Free llms.txt Validator & Syntax Checker
Validate your llms.txt or llms-full.txt files against the official specification. Paste your raw Markdown or fetch by URL to verify heading hierarchy, syntax formats, and token budgets.
What is an llms.txt validator?
An llms.txt validator is a developer utility that checks whether a website's llms.txt file adheres to the official Markdown specification for AI web crawlers. It verifies heading structures, blockquote summaries, UTF-8 encoding, and link formatting to ensure Large Language Models can accurately index site content.
What It Checks
The validator runs 9 specification checks against your llms.txt file. A file must pass all 9 checks to be considered fully compliant.
| Check | What It Tests | Pass Condition |
|---|---|---|
| File Accessibility | Is llms.txt reachable at root domain? | HTTP 200 response at /llms.txt |
| Single H1 Rule | Does file have exactly one H1? | Exactly 1 H1 heading found |
| Blockquote Summary | Is there a blockquote after H1? | `>` blockquote present |
| H2 Sections | Are sections organized with H2? | At least 1 H2 section |
| No H3+ Headings | No deep heading levels | Zero H3/H4/H5/H6 tags |
| Absolute URLs | Are all links absolute? | All links start with https:// |
| Markdown Links | Correct link format? | `[Title](URL)` format used |
| File Size | Under 50KB? | File ≤ 50,000 bytes |
| Bullet Format | Links in bullet lists? | `-` or `*` bullets used |
What Does a Valid llms.txt Look Like?
Here is a side-by-side comparison of a spec-compliant file and one with common errors.
# My Awesome Blog > A curated collection of guides about web development. ## Featured Posts - [How to Build a React App](https://myblog.com/react-guide): Walkthrough - [CSS Grid Mastery](https://myblog.com/css-grid): Reference
# My Awesome Blog ## Featured Posts (missing blockquote!) - [How to Build a React App](/react-guide) (relative URL!) ### Deep Dive (H3 heading — not allowed!) # Additional H1 (duplicate H1 — only one allowed!)
The Most Common Validation Errors
1. Mismatched or Broken URLs
Fix: Verify every URL in your file. Use absolute paths starting with https://.
2. Multiple H1 Headings
Fix: Use exactly one H1 at the top. Convert additional headings to H2.
3. Sub-directory Placement
Fix: Move the file to your domain root (https://yourdomain.com/llms.txt).
Frequently Asked Questions
How is llms.txt different from robots.txt and sitemap.xml?▼
They perform three distinct jobs. `robots.txt` acts as an access control protocol telling crawlers what paths they cannot fetch. `sitemap.xml` provides a complete URL inventory for standard indexing. `llms.txt` acts as a curated Markdown summary designed specifically to feed structured context to Large Language Models and AI agents.
Does Google Lighthouse check for an llms.txt file?▼
Yes. Google Chrome Developers integrated an llms.txt check into Google Lighthouse under the Agentic Browsing category. While currently marked as optional, verifying your file prevents Lighthouse 404 fetch flags and ensures your site is ready for AI agent discovery.
What is the difference between llms.txt and llms-full.txt?▼
`llms.txt` is a lightweight, curated index containing category links and one-line summaries for agents that fetch pages selectively. `llms-full.txt` embeds your full plaintext content inline into one large file, designed for RAG pipelines and high-context AI ingestion to avoid network round-trips.
What are the strict syntax rules required for llms.txt?▼
According to the v1.7.0 spec, the file must be hosted at the root domain (`https://example.com/llms.txt`) served as `text/plain; charset=utf-8`. It must contain exactly one `# H1 Title` at the very top, followed immediately by a `> Blockquote` site summary, and format all category links strictly as: `- [Title](Absolute-URL): Description`.
Ready to verify your site's agentic readiness? Scroll up to run the free llms.txt checker.
Integrate into GitHub Actions / CI/CD Pipelines
curl -X POST https://llmstxtapp.com/api/validate \
-H "Content-Type: application/json" \
-d '{"markdown": "# Your Brand\n> Site description\n## Links\n- [Docs](https://docs.site.com)"}'Designed for Google Lighthouse & Agentic Browsing Compliance
AI agents, search spiders, and RAG parsers consume site manifests differently. Our validator runs rigorous technical audits designed to meet compliance standards of modern browsing bots.
Lighthouse Agentic Audits
Verifies heading structure and markdown outlines. Ensures that automated agents running accessibility and performance audits can parse your site index without experiencing semantic layout issues.
OpenAI GPTBot Discovery
Tests HTTP status codes and strict MIME-type constraints. Compares headers against strict GPT-4o browsing vectors to guarantee your domain gets cataloged by GPTBot during system crawls.
Anthropic Claude Parsing
Audits token limits and sectioning rules (single H1 with direct summary). Prevents prompt context window overflow when Claude's RAG system retrieves lists under H2 headings.