diff --git a/src/planai/llm_interface.py b/src/planai/llm_interface.py
index 5de378f..3c651b8 100644
--- a/src/planai/llm_interface.py
+++ b/src/planai/llm_interface.py
@@ -245,6 +245,9 @@ def generate_pydantic(
             if extra_validation:
                 extra_error_message = extra_validation(response)
                 if extra_error_message:
+                    if self.support_structured_outputs:
+                        # the raw response was a pydantic object, so we need to dump it to a string
+                        raw_response = raw_response.model_dump_json()
                     messages.extend(
                         [
                             {"role": "assistant", "content": raw_response},