cua-driver-rs

name	cua-driver-rs
description	Drive a native GUI app (macOS, Windows, Linux) via the cua-driver CLI (default) or MCP server — snapshot its accessibility tree, click/type/scroll by element_index or pixel coords, verify via re-snapshot, all without bringing the target to the foreground. Use when the user asks you to operate, drive, automate, or perform a GUI task in a real application on the host.

cua-driver-rs

Orchestrates cross-platform app automation via cua-driver. Whenever a user asks to drive a native app, follow the loop in this skill rather than calling tools ad-hoc — the snapshot-before-action invariant is not optional and silently breaks if you skip it.

Platform-specific reading — read this first

This file is the cross-platform core: snapshot invariant, CLI vs MCP choice, tool surface naming, behavior matrix, canonical loop, pixel-click contract, common failure modes. The platform-specific material (forbidden-list, accessibility tree implementation, launch semantics, click dispatch) lives in companion files in this same directory:

• macOS — read MACOS.md (no-foreground contract, forbidden open/osascript/cliclick invocations, AXMenuBar navigation, SkyLight pixel-click dispatch, Apple-Events JS bridge).
• Windows — read WINDOWS.md (UIA tree vs AX, UWP / ApplicationFrameHost hosting, layered UIA+PostMessage click chain, Session 0 isolation, Windows-specific focus-steal vectors).
• Linux — read LINUX.md (X11 background input via AT-SPI + XSendEvent, recording, Wayland opt-in/preview).

Cross-cutting topics also have their own files:

• WEB_APPS.md — browser / Electron / Tauri specifics (sparse AX trees, omnibox navigation, the set_value workaround for minimized windows, tabs-vs-windows guidance).
• RECORDING.md — session recording + replay_trajectory.
• TESTS.md — internal test surface.

Use whichever combination matches the host. When in doubt, run cua-driver doctor — it reports the platform and the right entry point.

The no-foreground principle (cross-platform)

The user's frontmost app MUST NOT change. Every platform has its own list of forbidden commands; the principle is universal:

• macOS: any open invocation, any osascript that mutates GUI state, cliclick, cghidEventTap writes targeting another app's window. Full list in MACOS.md.
• Windows: any Start-Process that triggers a ShowWindow/SetForegroundWindow on the target, WScript.Shell.AppActivate, attaching to the foreground thread for input forwarding. Full list in WINDOWS.md.

If you reach for a command that says "activate", "foreground", "raise", or "make key", stop and translate to the cua-driver tool that does the same intent without focus-stealing.

Defaults — always prefer cua-driver over shell shims

Default transport is the cua-driver CLI — Bash shelling out to cua-driver <tool-name> '<JSON-args>'. MCP tools (prefix mcp__cua-driver__*) only when the user explicitly asks for them. CLI wins because it picks up rebuilds instantly, failures are easier to diagnose, and there's no per-tool schema-load overhead.

Every reference to click(...), get_window_state(...) etc. in this skill means cua-driver click '{...}' — translate to MCP form only when MCP is requested.

Claude Code computer-use compatibility mode

For normal Claude Code use, keep the default CLI or cua-driver MCP server path above. If the user explicitly wants Claude Code's vision/computer-use-style flow, they can register:

cua-driver mcp-config --client claude   # then paste + run the printed line

Observation: Claude Code vision flows appear to treat a screenshot MCP tool as the image-grounding anchor. This compatibility mode keeps the normal CuaDriver tools and changes only screenshot. The compatibility screenshot requires pid and window_id, captures only that target window, and returns the window-local pixel coordinate frame. Start with launch_app or list_windows, then call screenshot({pid, window_id}); do not assume desktop coordinates or a full-screen capture.

Use MCP for this Claude Code vision/computer-use-style path. Do not shell out to cua-driver screenshot as a substitute: CLI screenshots still work as CuaDriver calls, but they do not expose the mcp__cua-computer-use__screenshot tool name that Claude Code appears to use as the image-grounding cue.

Using cua-driver from the shell

Tool names are snake_case, management subcommands are kebab-case — no ambiguity. Tools invoked as cua-driver <tool-name> '<JSON-args>'. Management subcommands:

• cua-driver serve — start persistent daemon (required for element_index workflows; without it each CLI invocation spawns a fresh process and the per-pid element cache dies between calls). macOS users: see MACOS.md for the LaunchServices-routed launch form.
• cua-driver stop / status
• cua-driver list-tools, describe <tool>
• cua-driver recording start|stop|status — see RECORDING.md
• cua-driver check-update [--json] [--no-cache] — read-only "is a newer release available?" probe. Same payload as the check_for_update MCP tool; pair with cua-driver update --apply to install.

