Summary

When an assistant message errors mid-stream during a tool_use call (e.g., web_fetch timing out), the transcript repair logic can produce an orphaned tool_result block with no matching tool_use in the preceding assistant message. This causes every subsequent API request to fail with a 400 error, permanently breaking the session until manual JSONL surgery is performed.

Environment

Clawdbot version: 2026.1.24-3
Provider: Anthropic (claude-opus-4-5)
Channel: Discord
OS: Ubuntu Linux (Hetzner VPS)

Steps to Reproduce

Agent calls web_fetch (or any tool) during a response
The API call errors mid-stream (stopReason: "error")
The assistant message with the tool_use block is partially persisted to the session JSONL
Clawdbot's transcript repair injects a synthetic tool_result for the errored call
On next turn, the Anthropic API sees the tool_result referencing toolu_XXXX but the corresponding tool_use block is missing from the assistant message (because it was only partially written)
API returns 400: unexpected tool_use_id found in tool_result blocks
Every subsequent message hits the same 400 — the session is permanently broken

Expected Behavior

The transcript hygiene / sanitization logic should:
Detect orphaned tool_result blocks (tool_result with no matching tool_use in the preceding assistant message)
Either remove the orphaned tool_result, or inject a synthetic tool_use to match it
Prevent the session from entering an unrecoverable state

Actual Behavior

The session enters a permanent error loop. The only recovery is manual truncation of the session JSONL file.

Workaround

Locate the session file at ~/.clawdbot/agents/main/sessions/.jsonl, find the line number of the broken entry, and truncate:

cp session.jsonl session.jsonl.bak
head -n session.jsonl.bak > session.jsonl

Notes

The transcript hygiene docs indicate that Anthropic provider policy includes "tool result pairing repair and synthetic tool results" — but this appears to only handle the case of missing tool_results (tool_use without a result), not the reverse case of missing tool_use (orphaned tool_result). The reverse case should be covered too.

This issue was written collaboratively with my Clawdbot agent (Claude). The bug was encountered and manually debugged by a human; the writeup was drafted by the agent based on the diagnosis.