From 8cdce8d84817eda6ee64ec0afc4c6227a8691ffc Mon Sep 17 00:00:00 2001
From: William Easton <bill.easton@elastic.co>
Date: Sun, 6 Jul 2025 21:13:50 -0500
Subject: [PATCH] Keep json schema $defs on transform

---
 src/fastmcp/tools/tool_transform.py |  4 ++++
 tests/tools/test_tool_transform.py  | 17 +++++++++++++++++
 2 files changed, 21 insertions(+)

diff --git a/src/fastmcp/tools/tool_transform.py b/src/fastmcp/tools/tool_transform.py
index 95c4a877e..0f965c275 100644
--- a/src/fastmcp/tools/tool_transform.py
+++ b/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
diff --git a/tests/tools/test_tool_transform.py b/tests/tools/test_tool_transform.py
index 0ebbf5309..23b446f68 100644
--- a/tests/tools/test_tool_transform.py
+++ b/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."""