[BUG] Conversation Compaction Corrupts Tool Use/Result Pairs

Preflight Checklist

• [x] I have searched existing issues and this hasn't been reported yet
• [x] This is a single bug report (please file separate reports for different bugs)
• [x] I am using the latest version of Claude Code

What's Wrong?

Description

After conversation compaction occurs, subsequent API calls fail with a 400 error indicating orphaned tool_result blocks without corresponding tool_use blocks in the previous message.

Error Message

API Error: 400 {"type":"error","error":{"type":"invalid_request_error","message":"messages.2.content.3: unexpected `tool_use_id` found in `tool_result` blocks: toolu_01Mhh3CD3BeVEU2mUPenc1X3. Each `tool_result` block must have a corresponding `tool_use` block in the previous message."},"request_id":"req_011CTeiXF7oECCgLbvci5oey"}

Steps to Reproduce

1. Start a Claude Code conversation
2. Perform many tool uses (Read, Write, Bash, etc.) over an extended conversation
3. Wait for automatic conversation compaction to trigger (likely based on token threshold)
4. After compaction, attempt to continue the conversation
5. Observe API 400 error about mismatched tool_use_id

Expected Behavior

Conversation compaction should maintain the integrity of tool_use/tool_result pairs by either:

• Removing both the tool_use AND corresponding tool_result blocks together, OR
• Keeping both blocks intact

Actual Behavior

Compaction appears to remove tool_use blocks while leaving orphaned tool_result blocks, which violates the Claude API requirement that every tool_result must have a corresponding tool_use in the previous message.

Impact

• Forces users to start a new conversation, losing context and flow
• Conversation history becomes corrupted and unusable
• Disrupts multi-step workflows and long-running tasks

Workaround

Start a new conversation when this error occurs. There is no way to recover the corrupted conversation.

Environment

• Claude Code Version: [Current version]
• Platform: Linux
• OS Version: Linux 6.8.12-11-pve
• Date Observed: 2025-09-30

Additional Context

This issue appears to be related to the conversation compaction algorithm that reduces token usage. The compaction logic needs to ensure atomic removal/retention of tool_use/tool_result pairs to maintain API contract compliance.

Suggested Fix

The compaction algorithm should:

1. Identify tool_use/tool_result pairs by matching tool_use_id
2. Remove pairs atomically (both blocks together or neither)
3. Validate message structure after compaction to ensure no orphaned tool_result blocks remain

What Should Happen?

Conversation compaction should maintain the integrity of tool_use/tool_result pairs. When compacting a conversation to reduce token usage, the algorithm must either:

1. Remove both the tool_use block AND its corresponding tool_result block atomically (as a matched pair), OR
2. Keep both blocks intact

After compaction, the conversation should remain valid and usable, with all tool_result blocks having their corresponding tool_use blocks in the previous message. The conversation should continue without API errors.

Error Messages/Logs

Steps to Reproduce

1. Start a new Claude Code conversation
2. Perform multiple tasks that require many tool uses (Read, Write, Bash, Edit, etc.) - approximately 50+ tool calls
3. Continue the conversation until it reaches the token threshold that triggers automatic conversation compaction
4. After compaction occurs (you may see a brief pause or indication), attempt to continue the conversation with another request
5. Observe the API 400 error: "unexpected tool_use_id found in tool_result blocks: [id]. Each tool_result block must have a corresponding tool_use block in the previous message"

Note: The exact token threshold for compaction is unknown, but this typically occurs in longer conversations with extensive tool usage.

Claude Model

None

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Claude Code version 2.0.1

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Non-interactive/CI environment

Additional Information

_No response_