Context compaction breaks tool_use/tool_result pairing causing API errors

Bug Description

During automatic context compaction in long sessions, Claude Code sometimes removes a tool_use block while keeping its corresponding tool_result block. This breaks the required API pairing and causes the conversation to become corrupted.

Error Message

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.44.content.1: unexpected `tool_use_id` found in `tool_result` blocks: toolu_01KczKp1RFJ7iagyYRDGUPYS. Each `tool_result` block must have a corresponding `tool_use` block in the previous message."}}

Steps to Reproduce

1. Start a Claude Code session
2. Perform many tool calls (100+ over an extended session)
3. Context compaction triggers automatically when context size grows
4. API error occurs when the compacted conversation contains orphaned tool_result blocks

Expected Behavior

Context compaction should treat tool_use and tool_result blocks as atomic pairs:

• Either keep both blocks together
• Or remove both blocks together
• Never orphan a tool_result without its corresponding tool_use

Actual Behavior

The compaction algorithm sometimes removes the tool_use block but keeps the tool_result, creating an invalid conversation structure that the API rejects.

Impact

• Session becomes unusable and must be restarted
• User loses conversation context
• No way to recover the session without starting fresh

Environment

• Claude Code CLI
• macOS (Darwin 25.1.0)
• Long sessions with many tool calls

Workarounds

Currently, the only workaround is to:

1. Start a new session (/clear or restart)
2. Use memory persistence systems to preserve state between sessions
3. Keep sessions shorter to avoid triggering compaction

Suggested Fix

Modify the context compaction algorithm to:

1. Identify all tool_use/tool_result pairs by tool_use_id
2. Treat each pair as an atomic unit during summarization decisions
3. If space constraints require removal, remove the entire pair, not just one half

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Reported via Claude Code session