Canonical multi-step workflow (example shape — platform-specific launch idioms in the per-OS companion file):

cua-driver serve
cua-driver launch_app '{"bundle_id":"..."}'
# → {pid: 844, windows: [{window_id: 10725, ...}]}
cua-driver get_window_state '{"pid":844,"window_id":10725}'
cua-driver click '{"pid":844,"window_id":10725,"element_index":14}'
cua-driver stop

Agent cursor overlay

Visual cursor overlay for demos and screen recordings. Default: enabled — you do NOT need to enable it. Toggle with cua-driver set_agent_cursor_enabled '{"enabled":true|false}' only to hide or re-show it. A triangle pointer Bezier-glides to each click target, ring-ripples on landing, idle-hides after ~1.5s. Motion knobs: set_agent_cursor_motion takes any subset of start_handle, end_handle, arc_size, arc_flow, spring — tuneable at runtime, persisted to config.

Per-session cursors. Each MCP session automatically owns its own cursor, keyed by the session's id (the proxy mints one session id per MCP connection and the daemon scopes the cursor, config overrides, and recording to it). You normally pass nothing — the session key is wired through for you. Pass an explicit cursor_id only to deliberately share one cursor across sessions. When a session ends (the MCP client disconnects) its cursor is removed automatically.

Visibility caveat (AX runs). On a pure accessibility-action run (clicking by element_index), the first action seeds the cursor on-screen a short distance from the target and plays a brief glide + pulse — not the long Bezier sweep a cursor already on-screen would trace from its previous spot. It's subtle and easy to miss in a recording. If you want a clearly gliding cursor for a demo or screen recording, do a pixel click (click({pid,x,y})) or a move_cursor first to put the cursor on-screen; subsequent AX actions then glide the full path normally.

Requires the daemon process's UI runloop, which cua-driver serve / mcp bootstraps. One-shot CLI invocations skip the overlay entirely.

The core invariant — snapshot before AND after every action

Every action MUST be bracketed by get_window_state(pid, window_id):

• Before — the pre-action snapshot resolves the element_index you're about to use. Indices from previous turns are stale; the server replaces the element index map on every snapshot, keyed on (pid, window_id). Indices from turn N don't resolve in turn N+1, and indices from window A don't resolve against window B of the same app. Skip this and element-indexed actions fail with No cached AX state.
• After — the post-action snapshot verifies the action actually landed. Without it you can't tell a silent no-op from a real effect. The accessibility-tree change (new value, new window, disappeared menu, disabled button, etc.) is your evidence that the action fired. If nothing changed, the action probably failed silently — say so, don't assume success.

This applies to pixel clicks too — re-snapshot after to confirm the click landed on the intended target.

Why window selection is the caller's job now

get_app_state used to pick a window for you via a max-area heuristic that returned the wrong surface on apps with large off-screen utility panels. Concrete reproducer: IINA's OpenSubtitles helper (600×432 off-screen) out-area'd the visible 320×240 player window, so get_app_state(pid) screenshot'd the invisible panel and clicks landed there silently. The new get_window_state(pid, window_id) makes the caller name the window explicitly — the driver validates that the window belongs to the pid and is on the current Space/desktop, then snapshots exactly what was asked for. Enumerate candidates via list_windows or read the windows array launch_app already returns.

Behavior matrix

Perception is mode-agnostic — get_window_state returns BOTH

get_window_state(pid, window_id) returns both the accessibility tree AND a screenshot by default. There is no capture mode to pick and nothing to configure — you ground on the tree and the screenshot together, and you cross-check one against the other. This matters because the tree lies on some surfaces:

• Electron echo-confirms a set_value / type_text against the AX shim while the rendered text view never changed.
• Catalyst (iOSAppOnMac) exposes null / placeholder AXValues.
• Virtualized / off-viewport list rows report bogus frames (an h:1 height, an off-screen origin) for rows that aren't actually laid out.

A grounding screenshot is present by default, so when the tree looks wrong you look at the pixels in the same response — no second capture, no mode flip.

