Is an llms.txt checker the same as a validator or tester?

In practice checker, validator, tester and linter all describe the same intent. LLMs.txt Checker & Validator covers both sides: it checks whether the file is really being served, and validates the Markdown structure against the llmstxt.org proposal.

What makes an llms.txt valid?

Under the v2 proposal, an H1 naming the site is the only required element. Everything else, from the blockquote summary to per-link descriptions, is a recommendation that makes the file more useful, which is why the report separates errors from warnings and recommendations.

Can I check a draft before publishing?

Yes. Open the paste option and drop in the file content. The same structure tests and Lighthouse criteria run on it, though delivery and link checks need a live URL.

Why does my file fail the link check?

Almost always because the links are plain URLs instead of Markdown links. Only the [Title](URL) form counts.

Does a missing llms.txt lower my Lighthouse result?

No. For that audit a missing file is treated as Not Applicable and drops out of the calculation. A published file that fails the audit can be reported as a failure.

Can the checker validate a file in a subdirectory?

Yes. Paste the full address, for example example.com/docs/llms.txt. The v2 proposal allows files at any path, and the checker also flags links that fall outside the path such a file describes.

Yes. LLMs.txt Checker & Validator is free and needs no account. It reads one public file and the pages it links to.

LLMs.txt Checker & Validator

Enter a domain, a full llms.txt URL, or paste a draft. LLMs.txt Checker & Validator fetches the live llms.txt, validates its Markdown syntax against the llmstxt.org proposal, mirrors the relevant Chrome Lighthouse criteria and tests the URLs it contains. You get a report that separates errors, warnings and recommendations. Checks run server-side, so there are no CORS limits on which sites can be tested. Free, no signup.

What the llms.txt checker validates

File delivery

Plenty of files look fine but are never actually served. The checker reports the HTTP status and content type, and detects the soft 404, where a server answers /llms.txt with an HTML page instead of a plain-text file. LLMs.txt Checker & Validator also reads robots.txt, because a file blocked there stays out of reach for agents that honour it, and notes whether an llms-full.txt sits alongside.

Markdown structure

Against the llmstxt.org proposal the H1 naming the site is the only required element, so that is the one thing reported as an error. The blockquote summary, H2 sections, absolute URLs, per-link descriptions, duplicate URLs, cross-domain links and file size are reported as warnings or recommendations, because they affect usefulness rather than validity. A file served from a subpath is checked against the path it describes, and a leading byte order mark is accepted rather than mistaken for a missing heading.

Getting the format right is the hard part. In a benchmark of the top 1,000 sites, 186 served an llms.txt and only 64 of those passed validation - most files that exist have something wrong with them.

Chrome Lighthouse criteria

The checker mirrors the three criteria from the experimental Agentic Browsing audit in Lighthouse 13.3 and shows the outcome out of three. In practice one fails far more often than the rest: links written as bare URLs instead of Markdown, the classic defect of hand-written files.

This is a structural audit, not a ranking signal in Google Search. For the audit a missing file is treated as Not Applicable, while a published file that fails it can be reported as a failure.

Link health

Every linked address gets a request, and the ones returning an error are listed with their HTTP status. Redirects are reported separately with the destination, so a file still pointing at old URLs shows up as stale rather than broken.

Common llms.txt syntax errors and how to fix them

Missing H1

The first line must be a Markdown heading with your site name, for example # Acme Inc.. Plain text or an H2 does not count. This is the only required element of the format.

Links are not Markdown

Write - [Pricing](https://example.com/pricing): What each plan includes. rather than a bare URL.

Relative URLs

Use absolute links. A model reading the file has no base URL to resolve /pricing against.

Missing descriptions

A link without a description tells a model nothing beyond the title. Add one factual sentence after the colon.

Broken links appear after a site restructure, so regenerate the file whenever important URLs change. The llms.txt generator builds a validated file for you.

Frequently asked questions

Is an llms.txt checker the same as a validator or tester?

In practice checker, validator, tester and linter all describe the same intent. LLMs.txt Checker & Validator covers both sides: it checks whether the file is really being served, and validates the Markdown structure against the llmstxt.org proposal.
What makes an llms.txt valid?

Under the v2 proposal, an H1 naming the site is the only required element. Everything else, from the blockquote summary to per-link descriptions, is a recommendation that makes the file more useful, which is why the report separates errors from warnings and recommendations.
Can I check a draft before publishing?

Yes. Open the paste option and drop in the file content. The same structure tests and Lighthouse criteria run on it, though delivery and link checks need a live URL.
Why does my file fail the link check?

Almost always because the links are plain URLs instead of Markdown links. Only the [Title](URL) form counts.
Does a missing llms.txt lower my Lighthouse result?

No. For that audit a missing file is treated as Not Applicable and drops out of the calculation. A published file that fails the audit can be reported as a failure.
Can the checker validate a file in a subdirectory?

Yes. Paste the full address, for example example.com/docs/llms.txt. The v2 proposal allows files at any path, and the checker also flags links that fall outside the path such a file describes.
Is the checker free?

Yes. LLMs.txt Checker & Validator is free and needs no account. It reads one public file and the pages it links to.

Check your llms.txt

Enter a domain and see whether your file is served correctly, follows the structure and is free of dead links. Need a new file, or want to rebuild a failing one? Use the llms.txt generator.