[BUG] URGENT! EVERY CONVERSATION SEVERED -- Cowork: Live session severs mid-conversation — error on reply, then Claude receives message with zero prior context (macOS)

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
During an active Cowork session (not across restarts, not across tab switches), replying to Claude's most recent message produces an error indicating the conversation cannot be found or recognized. After dismissing the error and resubmitting the same message, Claude receives it as the first message of a brand new session. All prior conversation context is gone. Claude has no memory of anything said in the session before that point.
This is not a UI rendering issue. The conversation history is not hidden; it is severed. Claude explicitly confirms it has no record of anything prior to the resubmitted message. The session identity itself is lost mid-conversation.

What Should Happen?

The reply should be appended to the existing conversation. Claude should receive it with the full conversation history intact and respond in context.

Error Messages/Logs

"No conversation found with session ID: a8201067-b6a8-4d39-a6be-e7c604124935

You can restart the conversation from an earlier message."

I now get an error like this with *EVERY SINGLE CONVERSATION* !!

Steps to Reproduce

1. Open a Cowork session with a mounted project folder (macOS).
2. Have a multi-turn conversation with substantial tool use (file reads, subagent delegation, memory writes, file creation). Session was approximately 60-90 minutes in.
3. Claude sends a long response with analysis and file creation.
4. User types a reply to that response.
5. UI throws an error indicating the conversation is not recognized / cannot be found.
6. User resubmits the same message.
7. Claude receives the message with zero prior context and treats it as the start of a new session.

Claude Model

Opus

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Can't find it

Platform

Anthropic API

Operating System

macOS

Terminal/Shell

Terminal.app (macOS)

Additional Information

Compound Failure with #42890 / #37273
This bug compounds with the known UI truncation issue (#42890) and compaction-triggered scrollback loss (#37273). In sessions where both occur:

The user has already lost visibility into the earlier portion of the conversation (UI truncation / compaction).
Claude still had that history and could be asked about it (a viable workaround).
When the session identity severs, Claude also loses the history.
Neither party retains the conversation. The data is unrecoverable.

This is a full data loss scenario, not a rendering bug. The workaround for #42890 (asking Claude to recall what the user can no longer see) is destroyed by this bug.