Claude Prefill Deprecated: Migrate to Structured Outputs

In earlier Claude 3 models, engineers prefilled the assistant turn with { to force JSON output. Per Anthropic's official migration guide, starting with Claude 4.6 and preview models, prefilling assistant turns is unsupported and returns a 400 error. Replace JSON prefills by combining a zero-preamble system rule with Structured Outputs or tool-calling enums.

System: Respond directly without preamble. Do not start with phrases like "Here is..." or "Based on..." Constrain your output strictly to the provided JSON schema. User: Extract all entity relationships from <text>{{INPUT}}</text>.