Perf opt-out — include_screenshot. include_screenshot (boolean, default true) is the one knob, and it is a perf knob, not a modality choice. Default returns both (grounding-first). Pass include_screenshot:false to skip the screen grab and get the tree only — the cheap path when you're just re-indexing before an element ax action and don't need to re-ground on pixels. The ax/px decision still lives at action time, not here.

capture_mode is DEPRECATED and ignored. It is still accepted on get_window_state so old callers don't error, but it has no effect — both the tree and the screenshot come back regardless of what you pass (ax, vision, som, anything). There is no ax/vision/som capture choice anymore. Drop the word "vision" for perception entirely. (The tool named screenshot is separate — raw PNG, no AX walk — and unrelated.)

The modality is chosen at ACTION time — ax vs px

You don't pick a capture mode; you pick how you address the target on the action call, and that one choice selects the rung:

• element ax action — pass element_index / element_token. Dispatches through the accessibility rung: AXPress (macOS) / UIA Invoke (Windows) / AT-SPI doAction (Linux). Backgroundable, z-order-independent, and the only driver-verifiable rung.
• element px action — pass x, y. Dispatches through the pixel rung, reading the coordinate straight off the screenshot that's already in the get_window_state response. Best-effort; the caller confirms the effect.

ax↔element_index, px↔pixel x,y. We retired the word "vision" for the dispatch path — it conflated perception with dispatch. Perception is always both; dispatch is ax or px.

The keyboard family has both forms too. type_text, press_key, and hotkey take element_index (ax) or x,y (px) — mutually exclusive, same as the pointer tools. The px form pixel-clicks at (x,y) to establish real renderer focus, then delivers the keystroke(s) to the now-focused element (it reuses click's coordinate translation + delivery_mode). That gives e.g. type_text({pid, window_id, x, y, text}) as a one-call focus-then-type for Chromium/Electron inputs the AX path can't reach, and hotkey({pid, x, y, keys:["cmd","v"]}) to paste into a specific field.

Typing default (the ladder). Call type_text directly with element_index (ax) — it targets the field, no pre-click. On Electron/Catalyst the AX layer echoes the write without rendering it, so the driver returns effect:"unverifiable" + escalation:"px" there (never a false verified:true) — follow it, and cross-check the screenshot in the response (the only ground truth). Escalate to the px form — type_text({pid, window_id, x, y, text}) — which pixel-clicks to focus, then types. If the target control is closed (a search button, a collapsed field), AX-press to open it first (AX actions work in the background): a px focus-click won't reliably open and focus a closed control, so the text leaks into whatever's already focused. Escalate to delivery_mode:"foreground" only if it still drops.

set_value stays AX-only by design — it's for non-text controls (dropdown / AXPopUpButton, checkbox, slider, stepper). Its pixel counterpart is a click/drag on the control, not a "set value at a pixel." So: text → type_text (ax+px); non-text control values → set_value; pixel-manipulate a control → click/drag.

Action responses carry an effect/escalation verdict

Every action response keeps verified (did the driver read back a post-condition?) and adds two machine-readable fields so you know whether — and where — to climb the ladder:

• effect: one of
  • "confirmed" — the driver read back the effect (ax rung only).
  • "unverifiable" — dispatched, but the driver has no handle to read back (every px/CGEvent path; foreground rung). You confirm it off the screenshot — it is not a failure.
  • "suspected_noop" — the ax action likely did nothing (the element didn't actually advertise the action, or you hit a passive label). This is the explicit "cross to px" trigger.
• escalation: {recommended, reason} when the driver thinks you should change rung —
  • "px" — the element isn't really actionable in ax; do an element px action off the screenshot you already have.
  • "foreground" — a background insert/click was dropped on delivery; re-call the same action with delivery_mode:"foreground".

get_window_state itself, when the AX tree comes back empty (a non-AX surface like Electron/Chromium/canvas), returns degraded: true plus the same escalation hint — normally pointing at px (you still have the screenshot from the same call to click off).

Platform nuance for escalation. On Wayland an unfocused window cannot be pixel-targeted in the background (libei → background_unavailable), so there the recommendation is foreground, not px. macOS, X11, and most Windows surfaces can pixel-target in the background, so they recommend px. See LINUX.md / WINDOWS.md.

