[Candidate / unverified] Solution for 074ab78b-3e46-4e50-8a9f-e6460eee132e: Remove assistant prefill: end the conversation with a user message and steer for

## Support semantics This is a **candidate / unverified** Solution from the KFA Solution Factory Tier-2 gate. It is source-grounded research material, not an Outcome, and does **not** claim independent reproduction or confirmed field success. ## Cause Prefilling assistant messages is not supported on Claude 4.6 and later models; the API requires the conversation to end with a user turn. ## Steps 1. Delete trailing assistant prefill messages from the payload. 2. Add formatting constraints via structured outputs or system prompt rather than starting the assistant reply. 3. Ensure the final message in messages is role user before messages.create. ## Expected result Request validates without the prefill-not-supported 400. ## Misleading approaches - Keeping a trailing assistant message with partial JSON to force structure. - Assuming prefill still works after upgrading from Claude 4.5. ## Limitations - Structured outputs availability and schema constraints vary by model; verify model support separately. - Migration from prefill may require prompt redesign for strict JSON shaping. ## Unknowns - Whether specific 4.6.x model aliases beyond those named in docs share identical prefill restriction (assume yes until docs differ). ## Evidence claims - (documented_platform_behavior) Claude 4.6+ and Claude Mythos Preview reject requests whose last message is an assistant prefill with the documented 400 prefill error string. — https://platform.claude.com/docs/en/api/errors - (documented_workaround) Official errors doc points integrators to structured outputs or system prompt instructions instead of prefill for these models. — https://platform.claude.com/docs/en/api/errors