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Web Accessibility MCP Server

by bilhasry-deriv
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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

  1. Install dependencies:

npm install

  1. 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.

Available Tools

2 tools
check_accessibilityC

Check web accessibility of a given URL using axe-core

ParametersJSON Schema
NameRequiredDescriptionDefault
urlYesURL to analyze
waitForSelectorNoOptional CSS selector to wait for before analysis
userAgentNoOptional user agent string to use for the request

TDQS

C2.9/5.0
Behavior2/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

With no annotations provided, the description carries the full burden of behavioral disclosure. It states what the tool does but doesn't describe how it behaves: it doesn't mention whether this is a read-only analysis, what the output format might be, potential rate limits, authentication requirements, or error conditions. For a tool that performs web analysis, this leaves significant behavioral gaps.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a single, efficient sentence that communicates the core purpose without any wasted words. It's appropriately sized for a tool with a clear, focused function and is front-loaded with the essential information.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness2/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given that there are no annotations and no output schema, the description should provide more complete context for this accessibility checking tool. It doesn't explain what kind of results to expect, what accessibility standards are checked, whether the analysis is comprehensive or limited, or how the tool handles dynamic content. For a tool with 3 parameters and no structured output documentation, this is insufficient.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The input schema has 100% description coverage, so all parameters are documented in the schema itself. The description doesn't add any parameter-specific information beyond what's already in the schema descriptions. According to the scoring rules, when schema_description_coverage is high (>80%), the baseline is 3 even with no param info in the description.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool's purpose with a specific verb ('Check') and resource ('web accessibility of a given URL'), and mentions the technology used ('axe-core'). However, it doesn't explicitly differentiate from its sibling tool 'simulate_colorblind', which appears to be a related but distinct accessibility function.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines2/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides no guidance on when to use this tool versus its sibling 'simulate_colorblind' or other alternatives. It doesn't mention prerequisites, typical use cases, or exclusions, leaving the agent with no contextual usage information beyond the basic purpose.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

simulate_colorblindC

Simulate how a webpage looks for colorblind users

ParametersJSON Schema
NameRequiredDescriptionDefault
urlYesURL to capture
typeYesType of color blindness to simulate
outputPathNoOptional path to save the screenshot
userAgentNoOptional user agent string to use for the request

TDQS

C2.9/5.0
Behavior2/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

No annotations are provided, so the description carries the full burden of behavioral disclosure. It states the tool simulates colorblind views but doesn't describe how (e.g., generates a screenshot, modifies display, or returns data), what the output is (e.g., image file, visual report), or any behavioral traits like performance, rate limits, or side effects. This leaves significant gaps for an agent to understand the tool's operation.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a single, clear sentence: 'Simulate how a webpage looks for colorblind users.' It is front-loaded with the core purpose, has zero wasted words, and is appropriately sized for the tool's complexity. Every part of the sentence earns its place by conveying essential information efficiently.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness2/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

Given the tool's moderate complexity (4 parameters, no output schema, no annotations), the description is incomplete. It lacks details on output (e.g., what is returned or saved), behavioral context (e.g., how simulation works, any limitations), and usage guidelines. While the schema covers parameters well, the description doesn't compensate for missing annotations or output schema, leaving the agent with insufficient context for effective use.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters3/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The input schema has 100% description coverage, clearly documenting all four parameters (url, type, outputPath, userAgent) with details like enum values for 'type.' The description doesn't add any parameter-specific information beyond what the schema provides, such as explaining the simulation process or output format. Given the high schema coverage, a baseline score of 3 is appropriate as the schema handles the heavy lifting.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool's purpose: 'Simulate how a webpage looks for colorblind users.' It specifies the action (simulate) and resource (webpage appearance for colorblind users), making it easy to understand. However, it doesn't explicitly differentiate from its sibling tool 'check_accessibility,' which might also involve accessibility testing, though the focus here is specifically on colorblind simulation.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines2/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides no guidance on when to use this tool versus alternatives. It doesn't mention the sibling tool 'check_accessibility' or any other tools, nor does it specify prerequisites, contexts, or exclusions. Usage is implied from the purpose but lacks explicit direction.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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TDQS

B3.1/5.0
Disambiguation5/5

The two tools have clearly distinct purposes: one checks general web accessibility using axe-core, while the other specifically simulates colorblindness effects on a webpage. There is no overlap or ambiguity between them.

Naming Consistency5/5

Both tools follow a consistent verb_noun pattern (check_accessibility, simulate_colorblind) with clear, descriptive names. The naming style is uniform and predictable throughout the set.

Tool Count2/5

With only two tools, the server feels thin for a web accessibility domain. While the tools are useful, typical accessibility testing involves more operations like checking screen reader compatibility, keyboard navigation, or ARIA attributes, suggesting notable gaps in coverage.

Completeness2/5

The tool set is severely incomplete for web accessibility. It lacks core operations such as validating HTML structure, testing screen reader output, assessing keyboard accessibility, or generating accessibility reports, which are essential for comprehensive accessibility evaluation.

Maintenance

ActivityInactive
ResponsivenessSyncing

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