[BUG] Conversation compaction intermittently corrupts conversation state: “Compaction failed unexpectedly” and silent prompt submission failures

[avrilelvina-web]

Preflight Checklist

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

What's Wrong?

Conversation compaction intermittently fails with the error
"Compaction failed unexpectedly".

After the first compaction event, affected conversations enter a broken state:

• New prompts fail to execute and revert to draft
• Prompts with attachments also fail to submit
• Issue reproduces across iOS, web, and desktop
• Issue is conversation-specific (other chats may work normally)

This appears to be a backend conversation-state corruption rather than a client-side or rollout issue.

What Should Happen?

Conversation compaction should complete successfully and allow continued prompt execution.

If compaction fails, the conversation should either:

• Retry safely, or
• Gracefully recover/reset state without blocking further prompts

Error Messages/Logs

UI error: "Compaction failed unexpectedly"

Observed behavior:
- Prompt submits briefly, then reverts to draft
- No execution occurs
- Occurs after compaction event

Steps to Reproduce

1. Start a new long-running conversation.
2. Continue the conversation until automatic compaction triggers.
3. Observe "Compaction failed unexpectedly".
4. Attempt to submit a new prompt:
   • Prompt reverts to draft and does not execute.
5. Attempt to submit a prompt with an attachment:
   • Same failure occurs.
6. Open the same conversation on iOS, web, and desktop:
   • Issue reproduces across all platforms.
7. Start a new conversation:
   • New conversation may work normally.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Before Jan 14–15, 2026 (prior to compaction incident)

Claude Code Version

Unknown (issue reproduced across web/iOS/desktop)

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Windows Terminal

Additional Information

This issue reproduces across platforms:

• Initially observed on iOS (Claude iOS app)
• Then reproduced on web and Claude Desktop
• Same affected conversation fails across all platforms

Behavior:

• After first automatic compaction event, the conversation enters a broken state
• Error shown: "Compaction failed unexpectedly"
• New prompts revert to draft and never execute
• Prompts with attachments also fail to submit
• New conversations may work normally

This suggests conversation-level backend state corruption related to the compaction pipeline, not a client or OS-specific issue.

Issue began around the Jan 15, 2026 compaction incident. Community reports indicate similar intermittent failures.

[github-actions]

Found 3 possible duplicate issues:

1. Both /compact command and auto-compaction broken - 'conversation too long' error #18211
2. [Bug] Auto-Compact returns stale conversation state instead of current context #18089
3. [BUG] Failed to generate conversation summary - response did not contain valid text content #18168

This issue will be automatically closed as a duplicate in 3 days.

• If your issue is a duplicate, please close it and 👍 the existing issue instead
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🤖 Generated with Claude Code

[dwight000]

This is not a duplicate, it is slightly different than the others. From the ticket: "Then reproduced on web and Claude Desktop", there is a fix for Claude code.

[dwight000]

This is still happening 9:43PM EST 1/16/2026. Regarding the error message reported in this ticket, it no longer shows on the UI since a few days ago, but the problem still happens on claude.ai (web) right now, Claude does not accept the message on web. Thinking ON, and Image attachments cause the issue to be reproduced a lot easier.

[braidwalkers]

Happening in Claude chat in projects. Web, desktop app, mobile. Can't access project files.

[avrilelvina-web]

Clarifying: the v2.1.11 fix resolves Claude Code (MCP) compaction, but this issue tracks ongoing conversation-state corruption on claude.ai web, Desktop, and mobile (esp. with Thinking ON and image attachments). Multiple users are still reproducing post-fix. This is not resolved.