Chrome Debugging and Browser Manipulation via Remote Debugging Protocol

Overview

Chrome DevTools Protocol (CDP) enables remote browser automation and debugging through mcporter.

Key capabilities:

• Live browser debugging alongside Agent conversations

• Automated form filling and interaction testing

• Visual feedback via screenshots

• Console log and network request inspection

• JavaScript execution in page context

Prerequisites [CRITICAL]

Before using Chrome DevTools, ensure:

• Chrome/Chromium is running with remote debugging enabled

• The browser is listening on port 9222 (default)

• Test connection with:

mise x node@20 -- mcporter call chrome-devtools.list_pages

If this fails:

• Start Chrome: google-chrome --remote-debugging-port=9222

• Check no other process is using port 9222

• Get a human to help with browser setup

Available Tools

Tool Purpose

list_pages

List all open pages/tabs

select_page

Select a specific page/tab to work with

new_page

Create a new browser page/tab

close_page

Close a browser page/tab

navigate_page

Navigate to a URL, back, forward, or reload

take_snapshot

Take a DOM snapshot for inspection (returns UIDs)

take_screenshot

Capture a screenshot of the current page

click

Click an element on the page

fill

Fill input fields with text

hover

Hover over an element

press_key

Press keyboard keys (Enter, Tab, Escape, etc.)

evaluate_script

Execute JavaScript code in the page context

wait_for

Wait for elements, navigation, or conditions

list_console_messages

Get all console messages (logs, errors, warnings)

list_network_requests

Get all network requests made by the page

emulate

Emulate device settings (network, CPU throttling)

resize_page

Resize the browser viewport

performance_start_trace

Start performance tracing

performance_stop_trace

Stop performance tracing and get results

[!TIP] Get full tool list: mcporter list chrome-devtools --json | jq -r '.tools[] | [.name, .description] | @tsv' | column -t -s $'\t'

Core Concepts

1. Page Selection Model

• Chrome DevTools works with multiple pages/tabs

• Use list_pages to see all open pages

• Use select_page to choose which page to work with

• All subsequent commands operate on the selected page

2. UID-Based Element Model [CRITICAL]

• You CANNOT interact with elements using CSS selectors directly

• Must first call take_snapshot to get accessibility tree with UIDs

• UIDs are temporary identifiers for elements (e.g., "5", "12", "42")

• UIDs are invalidated on navigation - take new snapshot after nav

3. JSON Arguments Required

• All mcporter commands require --args with JSON object

• Property names are camelCase (e.g., filePath , fullPage , pageIdx )

• Never use individual flags like --file-path or --full-page

4. Function-Based Script Evaluation

• evaluate_script requires a function declaration, not plain code

• Return values must be JSON-serializable

• Can pass element arguments via args array with UIDs

Quick Reference

Essential commands in bash-friendly format:

List and select page

mise x node@20 -- mcporter call chrome-devtools.list_pages mise x node@20 -- mcporter call chrome-devtools.select_page --args '{"pageIdx":0}'

Take snapshot to get UIDs

mise x node@20 -- mcporter call chrome-devtools.take_snapshot

Take screenshot

mise x node@20 -- mcporter call chrome-devtools.take_screenshot --args '{"filePath":"./screen.png"}'

Take full-page screenshot

mise x node@20 -- mcporter call chrome-devtools.take_screenshot --args '{"filePath":"./full.png","fullPage":true}'

Navigate to URL

mise x node@20 -- mcporter call chrome-devtools.navigate_page --args '{"type":"url","url":"http://localhost:3000"}'

Navigate back/forward/reload

mise x node@20 -- mcporter call chrome-devtools.navigate_page --args '{"type":"back"}' mise x node@20 -- mcporter call chrome-devtools.navigate_page --args '{"type":"reload"}'

Click element (requires UID from snapshot)

mise x node@20 -- mcporter call chrome-devtools.click --args '{"uid":"12"}'

Fill input field

mise x node@20 -- mcporter call chrome-devtools.fill --args '{"uid":"5","value":"test@example.com"}'

Hover element

mise x node@20 -- mcporter call chrome-devtools.hover --args '{"uid":"8"}'

Press key

mise x node@20 -- mcporter call chrome-devtools.press_key --args '{"key":"Enter"}'

Run JavaScript

mise x node@20 -- mcporter call chrome-devtools.evaluate_script --args '{"function":"() => { return document.title }"}'

Run JS with element argument

mise x node@20 -- mcporter call chrome-devtools.evaluate_script --args '{"function":"(el) => { return el.innerText }","args":[{"uid":"12"}]}'

List console messages

mise x node@20 -- mcporter call chrome-devtools.list_console_messages

List only errors

mise x node@20 -- mcporter call chrome-devtools.list_console_messages --args '{"types":["error"]}'

