diff --git a/python/src/nemo_fabric/models.py b/python/src/nemo_fabric/models.py
index d483fc9cc..d5ab3a95e 100644
--- a/python/src/nemo_fabric/models.py
+++ b/python/src/nemo_fabric/models.py
@@ -982,11 +982,18 @@ def add_definition(
 
         if not name.strip():
             raise ValueError("tool definition names must not be empty")
+        extras = dict(extra_fields or {})
+        overlap = {"kind", "ref", "settings"}.intersection(extras)
+        if overlap:
+            raise ValueError(
+                "extra_fields duplicates known fields: "
+                + ", ".join(sorted(overlap))
+            )
         value = {
             "kind": kind,
             "ref": ref,
             "settings": dict(settings or {}),
-            **dict(extra_fields or {}),
+            **extras,
         }
         self.definitions[name] = ToolDefinitionConfig.model_validate(value)
         return self
diff --git a/tests/python/test_sdk_contract.py b/tests/python/test_sdk_contract.py
index 6fe5b8b73..5c7c878cc 100644
--- a/tests/python/test_sdk_contract.py
+++ b/tests/python/test_sdk_contract.py
@@ -763,6 +763,19 @@ def test_typed_tool_definition_omits_empty_settings():
     }
 
 
+@pytest.mark.parametrize("field", ["kind", "ref", "settings"])
+def test_typed_tool_definition_rejects_known_extra_field_collisions(field: str):
+    tools = ToolsConfig()
+
+    with pytest.raises(ValueError, match="extra_fields duplicates known fields"):
+        tools.add_definition(
+            "web",
+            kind="function_group",
+            ref="web_tools",
+            extra_fields={field: "replacement"},
+        )
+
+
 def test_run_plan_snapshot_removes_named_definition():
     config = _FabricConfigSnapshot.from_mapping(
         {