diff --git a/src/agents/tool.py b/src/agents/tool.py
index 968928b792..974115f1aa 100644
--- a/src/agents/tool.py
+++ b/src/agents/tool.py
@@ -401,7 +401,9 @@ def __post_init__(self):
         if callable(bind_to_function_tool):
             self.on_invoke_tool = bind_to_function_tool(self)
         if self.strict_json_schema:
-            self.params_json_schema = ensure_strict_json_schema(self.params_json_schema)
+            self.params_json_schema = ensure_strict_json_schema(
+                copy.deepcopy(self.params_json_schema)
+            )
         _validate_function_tool_timeout_config(self)
 
     def __copy__(self) -> FunctionTool:
diff --git a/tests/test_function_tool.py b/tests/test_function_tool.py
index 300d1ab3b9..bf410971e3 100644
--- a/tests/test_function_tool.py
+++ b/tests/test_function_tool.py
@@ -732,6 +732,27 @@ def echo(value: str) -> str:
         )
 
 
+def test_function_tool_does_not_mutate_params_json_schema() -> None:
+    async def noop(ctx: ToolContext[Any], input: str) -> str:
+        return ""
+
+    schema = {"type": "object", "properties": {"x": {"type": "string"}}}
+    schema_snapshot = copy.deepcopy(schema)
+
+    tool = FunctionTool(
+        name="t",
+        description="d",
+        params_json_schema=schema,
+        on_invoke_tool=noop,
+        strict_json_schema=True,
+    )
+
+    assert schema == schema_snapshot
+    assert tool.params_json_schema is not schema
+    assert tool.params_json_schema["additionalProperties"] is False
+    assert tool.params_json_schema["required"] == ["x"]
+
+
 @pytest.mark.asyncio
 async def test_default_failure_error_function_survives_deepcopy() -> None:
     def boom() -> None: