Web Accessibility MCP Server

An MCP (Model Context Protocol) server that provides web accessibility analysis capabilities using axe-core and Puppeteer.

Features

• Analyze web accessibility of any URL using axe-core

• Simulate color blindness (protanopia, deuteranopia, tritanopia) using color matrices

• Detailed reporting of accessibility violations

• Support for custom user agents and selectors

• Debug logging for troubleshooting

• Comprehensive accessibility checks based on WCAG guidelines

Related MCP server: Cursor A11y MCP

Prerequisites

• Node.js (v14 or higher)

• npm

Installation

Installing via Smithery

To install Web Accessibility MCP Server for Claude Desktop automatically via Smithery:

npx -y @smithery/cli install @bilhasry-deriv/mcp-web-a11y --client claude

Manual Installation

1. Clone the repository:

git clone [repository-url]
cd mcp-web-a11y

2. Install dependencies:

npm install

3. Build the server:

npm run build

Configuration

Add the server to your MCP settings file (typically located at ~/Library/Application Support/Code/User/globalStorage/saoudrizwan.claude-dev/settings/cline_mcp_settings.json):

{
  "mcpServers": {
    "web-a11y": {
      "command": "node",
      "args": ["/path/to/mcp-web-a11y/build/index.js"],
      "disabled": false,
      "autoApprove": [],
      "env": {
        "MCP_OUTPUT_DIR": "/path/to/output/directory"
      }
    }
  }
}

Environment Variables

• MCP_OUTPUT_DIR: Directory where screenshot outputs will be saved

  • Required for the simulate_colorblind tool

  • If not specified, defaults to './output' relative to the current working directory

  • Must be an absolute path when configured in MCP settings

Usage

The server provides two tools: check_accessibility for analyzing web accessibility and simulate_colorblind for simulating color blindness.

Tool: check_accessibility

Checks the accessibility of a given URL using axe-core.

Parameters

• url (required): The URL to analyze

• waitForSelector (optional): CSS selector to wait for before analysis

• userAgent (optional): Custom user agent string for the request

Example Usage

<use_mcp_tool>
<server_name>mcp-web-a11y</server_name>
<tool_name>check_accessibility</tool_name>
<arguments>
{
  "url": "https://example.com",
  "waitForSelector": ".main-content",
  "userAgent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/120.0.0.0 Safari/537.36"
}
</arguments>
</use_mcp_tool>

Tool: simulate_colorblind

Simulates how a webpage appears to users with different types of color blindness using color matrix transformations.

Color Blindness Types

The tool supports three types of color blindness simulation:

1. Protanopia (red-blind) - Uses matrix:

   0.567, 0.433, 0
   0.558, 0.442, 0
   0, 0.242, 0.758

2. Deuteranopia (green-blind) - Uses matrix:

   0.625, 0.375, 0
   0.7, 0.3, 0
   0, 0.3, 0.7

3. Tritanopia (blue-blind) - Uses matrix:

   0.95, 0.05, 0
   0, 0.433, 0.567
   0, 0.475, 0.525

Parameters

• url (required): The URL to capture

• type (required): Type of color blindness to simulate ('protanopia', 'deuteranopia', or 'tritanopia')

• outputPath (optional): Custom path for the screenshot output

• userAgent (optional): Custom user agent string for the request

Example Usage

<use_mcp_tool>
<server_name>mcp-web-a11y</server_name>
<tool_name>simulate_colorblind</tool_name>
<arguments>
{
  "url": "https://example.com",
  "type": "deuteranopia",
  "outputPath": "colorblind_simulation.png"
}
</arguments>
</use_mcp_tool>

Response Format

check_accessibility Response

{
  "url": "analyzed-url",
  "timestamp": "ISO-timestamp",
  "violations": [
    {
      "impact": "serious|critical|moderate|minor",
      "description": "Description of the violation",
      "help": "Help text explaining the issue",
      "helpUrl": "URL to detailed documentation",
      "nodes": [
        {
          "html": "HTML of the affected element",
          "failureSummary": "Summary of what needs to be fixed"
        }
      ]
    }
  ],
  "passes": 42,
  "inapplicable": 45,
  "incomplete": 3
}

simulate_colorblind Response

{
  "url": "analyzed-url",
  "type": "colorblind-type",
  "outputPath": "path/to/screenshot.png",
  "timestamp": "ISO-timestamp",
  "message": "Screenshot saved with [type] simulation"
}

Error Handling

The server includes comprehensive error handling for common scenarios:

• Network errors

• Invalid URLs

• Timeout issues

• DNS resolution problems

Error responses will include detailed messages to help diagnose the issue.

Development

Project Structure

mcp-web-a11y/
├── src/
│   └── index.ts    # Main server implementation
├── build/          # Compiled JavaScript
├── output/         # Generated screenshots
├── package.json    # Project dependencies and scripts
└── tsconfig.json   # TypeScript configuration

Building

npm run build

This will:

1. Compile TypeScript to JavaScript

2. Make the output file executable

3. Place the compiled files in the build directory

Debugging

The server includes detailed debug logging that can be observed in the console output. This includes:

• Network requests and responses

• Page loading status

• Selector waiting status

• Any console messages from the analyzed page

• Color simulation progress

Common Issues and Solutions

1. Timeout Errors

   • Increase the timeout value in the code

   • Check network connectivity

   • Verify the URL is accessible

2. DNS Resolution Errors

   • Verify the URL is correct

   • Check network connectivity

   • Try using the www subdomain

3. Selector Not Found

   • Verify the selector exists on the page

   • Wait for dynamic content to load

   • Check the page source for the correct selector

4. Color Simulation Issues

   • Ensure the page's colors are specified in a supported format (RGB, RGBA, or HEX)

   • Check if the page uses dynamic color changes (may require additional wait time)

   • Verify the screenshot output directory exists and is writable

Contributing

1. Fork the repository

2. Create a feature branch

3. Commit your changes

4. Push to the branch

5. Create a Pull Request

License

This project is licensed under the MIT License - see the LICENSE file for details.