dev-browser

description	Fallback browser automation with persistent Chrome state. Use only when Browser Use is unavailable or blocked.
name	dev-browser
metadata	{"skiller":{"source":".agents/rules/dev-browser.mdc"}}

Dev Browser

Use this only as the fallback browser path when [@browser-use](plugin://browser-use@openai-bundled) is unavailable or blocked.

Do not substitute Puppeteer, standalone Playwright, or raw Chrome DevTools for this fallback path.

Installation

npm install -g dev-browser
dev-browser install

Run dev-browser --help to learn more.

Plate Defaults

• Use dev-browser --connect http://127.0.0.1:9222 by default. Do not preflight 9222 first.
• Only inspect 9222 after a direct dev-browser --connect http://127.0.0.1:9222 attempt fails.
• Reuse one persistent debug Chrome on 127.0.0.1:9222. Do not spin up disposable browser instances unless the user asks.
• Use a dedicated Chrome --user-data-dir for that debug browser, not the user's normal daily Chrome data dir.
• Clone the signed-in Chrome profile into the dedicated debug dir, then launch the debug browser from that clone.
• On macOS, launch the debug browser with open -na "Google Chrome" --args ... --remote-debugging-port=9222 so it opens as a separate Chrome instance without hijacking the user's normal window.
• Do not close or stop the user's connected debug browser. Leave that debug window open and reuse it. Close named pages only when needed.
• Keep scripts small and direct. Prefer browser.getPage("persistent-main") for the main app.
• Use dev-browser instead of agent-browser or next-devtools browser_eval.
• For Plate registry/browser proof, prefer /blocks/[id]-demo over docs wrappers when that standalone demo route exists.
• If dev-browser gets blocked by a human prompt or loops on the same step, stop and ask the user to unblock.

Fallback Setup

Use this only after dev-browser --connect http://127.0.0.1:9222 fails because no reusable debug Chrome is available or the CDP endpoint is broken.

Rules

• Prefer one permanent debug browser/profile over disposable automation browsers.
• Treat a custom --user-data-dir as mandatory, not optional. Chrome 136+ expects remote debugging to happen from a dedicated profile.
• Keep auth in that profile. Do not fall back to cookie dumps or state files unless the user asks.
• Use a separate signed-in Chrome profile for browser work, like dev. Do not use the user's normal daily Default profile as the source profile.
• Clone that separate signed-in Chrome profile into the dedicated debug --user-data-dir; do not point 9222 straight at the user's daily Chrome data dir.
• On macOS, use open -na "Google Chrome" --args ... for the debug browser. That starts a separate Chrome instance with the dedicated debug profile without touching the user's normal Chrome window.

Preferred Shape

Use a dedicated browser/profile with:

• --remote-debugging-address=127.0.0.1
• --remote-debugging-port=9222
• a persistent --user-data-dir=<debug-profile-dir>

Sign in once in that dedicated browser and keep reusing it for agent work.

Quick sanity check:

curl -sS http://127.0.0.1:9222/json/version

Healthy output includes a JSON object with webSocketDebuggerUrl. Empty output or 404 means the wrong process owns 9222.

Then verify:

dev-browser --connect http://127.0.0.1:9222 <<'EOF'
const page = await browser.getPage("persistent-main");
console.log(await page.title());
EOF

If direct connect still cannot resolve CDP even though /json/version is healthy, connect with the exact websocket URL:

WS=$(curl -sS http://127.0.0.1:9222/json/version | jq -r '.webSocketDebuggerUrl')

dev-browser --connect "$WS" <<'EOF'
const page = await browser.getPage("persistent-main");
console.log(await page.title());
EOF

Google Chrome Path

Default setup on macOS:

1. Pick a separate signed-in Chrome profile for agent work, like dev, not the daily Default profile.
2. Map that human-facing Chrome profile name to the real folder in Local State.
3. Clone that profile into the dedicated debug dir.
4. Launch a separate Chrome instance on 9222.
5. Leave that debug window open and reuse it.

python3 - <<'PY'
import json, pathlib
p = pathlib.Path('~/Library/Application Support/Google/Chrome/Local State').expanduser()
obj = json.loads(p.read_text())
for key, val in obj.get('profile', {}).get('info_cache', {}).items():
    print(f"{key}\tname={val.get('name')}\tgaia_name={val.get('gaia_name')}")
PY

# Example: if `dev` maps to `Profile 1`, clone `Profile 1`.
mkdir -p "$HOME/.config/google-chrome-debug-profile/Default"
rsync -a --delete \
  --exclude='Singleton*' \
  --exclude='DevToolsActivePort' \
  --exclude='lockfile' \
  "$HOME/Library/Application Support/Google/Chrome/Profile 1/" \
  "$HOME/.config/google-chrome-debug-profile/Default/"
cp "$HOME/Library/Application Support/Google/Chrome/Local State" \
  "$HOME/.config/google-chrome-debug-profile/Local State"

open -na "Google Chrome" --args \
  --user-data-dir="$HOME/.config/google-chrome-debug-profile" \
  --profile-directory="Default" \
  --remote-debugging-address=127.0.0.1 \
  --remote-debugging-port=9222

Do not point 9222 at the normal daily Default Chrome profile.

If the wrong Chrome steals 9222, identify it with:

lsof -nP -iTCP:9222 -sTCP:LISTEN

Kill that listener and relaunch the dedicated debug browser. Do not keep debugging against a stale 404 or empty /json/version owner.