API Error: orphaned tool_result blocks after context truncation

Bug Description

Getting a 400 error from the Claude API due to orphaned tool_result blocks in the conversation history:

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.12.content.4: unexpected `tool_use_id` found in `tool_result` blocks: toolu_01NTC5Dcxuc9LdQFXHLeUMW7. Each `tool_result` block must have a corresponding `tool_use` block in the previous message."},"request_id":"req_011CW6srrMDmuwnnoYQSVLUU"}

Root Cause

This appears to happen when context truncation or summarization removes a tool_use block from an assistant message, but the corresponding tool_result block in the following user message is preserved. The Claude API then rejects the request because the tool_use_id reference is dangling.

Suggested Fix

Add a sanitization function that runs before every API call to:

1. Collect all tool_use_id values from tool_use blocks in assistant messages
2. Remove any tool_result blocks from user messages whose tool_use_id doesn't have a matching tool_use block in the immediately preceding assistant message

Environment

• Claude Code CLI
• Error occurs during normal conversation flow after extended usage