List network requests

mise x node@20 -- mcporter call chrome-devtools.list_network_requests

Filter network by type

mise x node@20 -- mcporter call chrome-devtools.list_network_requests --args '{"types":["fetch","xhr"]}'

Wait for text to appear

mise x node@20 -- mcporter call chrome-devtools.wait_for --args '{"text":"Success"}'

Emulate network conditions

mise x node@20 -- mcporter call chrome-devtools.emulate --args '{"networkConditions":"Slow 3G"}'

Common Workflows

Basic Element Interaction

1. Select page and take snapshot

mise x node@20 -- mcporter call chrome-devtools.list_pages mise x node@20 -- mcporter call chrome-devtools.select_page --args '{"pageIdx":0}' SNAPSHOT=$(mise x node@20 -- mcporter call chrome-devtools.take_snapshot) echo "$SNAPSHOT"

2. Find element UID in snapshot output

Example: uid=12 input type="email"

3. Interact with element using its UID

mise x node@20 -- mcporter call chrome-devtools.fill --args '{"uid":"12","value":"user@example.com"}' mise x node@20 -- mcporter call chrome-devtools.click --args '{"uid":"15"}'

Screenshot Workflow

Take viewport screenshot

mise x node@20 -- mcporter call chrome-devtools.take_screenshot --args '{"filePath":"./screen.png"}'

Take full-page screenshot

mise x node@20 -- mcporter call chrome-devtools.take_screenshot --args '{"filePath":"./full.png","fullPage":true}'

Screenshot specific element

mise x node@20 -- mcporter call chrome-devtools.take_screenshot --args '{"uid":"20","filePath":"./button.png"}'

Debug JavaScript Errors

Check console for errors

mise x node@20 -- mcporter call chrome-devtools.list_console_messages --args '{"types":["error","warn"]}'

Check network requests

mise x node@20 -- mcporter call chrome-devtools.list_network_requests --args '{"types":["fetch","xhr"]}'

Run Performance Tests

Execute JavaScript to get performance metrics

mise x node@20 -- mcporter call chrome-devtools.evaluate_script
--args '{"function":"() => { const perf = performance.getEntriesByType("navigation")[0]; return { loadTime: perf.loadEventEnd - perf.fetchStart, domInteractive: perf.domInteractive - perf.fetchStart }; }"}'

Important Reminders

UID Workflow is Mandatory

❌ WRONG - CSS selectors don't work

mise x node@20 -- mcporter call chrome-devtools.click --selector "#login-button"

✅ CORRECT - Use UIDs from snapshot

mise x node@20 -- mcporter call chrome-devtools.take_snapshot # Get UIDs first mise x node@20 -- mcporter call chrome-devtools.click --args '{"uid":"12"}'

UIDs Expire on Navigation

After navigation, UIDs are invalid

mise x node@20 -- mcporter call chrome-devtools.navigate_page --args '{"type":"url","url":"..."}'

Take fresh snapshot to get new UIDs

mise x node@20 -- mcporter call chrome-devtools.take_snapshot

Always Use --args with JSON

❌ WRONG - Individual flags don't work

mise x node@20 -- mcporter call chrome-devtools.take_screenshot --file-path "./screen.png"

✅ CORRECT - Use --args with JSON

mise x node@20 -- mcporter call chrome-devtools.take_screenshot --args '{"filePath":"./screen.png"}'

Quick Troubleshooting

Error Solution

"Element not found" / "Invalid UID" Take fresh snapshot: take_snapshot

"No page selected" Select page: select_page --args '{"pageIdx":0}'

"Connection refused" Start Chrome: google-chrome --remote-debugging-port=9222

Screenshot not created Ensure directory exists and use --args format

UIDs not working UIDs expired after navigation - take new snapshot

Additional Resources

Lazy-load these references based on your needs:

Reference When to Use

Element Interaction When working with UIDs, clicking, hovering, or measuring elements

Form Filling When filling forms, submitting data, or handling keyboard input

Screenshots When capturing screenshots, visual testing, or documenting state

Performance When measuring page performance, network timing, or emulating conditions

Debugging When investigating console errors, network failures, or script evaluation

Navigation When navigating pages, managing tabs, or handling viewports

Troubleshooting When encountering errors or unexpected behavior

[!IMPORTANT] Load references only when needed - Don't read all files upfront. Read the specific reference that matches your current task.

Real-World Impact

Integrating Chrome DevTools Protocol enables:

• Live browser debugging alongside Agent conversations

• Automated form filling and interaction testing

• Visual feedback on application behavior

• Immediate error diagnostics from console logs

• Screenshot-based validation workflows

Without this integration, debugging web applications requires constant context-switching between browser and Agent.