Conciseness

The body is lean: command blocks with terse inline comments, one line of necessary context ('A sandbox is a disposable computer: a container or VM...'), and dense parameter documentation ('`--on` is where (`local`, `cloud`, `direct:<addr>`)...'). No concept Claude already knows is explained, and every token carries skill-specific information — matching 'Lean and efficient; assumes Claude's competence; every token earns its place'.

Actionability

Nearly everything is copy-paste ready: complete CLI commands for create/exec/shell/screenshot/vnc/cleanup, a fully executable Python example (including asyncio.run), and concrete MCP tool calls with argument names (e.g. `sandbox_create {"browser": true, "url": ...}`). Specific examples cover the common cases across CLI, SDK, and MCP, matching the top anchor.

Workflow Clarity

The lifecycle is clearly sequenced (Before you start → Create → Use → Clean up) with pre-flight checks (`auth status`, `runtime doctor`), documented error feedback ('An impossible combination fails with `invalid placement` and lists the valid values'), a re-read loop ('read again after every page change: refs are per snapshot'), and an explicit consent checkpoint before `teleport_browser_session`. Destructive operations here target disposable sandboxes — the isolation mechanism itself — so the missing-validation cap for destructive/batch operations does not apply; the content matches 'Clear sequence with explicit validation steps; feedback loops for error recovery'.

Progressive Disclosure

Sections are clear and flat (Create, Use, Clean up, From code, MCP, Browse the web, Rules) with no nested references and one clearly signaled cross-skill pointer ('see the gui-automation skill'), so it is well above anchor 3. It falls short of anchor 5 because there are no bundle reference files at all — everything, including the sizable SDK/MCP/browser detail, is inlined in a ~134-line body that could plausibly be split into one-level-deep references, and the under-50-line simple-skill exception does not apply.

Specificity

The description lists multiple concrete actions — 'Create, use and clean up cua sandboxes', 'browse the web inside one' — and covers both interfaces ('the `cua` CLI or the cua SDK') and both environments ('locally or in the Cua cloud'), giving comprehensive lifecycle coverage. It fits the anchor 'Lists multiple specific concrete actions; comprehensive coverage' rather than anchor 4, since no meaningful capability of the skill is left unmentioned.

Completeness

It explicitly answers both questions: the 'what' ('Create, use and clean up cua sandboxes... and browse the web inside one') and the 'when' via a concrete 'Use when a task needs an isolated machine...' clause with specific trigger scenarios. This is the anchor-5 pattern of clearly and explicitly answering both what AND when with concrete trigger phrases.

Trigger Term Quality

'run code, test an app, drive a desktop GUI, browse or test a website, fill a web form, take a web page screenshot, or reproduce something without touching the user's own computer' comprehensively covers natural phrases a user would actually say, including synonyms ('browse or test a website'). This matches the top anchor for natural-term coverage including variations.

Distinctiveness Conflict Risk

The sandbox/isolated-machine niche is distinct, and potentially overlapping terms (browsing, GUI, screenshots) are explicitly scoped to running 'inside one' sandbox, so conflict with generic browser or GUI-automation skills is minimal. It matches 'Clear niche with distinct triggers; minimal conflict risk'.