diff --git a/python/packages/anthropic/agent_framework_anthropic/_chat_client.py b/python/packages/anthropic/agent_framework_anthropic/_chat_client.py
index b3b0458457c..b6f471f8397 100644
--- a/python/packages/anthropic/agent_framework_anthropic/_chat_client.py
+++ b/python/packages/anthropic/agent_framework_anthropic/_chat_client.py
@@ -847,7 +847,8 @@ def _prepare_response_format(self, response_format: type[BaseModel] | dict[str,
                 schema = response_format
 
             if isinstance(schema, dict):
-                schema["additionalProperties"] = False
+                # Copy, so the caller's response_format is not changed for later requests.
+                schema = {**schema, "additionalProperties": False}
 
             return {
                 "type": "json_schema",
diff --git a/python/packages/anthropic/tests/test_anthropic_client.py b/python/packages/anthropic/tests/test_anthropic_client.py
index 24597877291..ca3bed0700b 100644
--- a/python/packages/anthropic/tests/test_anthropic_client.py
+++ b/python/packages/anthropic/tests/test_anthropic_client.py
@@ -3051,6 +3051,19 @@ def test_prepare_response_format_openai_style(mock_anthropic_client: MagicMock)
     assert result["schema"]["properties"]["name"]["type"] == "string"
 
 
+def test_prepare_response_format_does_not_modify_caller_schema(mock_anthropic_client: MagicMock) -> None:
+    """The caller's schema dict keeps its own additionalProperties for later requests."""
+    client = create_test_anthropic_client(mock_anthropic_client)
+
+    schema = {"type": "object", "properties": {"name": {"type": "string"}}, "additionalProperties": True}
+    response_format = {"json_schema": {"schema": schema}}
+
+    result = client._prepare_response_format(response_format)
+
+    assert result["schema"]["additionalProperties"] is False
+    assert schema["additionalProperties"] is True
+
+
 def test_prepare_response_format_direct_schema(
     mock_anthropic_client: MagicMock,
 ) -> None: