fix(anthropic): strip leaked think tags from messages

litellm/llms/anthropic/chat/transformation.py

@@ -1621,6 +1621,9 @@ class AnthropicConfig(AnthropicModelInfo, BaseConfig):
                 if thinking_content is not None:
                     reasoning_content += thinking_content
 
+        if "</think>" in text_content:
+            text_content = text_content.split("</think>", 1)[1].lstrip()
+
         return (
             text_content,
             citations,

litellm/llms/anthropic/experimental_pass_through/adapters/transformation.py

@@ -1249,10 +1249,14 @@ class LiteLLMAnthropicMessagesAdapter:
                 )
 
             # Handle text content
-            if choice.message.content is not None:
+            text_content = choice.message.content
+            if isinstance(text_content, str) and "</think>" in text_content:
+                text_content = text_content.split("</think>", 1)[1].lstrip()
+
+            if text_content:
                 new_content.append(
                     AnthropicResponseContentBlockText(
-                        type="text", text=choice.message.content
+                        type="text", text=text_content
                     ).model_dump()
                 )
             # Handle tool calls (in parallel to text content)

litellm/llms/anthropic/experimental_pass_through/messages/handler.py

@@ -45,6 +45,41 @@ def _should_route_to_responses_api(custom_llm_provider: Optional[str]) -> bool:
     return custom_llm_provider in _RESPONSES_API_PROVIDERS
 
 
+def _sanitize_think_tag_text_blocks(
+    response: Union[AnthropicMessagesResponse, AsyncIterator]
+) -> Union[AnthropicMessagesResponse, AsyncIterator]:
+    if not isinstance(response, dict):
+        return response
+
+    content = response.get("content")
+    if not isinstance(content, list):
+        return response
+
+    sanitized_content: List[Dict[str, Any]] = []
+    for block in content:
+        if not isinstance(block, dict):
+            sanitized_content.append(block)
+            continue
+
+        if block.get("type") != "text":
+            sanitized_content.append(block)
+            continue
+
+        text = block.get("text")
+        if not isinstance(text, str) or "</think>" not in text:
+            sanitized_content.append(block)
+            continue
+
+        cleaned_text = text.split("</think>", 1)[1].lstrip()
+        if cleaned_text:
+            sanitized_block = dict(block)
+            sanitized_block["text"] = cleaned_text
+            sanitized_content.append(sanitized_block)
+
+    response["content"] = sanitized_content
+    return response
+
+
 ####### ENVIRONMENT VARIABLES ###################
 # Initialize any necessary instances or variables here
 base_llm_http_handler = BaseLLMHTTPHandler()
@@ -286,7 +321,7 @@ async def anthropic_messages(
         response = await init_response
     else:
         response = init_response
-    return response
+    return _sanitize_think_tag_text_blocks(response)
 
 
 def validate_anthropic_api_metadata(metadata: Optional[Dict] = None) -> Optional[Dict]:
@@ -421,10 +456,12 @@ def anthropic_messages_handler(
             **kwargs,
         )
         if _should_route_to_responses_api(custom_llm_provider):
-            return LiteLLMMessagesToResponsesAPIHandler.anthropic_messages_handler(
-                **_shared_kwargs
+            return _sanitize_think_tag_text_blocks(
+                LiteLLMMessagesToResponsesAPIHandler.anthropic_messages_handler(
+                    **_shared_kwargs
+                )
             )
-        return (
+        return _sanitize_think_tag_text_blocks(
             LiteLLMMessagesToCompletionTransformationHandler.anthropic_messages_handler(
                 **_shared_kwargs
             )
@@ -441,20 +478,22 @@ def anthropic_messages_handler(
             params=local_vars
         )
     )
-    return base_llm_http_handler.anthropic_messages_handler(
-        model=model,
-        messages=messages,
-        anthropic_messages_provider_config=anthropic_messages_provider_config,
-        anthropic_messages_optional_request_params=dict(
-            anthropic_messages_optional_request_params
-        ),
-        _is_async=is_async,
-        client=client,
-        custom_llm_provider=custom_llm_provider,
-        litellm_params=litellm_params,
-        logging_obj=litellm_logging_obj,
-        api_key=api_key,
-        api_base=api_base,
-        stream=stream,
-        kwargs=kwargs,
+    return _sanitize_think_tag_text_blocks(
+        base_llm_http_handler.anthropic_messages_handler(
+            model=model,
+            messages=messages,
+            anthropic_messages_provider_config=anthropic_messages_provider_config,
+            anthropic_messages_optional_request_params=dict(
+                anthropic_messages_optional_request_params
+            ),
+            _is_async=is_async,
+            client=client,
+            custom_llm_provider=custom_llm_provider,
+            litellm_params=litellm_params,
+            logging_obj=litellm_logging_obj,
+            api_key=api_key,
+            api_base=api_base,
+            stream=stream,
+            kwargs=kwargs,
+        )
     )