diff --git a/src/agents/extensions/experimental/codex/codex_tool.py b/src/agents/extensions/experimental/codex/codex_tool.py
index 854aa65fc9..cf0eee08cd 100644
--- a/src/agents/extensions/experimental/codex/codex_tool.py
+++ b/src/agents/extensions/experimental/codex/codex_tool.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import asyncio
+import copy
 import dataclasses
 import inspect
 import json
@@ -694,7 +695,7 @@ def _resolve_output_schema(
         return _build_codex_output_schema(descriptor)
 
     if isinstance(option, Mapping):
-        schema = dict(option)
+        schema = copy.deepcopy(dict(option))
         if "type" in schema and schema.get("type") != "object":
             raise UserError('Codex output schema must be a JSON object schema with type "object".')
         return ensure_strict_json_schema(schema)
diff --git a/tests/extensions/experiemental/codex/test_codex_tool.py b/tests/extensions/experiemental/codex/test_codex_tool.py
index 042e05bc01..9bf650816b 100644
--- a/tests/extensions/experiemental/codex/test_codex_tool.py
+++ b/tests/extensions/experiemental/codex/test_codex_tool.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import asyncio
+import copy
 import dataclasses
 import importlib
 import inspect
@@ -1516,6 +1517,19 @@ def test_codex_tool_resolve_output_schema_validation_errors() -> None:
         codex_tool_module._resolve_output_schema({"type": "string"})
 
 
+def test_codex_tool_resolve_output_schema_does_not_mutate_input() -> None:
+    nested = {"type": "object", "properties": {"y": {"type": "string"}}}
+    option = {"type": "object", "properties": {"inner": nested}}
+    option_snapshot = copy.deepcopy(option)
+
+    result = codex_tool_module._resolve_output_schema(option)
+
+    assert option == option_snapshot
+    assert nested == {"type": "object", "properties": {"y": {"type": "string"}}}
+    assert result is not None
+    assert result["properties"]["inner"] is not nested
+
+
 def test_codex_tool_resolve_output_schema_descriptor() -> None:
     descriptor = {
         "title": "Report",