diff --git a/python/packages/anthropic/agent_framework_anthropic/_chat_client.py b/python/packages/anthropic/agent_framework_anthropic/_chat_client.py
index ab9b1d79a09..5cc91b1efed 100644
--- a/python/packages/anthropic/agent_framework_anthropic/_chat_client.py
+++ b/python/packages/anthropic/agent_framework_anthropic/_chat_client.py
@@ -746,7 +746,8 @@ def _prepare_options(
 
         # Handle user option -> metadata.user_id (Anthropic uses metadata.user_id instead of user)
         if user := run_options.pop("user", None):
-            metadata = run_options.get("metadata", {})
+            # Copy so the caller's metadata dict is not modified across requests.
+            metadata = dict(run_options.get("metadata") or {})
             if "user_id" not in metadata:
                 metadata["user_id"] = user
             run_options["metadata"] = metadata
diff --git a/python/packages/anthropic/tests/test_anthropic_client.py b/python/packages/anthropic/tests/test_anthropic_client.py
index 30de6f63193..fb35902fe74 100644
--- a/python/packages/anthropic/tests/test_anthropic_client.py
+++ b/python/packages/anthropic/tests/test_anthropic_client.py
@@ -3662,6 +3662,23 @@ def test_prepare_options_user_metadata_no_override(
     assert result["metadata"]["user_id"] == "existing_user"
 
 
+def test_prepare_options_user_does_not_mutate_caller_metadata(
+    mock_anthropic_client: MagicMock,
+) -> None:
+    """Test mapping user to metadata.user_id leaves the caller's metadata dict untouched."""
+    client = create_test_anthropic_client(mock_anthropic_client)
+
+    messages = [Message(role="user", contents=[Content.from_text("Hello")])]
+    metadata = {"trace": "t1"}
+
+    first = client._prepare_options(messages, {"user": "alice", "metadata": metadata})
+    second = client._prepare_options(messages, {"user": "bob", "metadata": metadata})
+
+    assert metadata == {"trace": "t1"}
+    assert first["metadata"] == {"trace": "t1", "user_id": "alice"}
+    assert second["metadata"] == {"trace": "t1", "user_id": "bob"}
+
+
 def test_process_stream_event_message_stop(mock_anthropic_client: MagicMock) -> None:
     """Test processing message_stop event."""
     client = create_test_anthropic_client(mock_anthropic_client)