[BUG] SendMessage tool missing/unregistered in desktop app engine 2.1.260 (cross-session messaging broken)

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?

Environment:

Claude Desktop app (Electron shell): 1.46388.2 (apt, already latest)
Affected internal engine version: 2.1.260 (self-updated by the desktop app on 2026-09-06, at ~/.config/Claude/claude-code/2.1.260/)
Working comparison: standalone CLI claude-code 2.1.236 (apt-installed) — not affected
OS: Ubuntu (Linux 7.0.0-31-generic)
What's broken:
In a session running on the desktop app's 2.1.260 internal engine, the SendMessage tool (used for cross-session messaging between concurrently running Claude Code sessions) is completely unavailable — not erroring, but explicitly listed as a deferred tool with no schema, and ToolSearch is told not to bother looking for it ("no longer available in this session"). This breaks all outbound cross-session communication for any session on this engine build.

Scope observed:
Every currently-running claude-desktop (GUI) session on this machine is on 2.1.260, and every one of them has lost SendMessage. A cli-entrypoint session on the older 2.1.236 engine still has it working fine and can send/receive normally.

Notably: receiving cross-session messages still works on the affected 2.1.260 sessions — a message sent from a working CLI session arrived normally. It's specifically the outbound SendMessage tool registration that's gone, not the underlying transport.

Actual behavior: SendMessage tool is completely absent/deferred-and-unreachable on 2.1.260.

What Should Happen?

SendMessage should be available on 2.1.260 the same as it is on 2.1.236.

Error Messages/Logs

Steps to Reproduce

Run any session through the current desktop app build and check its available tools — SendMessage won't be there.

Claude Model

Sonnet (default)

Is this a regression?

Yes, this worked in a previous version

Last Working Version

_No response_

Claude Code Version

Claude Desktop app (Electron shell): 1.46388.2 (apt, already latest)

Platform

Anthropic API

Operating System

Ubuntu/Debian Linux

Terminal/Shell

Terminal.app (macOS)

Additional Information

I have multiple sessions running at any given time and their cooperation with each other is crucial.