Unrecoverable session after dropped tool_result — rewind/restore/summarize all fail

When a tool_result is dropped mid-conversation, the session enters a permanently unrecoverable state. All built-in recovery mechanisms fail with the same 400 error.

What happened

1. A Bash tool call (git log --oneline | wc -l) executed but returned (No output) — the tool_result appears to have been dropped or malformed
2. Every subsequent message fails: API Error: 400 due to tool use concurrency issues
3. /rewind offers three options: Restore conversation, Summarize from here, Never mind
4. All three fail with the same 400 error
5. The session is permanently unusable — no recovery path works

Root cause

The Anthropic API requires every tool_use block to have a matching tool_result. When a result is dropped, the conversation history becomes structurally invalid. The recovery mechanisms (rewind, restore, summarize) reconstruct the API conversation from the same broken history, reproducing the invalid tool_use/tool_result sequence.

Even amputating 200+ entries from the session JSONL does not fix it, suggesting the broken pair may be embedded in compacted/summarized history that amputation cannot reach.

Expected behavior

At minimum one of these should work:

• /rewind "Restore conversation" should inject a synthetic tool_result for any orphaned tool_use blocks
• /rewind "Summarize from here" should discard the raw history and start fresh from a summary, skipping broken tool pairs
• Claude Code should detect orphaned tool_use blocks before sending to the API and auto-repair them

Impact

• Session is permanently dead — $50+ of context and work history lost
• All commits were on disk (no code lost), but session continuity is destroyed
• Related issues: #20555, #37452, #21321 — but those don't address the recovery failure

Environment

• Claude Code v2.1.84
• macOS
• Model: Opus 4.6