Keep json schema $defs on transform

src/fastmcp/tools/tool_transform.py

@@ -553,6 +553,7 @@ class TransformedTool(Tool):
         """
 
         # Build transformed schema and mapping
+        parent_defs = parent_tool.parameters.get("$defs", {})
         parent_props = parent_tool.parameters.get("properties", {}).copy()
         parent_required = set(parent_tool.parameters.get("required", []))
 
@@ -608,6 +609,9 @@ class TransformedTool(Tool):
             "required": list(new_required),
         }
 
+        if parent_defs:
+            schema["$defs"] = parent_defs
+
         # Create forwarding function that closes over everything it needs
         async def _forward(**kwargs):
             # Validate arguments

tests/tools/test_tool_transform.py

@@ -383,6 +383,23 @@ async def test_forward_raw_outside_context_raises_error():
     ):
         await forward_raw(new_x=1, old_y=2)
 
+def test_transform_args_with_parent_defaults():
+    """Test that transform_args with parent defaults works."""
+
+    class CoolModel(BaseModel):
+        x: int = 10
+
+    def parent_tool(cool_model: CoolModel) -> int:
+        return cool_model.x
+
+    tool = Tool.from_function(parent_tool)
+
+    new_tool = Tool.from_tool(
+        tool
+    )
+
+    assert new_tool.parameters["$defs"] == tool.parameters["$defs"]
+
 
 def test_transform_args_validation_unknown_arg(add_tool):
     """Test that transform_args with unknown arguments raises ValueError."""