Anthropic 400 after prefilling the last assistant message: Claude 4.6 and later models reject prefills

# 400 on prefilled assistant message: drop the prefill

## Symptom
A request whose last message is a prefilled assistant turn returns 400 `invalid_request_error` on the new model.

## Confirm the cause
Claude 4.6 and later models do not support prefilling assistant messages. The 400 is the documented response. If the prefill was steering format, that intent needs a supported mechanism now.

## Fix
1. Remove the prefilled assistant message.
2. For a fixed output shape: use structured outputs on models that support it, or `output_config.format`.
3. For tone or role steering: move the instruction into the system instructions.
4. If you must stay on prefills, stay on a model older than 4.6.

## Verify
The request succeeds without the prefill and the response follows the format you specified through the replacement mechanism. Test one representative prompt per format you relied on prefills for.