Browser DevTools MCP

A powerful Model Context Protocol (MCP) server that provides AI coding assistants with comprehensive browser automation and debugging capabilities using Playwright. This server enables both execution-level debugging (logs, network requests) and visual debugging (screenshots, ARIA snapshots) to help AI assistants understand and interact with web pages effectively.

Overview

Browser DevTools MCP exposes a Playwright-powered browser runtime to AI agents, enabling deep, bidirectional debugging and interaction with live web pages. It supports both visual understanding and code-level inspection of browser state, making it ideal for AI-driven exploration, diagnosis, and automation.

Key Capabilities

• Visual Inspection: Screenshots, ARIA snapshots, HTML/text extraction, PDF generation

• Design Comparison: Compare live page UI against Figma designs with similarity scoring

• DOM & Code-Level Debugging: Element inspection, computed styles, accessibility data

• Browser Automation: Navigation, input, clicking, scrolling, viewport control

• Execution Monitoring: Console message capture, HTTP request/response tracking

• OpenTelemetry Integration: Automatic trace injection into web pages, UI trace collection, and backend trace correlation via trace context propagation

• JavaScript Execution: Execute code in browser page context or in Node.js VM sandbox on the server

• Session Management: Long-lived, session-based debugging with automatic cleanup

• Multiple Transport Modes: Supports both stdio and HTTP transports

Features

Content Tools

• Screenshots: Capture full page or specific elements (PNG/JPEG) with image data

• HTML/Text Extraction: Get page content with filtering, cleaning, and minification options

• PDF Export: Save pages as PDF documents with customizable format and margins

Interaction Tools

• Click: Click elements by CSS selector

• Fill: Fill form inputs

• Hover: Hover over elements

• Press Key: Simulate keyboard input

• Select: Select dropdown options

• Drag: Drag and drop operations

• Scroll: Scroll the page viewport or specific scrollable elements with multiple modes (by, to, top, bottom, left, right)

• Resize Viewport: Resize the page viewport using Playwright viewport emulation

• Resize Window: Resize the real browser window (OS-level) using Chrome DevTools Protocol

Navigation Tools

• Go To: Navigate to URLs with configurable wait strategies

• Go Back: Navigate backward in history

• Go Forward: Navigate forward in history

Run Tools

• JS in Browser: Execute JavaScript code inside the active browser page (page context with access to window, document, DOM, and Web APIs)

• JS in Sandbox: Execute JavaScript code in a Node.js VM sandbox on the MCP server (with access to Playwright Page, console logging, and safe built-ins)

Observability (O11Y) Tools

• Console Messages: Capture and filter browser console logs with advanced filtering (level, search, timestamp, sequence number)

• HTTP Requests: Monitor network traffic with detailed request/response data, filtering by resource type, status code, and more

• Web Vitals: Collect Core Web Vitals (LCP, INP, CLS) and supporting metrics (TTFB, FCP) with ratings and recommendations based on Google's thresholds

• OpenTelemetry Tracing: Automatic trace injection into web pages, UI trace collection (document load, fetch, XMLHttpRequest, user interactions), and trace context propagation for backend correlation

• Trace ID Management: Get, set, and generate OpenTelemetry compatible trace IDs for distributed tracing across API calls

Synchronization Tools

• Wait for Network Idle: Wait until the page reaches a network-idle condition based on in-flight request count, useful for SPA pages and before taking screenshots

Accessibility (A11Y) Tools

• ARIA Snapshots: Capture semantic structure and accessibility roles in YAML format

• AX Tree Snapshots: Combine Chromium's Accessibility tree with runtime visual diagnostics (bounding boxes, visibility, occlusion detection, computed styles)

Stub Tools

• Intercept HTTP Request: Intercept and modify outgoing HTTP requests (headers, body, method) using glob patterns

• Mock HTTP Response: Mock HTTP responses (fulfill with custom status/headers/body or abort) with configurable delay, times limit, and probability (flaky testing)

• List Stubs: List all currently installed stubs for the active browser context

• Clear Stubs: Remove one or all installed stubs

Figma Tools

• Compare Page with Design: Compare the current page UI against a Figma design snapshot and return a combined similarity score using multiple signals (MSSIM, image embedding, text embedding)

React Tools

• Get Component for Element: Find React component(s) associated with a DOM element using React Fiber (best-effort)

• Get Element for Component: Map a React component instance to the DOM elements it renders by traversing the React Fiber graph

Important Requirements for React Tools:

• Persistent Browser Context: React tools work best with persistent browser context enabled (BROWSER_PERSISTENT_ENABLE=true)

• React DevTools Extension: For optimal reliability, manually install the "React Developer Tools" Chrome extension in the browser profile. The MCP server does NOT automatically install the extension.