The verify-then-escalate ladder (algorithm)

Every snapshot already hands you both the tree and the screenshot, so verifying never means "go take a screenshot" — it means cross-check the tree against the pixels you already have, and only change dispatch rung on a real signal. Walk the rungs:

# Rung 1 — element ax action, backgrounded (the cheap default)
get_window_state(pid, window_id)            # tree + screenshot, both, always
resp = click(pid, window_id, element_index) # or type_text / set_value / press_key
get_window_state(pid, window_id)            # re-snapshot — did the tree change?

if resp.effect == "confirmed" and tree changed:
    done                                    # driver-verified

# escalate only on a real signal
if resp.effect == "suspected_noop"
   or resp.escalation.recommended == "px"
   or get_window_state.degraded            # empty tree → non-AX surface
   or the tree looks wrong vs the screenshot:   # e.g. an h:1 / off-viewport row

    # Rung 2 — element px action off the SAME screenshot
    pick the target pixel from the screenshot already in the response
    click(pid, x, y)                        # background pixel — still no foreground
    get_window_state(pid, window_id)        # re-snapshot, eyeball the result
    if it landed: done

# Rung 3 — background delivery was dropped (insert/click never arrived)
if resp.escalation.recommended == "foreground"
   or the px action still did nothing:
    re-call the same action with delivery_mode:"foreground"
    # on Wayland this is the ONLY escalation — px-bg can't target an
    # unfocused window there; see LINUX.md
    verify again

The two ideas to hold onto: (1) the AX tree lies on canvas / web / Catalyst / virtualized surfaces, so an unchanged-or-bogus tree plus suspected_noop/degraded — or a tree that simply disagrees with the screenshot — is your cue to do an element px action off the screenshot you already have; (2) px is a conscious switch to the pixel addressing path, not a different capture.

Window state → what works

state	get_window_state	element-index click (AX/UIA)	press_key commit	pixel click
frontmost	✅	✅	✅	✅
backgrounded / visible	✅	✅	✅	✅
minimized	✅	✅ (actions fire in place)	❌ silent no-op — use set_value or click equivalent	❌ no on-screen bounds
hidden	✅	✅	depends	❌
on another desktop / Space	⚠️ tree may be stripped on some apps — response carries off_space: true so you can detect it	✅	✅	❌ not in current-desktop list

Critical cell — minimized + keyboard commit. The keystroke reaches the app but accessibility focus doesn't propagate to renderer focus on a minimized window. Workarounds in order of preference: set_value to write the field's entire value directly, or element-index-click a commit-equivalent button (Go, Submit, checkbox). Tell the user the window needs to un-minimize only as a last resort.

The canonical loop

start_session(session)            # once per run: declares this run's identity
launch_app(target)
  → pick window_id from the returned `windows` array
    (or call list_windows(pid) separately)
  → get_window_state(pid, window_id)
    → [act]  # every action also takes (pid, window_id) + your `session`
  → get_window_state(pid, window_id) → verify
end_session(session)              # when the run finishes

launch_app now returns a windows array alongside the pid, so the common case collapses to two calls (launch_app → get_window_state) without a separate list_windows hop.

Declare a session. A session is your run's identity — a stable id you choose ("research-1"), declared with start_session and passed as session on every action. It owns your agent cursor (a distinct colour per id), follows the run across any apps/windows, and is the same whether you drive over MCP, the CLI, or the socket. The cursor is opt-in: it appears only once you declare a session (anonymous actions run cursor-less). End with end_session (or the idle-TTL reclaims it).

Concurrent runs/subagents: launch_app is idempotent — two runs that launch the same app get the same instance (and on single-instance apps like Calculator, the same window), so they clobber each other. Give each run its own session (→ its own cursor) AND pass creates_new_application_instance: true to launch_app (→ its own window). The element cache is keyed on (pid, window_id) and the cursor on session, so distinct instances + distinct sessions keep the runs fully separated.

Parallelism vs. ordering. Distinct sessions give distinct cursors, not distinct connections. Subagents that share one cua-driver mcp (stdio) connection have their tool calls serialized by the transport — they take turns, not run in parallel. That's not a correctness problem (session + window isolation means they can't collide), just a throughput one. For genuinely parallel agents, give each its own connection: separate cua-driver mcp processes, or point each agent's MCP client at the daemon's HTTP endpoint (CUA_DRIVER_RS_MCP_HTTP_PORT → POST http://127.0.0.1:<port>/mcp). The daemon serves connections concurrently; per-connection ordering keeps each agent's own sequence (e.g. 3 → + → 1 → =) correct.

list_apps is for app-level discovery (answering "what's installed / running / frontmost?") — not part of the core action loop. Skip it in the loop. For window-level questions — "does this app have a visible window?", "which desktop is this window on?", "which of this pid's windows is the main one?" — call list_windows instead; the app record doesn't carry window state on purpose. In the common single-window case you can skip list_windows entirely and read the windows array that launch_app already returned.

Snapshot and act by element_index

Call get_window_state({pid, window_id}) with the window_id from launch_app's windows array (or a fresh list_windows({pid}) if you're interacting with a long-lived process). It returns the tree and the screenshot together by default, so you can both dispatch by element_index and ground on pixels from one call — no config change, no mode flip. When you're just re-indexing before an element ax action and don't need fresh pixels, pass include_screenshot:false to skip the grab (a perf knob, not a modality choice).

The response carries:

• tree_markdown — every actionable element tagged [N]. That N is the element_index. The tree can be very large (Finder is ~1600 elements, ~190 KB); when it exceeds token limits the MCP harness saves it to a file and returns the path. Use Bash + jq -r '.tree_markdown' + grep to pull the section you need.
• effect / escalation / degraded — the verify-then-escalate signals (see the behavior matrix above): degraded: true means the tree came back empty (non-AX surface), so you act by px off the screenshot in the same response.
• screenshot_file_path — present when the screenshot was written to disk instead of inlined (you passed screenshot_out_file, or the context-saving CLI path); otherwise the frame is inlined.
• screenshot_width / _height / _scale_factor — dimensions of the captured image. Present whenever a screenshot was taken (i.e. unless you passed include_screenshot:false).

Getting the screenshot as a file (CLI and context-constrained agents):

# write to file — stdout stays readable (AX/UIA tree / summary only, no base64)
cua-driver get_window_state '{"pid":N,"window_id":W,"screenshot_out_file":"/tmp/shot.jpg"}'

# CLI --screenshot-out-file flag is equivalent
cua-driver get_window_state '{"pid":N,"window_id":W}' --screenshot-out-file /tmp/shot.jpg

Pass screenshot_out_file when using get_window_state via CLI or from an agent whose context window can't absorb ~31 KB of inline base64 (e.g. OpenCode with a local Ollama model). The MCP image content block is omitted from the response when this param is set — the model receives only the tree and screenshot_file_path, then reads the image from disk.

The tree and the screenshot are complementary, not redundant — and they come from the same call. Each half carries signal the other can't, which is exactly why you cross-check them:

• The tree tells you what's clickable — roles, labels, element_index handles, advertised actions, parent-child structure. This is the ground truth for an element ax action.
• The screenshot tells you which one — the tree often has many buttons with similar or empty labels ("Delete", "OK", anonymous UUID-labeled buttons, repeated static-text), and visual context disambiguates. Captions, colors, layout relationships visible in pixels often don't show up in the tree at all (especially in Chromium / Electron / web content) — and the screenshot is where you catch the tree lying (an h:1/off-viewport row, a Catalyst null value).

Default to dispatching by element_index (the element ax action) — it's the verifiable, backgroundable rung. Do an element px action (x,y off the same screenshot) when the tree can't disambiguate (repeated/empty labels), when it's empty (degraded — non-AX surface), when an action came back suspected_noop, or when the tree disagrees with the pixels. You never re-capture to switch — the screenshot is already there; you just change how you address the target.

Reach for pixel coordinates only when the target is a canvas / video / WebGL / custom-drawn surface that isn't in the tree (see "Pixel-coordinate clicks" below).

The actions=[...] list on each element is advisory, not authoritative. cua-driver does not pre-flight check against it — click({pid, element_index}) always attempts the default action (or the action you pass) and surfaces whatever the target returns. Try the click first — pivot only on the returned error code.

Tool dispatch table

Every row assumes a (pid, window_id) pair from the last get_window_state; window_id is required alongside element_index, ignored on pixel-only forms unless you want to anchor the conversion against a specific window.

