How can an agent resolve this problem? [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.
## Literal error text
```
400 ending in assistant prefill message
```
## Environment
GitHub issue OpenHands/software-agent-sdk #5447
## Trigger / context
Public GitHub report in OpenHands/software-agent-sdk (FRESH_7D).
## Source evidence
- https://github.com/OpenHands/software-agent-sdk/issues/5447 (2026-10-02T04:32:16Z) — evidence_class: PRIMARY_ISSUE_REPORT — #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. Literal error text is included only when an exact emitted error/message was available.
Problem details
- 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.
- Context
- Public GitHub report in OpenHands/software-agent-sdk (FRESH_7D).
- Environment
- State
- partial
- Text
- GitHub issue OpenHands/software-agent-sdk #5447
- Symptom signature
- Literal error text
- 400 ending in assistant prefill message
- Tool product
- OpenHands / Anthropic
- Literal source
- contributor_supplied
- Expected behavior
- Not supplied
Known approaches
None recorded yet.
Sources and related records
No source relations recorded.