diff --git a/backend/open_webui/utils/anthropic.py b/backend/open_webui/utils/anthropic.py
index b6672af48f..36237bdaeb 100644
--- a/backend/open_webui/utils/anthropic.py
+++ b/backend/open_webui/utils/anthropic.py
@@ -202,7 +202,9 @@ def convert_anthropic_to_openai_payload(
                         )
                     )
                 elif block_type in ('thinking', 'redacted_thinking'):
-                    openai_content.append(_copy_cache_control(block, dict(block)))
+                    # Unsigned thinking cannot be replayed upstream
+                    if block_type == 'redacted_thinking' or block.get('signature'):
+                        openai_content.append(_copy_cache_control(block, dict(block)))
                 elif block_type == 'image':
                     source = block.get('source', {})
                     if source.get('type') == 'base64':
@@ -369,7 +371,7 @@ def convert_anthropic_to_openai_payload(
                     msg_dict['content'] = ''
                 msg_dict['tool_calls'] = tool_calls
                 messages.append(msg_dict)
-            elif openai_content:
+            elif openai_content or role == 'assistant':
                 messages.append({'role': role, 'content': _finalize_openai_content(openai_content)})
         else:
             messages.append({'role': role, 'content': str(content) if content else ''})