SendMessage tool missing in desktop Code tab session while ListAgents shows peers (2.1.258, Windows)

Summary

In a Claude Code desktop app (Windows, Code tab) session, ListAgents lists peer sessions and instructs "send with SendMessage({to: "<name>", message: "..."})", but the SendMessage tool is not present in the session's toolset. ToolSearch with select:SendMessage returns "No matching deferred tools found".

A sibling session on the same machine and same account ("Orchestration", cwd D:\dev\GitHub) did have SendMessage and used it successfully to message the same peer.

Environment

• Claude Code 2.1.258 (claude --version)
• Windows 11 Enterprise 10.0.26200
• Claude desktop app, Code tab (session reports "running within the Claude Agent SDK")
• Model: claude-fable-5-1
• ~/.claude/settings.json: permissions.defaultMode: "dontAsk", crossSessionInbound: "accept", no permissions.deny entries. No project settings gate the tool.

Steps to reproduce

1. Open the Claude desktop app, Code tab, start a session in D:\dev\GitHub.
2. Prompt: ListAgents then SendMessage aria-80 hello.
3. ListAgents succeeds and lists 21 peers, including aria-80 [056652] · interactive · idle.
4. The model has no SendMessage tool to call. ToolSearch("select:SendMessage") finds nothing.

Expected

SendMessage is available whenever ListAgents is available and reports peers, per https://code.claude.com/docs/en/cross-session-messaging ("messaging is on with nothing to enable" once on 2.1.234+ on Windows).

Actual

ListAgents present, SendMessage absent. The desktop-app send_message MCP tool (session_mgmt) is not a substitute: it requires a CCD session id and cannot resolve peer names such as aria-80.

Notes

• Reproduced twice in the same session (tool list does not change mid-session).
• Another session started on the same machine had the tool, so the gate appears to be per-session at startup, not version or settings.