Summary

When an assistant message has stopReason: "aborted" (in addition to "error"), the repairToolUseResultPairing() function incorrectly creates synthetic tool_result entries for incomplete tool calls. This causes all subsequent API requests to fail with:

unexpected tool_use_id found in tool_result blocks: toolu_XXXXX.
Each tool_result block must have a corresponding tool_use block in the previous message.

Root Cause

When a request is aborted mid-stream:

1. The assistant message is saved with stopReason: "aborted" and incomplete tool_use blocks (often with partialJson: true)
2. repairToolUseResultPairing() sees a tool_use without a matching tool_result
3. It creates a synthetic tool_result to "fix" the pairing
4. During API serialization, the incomplete tool_use may be filtered out
5. Result: orphaned tool_result → API rejects with 400 error

Evidence

A corrupted session had the following stopReason distribution:

  1 "stopReason":"aborted"    <-- caused the corruption
  3 "stopReason":"error"
121 "stopReason":"stop"
521 "stopReason":"toolUse"

The single "aborted" message corrupted the entire session.

Proposed Fix

Skip tool call extraction for assistant messages with stopReason: "error" OR stopReason: "aborted":

const stopReason = (assistant as { stopReason?: string }).stopReason;
if (stopReason === "error" || stopReason === "aborted") {
  out.push(msg);
  continue;
}

Related Issues

• Bug: Session tree branching causes tool_use_id mismatch error (LLM request rejected) #4553 - Session tree branching causes tool_use_id mismatch (related but different root cause)
• Synthetic tool_result for terminated tool_use causes all subsequent requests to fail #1826 - Same underlying issue
• fix(agents): skip extracting tool calls from errored assistant turns #1859 - Similar fix but only handles "error", not "aborted"
• [Bug] Anthropic API Error: Unexpected tool_use_id in tool_result blocks anthropics/claude-code#11064 - Same issue in claude-code

Impact

• Without fix: Any aborted request during a tool call permanently corrupts the session
• With fix: Sessions remain usable after interruptions

I have a PR ready with the fix and tests.