  • Chrome Web Store: https://chrome.google.com/webstore/detail/react-developer-tools/fmkadmapgofadopljbjfkapdkoienihi

  • The extension enables reliable root discovery and component search via __REACT_DEVTOOLS_GLOBAL_HOOK__

  • Without the extension, tools fall back to scanning DOM for __reactFiber$ pointers (best-effort, less reliable)

Prerequisites

• Node.js 18+

• An AI assistant (with MCP client) like Cursor, Claude (Desktop or Code), VS Code, Windsurf, etc.

Quick Start

This MCP server (using STDIO or Streamable HTTP transport) can be added to any MCP Client like VS Code, Claude, Cursor, Windsurf, GitHub Copilot via the browser-devtools-mcp NPM package.

No manual installation required! The server can be run directly using npx, which automatically downloads and runs the package.

CLI Arguments

Browser DevTools MCP server supports the following CLI arguments for configuration:

• --transport <stdio|streamable-http> - Configures the transport protocol (defaults to stdio).

• --port <number> – Configures the port number to listen on when using streamable-http transport (defaults to 3000).

MCP Client Configuration

Claude Desktop

Local Server

Add the following configuration into the claude_desktop_config.json file. See the Claude Desktop MCP docs for more info.

Remote Server (HTTP Transport)

First, start the server with HTTP transport:

Then, go to Settings > Connectors > Add Custom Connector in Claude Desktop and add the MCP server with:

• Name: Browser DevTools

• Remote MCP server URL: Point to where your server is hosted (e.g., http://localhost:3000/mcp if running locally, or https://your-server.com/mcp if hosted remotely)

Claude Code

Run the following command. See Claude Code MCP docs for more info.

Local Server

Remote Server

First, start the server with HTTP transport:

Then add the MCP server:

Replace <SERVER_URL> with your server URL (e.g., http://localhost:3000/mcp if running locally, or https://your-server.com/mcp if hosted remotely).

Cursor

Add the following configuration into the ~/.cursor/mcp.json file (or .cursor/mcp.json in your project folder). See the Cursor MCP docs for more info.

Local Server

Remote Server

First, start the server with HTTP transport:

Then add the configuration:

Replace <SERVER_URL> with your server URL (e.g., http://localhost:3000/mcp if running locally, or https://your-server.com/mcp if hosted remotely).

VS Code

Add the following configuration into the .vscode/mcp.json file. See the VS Code MCP docs for more info.

Local Server

Remote Server

First, start the server with HTTP transport:

Then add the configuration:

Replace <SERVER_URL> with your server URL (e.g., http://localhost:3000/mcp if running locally, or https://your-server.com/mcp if hosted remotely).

Windsurf

Add the following configuration into the ~/.codeium/windsurf/mcp_config.json file. See the Windsurf MCP docs for more info.

Local Server

Remote Server

First, start the server with HTTP transport:

Then add the configuration:

Replace <SERVER_URL> with your server URL (e.g., http://localhost:3000/mcp if running locally, or https://your-server.com/mcp if hosted remotely).

Copilot Coding Agent

Add the following configuration to the mcpServers section of your Copilot Coding Agent configuration through Repository > Settings > Copilot > Coding agent > MCP configuration. See the Copilot Coding Agent MCP docs for more info.

Local Server

Remote Server

First, start the server with HTTP transport:

Then add the configuration:

Replace <SERVER_URL> with your server URL (e.g., http://localhost:3000/mcp if running locally, or https://your-server.com/mcp if hosted remotely).

Gemini CLI

Add the following configuration into the ~/.gemini/settings.json file. See the Gemini CLI MCP docs for more info.

Local Server

Remote Server

First, start the server with HTTP transport:

Then add the configuration:

Replace <SERVER_URL> with your server URL (e.g., http://localhost:3000/mcp if running locally, or https://your-server.com/mcp if hosted remotely).

Smithery

Run the following command. You can find your Smithery API key here. See the Smithery CLI docs for more info.

HTTP Transport

To use HTTP transport, start the server with:

The server exposes the following endpoints:

• GET /health - Health check

• GET /ping - Ping endpoint

• GET /mcp - MCP protocol info

• POST /mcp - MCP protocol messages

• DELETE /mcp - Delete session

Important: When configuring remote MCP servers, use the actual URL where your server is hosted:

• If running locally: http://localhost:3000/mcp (or http://127.0.0.1:3000/mcp)

• If hosted remotely: https://your-server.com/mcp (replace with your actual server URL)

MCP Inspector

Test the server using the MCP Inspector:

Configuration

The server can be configured using environment variables:

Available Tools

Content Tools

Parameters:

• outputPath (string, optional): Directory path where screenshot will be saved (default: OS temp directory)

• name (string, optional): Screenshot name (default: "screenshot")

• selector (string, optional): CSS selector for element to capture

• fullPage (boolean, optional): Capture full scrollable page (default: false)

• type (enum, optional): Image format - "png" or "jpeg" (default: "png")

• quality (number, optional): The quality of the image, between 0-100. Not applicable to PNG images, only used for JPEG format (default: 100)

Returns:

• filePath (string): Full path of the saved screenshot file

• image (object): Screenshot image data with mimeType

Notes:

