[Feature]: "Restart thread" action to reload provider config (MCP servers, etc.) while keeping conversation context #6877

This would be sick as I have just run into this exact issue here!

➕ On this, would be really helpful

➕

Main now has part of this: Cmd+K "Restart agent session" from #14542 ends the provider session and the next message resumes the same conversation with fresh MCP servers. Thanks for that.

Three gaps remain in daily use:

1. It acts at once. On a running thread it detaches the session mid-turn, so the turn is lost. The Lineage menu's "Disconnect agent session" has the same problem.
2. It lives only in the palette. People look for it in the thread menu, and an agent whose MCP server came up late has no way to restart its own session.
3. Nothing confirms that the restart fixed anything.

I'd like to propose a small follow-up on top of #14542:

• Restarting a running thread marks it, and the next turn ends the old session right before it opens a new one. The current turn finishes first. If background tasks are still running in the session, the restart waits for them.
• "Restart agent session" in the sidebar and chat header thread menus and a /restart composer command, sharing one hook with the palette entry.
• A t3_thread_restart MCP tool and t3 thread restart <thread> CLI command that use the deferred path.
• On Claude, the first turn after a restart shows a short notice with MCP server changes, for example restart-probe 0 → 1 tool.

No setting and no changed default: nothing happens unless someone uses it.

We built this in our fork on current V2 and tested it with a small stdio MCP server that lists no tools until a key file exists. Claude and Codex threads both went from no tool to calling it after a restart. A Codex thread without the restart still had no tool. A headless browser run covered the thread menu, /restart, and the disabled state while a turn runs.

Not checked: Cursor, OpenCode and other providers, and mobile, which this leaves unchanged.

Would you take a PR for this? It is written and tested on our side, and I'll open it once you're happy with the direction and scope.

Written with Claude Opus 5.5 in Claude Code.