dsh-cua

Computer Use for DeepSeek Harness. Control macOS desktop applications from a DSH agent through a persistent JavaScript REPL, backed by the macOS Accessibility API.

The model gets two tools — js and js_reset — and drives everything through them. There is no tool per action; the tool surface stays fixed while capability lives in the sky API.

What it can do

• Read any application's accessibility tree as text, optionally with a screenshot
• Act — click elements or coordinates, type, paste, press key chords, scroll, drag, select text, set values, invoke accessibility actions
• Work incrementally — the tree comes back as a diff after the first read, so a long interaction stays cheap in tokens
• Read screenshots back into the conversation as images

Actions address elements by element_index, derived from the most recent state read. Coordinates are the fallback, not the default.

Requirements

• macOS 14.4 or newer (Apple Silicon or Intel)
• Node.js 20 or newer
• Accessibility permission — required; without it nothing works
• Screen Recording permission — optional; only needed for screenshots

Install

dsh plugin --profile web add dsh-cua

The package ships prebuilt, so nothing is compiled during install. Register the MCP server with your profile — it writes the patch row with this installation's absolute paths:

dsh-cua-setup --write     # add a profile row carrying this install's absolute paths
dsh-cua-setup             # check everything and report what is missing

The package declares dsh.bundle, so dsh plugin add dsh-mac-cua mounts the MCP server from the package's own cordis.patch.yml. That bundled row cannot carry absolute paths — the install location is not known when the package is built — and it cannot be extended from your profile either, because an id-targeted patch replaces a row's config wholesale rather than merging into it. dsh-cua-setup --write supplies the absolute paths, and dsh-cua-setup with no arguments reports whether that has been done.

The bundled row sets failOnStartupError: false on purpose: a misconfiguration then costs you the tool, not the application. The harness still boots.

Manual installs, or overriding the row

An install that has no bundle layer (a bare copy of the source, for instance) needs an explicit row carrying absolute paths. dsh-cua-setup --write adds one; --print shows it without writing; --remove takes it back out. The row goes into ~/Library/Application Support/dsh-desktop/harness/profiles/web/cordis.patch.yml:

- insert:
    - id: mcp-cua
      name: '@deepseek-ai/dsh-mcp-client'
      config:
        serverName: cua_repl
        transport: stdio
        command: node
        args:
          - '/path/to/dsh-cua/cua-repl/server.js'
        cwd: '/path/to/dsh-cua'
        env:
          DSH_CUA_NATIVE: '/path/to/dsh-cua/native/dsh_cua.node'
        toolCallTimeoutMs: 180000
        failOnStartupError: true

The row must nest under insert:. A top-level - id: row is an override of an existing entry, and the loader rejects it with entry "mcp-cua" not found.

Then grant Accessibility so the harness can read and operate app UI:

System Settings → Privacy & Security → Accessibility → add and enable DSH Desktop

For screenshots, also grant:

System Settings → Privacy & Security → Screen Recording → add and enable DSH Desktop

Restart DSH Desktop after granting either permission.

Verify with the agent:

Use mcp__cua_repl__js to run (await import("node:fs")).existsSync ? "ok" : "ok"

or ask it to read the state of an app. A permission problem is reported with the exact settings path rather than failing silently.

Configuration

The bundle ships this row, which mounts the MCP server. dsh-cua-setup --print shows the same row with absolute paths for a manual install:

- insert:
    - id: mcp-cua
      name: '@deepseek-ai/dsh-mcp-client'
      config:
        serverName: cua_repl
        transport: stdio
        command: node
        args:
          - '/path/to/dsh-cua/cua-repl/server.js'
        cwd: '/path/to/dsh-cua'
        env:
          DSH_CUA_NATIVE: '/path/to/dsh-cua/native/dsh_cua.node'
        toolCallTimeoutMs: 180000
        failOnStartupError: true

serverName determines the tool prefix, so the tools appear as mcp__cua_repl__js and mcp__cua_repl__js_reset.

The shipped row uses paths relative to the profile directory, so it works for every install location. To change a setting, add an override row to your profile patch — - id: mcp-cua plus only the fields you want to differ.

Usage

The agent writes JavaScript. State persists between calls.

// Read state. `sky` is injected as a global — do not import it.
var state = await sky.get_app_state({ app: "Finder" });
nodeRepl.write(state.text);

// Act on an element by its index, then read state again.
await sky.click({ app: "Finder", element_index: 42 });
nodeRepl.write((await sky.get_app_state({ app: "Finder" })).text);

// Read a screenshot back into the conversation.
const fs = await import("node:fs/promises");
const { fileURLToPath } = await import("node:url");
var state = await sky.get_app_state({ app: "Finder" });
if (state.screenshot) {
  await nodeRepl.emitImage({
    bytes: await fs.readFile(fileURLToPath(state.screenshot.url)),
    mimeType: "image/png",
  });
}

API

sky.get_app_state({ app, disableDiff?, screenshot? })  // -> AppState
sky.list_apps()                                       // -> App[]
sky.get_screenshot()                                  // -> { url, note }

sky.click({ app, element_index?, x?, y?, mouse_button?, click_count? })
sky.set_value({ app, element_index, value })
sky.select_text({ app, element_index, text, prefix?, suffix?, selection_type? })
sky.perform_secondary_action({ app, element_index, action })
sky.type_text({ app, text })
sky.press_key({ app, key })
sky.paste({ app, text, format })     // "text" | "md" | "html"
sky.scroll({ app, element_index?, x?, y?, direction, pages? })
sky.drag({ app, from_x, from_y, to_x, to_y })

app accepts a display name, bundle identifier, or full path: "Finder", "com.apple.finder", "/Applications/Safari.app".