• The quality parameter only applies to JPEG images. PNG images are always saved at full quality

• Lower quality values (e.g., 50-70) result in smaller file sizes but reduced image quality

• Quality value of 100 provides maximum quality but larger file sizes

Parameters:

• selector (string, optional): CSS selector to limit the HTML content to a specific container

• removeScripts (boolean, optional): Remove all script tags from the HTML (default: true)

• removeComments (boolean, optional): Remove all HTML comments (default: false)

• removeStyles (boolean, optional): Remove all style tags from the HTML (default: false)

• removeMeta (boolean, optional): Remove all meta tags from the HTML (default: false)

• cleanHtml (boolean, optional): Perform comprehensive HTML cleaning (default: false)

• minify (boolean, optional): Minify the HTML output (default: false)

• maxLength (number, optional): Maximum number of characters to return (default: 50000)

Returns:

• output (string): The requested HTML content of the page

Parameters:

• selector (string, optional): CSS selector to limit the text content to a specific container

• maxLength (number, optional): Maximum number of characters to return (default: 50000)

Returns:

• output (string): The requested text content of the page

Parameters:

• outputPath (string, optional): Directory path where PDF will be saved (default: OS temp directory)

• name (string, optional): PDF name (default: "page")

• format (enum, optional): Page format - "Letter", "Legal", "Tabloid", "Ledger", "A0" through "A6" (default: "A4")

• printBackground (boolean, optional): Whether to print background graphics (default: false)

• margin (object, optional): Page margins with top, right, bottom, left (default: "1cm" for each)

Returns:

• filePath (string): Full path of the saved PDF file

Interaction Tools

Parameters:

• selector (string, required): CSS selector for the element to click

Parameters:

• selector (string, required): CSS selector for the input field

• value (string, required): Value to fill

Parameters:

• selector (string, required): CSS selector for the element to hover

Parameters:

• key (string, required): Key to press (e.g., "Enter", "Escape", "Tab")

Parameters:

• selector (string, required): CSS selector for the select element

• value (string, required): Value to select

Parameters:

• sourceSelector (string, required): CSS selector for the source element

• targetSelector (string, required): CSS selector for the target element

Parameters:

• mode (enum, optional): Scroll mode - "by" (relative delta), "to" (absolute position), "top", "bottom", "left", "right" (default: "by")

• selector (string, optional): CSS selector for a scrollable container. If omitted, scrolls the document viewport

• dx (number, optional): Horizontal scroll delta in pixels (used when mode="by", default: 0)

• dy (number, optional): Vertical scroll delta in pixels (used when mode="by", default: 0)

• x (number, optional): Absolute horizontal scroll position in pixels (used when mode="to")

• y (number, optional): Absolute vertical scroll position in pixels (used when mode="to")

• behavior (enum, optional): Native scroll behavior - "auto" or "smooth" (default: "auto")

Returns:

• mode (string): The scroll mode used

• selector (string | null): The selector of the scroll container if provided; otherwise null (document viewport)

• behavior (string): The scroll behavior used

• before (object): Scroll metrics before the scroll action (x, y, scrollWidth, scrollHeight, clientWidth, clientHeight)

• after (object): Scroll metrics after the scroll action (x, y, scrollWidth, scrollHeight, clientWidth, clientHeight)

• canScrollX (boolean): Whether horizontal scrolling is possible

• canScrollY (boolean): Whether vertical scrolling is possible

• maxScrollX (number): Maximum horizontal scrollLeft

• maxScrollY (number): Maximum vertical scrollTop

• isAtLeft (boolean): Whether the scroll position is at the far left

• isAtRight (boolean): Whether the scroll position is at the far right

• isAtTop (boolean): Whether the scroll position is at the very top

• isAtBottom (boolean): Whether the scroll position is at the very bottom

Usage:

• Reveal content below the fold

• Jump to the top/bottom without knowing exact positions

• Bring elements into view before clicking

• Inspect lazy-loaded content that appears on scroll

Parameters:

• width (number, required): Target viewport width in CSS pixels (minimum: 200)

• height (number, required): Target viewport height in CSS pixels (minimum: 200)

Returns:

• requested (object): Requested viewport configuration (width, height)

• viewport (object): Viewport metrics observed inside the page after resizing:

  • innerWidth, innerHeight: window.innerWidth/innerHeight

  • outerWidth, outerHeight: window.outerWidth/outerHeight

  • devicePixelRatio: window.devicePixelRatio

Notes:

• This affects window.innerWidth/innerHeight, CSS media queries, layout, rendering, and screenshots

• This does NOT resize the OS-level browser window

• Runtime switching to viewport=null (binding to real window size) is not supported by Playwright

• If you need real window-driven responsive behavior, start the BrowserContext with viewport: null and use the window resize tool instead

Parameters:

• width (number, optional): Target window width in pixels (required when state="normal", minimum: 200)

• height (number, optional): Target window height in pixels (required when state="normal", minimum: 200)

