Conciseness

The body is lean and command-first: every section is copy-paste commands, flag definitions, or rules — e.g. "cua sb create linux --name dev # local (default): a gVisor container" and "`--on` is where (`local`, `direct:<addr>`)". It assumes Claude's competence (one-line definition of a sandbox, no explanation of what containers/VMs are), matching 'Lean and efficient; assumes Claude's competence; every token earns its place'. A 4 would require identifiable over-explanation, and none is present.

Actionability

Fully executable throughout: concrete CLI invocations ("cua sb exec dev -- uname -a"), a complete runnable Python example ending in asyncio.run(main()), MCP tool calls with argument shapes ("sandbox_create {\"browser\": true, \"url\": ...}"), and terminal equivalents ("cua sb create --browser --open https://example.com"). This matches 'Fully executable; copy-paste ready code or commands; specific examples cover the common cases' — create, use, cleanup, code, and MCP paths are all covered.

Workflow Clarity

The lifecycle is clearly sequenced (Before you start → Create → Use → Clean up) and the browser workflow is a numbered 5-step sequence with per-step guidance ("read again after every page change: refs are per snapshot"). Checkpoints are mostly present — "cua runtime doctor" preflight, a documented failure mode with remedy ("fails with `invalid placement` and lists the valid values"), user-consent gating for logged-in sites, and "open it once to check it works before sending" — but a few are implicit, e.g. no explicit step to verify a sandbox is healthy after create. That matches 'Clear sequence with most checkpoints present; minor validation gaps' rather than the fully explicit feedback-loop anchor at 5. The destructive/batch cap does not apply: deletion targets disposable sandboxes, not user data, and the skill's core rule is to keep destructive operations off the user's machine.

Progressive Disclosure

The bundle has no reference/script/asset files, and the single SKILL.md is well-organized into short, clearly headed sections at overview level, delegating GUI detail elsewhere ("use `cua do` (see the gui-automation skill)"). This matches 'Good structure; most content is appropriately placed... minor organization gaps': the ~30-line MCP browser-driving section is denser than the rest and could live in a reference file, and the gui-automation pointer is a bare mention rather than a well-signaled link. Not 5 because the over-50-line self-contained file with an inline MCP detail block doesn't fully meet 'content appropriately split', and not 3 because nothing that clearly belongs in a separate file is inlined.

Specificity

Quotes multiple concrete actions — "Create, use and clean up cua sandboxes", "browse the web inside one", and task-level actions like "take a web page screenshot" and "fill a web form" — with no filler. This matches the anchor 'Lists multiple specific concrete actions; comprehensive coverage' (like 'Extract text and tables from PDF files, fill forms, merge documents, convert pages to images'); a 4 would require minor gaps in coverage, but create/use/cleanup plus browsing covers the skill's full lifecycle.

Completeness

Explicitly answers both questions: what ("Create, use and clean up cua sandboxes... with the `cua` CLI or the cua SDK, and browse the web inside one") and when ("Use when a task needs an isolated machine to run code, test an app, drive a desktop GUI..."). This matches the anchor 'Clearly and explicitly answers both what AND when with concrete trigger phrases' — the 'Use when' clause is present and enumerates concrete triggers, so the missing-trigger cap of 3 does not apply and a 4 would require a less explicit 'when'.

Trigger Term Quality

Natural user phrases are well covered: "run code", "test an app", "drive a desktop GUI", "browse or test a website", "fill a web form", "take a web page screenshot", "isolated machine". Missing common synonyms users might say — "VM", "container", "disposable/throwaway machine", "clean environment" — so it fits 'Good keyword coverage; a few natural terms missing' rather than the comprehensive-synonyms anchor at 5.

Distinctiveness Conflict Risk

The niche is distinct — "cua sandboxes (disposable Linux or macOS computers) on this computer with the `cua` CLI or the cua SDK" and "without touching the user's own computer" clearly scope it away from generic skills. Minor overlap risk remains with browser-automation or screenshot skills via triggers like "browse or test a website" and "take a web page screenshot", matching 'Mostly distinct; minor overlap risk with closely related skills' rather than the minimal-conflict anchor at 5.