diff --git a/gns3server/api/routes/mcp/links.py b/gns3server/api/routes/mcp/links.py
index 2a2f6f0ef..b340f122a 100644
--- a/gns3server/api/routes/mcp/links.py
+++ b/gns3server/api/routes/mcp/links.py
@@ -141,6 +141,8 @@ def create_link_handler(params: dict[str, Any], gns3_ctx: dict[str, Any]) -> dic
         return {"error": "project_id is required"}
 
     fields = params.get("fields")
+    if fields is not None and not isinstance(fields, list):
+        return {"error": "fields must be a list, e.g. [\"link_id\", \"nodes\"]"}
 
     links = params.get("links")
     # Batch mode: links=[{nodes, link_type?, filters?, suspend?}]
diff --git a/gns3server/api/routes/mcp/nodes.py b/gns3server/api/routes/mcp/nodes.py
index c14542ec8..135872d3b 100644
--- a/gns3server/api/routes/mcp/nodes.py
+++ b/gns3server/api/routes/mcp/nodes.py
@@ -209,6 +209,8 @@ def create_node_handler(params: dict[str, Any], gns3_ctx: dict[str, Any]) -> dic
         return {"error": "project_id is required"}
 
     fields = params.get("fields")
+    if fields is not None and not isinstance(fields, list):
+        return {"error": "fields must be a list, e.g. [\"node_id\", \"name\"]"}
 
     nodes = params.get("nodes")
     # Batch mode: nodes=[{template_id?, x, y, name?, compute_id?}]