From 9f5a9f34d829e7af87217c1aaf3546f3373f819c Mon Sep 17 00:00:00 2001
From: Marvin Context Protocol
 <41898282+Marvin Context Protocol@users.noreply.github.com>
Date: Mon, 20 Jul 2026 11:42:14 +0000
Subject: [PATCH] Fix nondeterministic required order in transformed tool
 schemas

Co-authored-by: Bill Easton <6384545+strawgate@users.noreply.github.com>
---
 fastmcp_slim/fastmcp/tools/tool_transform.py |  8 +++++--
 tests/tools/tool_transform/test_schemas.py   | 23 ++++++++++++++++++++
 2 files changed, 29 insertions(+), 2 deletions(-)

diff --git a/fastmcp_slim/fastmcp/tools/tool_transform.py b/fastmcp_slim/fastmcp/tools/tool_transform.py
index 910b89262..c11f58eda 100644
--- a/fastmcp_slim/fastmcp/tools/tool_transform.py
+++ b/fastmcp_slim/fastmcp/tools/tool_transform.py
@@ -707,7 +707,9 @@ class TransformedTool(Tool):
         schema = {
             "type": "object",
             "properties": new_props,
-            "required": list(new_required),
+            # Order by property insertion order so the schema is deterministic
+            # across processes (a bare set() order depends on PYTHONHASHSEED).
+            "required": [name for name in new_props if name in new_required],
             "additionalProperties": False,
         }
 
@@ -899,7 +901,9 @@ class TransformedTool(Tool):
         result = {
             "type": "object",
             "properties": merged_props,
-            "required": list(final_required),
+            # Order by property insertion order so the schema is deterministic
+            # across processes (a bare set() order depends on PYTHONHASHSEED).
+            "required": [name for name in merged_props if name in final_required],
             "additionalProperties": False,
         }
 
diff --git a/tests/tools/tool_transform/test_schemas.py b/tests/tools/tool_transform/test_schemas.py
index 703dd7955..8b0d8069a 100644
--- a/tests/tools/tool_transform/test_schemas.py
+++ b/tests/tools/tool_transform/test_schemas.py
@@ -553,3 +553,26 @@ class TestInputSchema:
                 "additionalProperties": False,
             }
         )
+
+
+class TestRequiredOrdering:
+    def test_required_follows_property_order(self):
+        """Transformed `required` should match property order deterministically.
+
+        Regression test for a bug where `required` was built from a set, so its
+        order depended on PYTHONHASHSEED and differed across processes.
+        """
+
+        def base(alpha: int, beta: str, gamma: float, delta: bool, epsilon: int) -> str:
+            return "x"
+
+        transformed = Tool.from_tool(
+            Tool.from_function(base),
+            transform_args={"alpha": ArgTransform(name="a")},
+        )
+
+        properties = list(transformed.parameters["properties"])
+        required = transformed.parameters["required"]
+
+        assert required == [name for name in properties if name in set(required)]
+        assert required == ["a", "beta", "gamma", "delta", "epsilon"]