• state (enum, optional): Target window state - "normal", "maximized", "minimized", or "fullscreen" (default: "normal")

Returns:

• requested (object): Requested window change parameters (width, height, state)

• before (object): Window bounds before resizing (windowId, state, left, top, width, height)

• after (object): Window bounds after resizing (windowId, state, left, top, width, height)

• viewport (object): Page viewport metrics after resizing (innerWidth, innerHeight, outerWidth, outerHeight, devicePixelRatio)

Notes:

• Works best on Chromium-based browsers (Chromium/Chrome/Edge)

• Especially useful in headful sessions when running with viewport emulation disabled (viewport: null)

• If Playwright viewport emulation is enabled (viewport is NOT null), resizing the OS window may not change page layout

• On non-Chromium browsers (Firefox/WebKit), CDP is not available and this tool will fail

Navigation Tools

Parameters:

• url (string, required): URL to navigate to (must include scheme)

• timeout (number, optional): Maximum operation time in milliseconds (default: 0 - no timeout)

• waitUntil (enum, optional): When to consider navigation succeeded - "load", "domcontentloaded", "networkidle", or "commit" (default: "load")

Returns:

• url (string): Final URL after navigation

• status (number): HTTP status code

• statusText (string): HTTP status text

• ok (boolean): Whether navigation was successful (2xx status)

Run Tools

Parameters:

• script (string, required): JavaScript code to execute

Returns:

• result (any): Result of the evaluation. This value can be of any type, including primitives, arrays, or objects. It represents the direct return value of the JavaScript expression executed in the page context.

Notes:

• The code executes in the PAGE CONTEXT (real browser environment):

  • Has access to window, document, DOM, Web APIs

  • Can read/modify the page state

  • Runs with the same permissions as the loaded web page

• The code runs in an isolated execution context, but within the page

• No direct access to Node.js APIs

• Return value must be serializable

Typical use cases:

• Inspect or mutate DOM state

• Read client-side variables or framework internals

• Trigger browser-side logic

• Extract computed values directly from the page

Parameters:

• code (string, required): JavaScript code to run on the MCP server in a VM sandbox. The code is wrapped in an async IIFE, so await is allowed. Use return ... to return a value

• timeoutMs (number, optional): Max VM CPU time for synchronous execution in milliseconds (default: 5000, max: 30000)

Returns:

• result (any): Return value of the sandboxed code (best-effort JSON-safe). If user returns undefined but logs exist, returns { logs }. If error occurs, returns { error, logs }

Available bindings:

• page: Playwright Page (main interaction surface)

• console: captured logs (log/warn/error)

• sleep(ms): async helper

Safe built-ins:

• Math, JSON, Number, String, Boolean, Array, Object, Date, RegExp

• isFinite, isNaN, parseInt, parseFloat

• URL, URLSearchParams

• TextEncoder, TextDecoder

• structuredClone

• crypto.randomUUID()

• AbortController

• setTimeout / clearTimeout

NOT available:

• require, process, fs, Buffer

• globalThis

Notes:

• This runs on the MCP SERVER (not in the browser)

• This is NOT a security boundary. Intended for trusted automation logic

• The timeoutMs parameter limits synchronous execution time, but does not automatically time out awaited Promises

Observability (O11Y) Tools

Parameters:

• type (enum, optional): Filter by message level - "ERROR", "WARNING", "INFO", "DEBUG"

• search (string, optional): Text to search for in messages

• timestamp (number, optional): Start time filter (Unix epoch milliseconds)

• sequenceNumber (number, optional): Only return messages after this sequence number

• limit (object, optional): Limit results

  • count (number): Maximum number of messages

  • from (enum): "start" or "end" (default: "end")

Returns:

• messages (array): Array of console messages with type, text, location, timestamp, and sequence number

Parameters:

• resourceType (enum, optional): Filter by resource type (e.g., "document", "script", "stylesheet")

• status (object, optional): Filter by status code range

  • min (number): Minimum status code

  • max (number): Maximum status code

• ok (boolean, optional): Filter by success/failure (2xx = success)

• timestamp (number, optional): Start time filter (Unix epoch milliseconds)

• sequenceNumber (number, optional): Only return requests after this sequence number

• limit (object, optional): Limit results

  • count (number): Maximum number of requests

  • from (enum): "start" or "end" (default: "end")

Returns:

• requests (array): Array of HTTP requests with URL, method, headers, body, response, timing, and metadata

Parameters:

• waitMs (number, optional): Optional wait duration in milliseconds before reading metrics (default: 0, max: 30000). Useful to allow LCP/INP/CLS to settle after interactions

• includeDebug (boolean, optional): If true, returns additional debug details such as entry counts and LCP element hint (default: false)

Returns:

• url (string): Current page URL

• title (string): Current page title

• timestampMs (number): Unix epoch timestamp (ms) when the metrics were captured

• metrics (object): Raw metric values (null if unavailable):

  • lcpMs (number | null): Largest Contentful Paint in milliseconds

  • inpMs (number | null): Interaction to Next Paint in milliseconds (best-effort approximation)

