Bug Report

Version: 2026.3.7 (updated from 3.2 earlier today)
Platform: macOS (Darwin 25.3.0, arm64), Node v25.7.0
Model Provider: Anthropic (Claude Opus 4.6)

Summary

After a hook-triggered compaction, the API message reconstruction drops tool_result blocks while preserving their corresponding tool_use blocks. This creates a permanently malformed conversation history — every subsequent API call fails with:

messages.97: `tool_use` ids were found without `tool_result` blocks immediately after: call70362952. Each `tool_use` block must have a corresponding `tool_result` block in the next message.

The session becomes permanently unrecoverable. The only fix is /new or /reset, which loses the entire conversation.

Reproduction

This happened twice in one day on the same agent, both times after heavy sessions (~180K tokens) triggered hook-based compaction.

Sequence of events:

1. Session reaches high token count (~180K tokens before compaction)
2. Hook-triggered compaction fires (fromHook: true in compaction entry)
3. Compaction completes — JSONL data is intact (both tool_use and tool_result entries are present and correctly linked via toolCallId)
4. Next API request to Anthropic fails: the reconstructed messages array includes the tool_use block but drops its matching tool_result
5. Every subsequent message fails with the same error — session is permanently dead
6. From the user's perspective: ~1 hour of Discord messages "disappear" and the bot starts posting error messages

JSONL forensics:

• Line 529: Assistant message with tool_use blocks: memory_search(call_70362952) and session_status(call_03536860)
• Line 530: toolResult with toolCallId: call_70362952 (memory_search result) ✅ exists
• Line 531: toolResult with toolCallId: call_03536860 (session_status result) ✅ exists
• Line 523: Compaction event (firstKeptEntryId: 36559378, tokensBefore: 180355)
• Line 535: First failed assistant response — empty content, stopReason: "error"

The data integrity is fine. The bug is in how the kept entries are serialized into the Anthropic API messages array after compaction.

Suspected Cause

The 2026.3.7 release includes postCompactionSections — a new feature that modifies what gets injected after compaction. This is the exact code path where the tool_use/tool_result pairing breaks. Both occurrences today happened after updating to 2026.3.7; this was not observed on 2026.3.2.

Impact

High / Disruptive:

• Complete session loss (all conversation context gone)
• No graceful recovery — must /new or /reset
• User-visible: messages "disappear" from Discord timeline
• Happened 2x in one day on a single agent
• Any session that reaches compaction threshold is at risk

Environment

• Agent: big-brain (primary agent, Opus 4.6)
• Channel: Discord
• Compaction trigger: hook-based (automatic, not manual)
• Session JSONL preserved as .reset file for inspection

Workaround

None currently. /new or /reset is the only recovery, which loses all session context.