Intent	Tool	Notes
List an app's windows	list_windows({pid})	returns window_id, title, bounds, z_index, is_on_screen, on_current_space. Already included in launch_app's response — only call this for long-lived pids
Snapshot a window	get_window_state({pid, window_id})	returns tree_markdown + screenshot_*; populates the (pid, window_id) element_index cache
Left click	click({pid, window_id, element_index})	default action: "press". Pixel form: click({pid, x, y}) (window_id optional) — modifier: ["cmd"|"ctrl"]
Double-click / open	double_click({pid, window_id, element_index})	Default action when the element advertises one (Open on Finder items / openable rows), else stamped pixel double-click at the element's center
Right click / context menu	right_click({pid, window_id, element_index}) or click({pid, window_id, element_index, action: "show_menu"})	Chromium web-content coerces pixel right-click to left on macOS — see WEB_APPS.md
Type at cursor	type_text({pid, text, window_id, element_index}) (ax) or type_text({pid, text, window_id, x, y}) (px)	ax focuses the element then writes via the platform's text-set primitive; px pixel-clicks (x,y) to focus the renderer, then types — the one-call fix for Chromium/Electron inputs the AX path can't reach
Set whole non-text control value	set_value({pid, window_id, element_index, value})	AX-only by design — dropdown/AXPopUpButton, checkbox, slider, stepper; also the keyboard-commit workaround on minimized windows. For text use type_text; to pixel-manipulate a control use click/drag
Scroll	scroll({pid, direction, amount, by, window_id, element_index})	synthesizes per-pid PageUp/PageDown/arrows
Focus + send key	press_key({pid, key, window_id, element_index, modifiers}) (ax) or press_key({pid, key, x, y}) (px)	ax element_index sets focus then posts the key; px pixel-clicks (x,y) to focus, then sends the key
Send key to pid	press_key({pid, key, modifiers})	no focus change; key goes to pid's current focus
Modifier combo	hotkey({pid, keys}) (no focus) or hotkey({pid, x, y, keys}) (px)	e.g. ["cmd","c"] / ["ctrl","c"]; posted per-pid, not HID tap. px pixel-clicks (x,y) to focus a field first, e.g. ["cmd","v"] to paste into it

All keyboard/text primitives require pid. There is no frontmost-routed variant — every key goes to the named target via the platform's per-pid event-post path, so the driver cannot leak keystrokes into the user's foreground app.

Why element_index is the primary path: works on hidden / occluded / off-desktop windows, no focus steal, stable across rebuilds, labels tell you what you're clicking. Reach for pixel coordinates only when the accessibility tree can't.

Cross-platform parameter contract

The capture, dispatch, and addressing params — session, delivery_mode, capture_mode (deprecated/ignored — see the behavior matrix; still in the schema only so old callers don't error), scope, modifier, button, element_index, element_token — are a shared schema contract: identical shape (type/enum/items) on macOS, Windows, and Linux. They compose from canonical fragments in cua-driver-core::tool_schema (+ capture_mode), and a CI gate (schema_consistency_test) runs every tool's live tools/list through a structural checker on each platform, so the three surfaces can't silently drift. Contributor note: when you add or edit one of these shared params on a tool, pull from the fragment — don't re-hand-write the JSON, or the gate fails. (Descriptions may legitimately vary per tool; the gate compares shape, not prose.)

Two consequences for callers:

• session is accepted on every action and cursor tool, on all three platforms. It's cursor-wired where the platform glides a cursor and schema-accepted everywhere else — so the same session you pass on macOS is no longer rejected by Windows/Linux, which previously refused unknown keys via additionalProperties:false.
• delivery_mode ("background" default / "foreground") is on the whole input family — click, double_click, right_click, drag, scroll, type_text, press_key, hotkey — uniformly. The foreground rung briefly fronts the target, acts, then restores the prior frontmost: the explicit last resort when a background attempt didn't land. foreground is a reaction, never a prediction. Always fire the background default first and let the driver tell you it can't (a background_unavailable error or escalation.recommended == "foreground") — or observe a verified no-op — before you escalate. Do not reason "it's a GTK/Chromium/Electron app, so background will drop, so I'll front up-front": the toolkit lists in the tool schemas are the driver's internal detectors, not a checklist for you to front on a guess. (Concretely: GIMP's GTK toolbox accepts background pixel clicks fine — a preemptive foreground click there just steals the user's focus for nothing.) What each platform's background rung can actually carry differs (e.g. a Windows background click can't carry modifier state — see WINDOWS.md); the schema is uniform, the residual limits are per-OS.

Required-set contract. click requires nothing (required:[]), scroll requires ["direction"], zoom requires ["window_id","x1","y1","x2","y2"] — same on every platform. pid is conditionally required (needed unless a windowless desktop-scope call) and validated in code with a clear error, NOT pinned in the schema — so omitting pid for a desktop-scope action is no longer schema-rejected.