  • cls (number | null): Cumulative Layout Shift score

  • ttfbMs (number | null): Time to First Byte in milliseconds

  • fcpMs (number | null): First Contentful Paint in milliseconds

• ratings (object): Ratings computed from Google thresholds for each metric:

  • lcp, inp, cls, ttfb, fcp: Each contains:

    • rating (enum): "good", "needs_improvement", "poor", or "not_available"

    • value (number | null): Metric value

    • unit (enum): "ms" or "score"

    • thresholds (object): Thresholds used for rating (good, poor)

• recommendations (object): Recommendations based on measured values:

  • coreWebVitalsPassed (boolean): True if all Core Web Vitals are rated "good"

  • summary (array): High-level summary and prioritization guidance

  • lcp, inp, cls, ttfb, fcp (array): Specific recommendations for each metric

  • general (array): General measurement and debugging notes

• notes (array): Notes about metric availability, browser limitations, and interpretation

• debug (object, optional): Optional debug details (when includeDebug=true):

  • waitMs (number): Actual wait duration used

  • entries (object): Counts of PerformanceEntry types used to compute metrics

  • lastLcpSelectorHint (string | null): Best-effort selector hint for the last LCP element

  • lastLcpTagName (string | null): Tag name of the last LCP element

Core Web Vitals Thresholds:

• LCP (Largest Contentful Paint): good <= 2500ms, poor > 4000ms

• INP (Interaction to Next Paint): good <= 200ms, poor > 500ms

• CLS (Cumulative Layout Shift): good <= 0.1, poor > 0.25

Supporting Metrics Thresholds:

• TTFB (Time to First Byte): good <= 800ms, poor > 1800ms

• FCP (First Contentful Paint): good <= 1800ms, poor > 3000ms

Usage:

• Call after navigation and after user actions

• If you need more stable LCP/CLS/INP, pass waitMs (e.g., 1000-3000ms)

• Some metrics may be unavailable depending on browser support and whether interactions occurred

Parameters:

• No input parameters

Returns:

• traceId (string, optional): The OpenTelemetry compatible trace id of the current session if available

Note: Requires OpenTelemetry to be enabled (OTEL_ENABLE=true).

Parameters:

• No input parameters

Returns:

• traceId (string): The generated new OpenTelemetry compatible trace id

Note: Requires OpenTelemetry to be enabled (OTEL_ENABLE=true). The new trace ID is automatically set and will be used for all subsequent traces in the session.

Parameters:

• traceId (string, optional): The OpenTelemetry compatible trace id to be set. Leave it empty to clear the session trace id, so no OpenTelemetry trace header will be propagated from browser throughout the API calls

Returns:

• No return value

Note: Requires OpenTelemetry to be enabled (OTEL_ENABLE=true). When a trace ID is set, it will be propagated in HTTP headers (traceparent) for all API calls, enabling correlation with backend traces.

Synchronization Tools

Parameters:

• timeoutMs (number, optional): Maximum time to wait before failing (milliseconds, default: 30000)

• idleTimeMs (number, optional): How long the network must stay idle continuously before resolving (milliseconds, default: 500)

• maxConnections (number, optional): Idle threshold - network is considered idle when in-flight requests <= maxConnections (default: 0)

• pollIntervalMs (number, optional): Polling interval used to sample the in-flight request count (milliseconds, default: 50)

Returns:

• waitedMs (number): Total time waited until the network became idle or the tool timed out

• idleTimeMs (number): Idle duration required for success

• timeoutMs (number): Maximum allowed wait time

• maxConnections (number): Idle threshold used

• pollIntervalMs (number): Polling interval used

• finalInFlightRequests (number): The last observed number of in-flight requests

• observedIdleMs (number): How long the in-flight request count stayed <= maxConnections

Usage:

• Use before interacting with SPA pages that load data asynchronously

• Use before taking screenshots or AX tree snapshots for more stable results

• Use after actions that trigger background fetch/XHR activity

Note: This tool uses server-side tracking, so it works reliably even with strict CSP. It does NOT rely on window globals or page-injected counters.

Accessibility (A11Y) Tools

Parameters:

• selector (string, optional): CSS selector for element to snapshot

Returns:

• output (string): Includes the page URL, title, and a YAML-formatted accessibility tree

Usage:

• Use in combination with accessibility_take-ax-tree-snapshot for comprehensive UI analysis

• Provides semantic structure and accessibility roles

• Helps identify accessibility issues and page hierarchy problems

Parameters:

• roles (array, optional): Optional role allowlist (button, link, textbox, checkbox, radio, combobox, switch, tab, menuitem, dialog, heading, listbox, listitem, option). If omitted, a built-in set of interactive roles is used

• includeStyles (boolean, optional): Whether to include computed CSS styles for each node (default: true)

• includeRuntimeVisual (boolean, optional): Whether to compute runtime visual information (bounding box, visibility, viewport) (default: true)

• checkOcclusion (boolean, optional): If true, checks whether each element is visually occluded by another element using elementFromPoint() sampled at multiple points (default: false)

• onlyVisible (boolean, optional): If true, only visually visible nodes are returned (default: false)

• onlyInViewport (boolean, optional): If true, only nodes intersecting the viewport are returned (default: false)

• textPreviewMaxLength (number, optional): Maximum length of the text preview extracted from each element (default: 80)

• styleProperties (array, optional): List of CSS computed style properties to extract (default: includes display, visibility, opacity, position, z-index, colors, fonts, etc.)

