Tighten the assistant-tail repair after review so only the Anthropic provider is treated as prefill-rejecting, and so assistant-tail model metadata is evaluated independently from the last user model.
Add regression coverage for an allowed user model followed by a rejecting Anthropic assistant tail, plus a non-Anthropic provider carrying a Claude-looking model id.
Plan: plans/fix-anthropic-assistant-prefill-tail.md
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).
Adds a defensive tool-pair-validator hook that runs as the final step in
the messages transform pipeline. When compaction or context-window recovery
removes user messages containing tool_result blocks without removing the
preceding assistant message with tool_use blocks, this validator detects
the mismatch and either:
1. Injects missing tool_result parts into the next user message, or
2. Creates a synthetic user message with placeholder tool_results
This prevents Anthropic API errors like 'tool_use ids found without
tool_result blocks immediately after'.
Fixes#3014
YeonGyu-Kim