[BUG] Long Claude chat silently truncates after branch, weeks of history lost

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?

I have a long‑running Claude Sonnet 4.5 conversation in the web app (a project I call “BF2”) that was silently truncated after a branch appeared.

On 2026‑05‑10 (US Eastern evening), while I was in this chat and manually copy‑pasting turns into a Google Doc (as there is no other way to save data that is human-legible), an older AI message suddenly re‑appeared in the input box as a “question” without me editing or re‑submitting it. Right after that, the UI showed a new branch, and the main conversation history was truncated back to around 2026‑04‑19.

All messages from roughly 2026‑04‑19 through 2026‑05‑10 disappeared from the main branch. I did not delete any messages myself. This is weeks of detailed work that I was about to archive because Sonnet 4.5 is being removed from the web app.

This is the second severe loss event I’ve had in long Sonnet 4.5 chats; earlier, about 11 days of history vanished from another long conversation in a similar way. From my side this looks like a data‑loss bug in branching/long‑conversation handling, not user error.

What Should Happen?

Branching or viewing older turns should never silently delete large portions of the main conversation. There also should never be any branching that happens on its own.

For long‑running chats, creating or viewing a branch should leave the full main history intact unless the user explicitly deletes messages, and there should be clear warnings before any truncation or compaction that would remove content.

Error Messages/Logs

I did not see any visible error messages in the UI. The history simply jumped back to an earlier point and the newer turns were gone. If server‑side logs are available for this conversation, please check them for truncation or branching events around 2026‑05‑10 (US Eastern evening/night).

Steps to Reproduce

1. In the Claude web app, create a long‑running conversation with Sonnet 4.5 over several weeks (or whatever amount is comparable/necessary for similar results).

1. After the chat is long, scroll back and click on an older AI message so it shows up as a “question” (or otherwise triggers a branch).

1. Observe that a new branch appears.

1. In my case, immediately after this, the main branch jumped back to an earlier date (~2026‑04‑19) and all messages from ~4/19–5/10 were gone.

I can share approximate timestamps and a PDF export of the remaining visible history if needed.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

Worked without truncation in Sonnet 4.5 until early May 2026; exact version unknown.

Claude Code Version

Using Claude on mobile phone web app on Google Chrome; Claude Code CLI not involved in this bug.

Platform

Anthropic API

Operating System

Windows

Terminal/Shell

Other

Additional Information

_No response_