class TestMarkerListener:
         assert ev["node_id"] == "n1"
         assert ev["link_id"] == "l1"
         assert ev["filter"] == "f1"
-        assert ev["tag"] == "7"
+        # The event tag is normalized to int to match the REST schema — the
+        # signal echoes the decimal we installed, so str and int variants of
+        # the same tag must never reach consumers as different keys.
+        assert ev["tag"] == 7
+        assert isinstance(ev["tag"], int)
         assert ev["ts"] == pytest.approx(1700000000.123456)
         assert ev["len"] == 98
         # No dir= in the signal (legacy uBridge) → undirected.
@@ -163,6 +167,24 @@ class TestMarkerListener:
         lis.datagram_received(b"MARK 2.0 node=n filter=f tag=- len=20\n", None)
         assert fmgr.events[0][1]["tag"] == 42
 
+    def test_non_numeric_signal_tag_falls_back_to_registered(self):
+        # A corrupt/unknown signal value must not leak a str tag into the
+        # event stream — the registry's int wins.
+        fmgr = FakeMarkerManager()
+        fmgr.register("p", "n", "f", "l", tag=42)
+        lis = MarkerListener(fmgr)
+        lis.connection_made(None)
+        lis.datagram_received(b"MARK 2.0 node=n filter=f tag=oops len=20\n", None)
+        assert fmgr.events[0][1]["tag"] == 42
+
+    def test_no_tag_anywhere_is_none(self):
+        fmgr = FakeMarkerManager()
+        fmgr.register("p", "n", "f", "l", tag=None)
+        lis = MarkerListener(fmgr)
+        lis.connection_made(None)
+        lis.datagram_received(b"MARK 2.0 node=n filter=f tag=- len=20\n", None)
+        assert fmgr.events[0][1]["tag"] is None
+
     def test_link_in_signal_overrides_registry_link(self):
         # Per-link attribution (contract §3.2/§3.3): the signal's `link=` is
         # authoritative and must disambiguate links sharing a node+filter —