Question

A developer prefills the assistant response with an opening brace to force Claude to return only a JSON object. However, the task also requires Claude to reason through several conditional pricing rules before deciding the final values. After adding the prefill, output quality drops noticeably — Claude picks wrong values more often than before. What is the most likely cause, and what should the developer do?

Accepted Answer

Prefilling with an open brace suppresses Claude's ability to reason step by step first; move the reasoning into a chain-of-thought step before the JSON, or use prompt chaining so reasoning happens in a separate call.

Answer

Prefilling always reduces accuracy, so the developer should remove the prefill entirely and rely on instructions asking for JSON-only output.

Answer

The temperature is too low; raising it will let Claude explore the pricing rules more thoroughly before answering.

Answer

The model is too small for the task; switching to a larger model is the only reliable fix for reasoning-heavy JSON generation.