llms-txt-validator

Validator for llms.txt — checks parser conformance against the reference spec, link reachability, and missing or malformed sections.

Ships as:

• A hosted web tool at bridgetoagent.com/tools/llms-txt-validator — paste, upload, or fetch by URL. No signup, no email gate.
• A CLI for local files and CI pipelines.
• A JavaScript / TypeScript library for embedding in your own validation flow.

MIT licensed. No telemetry. No external dependencies beyond fetch.

Install

npm install --save-dev @bridgetoagent-com/llms-txt-validator
# or
pnpm add -D @bridgetoagent-com/llms-txt-validator
# or
yarn add -D @bridgetoagent-com/llms-txt-validator

Requires Node 18 or newer (uses the native fetch API).

Quick start

CLI

# Validate a local file
npx @bridgetoagent-com/llms-txt-validator ./llms.txt

# Fetch and validate from a URL
npx @bridgetoagent-com/llms-txt-validator https://example.com/llms.txt

# Also verify every link is reachable
npx @bridgetoagent-com/llms-txt-validator ./llms.txt --check-links

# Machine-readable output for CI
npx @bridgetoagent-com/llms-txt-validator ./llms.txt --json

Exit codes:

| Code | Meaning | | ---- | ------- | | 0 | Valid (status: pass) | | 1 | Warnings only (status: pass_with_warnings) | | 2 | Errors found (status: fail) | | 64 | Bad command-line usage | | 66 | Could not read input |

Library

import { validate } from "@bridgetoagent-com/llms-txt-validator";

const source = await fs.readFile("./llms.txt", "utf8");
const report = await validate(source);

console.log(report.status);     // "pass" | "pass_with_warnings" | "fail"
console.log(report.summary);    // { errors, warnings, infos, sections, links }
console.log(report.issues);     // [{ severity, code, message, line, ... }, ...]
console.log(report.parsed);     // parsed document tree

Enable reachability checking:

const report = await validate(source, {
  checkReachability: true,
  concurrency: 8,       // default
  timeoutMs: 5000,      // default
  slowThresholdMs: 3000 // default
});

console.log(report.reachability); // { checked, failed, slow }

Parser only (no validation, no I/O):

import { parse } from "@bridgetoagent-com/llms-txt-validator";

const doc = parse(source);
// doc.title, doc.description, doc.sections[].links, etc.

What gets checked

Structure (always on)

• # Title is present and is the first non-blank content
• No duplicate H1 headings
• Optional > blockquote description captured immediately after the title
• ## Section headings used for resource groups (H2 level)
• H3+ headings flagged (info — uncommon in llms.txt)
• Empty sections flagged

Link bullets

• - [text](url) or - [text](url): description syntax
• Malformed Markdown links flagged
• Empty link text or URL
• Relative URLs (/docs/foo) — llms.txt is consumed by external agents, must be absolute
• Fragment-only URLs (#anchor)
• mailto: URLs flagged as unusual
• http:// URLs flagged (prefer https://)
• Duplicate URLs (warning) and duplicate link text (info)

Reachability (opt-in)

Pass --check-links (CLI) or { checkReachability: true } (library) to enable network checks:

• Bounded-concurrency HEAD requests against every link
• Falls back to GET when HEAD returns 405 or 501
• link-non-2xx for 4xx/5xx responses
• link-unreachable for network errors and timeouts
• link-slow warning for links above the slow threshold (default 3s)

User-agent: bridgetoagent-llms-txt-validator/0.1 (+https://github.com/bridgetoagent/llms-txt-validator) — identifies itself so server logs aren't anonymous.

Issue codes

Stable identifiers for every kind of finding — pin on these in CI or tooling.

| Code | Severity | Meaning | | ---- | -------- | ------- | | missing-title | error | No # Title heading found | | title-not-first | error | Content appears before the title | | title-not-h1 | error | First heading is not H1 | | duplicate-title | warning | Multiple H1 headings | | section-wrong-level | info | H3+ heading where H2 is conventional | | empty-section | warning | Section has no link bullets | | malformed-link | error | Bullet line is not a valid Markdown link | | link-missing-url | error | Link [text]() has no URL | | link-empty-text | warning | Link [](url) has no display text | | link-relative-url | warning | Root-relative URL — must be absolute | | link-hash-only | warning | Fragment-only URL — meaningless to external agents | | link-mailto | info | mailto: URL — unusual in llms.txt | | link-non-https | warning | http:// — prefer https:// | | duplicate-url | warning | Same URL appears more than once | | duplicate-link-text | info | Same link text appears more than once | | link-unreachable | error | Network error or timeout (reachability mode) | | link-non-2xx | error | HTTP 4xx or 5xx response (reachability mode) | | link-slow | warning | Response slower than threshold (reachability mode) | | no-content-after-title | warning | Title exists but nothing else | | trailing-whitespace | info | Line has trailing whitespace | | tabs-instead-of-spaces | info | Line contains tab characters |

CI / pre-commit usage

GitHub Actions

- name: Validate llms.txt
  run: npx @bridgetoagent-com/llms-txt-validator ./public/llms.txt --check-links

Husky / lint-staged

{
  "lint-staged": {
    "public/llms.txt": "@bridgetoagent-com/llms-txt-validator"
  }
}

Standalone script

#!/usr/bin/env bash
set -euo pipefail
npx @bridgetoagent-com/llms-txt-validator ./public/llms.txt --check-links --json > .llms-txt-report.json

How this compares to other tools

This validator focuses on conformance to the llmstxt.org reference spec plus link health.

It does not:

• Generate llms.txt from a site's DOM (see BridgeToAgent for that)
• Score how good the content of your llms.txt is for any particular agent
• Crawl the URLs it references to validate the linked content

Generating a complete agent-readiness kit from your real site is a separate problem — that's what the BridgeToAgent kit at bridgetoagent.com does ($49, generates agents.json + llms.txt + agent-instructions.md together).

Contributing

Bug reports, edge cases, and PRs welcome. See CONTRIBUTING.md.

Good first issues:

• Real-world llms.txt files we miss-parse (open an issue with the source URL)
• Additional issue codes for spec corner cases
• Performance improvements on large files

License

MIT — copyright 2026 BridgeToAgent editorial.

See also

• The llmstxt.org reference specification
• BridgeToAgent's agent-readiness blog coverage
• The Lighthouse Agentic Browsing audit suite, which includes the llms-txt-well-formed audit this validator helps you pass