Dropped items the composer can't attach should reach the agent on any environment

[YehudaGurovich]

Area

apps/web, apps/desktop, apps/server

Problem or use case

I drag folders and files from my file manager into the composer all day. On the local environment, a dropped folder becomes a path chip (#12001). On a remote environment, the same drop shows "Folders can only be referenced on a server running on this computer" and nothing reaches the agent. Files over 50 MB and some file types are refused on every environment (#17471).

The workaround is to leave T3 Code, copy the item to the host with scp, rsync or Taildrop, and type the remote path by hand. Hosts reached only through T3 Connect often have no inbound SSH, so there is no workaround at all (#31869).

Proposed behavior

One rule for every dropped item that the composer can't attach as an upload (a folder, a file over the attachment limit, or an unsupported type):

• The server can read the path (the client and the server are on the same machine): insert a path chip. No copy. This is today's local folder behavior, extended to large files and unsupported types, which also covers [Bug]: The composer refuses files that Claude Code and Codex accept by path #17471.
• The server can't read the path (any remote environment): the client sends a copy to the server, then inserts a path chip that points to the copy.

The copy:

• Goes over the environment connection that is already open, with the same authorization as attachment uploads. It doesn't depend on how the client reaches the host: local, direct URL (LAN, Tailscale, public), SSH tunnel, or T3 Connect relay.
• Is sent in chunks with a progress chip in the composer and a Cancel action. After a disconnect, it resumes from the last confirmed chunk. Send waits until the copy is complete.
• Is stored next to the existing attachments on the host, under one folder per upload, so agents can already read it. It is cleaned up with the same retention as attachments.
• Skips .git, node_modules and files that the folder's .gitignore excludes. The client asks before it sends more than a set size.
• Works from the desktop app (reads the folder from disk) and from the web client (reads the dropped folder's contents through webkitGetAsEntry, so it doesn't need a path).

Smallest useful scope

Composer drops only, on desktop and web, for every connection type. Out of scope for the first version: drops on the Files pane (#32383) and the terminal (#31869), the mobile app, and sending only the changed files when the same folder is dropped again. These could reuse the same transfer later.

Related

#11961, #12001, #17471, #31869, #32383