Returns:

• url (string): The current page URL at the time the AX snapshot was captured

• title (string): The document title of the page at the time of the snapshot

• axNodeCount (number): Total number of nodes returned by Chromium Accessibility.getFullAXTree before filtering

• candidateCount (number): Number of DOM-backed AX nodes that passed role filtering before enrichment

• enrichedCount (number): Number of nodes included in the final enriched snapshot output

• truncatedBySafetyCap (boolean): Indicates whether the result set was truncated by an internal safety cap

• nodes (array): List of enriched DOM-backed AX nodes combining accessibility metadata with visual diagnostics, including:

  • axNodeId, parentAxNodeId, childAxNodeIds: Tree structure

  • role, name, ignored: Accessibility properties

  • backendDOMNodeId, domNodeId, frameId: DOM references

  • localName, id, className, selectorHint: Element identification

  • textPreview: Short preview of rendered text content

  • styles: Computed CSS styles (if includeStyles is true)

  • runtime: Visual diagnostics including boundingBox, isVisible, isInViewport, and optional occlusion data

Usage:

• Use to detect UI issues like elements that exist semantically but are visually hidden or off-screen

• Identify wrong layout/geometry, styling issues, and overlap/stacking/occlusion problems

• ALWAYS use checkOcclusion: true when investigating UI/layout problems

• Use alongside a11y_take-aria-snapshot tool for complete UI analysis

Stub Tools

Parameters:

• pattern (string, required): Glob pattern matched against the full request URL (picomatch)

• modifications (object, optional): Request modifications to apply

  • headers (object, optional): Headers to merge into the outgoing request headers

  • body (string | object, optional): Override request body. If object/array, it will be JSON-stringified

  • method (string, optional): Override HTTP method (e.g., POST, PUT)

• delayMs (number, optional): Artificial delay in milliseconds before continuing the request (default: 0)

• times (number, optional): Apply only N times, then let through. Omit for infinite

Returns:

• stubId (string): Unique id of the installed stub

• kind (string): Stub kind (always "intercept_http_request")

• pattern (string): Glob pattern used

• enabled (boolean): Whether the stub is enabled

• delayMs (number): Applied artificial delay in milliseconds

• times (number): Max applications (-1 means infinite)

Use cases:

• A/B testing / feature flags (inject headers)

• Security testing (inject malformed headers / payload)

• Edge cases (special characters, large payload)

• Auth simulation (add API keys / tokens in headers)

Notes:

• Pattern is a glob matched against the full request URL (picomatch)

• This modifies requests; it does not change responses

• Times limits how many times the interceptor applies (-1 means infinite)

Parameters:

• pattern (string, required): Glob pattern matched against the full request URL (picomatch)

• response (object, required): Mock response configuration

  • action (enum, optional): "fulfill" or "abort" (default: "fulfill")

  • status (number, optional): HTTP status code (used when action="fulfill", range: 100-599)

  • headers (object, optional): HTTP headers for the mocked response

  • body (string | object, optional): Response body. If object/array, it will be JSON-stringified

  • abortErrorCode (string, optional): Playwright abort error code (used when action="abort"), e.g., "timedout"

• delayMs (number, optional): Artificial delay in milliseconds before applying the stub (default: 0)

• times (number, optional): Apply only N times, then let through. Omit for infinite

• chance (number, optional): Probability (0..1) to apply the stub per request (flaky testing)

Returns:

• stubId (string): Unique id of the installed stub (use it to clear later)

• kind (string): Stub kind (always "mock_http_response")

• pattern (string): Glob pattern used

• enabled (boolean): Whether the stub is enabled

• delayMs (number): Applied artificial delay in milliseconds

• times (number): Max applications (-1 means infinite)

• chance (number, optional): Apply probability (omit means always)

• action (string): Applied action ("fulfill" or "abort")

• status (number, optional): HTTP status (present when action="fulfill")

Use cases:

• Offline testing (return 200 with local JSON)

• Error scenarios (force 500/404 or abort with timedout)

• Edge cases (empty data / huge payload / special characters)

• Flaky API testing (chance < 1.0)

• Performance testing (delayMs)

Notes:

• Pattern is a glob matched against the full request URL

• Stubs are evaluated in insertion order; first match wins

• Times limits how many times the stub applies (-1 means infinite)

Parameters:

• No input parameters

Returns:

• stubs (array): Array of installed stubs, each containing:

  • id (string): Stub id

  • kind (string): Stub kind ("intercept_http_request" or "mock_http_response")

  • enabled (boolean): Whether stub is enabled

  • pattern (string): Glob pattern (picomatch)

  • delayMs (number): Artificial delay in ms

  • times (number): Max applications (-1 means infinite)

  • usedCount (number): How many times it has been applied

  • action (string, optional): For mock_response: "fulfill" or "abort"

  • status (number, optional): For mock_response: HTTP status (if set)

Usage:

• Useful to debug why certain calls are being mocked/intercepted

• Check stub status and usage statistics

• Verify stub configuration before debugging issues

Parameters:

• stubId (string, optional): Stub id to remove. Omit to remove all stubs

Returns:

• clearedCount (number): Number of stubs removed

Usage:

• Remove specific stub by ID when no longer needed

• Clear all stubs to reset the browser context

• Useful after testing or debugging sessions

Figma Tools

Parameters:

• figmaFileKey (string, required): Figma file key (the part after /file/ in Figma URL)

• figmaNodeId (string, required): Figma node id (frame/component node, usually looks like "12:34")

• selector (string, optional): Optional CSS selector to screenshot only a specific element instead of the whole page

• fullPage (boolean, optional): If true, captures the full scrollable page. Ignored when selector is provided (default: true)

• figmaScale (number, optional): Optional scale for Figma raster export (e.g., 1, 2, 3)

• figmaFormat (enum, optional): Optional format for Figma export - "png" or "jpg" (default: "png")

• weights (object, optional): Optional weights for combining signals. Missing/inactive signals are ignored and weights are renormalized:

  • mssim (number, optional): Weight for MSSIM signal

  • imageEmbedding (number, optional): Weight for image embedding signal

  • textEmbedding (number, optional): Weight for vision→text→text embedding signal

• mssimMode (enum, optional): MSSIM mode - "raw" (stricter) or "semantic" (more layout-oriented, default: "semantic")

• maxDim (number, optional): Optional preprocessing max dimension forwarded to compare pipeline

• jpegQuality (number, optional): Optional JPEG quality forwarded to compare pipeline (used only when JPEG encoding is selected internally, range: 50-100)

Returns:

• score (number): Combined similarity score in the range [0..1]. Higher means more similar

• notes (array): Human-readable notes explaining which signals were used and their individual scores

• meta (object): Metadata about what was compared:

  • pageUrl (string): URL of the page that was compared

  • pageTitle (string): Title of the page that was compared

  • figmaFileKey (string): Figma file key used for the design snapshot

  • figmaNodeId (string): Figma node id used for the design snapshot

  • selector (string | null): Selector used for page screenshot, if any. Null means full page

  • fullPage (boolean): Whether the page screenshot was full-page

  • pageImageType (enum): Image type of the captured page screenshot ("png" or "jpeg")

  • figmaImageType (enum): Image type of the captured Figma snapshot ("png" or "jpeg")

How it works:

1. Fetches a raster snapshot from Figma (frame/node screenshot)

2. Takes a screenshot of the live browser page (full page or a specific selector)

3. Computes multiple similarity signals and combines them into one score:

   • MSSIM (structural similarity; always available)

   • Image embedding similarity (optional; may be skipped if provider is not configured)

   • Vision→text→text embedding similarity (optional; may be skipped if provider is not configured)

Usage:

• Prefer 'semantic' MSSIM mode when comparing Figma sample data vs real data (less sensitive to text/value differences)

• Use 'raw' MSSIM mode only when you expect near pixel-identical output

• If you suspect layout/structure mismatch, run with fullPage=true first, then retry with a selector for the problematic region

• Notes explain which signals were used or skipped; skipped signals usually mean missing cloud configuration (e.g., AWS_REGION, inference profile, etc.)

Use cases:

• UI regression checks

• Design parity validation

• "Does this page still match the intended layout?" validation

• Automated visual testing

Architecture

Session Management

The server uses session-based architecture where each MCP client connection gets its own browser context and page. Sessions are automatically cleaned up when:

• The client disconnects

• The session becomes idle (configurable timeout)

• The session is explicitly closed

Browser Support

The server supports multiple browser engines:

• Chromium (default)

• Firefox

• WebKit

Browser Configuration:

• Headless Mode: By default, browsers run in headless mode (BROWSER_HEADLESS_ENABLE=true). Set to false to see the browser window.

• Persistent Context: When enabled (BROWSER_PERSISTENT_ENABLE=true), browser contexts persist across sessions, preserving:

  • Cookies and session data

  • LocalStorage and IndexedDB

  • Browser extensions and settings

  • User preferences

  Persistent contexts are shared across sessions and are not automatically closed when sessions end.

  Important for React Tools: React tools work best with persistent browser context enabled. This allows you to manually install the React DevTools extension in the browser profile, which enables reliable root discovery and component search via __REACT_DEVTOOLS_GLOBAL_HOOK__.

