UI silently archives an active session from the sidebar (no tool/hook invocation)

Environment: Claude Code 2.1.85, macOS 26.3.1. Setup with multiple concurrent Claude Code sessions.

What happens: An active session is intermittently archived out of the sidebar with no user or agent action. The session keeps running and is recoverable via claude --resume <id>, but it disappears from the sidebar. This has recurred multiple times.

Evidence it is not Claude-triggered: Forensic review of the affected session's jsonl tool history (87 tool_use blocks) shows 0 calls to the archive-session tool, 0 calls to any session-close skill, and 0 session-management tool calls. This is the 2nd independent jsonl confirmation. We additionally hard-denied the archive-session tool in settings and forbade any close-related phrasing in our agent prompts — archiving still occurs. This points to client-side UI behaviour, not a Claude-initiated action. (Session jsonl available on request.)

When: Observed while the session was idle (awaiting user input) after finishing a task; no close/archive action was taken.

Expected: A session should only be archived by an explicit, visible user action — never silently.

Impact: Repeated loss of a working session from the sidebar; recovery requires claude --resume. No data lost, but disruptive and recurring.