Genuinely platform-specific params stay OUT of the shared contract by design (launch-app identifiers, the Windows-only debug_window_info, the macOS-only check_permissions.prompt). The per-OS files list the residuals that matter when you drive on that platform.

Pixel-coordinate clicks

The pixel path (click({pid, x, y})) is for surfaces the accessibility tree doesn't reach — canvases, video players, WebGL, custom-drawn controls. Coords are window-local screenshot pixels (same space as the PNG get_window_state returns). Top-left origin, y-down. The driver handles screen-point conversion internally. Passing window_id alongside x, y is optional but recommended — it pins the coordinate conversion to the window whose screenshot produced the pixel.

PNGs returned by get_window_state are capped at 1568 px long-side by default (max_image_dimension config), matching Anthropic's multimodal-vision downsampling limit. The image the model reasons over and the image the click tool's coordinate system lives in are the same resolution — just look at the PNG, pick a pixel, click at that pixel. No scaling math.

This is the default because the mismatch between "rendered thumbnail" and "native PNG" was a recurring coord-estimation footgun. If you opt out (explicit max_image_dimension=0 for pixel-perfect verification flows), the old rule applies: don't eyeball coords from whatever your client renders — it may be 2-4× smaller than the PNG on disk, and a 2% error in thumbnail space becomes ~80 px in the real image.

For precise targeting on small / dense UIs:

1. get_window_state({pid, window_id}) → image capped at 1568 long-side plus screenshot_width / screenshot_height. Write to disk via --screenshot-out-file <path>.
2. Look at the PNG. Since it matches what you see, pick the target pixel directly.
3. When precision matters, draw a crosshair on the image (do not crop — cropping loses the coordinate system) and verify before clicking:

from PIL import Image, ImageDraw
img = Image.open('/tmp/shot.png')
draw = ImageDraw.Draw(img)
x, y = <your_coordinate>
r = 18
draw.ellipse([x-r, y-r, x+r, y+r], outline='red', width=4)
draw.line([x-30, y, x+30, y], fill='red', width=3)
draw.line([x, y-30, x, y+30], fill='red', width=3)
img.save('/tmp/shot_annotated.png')

4. Only dispatch the click after the user (or your own re-read of the annotated image) confirms the crosshair is on target.

Addressing variants:

• click({pid, x, y}) — single left-click.
• click({pid, x, y, count: 2}) — double-click.
• click({pid, x, y, modifier: ["cmd"\|"ctrl"]}) — modifier click. Accepts any subset of cmd/shift/option/alt/ctrl.
• right_click({pid, x, y}) — also takes modifier.

The pixel path animates the agent cursor overlay but never warps the real cursor (the per-pid event paths the driver uses on macOS and Windows route around HID synthesis). If the pid has no on-screen window the call errors with pid X has no on-screen window — you need a visible window to anchor the conversion. Dispatch details (SkyLight on macOS, layered UIA+PostMessage on Windows) are in the per-OS companion files.

Web-rendered apps (browsers, Electron, Tauri)

For Chrome / Edge / Brave / Arc / Safari, Electron apps (Slack, VSCode, Notion, Discord), and Tauri apps — see WEB_APPS.md.

Covers: sparse accessibility tree population (retry-once pattern for Chromium), URL navigation via omnibox suggestions, the set_value workaround for keyboard commits on minimized windows (Return silently no-ops — symptom is a system bell; use set_value or click a clickable equivalent), scrolling via synthetic PageUp/Down keystrokes, in-page clicks, and typing into web inputs.

Browser JS primitives are now cross-platform via the page tool — macOS uses Apple Events for Chrome/Brave/Edge/Safari + CDP for Electron (see MACOS.md); Windows + Linux use UIA / AT-SPI for get_text / query_dom and the shared CDP client for execute_javascript (browser must be launched with --remote-debugging-port=N and the port exported as CUA_DRIVER_CDP_PORT).

Re-snapshot and verify — mandatory

Always call get_window_state({pid, window_id}) after the action. This isn't optional verification — it's the second half of the snapshot invariant.

Check the tree diff: a changed value, a new element, a new window, or the disappearance of the thing you just clicked (menus collapse after selection, buttons may become disabled, etc.). The re-snapshot gives you both the tree and the screenshot, so you cross-check the tree diff against the pixels in one call — and when you're only confirming a tree change, include_screenshot:false skips the grab.