Behaviour worth knowing

Element indices only mean something for the snapshot that produced them. Every get_app_state publishes a fresh map and invalidates the previous one. A stale index is refused with a message telling the model to re-read — it is never applied to whatever now occupies that position.

paste restores your clipboard. It writes to the pasteboard, presses Cmd+V, then puts your previous clipboard contents back.

\n in type_text presses Return, which submits forms and sends messages in many apps. Use paste for multi-line text.

Typing waits for keyboard focus and reads the field back, so a mismatch is reported instead of silently lost.

Architecture

DSH agent
  │  mcp__cua_repl__js { code }
  ▼
dsh-mcp-client  (ships with DSH; mounted by this package's dsh.bundle)
  │  MCP over stdio, newline-delimited JSON-RPC
  ▼
cua-repl/server.js        (MCP server: js, js_reset)
  │
cua-repl/repl.js          (persistent vm context, completion values)
  │
lib/sky.js                (the sky API)
  │
native/dsh_cua.node       (Swift accessibility core + C N-API binding)
  │  Accessibility API · CGEvent · ScreenCaptureKit · NSWorkspace
  ▼
macOS applications

Layer	Responsibility
native/axcore.swift	AX tree walk, element indexing, text rendering, diffing
native/actions.swift	AX actions, index resolution, focus handling
native/input.swift	CGEvent keys and mouse, clipboard-preserving paste
native/screenshots.swift	ScreenCaptureKit capture, app enumeration, window geometry
native/abi.swift	JSON C ABI consumed by the binding
native/addon.c	N-API binding; every operation runs on a worker thread

Nothing uses AppleScript, JXA, or System Events — they are a different permission path and behave inconsistently.

Building from source

A prebuilt native/dsh_cua.node ships with the package, so this is only needed after editing the Swift sources or if the binary does not match your platform.

node scripts/build-native.js

Needs the Xcode Command Line Tools and Node headers. Point at the headers explicitly when they are not next to your Node binary:

NODE_INCLUDE=/path/to/include/node node scripts/build-native.js

The Swift sources compile with -wmo; cross-file references to internal symbols do not resolve otherwise, because swiftc emits one object per input file when building incrementally.

Tests

node test/run-tests.js    # native module, against a controlled host app
node test/mcp-tests.js    # MCP server, over the real stdio protocol
node test/setup-tests.js  # setup command, bundle patch and profile editing

78 tests in total. The native suite exercises every public sky method directly, including the ones that are easy to leave untested — select_text's caret modes and its prefix/suffix disambiguation, and paste's clipboard restoration.

The native suite drives write actions only against a purpose-built host application (test/host.swift), never against your real apps or data. Checks against real applications are read-only.

Troubleshooting

Tools do not appear. Run dsh-cua-setup first: it reports whether the dependency, the bundle layer, the native module and the permissions are each in place, and names the fix. A config that fails to compose reports entry "<id>" not found, which means a row was written as a bare - id: override instead of nesting under insert:.

"Accessibility is NOT available". Grant it to DSH Desktop under Privacy & Security → Accessibility, then restart DSH Desktop.

Screenshots are null. Read state.screenshotNote. It distinguishes the three causes: the screen is locked (macOS blocks capture of a locked session), the display is asleep (no active display), or Screen Recording is not granted. Only the last needs a settings change.

Screenshots return while the screen is locked? They do not, by design — the note says so. Accessibility-based operation is unaffected.

A click does not register. The tree may be stale, or the element exposes no AXPress. The result message says which path was used, and clicking falls back to the element's centre point automatically.

Some apps expose a poor accessibility tree. Games, canvas editors, and some Electron apps. Get a screenshot and fall back to coordinates; the agent is instructed to tell you when it had to work that way.

Security

This plugin gives the agent the ability to read and operate your desktop UI — the same class of access a screen reader has. Granting Accessibility to DSH Desktop means any process the harness spawns can control your Mac. Grant it only if you intend to use Computer Use.

The agent is instructed by instructions/computer-policy.md to confirm before actions with external side effects: deleting data, sending messages, submitting forms, financial transactions, installing software, changing system settings, and transmitting sensitive data. That policy is prompt-level guidance, not a kernel enforcement boundary — treat the agent as a capable assistant that can act on your machine, not as a sandboxed one.

License

MIT