Visual Test Agent Skill

Visual Test a Component

Visually verify $ARGUMENTS by launching Storybook and capturing a screenshot with playwright-cli.

Prerequisites

Run playwright-cli via npx so nothing is installed globally on the user's box. The first invocation downloads @playwright/[email protected] into the npx cache; subsequent calls are cached. Every command below uses this form:

npx -y @playwright/[email protected] <command>

Critical: use the per-component Storybook only

Always boot the per-component stories package (react-<component>-stories) via nx storybook target, which only imports its own component's stories and dependencies.

Steps

• Find the component's stories package. Each v9 component has a dedicated stories package named react-<component>-stories:

   yarn nx show project react-<lowercase-component-name>-stories --json

If nx returns nothing with output of Could not find project react-<component>-stories, the component doesn't have its own stories package — check for a preview package (react-<component>-preview-stories) or ask before proceeding.

• Start the component's Storybook dev server. Use the storybook target on the stories project directly — it's the most portable, since library aliases like react-<component>:start were only added in April 2026 and may not exist in older workspace snapshots:

   yarn nx run react-<component>-stories:storybook &

• Find the storybook port. Three quirks to know:

• Storybook picks a random high port on first boot (e.g. 49360), not the Storybook default 6006. Don't assume.
• The nx wrapper process often exits 0 after delegating to storybook, leaving the actual server running as a child. So the nx PID isn't the storybook PID.
• The storybook child opens two listening sockets: one for HTTP content, one for the webpack HMR event-stream. They are not ordered — either one can be numerically lower. Picking by port number is unreliable; pick by Content-Type.

Reliable detection — target the storybook node child (not the yarn wrapper), then probe each listening socket until one returns text/html:

   # Wait up to 180s for the storybook child to bind an HTTP port.
   # Pattern matches the node child specifically, not `yarn storybook dev` (the wrapper has no sockets).
   for i in $(seq 1 180); do
     SB_CHILD=$(pgrep -f "node.*\.bin/storybook dev" | head -1)
     if [ -n "$SB_CHILD" ]; then
       for port in $(lsof -a -p "$SB_CHILD" -i -P -sTCP:LISTEN 2>/dev/null | awk 'NR>1 {print $9}' | sed 's/.*://'); do
         CT=$(curl -sI --max-time 2 "http://localhost:$port/" 2>/dev/null | grep -i '^content-type:' | grep -i 'text/html')
         if [ -n "$CT" ]; then SB_PORT=$port; break; fi
       done
       if [ -n "$SB_PORT" ]; then break; fi
     fi
     sleep 1
   done
   echo "Storybook child PID=$SB_CHILD on port $SB_PORT"

Then wait for Storybook to finish compiling stories — the HTTP port answers before index.json is populated:

   for i in $(seq 1 60); do
     N=$(curl -s --max-time 2 "http://localhost:$SB_PORT/index.json" 2>/dev/null \
       | python3 -c "import json,sys; print(len(json.load(sys.stdin).get('entries', {})))" 2>/dev/null || echo 0)
     if [ "$N" -gt 0 ]; then break; fi
     sleep 2
   done

If no port turns up, or index.json never populates — do not fall back to the workspace-wide Storybook; read the nx output log and debug the per-component boot. The most common real failure is missing build artifacts for unstable re-export deps (see troubleshooting below).

• Open the page with playwright-cli:

   npx -y @playwright/[email protected] open "http://localhost:$SB_PORT"

• Navigate to the specific story iframe and capture a screenshot.

Use the iframe URL for a clean render without Storybook chrome:

   npx -y @playwright/[email protected] goto "http://localhost:$SB_PORT/iframe.html?id=components-<component>--default&viewMode=story"
   npx -y @playwright/[email protected] screenshot --filename=/tmp/visual-test-$ARGUMENTS.png

• View the screenshot using the Read tool to visually inspect the rendered component.

• Use snapshot to get the accessibility tree and find interactive element refs:

   npx -y @playwright/[email protected] snapshot

Then interact with elements by ref (e.g., click, hover) before taking more screenshots.

• If the component doesn't look right, go back to the code, fix the issue, and repeat from step 4 (Storybook hot-reloads changes).

• Clean up when done:

   npx -y @playwright/[email protected] close
   # Kill storybook — the nx wrapper may already be gone, so target the child
   [ -n "$SB_CHILD" ] && kill "$SB_CHILD" 2>/dev/null
   lsof -i :$SB_PORT -t 2>/dev/null | xargs kill 2>/dev/null

Troubleshooting

yarn nx run react-<component>-stories:storybook says the target doesn't exist.

The workspace graph may be stale (recent reparent). Run yarn nx reset then retry. If stroybook aliases still don't exist, use the direct yarn invocation:

cd packages/react-components/react-<component>/stories && yarn storybook dev --port 0 &
# --port 0 asks Storybook to pick a free port; detect it via the pgrep/lsof pattern above

Story ID Pattern

Story IDs follow the pattern <category>-<component>--<story>:

# Default story for Button
components-button--default

# Appearance variant
components-button--appearance

# Default story for Menu
components-menu--default

To discover exact story IDs, open the Storybook sidebar and use snapshot to find navigation links,

or check the story file's export default { title: '...' } metadata.

Iframe URL Format

# Local storybook (replace $SB_PORT with the actual port)
http://localhost:$SB_PORT/iframe.html?id=components-button--default&viewMode=story

# Dark theme
http://localhost:$SB_PORT/iframe.html?id=components-button--default&viewMode=story&globals=theme:webDarkTheme

The /iframe.html URL gives a clean render without Storybook chrome — always prefer this for screenshots.

Tips

• Use npx -y @playwright/[email protected] snapshot to get an accessibility tree — useful for verifying ARIA attributes and finding interactive elements.
• Use npx -y @playwright/[email protected] click <ref> to interact with the component (test hover states, open menus, etc.) before taking a screenshot.
• Use npx -y @playwright/[email protected] resize <width> <height> to test responsive behavior.
• For multiple story variants, take a screenshot of each: Default, Appearance, Size, Disabled, etc.