dsh-cua

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An MCP server + agent skill for computer use on Windows: accessibility element actions come first and screenshots are only the fallback. It ships a cross-session arbiter — when several agents share one machine it serializes them, and it yields while you are actually using the computer yourself.

This repository contains only the MCP server and the skill. It stays neutral toward any stdio MCP client (dsh / Claude Code / Codex / Cursor / Cline / ZCode …) — nothing here requires dsh.

Platform semantics (0.3.1 and later): the tools only work on Windows — they drive user32/kernel32 and UI Automation. But the package also imports elsewhere and the server starts there, answering tools/list as usual, so any client or directory crawler can enumerate all 19 tools with their full schemas; actually calling a tool returns a clear "requires Windows" error rather than the process failing to start at all. In 0.3.0 the import itself raised, which made such crawlers unable to see the server at all (tests/linux-handshake.py is the regression test for this property, and CI runs it on ubuntu-latest).

What it is

A stdio MCP server exposing 19 tools. Every tool name is prefixed tool_, exactly as tools/list returns it.

• Observe (read-only, callable at any time): tool_skyshot (reads a window as a compact, diffable text tree — three orders of magnitude smaller than a screenshot), tool_element_at_point, tool_read_element, tool_find_elements, tool_capture_window (DPI-aware, cropped to the client area), tool_list_windows / tool_find_window / tool_get_window_rect, tool_list_displays, tool_cursor_position, tool_clipboard_read, tool_coexistence_status
• Element actions (soft gate: serialized across agents, no physical input injected): tool_element_action / tool_element_action_at — press / set_value / select / toggle / expand / collapse / scroll_into_view / focus, delivered straight to the UIA element, so they never steal focus and never care about z-order
• Other mutating calls (soft gate too: the mutex, but no human-contention yield): tool_type_text (targeted PostMessage with effect_verified), tool_clipboard_write, tool_open_application. They synthesize no physical input, so they do not wait for you to stop working — and a clipboard write still destroys whatever you last copied, so announce it when you do.
• Physical input (hard gate: serialized across agents and yields to the human): exactly two things share your one cursor and one keyboard — tool_click_at's raw_event path and tool_send_keys' global hotkeys. The gate waits for the machine to go input-quiet, then refuses with user-active rather than fight you for the cursor. tool_click_at tries its element path first (ax_press), which injects no physical input and therefore takes the mutex only; the receipt's method field says which path actually ran.

"Read-only" here means it takes no mutating action and synthesizes no input, so it is safe to call while someone is using the machine. Two of them have a side effect worth knowing: tool_capture_window writes the screenshot to disk (save_path; a temp file when omitted), and tool_skyshot updates the server-side diff baseline it diffs the next shot against.

A small tree is not proof of an empty window. tool_skyshot and tool_find_elements report minimized, because a minimized window may be showing "nothing is displayed" rather than "nothing is there" — and how much it hides depends on the application, which is why the receipt reports the state instead of guessing at the cause. Measured here: an Edge window minimized before anything read it exposes its browser chrome and no page at all (and include_offscreen does not recover it), Explorer falls from 40 elements to 8 whenever it is minimized, and Notepad is unaffected. Restore the window once and read it again if the result matters.

Every action returns a receipt rather than a self-reported success: action_sent / effect_verified / foreground_changed / user-active / arbiter-busy. "The call was accepted" and "the effect happened" are two different things, and the tool separates them for the agent.

How it differs

There are already several mature open-source Windows implementations. dsh-cua's differences are concentrated on one thing: sharing a machine with a human.

The key distinction is two things that are routinely conflated:

• "No focus steal" is a mechanism guarantee — either a UIA pattern or a targeted PostMessage, so the cursor and keyboard focus are physically never touched. cua-driver has it (ax mode). ahk-mcp does not, and the distinction is narrower than "no UIA": it reads through UIA (ahk_uia_tree / ahk_uia_find / ahk_uia_url), but it has no UIA pattern action — per its README it acts with coordinate clicks or synthetic keys, so an action does move the real cursor.
• "Yield the moment you move" is a timing guarantee — it reads the age of the human's last input via GetLastInputInfo, waits when it sees you using the machine, and on timeout refuses (user-active) instead of barging in. As of 2026-09-25 a pattern search across the other three codebases in that table found no equivalent — that is search evidence, not proof, and it covers input-age detection only: cua-driver does have human-facing guards of a different kind (a consent requirement, and foreground-steal detection with restore).

effect_verified is likewise something the alternatives lack: it splits "the call was accepted" from "the effect happened" and gives three states (true changed as expected / false accepted but unchanged, downgraded to a failure / null no comparable state, i.e. unconfirmed). The usual alternative is to re-observe once after the action and leave the judgement to the model.

What dsh-cua does not do (stated up front to avoid misunderstanding): no grounding of its own — the server does not analyse pixels, so a text-only model cannot drive interfaces that a tree cannot express (canvas, games, remote desktop). With a vision-capable model the pixel path is supported end to end: capture_window returns the image together with a verified image→screen mapping (bounds, scale, dpi_verified), and the model supplies the grounding. Also not provided: record-and-replay, and an isolation sandbox. There are better-suited tools for those.

FAQ

Why not run the agent on a second desktop or a virtual display, so it never touches mine?