Let the agent know which device the user is typing from

[mimen]

Problem

I run T3 Code remotely most of the time. The desktop app on one laptop connects to a server on another laptop, and sometimes I use the phone or a browser instead. The agent only knows the machine it runs on. It has no idea which device I'm typing from.

That gets expensive whenever the agent does something that needs the person physically present:

• Opening a browser for an OAuth or CLI login (gh auth login, cloud CLIs) opens it on the host, not in front of me.
• OS permission prompts (macOS TCC, password-manager biometric confirmations) show up on a machine nobody is sitting at, and the turn stalls.
• Computer use, iPhone Mirroring, screenshots of "my screen", pbcopy, and say all act on the wrong machine.
• localhost and file:// links in replies only work when the device is the host.
• From a phone, the agent can't tell it should avoid keyboard-bound flows or keep replies short.
• Scheduled and agent-originated turns look the same as a turn where the user is present.

What the server already knows

Every client connection is an authenticated session with device metadata (AuthClientMetadata: label, device type, OS, IP). The WebSocket handler knows which session each message.dispatch arrives on. The user message only records createdBy: "user", so that information never reaches the turn.

Proposal

1. When a user message is dispatched over a client socket, the server stamps it with an optional sender: the label, device type, OS, and surface of the session it came from, plus sameMachine (loopback connection) and a best-effort hostName (reverse DNS of the client IP, which yields the machine name on Tailscale and most LANs). Clients can't set it. Server, MCP, scheduled, and agent-originated messages never carry it.
2. At turn start, the provider text gets one line ahead of the message, for example <user_device>The user sent this message from "T3 Code Desktop" (macOS desktop, desktop) on another machine (m3.tailnet.ts.net).</user_device>. It is per turn, so it stays correct when the user switches devices mid-thread. The UI is unchanged.

That is enough for agents, skills, and AGENTS.md rules to choose correctly ("only run computer use when the user is on this machine", "link the tailnet URL instead of localhost when the user is remote").

Out of scope: the desktop client presents a fixed label (T3 Code Desktop), so remote desktops are told apart by host name rather than label. Letting the desktop present the OS host name would be a small follow-up if you want it.

I have a working implementation and will link it as a draft PR here. Happy to reshape or drop it if you want a different direction.

[mimen]

Draft implementation: #20112. It stays a draft until the direction here is approved.

[fagnersales]

Up! This can be very useful with little technical debt imo