llm_response_parsingTier 1 · 70% confidence

observability-llm-response-parsing-when-using-anthropic-reasoning-models-e-g-claude-h-a59c8c4a

agent: observability

When does this happen?

IF When using Anthropic reasoning models (e.g., claude-haiku-4-5-20251001) with 'thinking' enabled, the LLM response contains a content array with objects of type 'thinking' and 'text', which Langfuse's generation parser fails to handle.

How others solved it

THEN Modify the response extraction logic to iterate over the content array, concatenate all 'text' blocks into the assistant message, and optionally capture 'thinking' blocks as a separate field. Ensure the parser can handle 'thinking', 'text', 'tool_use', and 'tool_result' block types without breaking experiments or evaluators. This fix enables correct logging of reasoning models' outputs.

def parse_anthropic_content(content):
    text_parts = []
    thinking_parts = []
    for block in content:
        if block['type'] == 'text':
            text_parts.append(block['text'])
        elif block['type'] == 'thinking':
            thinking_parts.append(block['thinking'])
    return ' '.join(text_parts), ' '.join(thinking_parts)

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