diff --git a/gns3server/agent/gns3_copilot/gns3_client/api_handlers.py b/gns3server/agent/gns3_copilot/gns3_client/api_handlers.py
index dc674ccb8..9c31d07d0 100644
--- a/gns3server/agent/gns3_copilot/gns3_client/api_handlers.py
+++ b/gns3server/agent/gns3_copilot/gns3_client/api_handlers.py
@@ -900,7 +900,7 @@ def link_marker_handler(params: dict[str, Any], gns3_ctx: dict[str, Any]) -> dic
         if not bpf:
             return {"error": "bpf is required for create action"}
         body: dict[str, Any] = {"bpf": bpf}
-        for opt in ("name", "tag", "capture_node_id", "color", "highlight_duration"):
+        for opt in ("name", "tag", "capture_node_id", "color", "highlight_duration", "data_link_type"):
             if params.get(opt) is not None:
                 body[opt] = params[opt]
         # direction: "tx"/"rx" set a one-way filter; "both"/omitted = no filter.
diff --git a/gns3server/agent/mcp/__init__.py b/gns3server/agent/mcp/__init__.py
index 191d819cf..b04a764ff 100644
--- a/gns3server/agent/mcp/__init__.py
+++ b/gns3server/agent/mcp/__init__.py
@@ -1035,6 +1035,7 @@ async def link_marker(
     capture_node_id: Annotated[str | None, Field(description="UUID of the endpoint whose uBridge hosts the marker (the observer; tx/rx are from its perspective). Must be a link endpoint and marker-capable. Omit to auto-pick.")] = None,
     color: Annotated[str | None, Field(description="Hex color for UI highlight, e.g. '#ff5722'")] = None,
     highlight_duration: Annotated[int | None, Field(description="UI highlight duration in milliseconds")] = None,
+    data_link_type: Annotated[str | None, Field(description="pcap link-layer type for serial links (create-only): DLT_C_HDLC / DLT_PPP_SERIAL / DLT_FRELAY / DLT_ATM_RFC1483, matching the encapsulation on the serial link. Omit = DLT_EN10MB (Ethernet). Ignored on update — changing it would invalidate the capture file.")] = None,
 ) -> list[dict[str, Any]]:
     """Manage traffic-insight markers on a link.
 
@@ -1051,7 +1052,7 @@ async def link_marker(
     and cannot be modified or deleted via this tool.
     """
     params = {"project_id": project_id, "link_id": link_id, "action": action}
-    for opt in ("bpf", "marker_name", "name", "tag", "enabled", "direction", "capture_node_id", "color", "highlight_duration"):
+    for opt in ("bpf", "marker_name", "name", "tag", "enabled", "direction", "capture_node_id", "color", "highlight_duration", "data_link_type"):
         val = locals().get(opt)
         if val is not None:
             params[opt] = val
diff --git a/tests/agent/mcp/test_handlers.py b/tests/agent/mcp/test_handlers.py
index 42738dc37..0a85e0256 100644
--- a/tests/agent/mcp/test_handlers.py
+++ b/tests/agent/mcp/test_handlers.py
@@ -437,6 +437,33 @@ class TestLinkMarker:
                 json_data={"bpf": "icmp"},
             )
 
+    def test_create_data_link_type_passthrough(self, ctx):
+        """create passes a serial WAN encapsulation through; update ignores it."""
+        from gns3server.agent.gns3_copilot.gns3_client.api_handlers import link_marker_handler
+        with patch(f"{AH}._get_connector") as m:
+            conn = _mock_conn({"name": "icmp"})
+            m.return_value = conn
+            link_marker_handler(
+                {"project_id": "p", "link_id": "l", "action": "create",
+                 "bpf": "icmp", "data_link_type": "DLT_C_HDLC"}, ctx,
+            )
+            conn.http_call.assert_called_with(
+                "post", "http://192.168.1.3:3080/v3/projects/p/links/l/markers",
+                json_data={"bpf": "icmp", "data_link_type": "DLT_C_HDLC"},
+            )
+
+            conn = _mock_conn({"name": "icmp"})
+            m.return_value = conn
+            link_marker_handler(
+                {"project_id": "p", "link_id": "l", "action": "update",
+                 "marker_name": "icmp", "tag": 1, "data_link_type": "DLT_PPP_SERIAL"}, ctx,
+            )
+            # create-only: dropped from the update body
+            conn.http_call.assert_called_with(
+                "put", "http://192.168.1.3:3080/v3/projects/p/links/l/markers/icmp",
+                json_data={"tag": 1},
+            )
+
     def test_create_direction_tx(self, ctx):
         from gns3server.agent.gns3_copilot.gns3_client.api_handlers import link_marker_handler
         with patch(f"{AH}._get_connector") as m: