Claude Opus 4.7 rejects requests whose final message is an assistant turn, because Anthropic treats that as unsupported assistant prefill. Add a transform guard that appends the existing synthetic user recovery turn only for Anthropic prefill-rejecting model IDs, while preserving assistant-prefill behavior for supported and unknown models.
Tests cover Opus 4.7, Opus 4.6 assistant metadata fallback, direct Sonnet 4.6 model fields, allowed OpenAI/Sonnet 4.5 tails, missing metadata, and compaction continuation compatibility.
Plan: plans/fix-anthropic-assistant-prefill-tail.md
Previously each transform hook was awaited sequentially without per-hook
error handling. If contextInjectorMessagesTransform or thinkingBlockValidator
threw, toolPairValidator was silently skipped, leaving orphaned tool_use
blocks in the post-compaction API payload and producing
"messages.N: tool_use ids were found without tool_result blocks immediately
after" 400s from Anthropic.
Wraps each hook in runHookSafely so an upstream throw is logged but the
chain continues. Adds regression tests covering the isolation contract and
the consecutive-assistants compaction tail case (ses_22bd806).
YeonGyu-Kim