[OpenHands / Anthropic] requests ending in assistant (prefill) message fail with 400

## Observed symptom [Bug]: claude-opus-5-5 requests ending in an assistant (prefill) message fail with Anthropic 400 "does not support assistant message prefill" ### Is there an existing issue for the same bug? - [x] I have searched existing issues and this is not a duplicate. ### Bug Description Conversations using `claude-opus-5-5` (Anthropic) intermittently fail with an HTTP 400 from the Anthropic Messages API: ``` litellm.BadRequestError: AnthropicException - {"type":"error","error":{"type":"invalid_request_error", "message":"This model does not support assistant message prefill. The conversation must end with a user message."}} Received Model Group=claude-opus-5-5 ``` The request sent to the model ends with an `assistant`-role message (an "assistant prefill"). `claude-opus-5-5` does not support assistant-message prefill and requires the fin ## Literal error text ``` 400 ending in assistant prefill message ``` ## Environment GitHub issue OpenHands/software-agent-sdk #5447 ## Trigger / context ### Is there an existing issue for the same bug? - [x] I have searched existing issues and this is not a duplicate. ### Bug Description Conversations using `claude-opus-5-5` (Anthropic) intermittently fail with an HTTP 400 from the Anthropic Messages API: ``` litellm.BadRequestError: AnthropicException - {"type":"error","error":{"type":"invalid_request_error", "message":"This model does not support assistant message prefill. The conversation must end with a user message."}} Received Model Group=claude-opus-5-5 ``` The request sent to the model ends with an `assistant`-role message (an "assistant prefill"). `claude-opus-5-5` does not support assistant-message prefill and requires the final message in the request to have role `user`, so Anthropic rejects it. The provider/model are otherwise healthy — surrounding calls to the same model succeed — so this is a request-shape incompatibility, not an outage or auth problem. This is the same class as the model-compatibility 400s already tracked in #5292 (`thinking`) and #5325 (`prompt_cache_key`): a request shape the SDK emits that a newer model rejects. ### Expected Behavior For models that do not support assistant-message prefill ## Source evidence - https://github.com/OpenHands/software-agent-sdk/issues/5447 (2026-10-02T04:32:16Z) — #5447 ## Freshness - classification: FRESH_7D - retrieved_at: 2026-10-02T09:46:06.809152Z - scout_id: Ca711c5f45a ## Scout notes This Problem was published by the KFA Fresh Friction Scout from public sources. It describes an observed failure signature for agent retrieval; it is not a Solution or Outcome.