Switch to an element px action only on a real signal: the action response carried effect:"suspected_noop", the re-snapshot came back degraded (empty tree → non-AX surface), the tree looks unchanged/unreadable or disagrees with the screenshot, or escalation.recommended points you there (px). That's the verify-then-escalate ladder in the behavior-matrix section. If the tree is unchanged AND the screenshot confirms nothing moved, the action likely failed silently — tell the user what you attempted and what you observed, don't paper over with "done" language (and consider delivery_mode:"foreground" when escalation.recommended == "foreground"). Agents that skip this step report success on silently-dropped actions — the single most common failure mode.

Recording trajectories

Session-scoped action recording + replay, for demos, regressions, and training data. Only invoke when the user explicitly asks to record a session — the skill does not auto-enable this. CLI surface: cua-driver recording start|stop|status; raw tools: start_recording / stop_recording. Video capture (main display → recording.mp4) is on by default; pass record_video: false to opt out.

See RECORDING.md for the full flow: enable/disable, turn folder contents, replay via replay_trajectory, and the element_index doesn't-survive-across-sessions caveat.

Common error patterns (cross-platform)

Error text	Meaning	Fix
No cached AX state for pid X window_id W	You either skipped get_window_state this turn, or passed a different window_id to the click than the one the snapshot cached against	Call get_window_state({pid: X, window_id: W}) first — the same window_id you intend to click in
Invalid element_index N for pid X window_id W	Index is stale or out of range	Re-run get_window_state with the same window_id, pick a fresh index from the new tree
window_id W belongs to pid P, not …	Passed a window_id that's owned by a different process	Use list_windows({pid: X}) to enumerate this pid's own windows
AX action … failed with code … / UIA invoke failed	Element doesn't support the default action	Try show_menu, confirm, cancel, pick, or fall through to a pixel click on the element's center
The user doesn't want to proceed with this tool use. The tool use was rejected …	The harness uses this exact string for BOTH a permission-prompt denial AND a manual interrupt (Esc / stop) of a running tool — they are indistinguishable from the tool result	Treat as "tool canceled, no result, await the user." Do NOT paraphrase ("you stopped me") — quote the literal message and name the canceled tool + its args, so the user can tell what was in flight vs. what landed

Platform-specific errors (TCC dialogs on macOS, Session 0 / UAC prompts on Windows, AT-SPI bus issues on Linux) live in their respective companion files.

Things to avoid

• Never reuse an element_index across a re-snapshot of the same pid.
• Don't conflate the two addressing modes. The tree gives you element_index handles; the screenshot (same call) gives you the pixel frame. An element ax action addresses by index, an element px action by x,y. Default to element_index and only do a px action on a real signal (suspected_noop / degraded / repeated labels / tree-disagrees-with-pixels). Don't pass an element_index you read off the screenshot, and don't pixel-click a coordinate you computed from the tree's (possibly lying) frame without checking it against the image.
• Prefer accessibility actions over pixels. click({pid, x, y}) works for canvas / WebView regions, but it lands blindly on raw coordinates. Exhaust accessibility paths (menu bars, cmd-k palettes, toolbar items, keyboard shortcuts) before dropping to coordinates. (The AX path does not skip the agent-cursor overlay — it seeds and pulses the session cursor and draws a focus rect on the targeted element; it just doesn't play a long glide on the very first action. See "Agent cursor overlay" for the demo-recording caveat.)
• Never drive destructive actions (delete files, close unsaved documents, send messages, submit forms) without explicit user intent for that specific destructive step.
• Never launch apps autonomously; confirm with the user first unless their original request clearly implies the launch.

Example end-to-end task

User: "Open the Downloads folder in the system file manager."

1. launch_app({bundle_id: "com.apple.finder", urls: ["~/Downloads"]}) on macOS, or launch_app({name: "explorer", args: ["%USERPROFILE%\\Downloads"]}) on Windows. Returns {pid, windows: [{window_id, title, ...}]}. Idempotent launch; the driver opens a hidden window via the platform's launch primitive — zero activation, no focus steal.
2. get_window_state({pid, window_id}) → verify the expected window title is present with a populated tree (sidebar, list view, files).
3. Done.

Platform-specific examples and edge cases (Finder menu navigation, Explorer ribbon, GNOME Files) live in the per-OS companion files.