Make transformed tool required order deterministic (#4564)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

fastmcp_slim/fastmcp/tools/tool_transform.py

@@ -707,7 +707,8 @@ class TransformedTool(Tool):
         schema = {
             "type": "object",
             "properties": new_props,
-            "required": list(new_required),
+            # Iterate props (not the set) for deterministic ordering
+            "required": [p for p in new_props if p in new_required],
             "additionalProperties": False,
         }
 
@@ -899,7 +900,11 @@ class TransformedTool(Tool):
         result = {
             "type": "object",
             "properties": merged_props,
-            "required": list(final_required),
+            # Iterate props (not the set) for deterministic ordering; keep any
+            # required names not present in properties (sorted) rather than
+            # silently dropping them.
+            "required": [p for p in merged_props if p in final_required]
+            + sorted(final_required - set(merged_props)),
             "additionalProperties": False,
         }
 

tests/tools/tool_transform/test_tool_transform.py

@@ -41,6 +41,23 @@ def test_tool_from_tool_no_change(add_tool):
     assert new_tool.description == add_tool.description
 
 
+def test_transformed_tool_required_order_is_deterministic():
+    """`required` must follow property order, not set iteration order.
+
+    Set iteration order varies with PYTHONHASHSEED, which broke snapshot
+    tests of tools/list output across processes.
+    """
+
+    def fn(alpha: int, beta: str, gamma: float, delta: bool, epsilon: int) -> str:
+        return "x"
+
+    base = Tool.from_function(fn)
+    transformed = Tool.from_tool(base, transform_args={"alpha": ArgTransform(name="a")})
+    props = list(transformed.parameters["properties"])
+    assert transformed.parameters["required"] == props
+    assert props == ["a", "beta", "gamma", "delta", "epsilon"]
+
+
 def test_from_tool_accepts_decorated_function():
     @tool
     def search(q: str, limit: int = 10) -> list[str]: