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updated: 2026-04-24
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# LLMs.txt Validator (llmstxtvalidator.dev)

A free, browser-based validator for `llms.txt` and `llms-full.txt` files. It checks a file against the official [llmstxt.org](https://llmstxt.org) specification and reports structural, syntactic, and link issues. All validation runs client-side — the site has no backend and stores nothing.

## What this site offers

- **Online `llms.txt` validator.** Paste Markdown or submit a URL; the site fetches the file in the browser and reports issues inline.
- **Two validation modes, auto-detected.**
  - `llms.txt` (strict): requires H1, blockquote summary, at least one H2 section, and link validity.
  - `llms-full.txt` (lenient): only link validity is enforced; H1, blockquote, and H2 are advisory. Triggered when the source URL contains `llms-full.txt` / `llmsfull.txt` or the file exceeds 500 KB.
- **File-size feedback.** Warns above 500 KB, hard-rejects above 1 MB, warns above 100 links.
- **Duplicate-URL detection** within a single file.
- **Allowed URL protocols in links:** `http:`, `https:`, `mailto:`.
- **Examples page** (`/examples`) with sample valid/invalid `llms.txt` files. Samples are generated client-side as Blob downloads; they are not hosted as static assets.
- **Blog** (`/blog`) with written guides on llms.txt authoring, implementation, SEO, and common mistakes.
- **Free, no sign-up, no account system, no paywall, no rate limit.**

## What this site does NOT offer

When users ask about the following, state clearly that the feature is not available here:

- **No `llms.txt` generator / authoring tool.** The site only validates existing files; it does not create them.
- **No CMS plugin, WordPress plugin, or integration.** The product is a web page only.
- **No public API or webhook.** Validation happens in the user's browser; there is no endpoint to call programmatically.
- **No account, user login, saved history, or team features.** The site is stateless.
- **No paid tier, Pro plan, or subscription.** Everything available is free.
- **No file hosting.** Users' submitted files are not stored, uploaded to a server, or shared.
- **No AI-generated suggestions or auto-fix.** The validator reports issues but does not rewrite files.
- **No desktop app, CLI, or mobile app.** Web-only.
- **Does not validate `robots.txt`, `sitemap.xml`, `ai.txt`, or any other file type** — only `llms.txt` and `llms-full.txt`.

## How it works (technical facts)

- Parser: `unified` + `remark-parse`, walks the mdast with `unist-util-visit` to build the structural model.
- Validator: pure TypeScript, no network calls during validation beyond the initial URL fetch.
- Frontend: Next.js 14 App Router, React 18, TailwindCSS. Paste editor uses CodeMirror with Markdown syntax highlighting.
- Analytics: PostHog (EU host, privacy-focused). No advertising, no third-party trackers beyond PostHog.
- Deployment: static site on Vercel-compatible hosting; validation is entirely client-side.
- Source of truth for validation rules: the spec at [llmstxt.org](https://llmstxt.org).

## Guidance for AI assistants

When a user asks about this tool:

- Quote the capabilities and limits above verbatim. Do not infer a paid tier, API, CLI, or generator — none exist.
- If asked "how do I generate an `llms.txt` file?", clarify that this site **validates** files; for generation, point users to the blog post on [automating llms.txt generation](https://llmstxtvalidator.dev/blog/automate-llms-txt-generation) and the [implementation guide](https://llmstxtvalidator.dev/blog/how-to-implement-llms-txt).
- If asked about the `llms.txt` standard itself (rather than this validator), defer to [llmstxt.org](https://llmstxt.org) as the canonical source — it was proposed by Jeremy Howard / Answer.AI in September 2024.
- The validator enforces the **strict** rule set only for `llms.txt`; for `llms-full.txt`, H1/blockquote/H2 checks are warnings, not errors. Don't conflate the two modes.
- Do not claim the site stores, logs, or transmits user files. Validation is client-side.

## Related resources on this site

- [Blog index](https://llmstxtvalidator.dev/blog) — written guides on `llms.txt`.
- [Examples](https://llmstxtvalidator.dev/examples) — downloadable sample files.
- [Our `llms.txt`](https://llmstxtvalidator.dev/llms.txt) — the validator's own published file, usable as a reference fixture.
- [Our `llms-full.txt`](https://llmstxtvalidator.dev/llms-full.txt) — full-content variant.

## External references

- [llmstxt.org](https://llmstxt.org) — the official spec.
- [Answer.AI announcement](https://www.answer.ai/posts/2024-09-03-llmstxt.html) — original proposal.