From d316f193a89964bf60ef2fbcae38913b0003f4e2 Mon Sep 17 00:00:00 2001
From: Jeremiah Lowin <153965+jlowin@users.noreply.github.com>
Date: Sat, 7 Mar 2026 11:41:07 -0500
Subject: [PATCH] Fix transform arg collisions with passthrough params (#3431)
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---
 src/fastmcp/tools/tool_transform.py              | 16 ++++++++++------
 .../tools/tool_transform/test_tool_transform.py  | 14 ++++++++++++++
 2 files changed, 24 insertions(+), 6 deletions(-)

diff --git a/src/fastmcp/tools/tool_transform.py b/src/fastmcp/tools/tool_transform.py
index 9fc63e6e3..251bc9229 100644
--- a/src/fastmcp/tools/tool_transform.py
+++ b/src/fastmcp/tools/tool_transform.py
@@ -554,12 +554,16 @@ class TransformedTool(Tool):
         # Additional validation: check for naming conflicts after transformation
         if transform_args:
             new_names = []
-            for old_name, transform in transform_args.items():
-                if not transform.hide:
-                    if transform.name is not NotSet:
-                        new_names.append(transform.name)
-                    else:
-                        new_names.append(old_name)
+            for old_name in parent_params:
+                transform = transform_args.get(old_name, ArgTransform())
+
+                if transform.hide:
+                    continue
+
+                if transform.name is not NotSet:
+                    new_names.append(transform.name)
+                else:
+                    new_names.append(old_name)
 
             # Check for duplicate names after transformation
             name_counts = {}
diff --git a/tests/tools/tool_transform/test_tool_transform.py b/tests/tools/tool_transform/test_tool_transform.py
index f6167a8ec..bcf999ae6 100644
--- a/tests/tools/tool_transform/test_tool_transform.py
+++ b/tests/tools/tool_transform/test_tool_transform.py
@@ -482,6 +482,20 @@ def test_transform_args_creates_duplicate_names(add_tool):
         )
 
 
+def test_transform_args_collision_with_passthrough_name(add_tool):
+    """Test that renaming to a passthrough parameter name raises ValueError."""
+    with pytest.raises(
+        ValueError,
+        match="Multiple arguments would be mapped to the same names: old_y",
+    ):
+        Tool.from_tool(
+            add_tool,
+            transform_args={
+                "old_x": ArgTransform(name="old_y"),
+            },
+        )
+
+
 def test_function_without_kwargs_missing_params(add_tool):
     """Test that function missing required transformed parameters raises ValueError."""