Web Validator by DigestSEO README
The full upstream README, mirrored here for reference. Install config, tool schemas, adoption signals, and an original overview live on the Web Validator by DigestSEO listing page.
Web Validator by DigestSEO
An MCP server for HTML and CSS validation, technical SEO and accessibility checks, JSON-LD syntax validation, broken-link checks, and responsive screenshots. It is part of the DigestSEO suite of open-source SEO tools.
- Product: digestseo.com/validator-mcp
- Engineering case study: DigestSEO MCP Suite — AI visibility, Search Console, web validation, and trend intelligence
- Documentation: this README
- Support: digestseo.com/support
- Privacy: digestseo.com/privacy
Choose the right surface
This repository contains two deliberately separate MCP surfaces:
| Surface | Transport | Best for | Access and side effects |
|---|---|---|---|
| Local npm server | stdio | Claude Desktop, Cursor, and other local MCP clients | Can read user-selected workspace files, contact validation and link targets, and write screenshot files. |
| Hosted app | Streamable HTTP | ChatGPT and remote MCP clients | Can fetch one authorized public HTML page, run a bounded sitemap-first site audit, or process supplied markup. It cannot access local files, authenticate, recursively crawl links, execute page JavaScript, or create screenshots. |
Local installation:
Hosted endpoint:
Requirements
- Node.js 22.12.0 or newer
- An MCP client that supports stdio, or a client that supports Streamable HTTP for the hosted endpoint
The validation, SEO, schema, link, and report tools do not download a browser during installation. The local screenshot.capture tool installs its pinned headless browser into Puppeteer's cache on first use, then reuses that browser for later screenshots. Set PUPPETEER_CACHE_DIR if you need a custom cache location.
Tools
The npm package exposes these exact runtime tool names:
| Tool | Purpose | Network or filesystem behavior |
|---|---|---|
html.local | Validate a local HTML or XHTML file. | Reads the selected file and submits its markup to the external W3C Nu HTML Checker at validator.w3.org/nu/; .xhtml paths use application/xhtml+xml parsing semantics. |
html.url | Validate the markup returned by a public URL. | Fetches the URL, then submits the returned markup to the external Nu checker. |
css.local | Validate a local CSS file. | Reads the selected file and submits its CSS to the external W3C Jigsaw CSS Validator. |
seo.metadata | Audit titles, descriptions, canonical tags, robots noindex directives, headings, viewport metadata, image alt attributes, and Open Graph metadata. | Processes supplied HTML locally. |
links.broken | Check links extracted from supplied HTML. | Sends bounded HTTP requests to eligible public links. |
schema.markup | Parse JSON-LD blocks and report JSON syntax errors. | Processes supplied HTML locally. |
report.validation | Combine HTML/XHTML, optional CSS, SEO, JSON-LD, and bounded link checks in a Markdown report. | Reads selected files, contacts the validators, and checks eligible public links found in the markup; .xhtml inputs use XHTML parsing semantics for Nu validation. |
screenshot.capture | Capture desktop, tablet, mobile, or custom viewport screenshots. | Opens a selected local file or eligible public URL and writes PNG files to the selected output directory; existing matching files may be replaced. |
Hosted app tools
The hosted app exposes eight tools:
| Tool | Purpose |
|---|---|
audit_public_webpage | Fetch one authorized public HTML page and run the combined HTML, SEO/accessibility-signal, and JSON-LD syntax audit. Link checks are optional. |
audit_public_site | Run a bounded sitemap-first audit of up to eight authorized, same-origin public pages. It respects robots.txt, returns compact page coverage and deduplicated findings, and does not run site-wide link checks. |
validate_html | Validate raw HTML markup already supplied in the conversation. |
validate_css | Parse supplied CSS for syntax errors inside the Worker. |
audit_seo_metadata | Audit supplied HTML for covered on-page SEO, robots noindex, and accessibility signals. |
validate_schema_markup | Check JSON-LD blocks in supplied HTML for JSON syntax errors. |
check_broken_links | Check up to 20 authorized public links extracted from supplied HTML. |
generate_validation_report | Combine the checks for supplied markup; base_url resolves relative links but does not fetch a page. |
audit_public_webpage follows at most three validated redirects, accepts a bounded text/html response, and records the final URL. It does not crawl additional pages, execute JavaScript, authenticate, or fetch linked stylesheets and assets. audit_public_site first locks the final public origin, then reads bounded robots.txt plus same-origin XML Sitemap, RSS 2.0, Atom 1.0, and plain-text sitemap representations before auditing at most eight eligible sitemap pages in one call. Robots discovery may follow up to five validated public redirects across authorities, as defined by the robots protocol; the resulting rules and relative Sitemap directives are still interpreted for the originally audited authority. Sitemap entries and audited pages remain same-origin, and private/reserved destinations, credentials, custom ports, service self-fetches, and HTTPS-to-HTTP redirect downgrades are rejected. The tool never follows HTML links or external sitemap entries; use page_offset when the response says more eligible pages remain. The fetched HTML is sent to https://html5.validator.nu/, the same external Nu HTML Checker used by validate_html and the report tool. CSS parsing, SEO analysis, and JSON-LD parsing run inside the DigestSEO Worker. Optional link checks make capped HEAD requests, with a bounded GET fallback where necessary, to eligible public HTTP(S) URLs; link redirects are reported but not followed. The site tool intentionally does not perform site-wide link checking. The hosted app does not retain tool inputs, fetched HTML, or results.
Privacy Policy
Only validate files, markup, and public webpages that you own or are authorized to inspect. Public page URLs can contain sensitive query values, so remove tokens and private identifiers before submitting them. Do not submit passwords, API keys, access tokens, payment data, health data, private source code, or other sensitive personal data.
The local server runs with the same operating-system permissions as its MCP client. Review tool inputs before approving file access, outbound validation, link checking, or screenshot creation. The hosted app has no access to your local filesystem.
See the published privacy policy for data-handling details and SECURITY.md for private vulnerability reporting.
Configure a local client
Claude Desktop
Add the server to claude_desktop_config.json:
- Windows:
%APPDATA%\Claude\claude_desktop_config.json - macOS:
~/Library/Application Support/Claude/claude_desktop_config.json
Restart Claude Desktop after saving the configuration.
Cursor and compatible clients
Create a command/stdio MCP server with:
This repository also includes a Cursor Marketplace plugin manifest. After the plugin is published, install Web Validator by DigestSEO from Cursor's Marketplace or add the repository from Cursor's plugin UI. The plugin bundles the same local stdio server and does not require API keys or environment variables.
OpenAI Codex
Codex CLI can add the published local stdio package directly:
Or add the existing hosted Streamable HTTP surface instead:
Verify either configuration with:
The local package includes workspace-file validation and responsive screenshot capture. The hosted surface is remote-only and cannot access local files or create screenshots.
Gemini CLI
Gemini CLI supports local stdio MCP servers. Add the published package with:
This writes the server definition to Gemini CLI's MCP configuration. No API keys or environment variables are required by Web Validator.
GitHub Copilot CLI
Add the published local stdio server with:
When Copilot CLI is started inside a clone of this repository, it can also
discover the checked-in root .mcp.json, which already points
mcp-web-validator at npx -y mcp-web-validator.
Kiro
This installs the local stdio package, preserving its workspace-file and screenshot capabilities. Node.js 22.12.0+ is required.
Development
Clone the repository and install the locked dependencies:
Run the local quality gates:
Run the local stdio server:
Validate the hosted Worker separately:
See CONTRIBUTING.md for contribution expectations. Deployment and marketplace-review notes live in chatgpt-plugin/SUBMISSION.md. For agent-assisted installation, see llms-install.md.
Ecosystem
License
Licensed under the MIT License.