Claude Certified Architect – Professional (CCA-P)

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Prompt Engineering & Structured Output

Context: An architect is building a system that requires Claude to classify user input into one of five categories and return the result in a strict JSON format: `{"category": "CATEGORY_NAME"}`. Problem: Despite a well-crafted system prompt and few-shot examples, the model occasionally adds conversational preamble (e.g., "Certainly, here is the classification:") before the JSON object, which breaks the downstream JSON parser. Question: Which Messages API technique provides the strongest guarantee that the model's response will begin directly with the opening brace `{` of the JSON object?

Add a final user message stating 'You must only respond with a single JSON object and no other text.'

Set the `temperature` parameter to 0.0 to make the output more deterministic.

Use the `cache_control` parameter with `type: 'ephemeral'` to ensure the system prompt is always processed correctly.

In the API call's `messages` array, add a final entry with `"role": "assistant"` and `"content": "{"` to prefill the beginning of Claude's turn.