WebMCP Kit — verify

Runtime check that a site's WebMCP tools register and work. Runs standalone, and is the ladder the implement skill uses in its Phase F.

Before you start

• The site must run locally. Use its own runbook (lifecycle commands, base URL, test identities) if it ships one; otherwise its own dev script. Never assume a harness exists.
• Use the browser driver shipped in this plugin. Do not ask for MCP configuration, a global chrome-devtools install, Playwright, or manual browser driving.

Packaged browser driver

Resolve the plugin root once. Use CLAUDE_PLUGIN_ROOT or PLUGIN_ROOT when the host exports one. Otherwise, take the absolute base directory shown for this skill (the directory containing this SKILL.md) and go up two directories. Do not guess from the repository checkout.

Run one driver session for the whole ladder:

plugin_root="${CLAUDE_PLUGIN_ROOT:-${PLUGIN_ROOT:-}}"
if [ -z "$plugin_root" ]; then
  skill_dir="<absolute base directory for this skill>"
  plugin_root="$(cd "$skill_dir/../.." && pwd -P)"
fi
webmcp="${plugin_root}/scripts/webmcp.sh"

"$webmcp" browser status
"$webmcp" browser start
"$webmcp" browser status
"$webmcp" browser new_page "<local URL>" --output-format=json
"$webmcp" browser list_webmcp_tools <pageId> --output-format=json
"$webmcp" browser execute_webmcp_tool <pageId> <toolName> --input '<JSON object>' --output-format=json
"$webmcp" browser take_snapshot <pageId> --output-format=json
"$webmcp" browser list_console_messages <pageId> --output-format=json
"$webmcp" browser list_network_requests <pageId> --output-format=json
"$webmcp" browser stop

On Windows, apply the same fallback in PowerShell and use the packaged command entry:

$pluginRoot = if ($env:CLAUDE_PLUGIN_ROOT) { $env:CLAUDE_PLUGIN_ROOT } elseif ($env:PLUGIN_ROOT) { $env:PLUGIN_ROOT } else { (Resolve-Path "<absolute base directory for this skill>\..\..").Path }
$webmcp = Join-Path $pluginRoot "scripts\webmcp.cmd"
& $webmcp browser status
& $webmcp browser start

Run the remaining ladder commands through & $webmcp browser ... and always end with & $webmcp browser stop.

start resolves the official chrome-devtools-mcp@latest, launches one host-session-scoped isolated headless Chrome 150+ with both --category-experimental-webmcp and --enable-features=WebMCP, and disables usage statistics and CrUX while redacting network headers. Set WEBMCP_BROWSER_SESSION_ID to any unique value when concurrent runs share one workspace; otherwise the entry scopes by the Claude/Codex session when available, then the workspace. It first uses a supported installed or cached browser; if none exists, on a platform published by Chrome for Testing, it downloads stable Chrome into the user cache and reuses it later. You may pass --executable-path <path> for a known safe test browser. On an unsupported host such as Linux arm64, install a native Chrome/Chromium 150+ and set WEBMCP_BROWSER_PATH instead.

If start fails, report its exact Node/npm, download, architecture, shared-library, or launch error and mark browser-dependent tools could-not-verify. Never use sudo, a system package manager, or a normal Chrome profile. After an interrupted run, use status, then stop, before retrying. Always stop the driver in cleanup, including after a failed rung.

Ladder

1. Boot. Site starts; the baseline page renders; no new console errors versus a clean load.
2. Discover. On each declared page and auth state — check anonymous and signed-in where relevant — list the registered tools. Confirm each tool appears where it should, is absent where it should not (logged-out account tools, wrong-role tools), and unregisters on its declared exits (empty cart, logout). Navigation is the expensive unit: visit each page × auth state once and settle every tool's expectations for that page in that single pass — never one navigation per tool.
3. Invoke read-only. Call read-only tools with sample inputs; check the returned data and the visible ui_effect. Batch by page: invoke a page's tools in the visit that discovered them.
4. Invoke state-changing. Only against local/dev/seeded data, with an explicit go-ahead. Never fire real POSTs at third-party or production services. No safe way to invoke → don't; report could-not-verify.

Report — one state per tool

• verified — registered and invoked as declared (note the rung reached).
• failed — did not register or errored; must be fixed or dropped, never shipped.
• could-not-verify — plausible but unproven (no browser, or no safe way to invoke); ships flagged.

Print a per-tool table and restate any could-not-verify items.