diff --git a/lib/active_agent/providers/anthropic/transforms.rb b/lib/active_agent/providers/anthropic/transforms.rb
index 747ed2e0..c28f1305 100644
--- a/lib/active_agent/providers/anthropic/transforms.rb
+++ b/lib/active_agent/providers/anthropic/transforms.rb
@@ -436,6 +436,7 @@ def cleanup_serialized_request(hash, defaults, gem_object = nil)
                 msg.delete(:id)
                 msg.delete(:model)
                 msg.delete(:stop_reason)
+                msg.delete(:stop_details)
                 msg.delete(:stop_sequence)
                 msg.delete(:type)
                 msg.delete(:usage)
diff --git a/test/providers/anthropic/transforms_test.rb b/test/providers/anthropic/transforms_test.rb
index 3c223a7b..8e0f915d 100644
--- a/test/providers/anthropic/transforms_test.rb
+++ b/test/providers/anthropic/transforms_test.rb
@@ -434,7 +434,7 @@ def transforms
       test "cleanup_serialized_request removes response-only fields from messages" do
         hash = {
           messages: [
-            { role: "assistant", content: "hello", id: "msg_123", model: "claude-3", stop_reason: "end_turn", type: "message", usage: { input_tokens: 10 } }
+            { role: "assistant", content: "hello", id: "msg_123", model: "claude-3", stop_reason: "end_turn", stop_details: nil, type: "message", usage: { input_tokens: 10 } }
           ]
         }
 
@@ -443,6 +443,7 @@ def transforms
         assert_nil result[:messages][0][:id]
         assert_nil result[:messages][0][:model]
         assert_nil result[:messages][0][:stop_reason]
+        assert_nil result[:messages][0][:stop_details]
         assert_nil result[:messages][0][:type]
         assert_nil result[:messages][0][:usage]
         assert_equal "hello", result[:messages][0][:content]