• System Browser: When enabled (BROWSER_USE_INSTALLED_ON_SYSTEM=true), the server uses the system-installed Chrome browser instead of Playwright's bundled browser. This is useful for:

  • Testing with the exact browser version users have

  • Using browser extensions installed on the system

  • Better compatibility with certain web applications

  Note: System browser support is currently only available for Chromium/Chrome.

React DevTools Extension Setup:

• React tools (react_get-component-for-element, react_get-element-for-component) work best when the React DevTools extension is installed in the browser profile

• The MCP server does NOT automatically install the extension - you must install it manually

• Installation Steps:

  1. Enable persistent browser context: BROWSER_PERSISTENT_ENABLE=true

  2. Start the MCP server (or navigate to a page in headful mode)

  3. Manually install the React Developer Tools extension from Chrome Web Store:

     • https://chrome.google.com/webstore/detail/react-developer-tools/fmkadmapgofadopljbjfkapdkoienihi

  4. The extension will be available in all subsequent sessions using the same persistent context

• Without the Extension: Tools will still work but use best-effort DOM scanning for __reactFiber$ pointers, which is less reliable than using the DevTools hook

Browser instances are shared across sessions for efficiency. Each session gets its own isolated browser context, unless persistent context is enabled (in which case contexts are shared).

Buffering & Filtering

Console messages and HTTP requests are buffered in memory with configurable buffer sizes. Both tools support advanced filtering:

• Level-based filtering: Filter by severity/type

• Text search: Search within message/request content

• Time-based filtering: Filter by timestamp

• Incremental retrieval: Use sequence numbers to fetch only new items

• Pagination: Limit results with start/end trimming

OpenTelemetry Integration

When enabled (OTEL_ENABLE=true), the server automatically injects OpenTelemetry instrumentation into all web pages navigated by the browser. This enables:

• Automatic Trace Collection: UI traces are automatically collected for:

  • Document load events

  • Fetch/XHR requests

  • User interactions (clicks, form submissions, etc.)

• Trace Context Propagation: Trace IDs are automatically propagated in HTTP headers (traceparent) for all API calls, enabling:

  • Correlation between frontend and backend traces

  • End-to-end distributed tracing across the entire application stack

• Trace ID Management: Tools allow you to:

  • Get the current session's trace ID

  • Generate new trace IDs

  • Set custom trace IDs (e.g., from backend trace context)

• Exporter Configuration: Traces can be exported to:

  • OTLP/HTTP: Send to OpenTelemetry collector (configure via OTEL_EXPORTER_HTTP_URL)

  • Console: Log traces to browser console (for debugging)

  • None: Collect traces but don't export (for testing)

The OpenTelemetry integration uses a proxy mechanism (/__mcp_otel/) to forward traces from the browser to the configured collector, ensuring proper CORS handling and trace context propagation.

Development

Prerequisites

• Node.js 18+

• npm or yarn

Setup

Scripts

• npm run build - Build TypeScript to JavaScript

• npm run start - Start server with stdio transport

• npm run start:http - Start server with HTTP transport

• npm run watch - Watch mode for development

• npm run inspector - Run MCP Inspector (stdio)

• npm run inspector:http - Run MCP Inspector (HTTP)

• npm run lint:check - Check code formatting

• npm run lint:format - Format code

Use Cases

For AI Coding Assistants

This server enables AI assistants to:

1. Debug Web Applications: Capture screenshots, inspect DOM, check console errors

2. Monitor Network Activity: Track API calls, analyze request/response patterns

3. Distributed Tracing: Enable OpenTelemetry to correlate frontend and backend traces for end-to-end debugging

4. Test User Flows: Automate navigation and interactions

5. Visual Verification: Compare visual states, verify UI changes

6. Design Comparison: Compare live page UI against Figma designs with automated similarity scoring

7. Content Extraction: Get HTML/text content with filtering and cleaning options

8. Accessibility Analysis: Use ARIA and AX tree snapshots to understand page structure and detect UI issues

9. Performance Analysis: Monitor HTTP request timing and failures

Example Workflow

1. Navigate to a web page using navigation_go-to

2. Wait for network idle with sync_wait-for-network-idle if needed (for SPA pages)

3. Take a screenshot with content_take-screenshot to see the current state

4. Check console messages with o11y_get-console-messages for errors

5. Monitor HTTP requests with o11y_get-http-requests to see API calls

6. Capture accessibility snapshots with a11y_take-aria-snapshot and accessibility_take-ax-tree-snapshot to understand page structure

7. Compare page with Figma design using compare-page-with-design to validate design parity

8. Interact with elements using interaction_click, interaction_fill, etc.

9. Extract content using content_get-as-html or content_get-as-text

10. Save the page as PDF using content_save-as-pdf for documentation

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

License

MIT License - see LICENSE file for details.

Author

Serkan Ozal

• Email: serkanozal86@gmail.com

• GitHub: @serkan-ozal

Acknowledgments

• Built with Playwright for browser automation

• Uses Model Context Protocol SDK for MCP implementation

• HTTP transport powered by Hono