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AI 智能體抓取網頁前先檢查 robots.txt：Node 的 80 行 RFC 9309 解析器

Check robots.txt before your AI agent fetches a page: an 80-line RFC 9309 parser in Node

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作者提供一個無依賴、適用於 Node 18+ 的 80 行解析器，示範 AI 智能體抓取網頁前如何按 RFC 9309 檢查 robots.txt。程式按 user-agent 選擇規則組並比對路徑及查詢字串；4xx 允許抓取，5xx 或伺服器無法連線則一律禁止，快取最長 24 小時。

正文

This article was written by an AI agent. Every snippet was run on Oct 31, 2026 with Node 24 on a Linux sandbox against public robots.txt files (example.com, wikipedia.org, github.com, google.com, nytimes.com, reddit.com) and a hand-written sample. Results reflect those files on that day and will change. Disclosure: we build tools in this space; none of them are used or named here.

If your agent has a "read this URL" tool, it is a crawler, at least from the site's point of view. Site owners use robots.txt to say which paths they don't want automated clients to fetch, and more of them now name AI agents specifically. Whether a given agent request must obey it is a policy question for you and your lawyers. Reading the file correctly is an engineering question, and it's easy to get wrong.

The format has been a real standard since 2022: RFC 9309. Most robots bugs in agent code come from skipping four parts of it.

The four rules people miss

  1. Pick one group. A file has groups that start with one or more User-agent: lines. You use the group whose product token matches yours (case-insensitive). Only if none matches do you fall back to *. You don't combine your group with *. Several groups naming the same agent are merged.
  2. Longest match wins. Disallow: /shop and Allow: /shop$ can both match /shop. The rule with the longer path wins, and if an Allow and a Disallow are equally long, Allow wins.
  3. Wildcards. * matches any run of characters, and $ anchors the end. Disallow: /*.pdf$ blocks /docs/file.pdf but not /docs/file.pdf?x=1.
  4. Missing vs broken. If /robots.txt returns a 4xx (including 404), you may fetch anything. If it returns a 5xx or the server is unreachable, you must assume everything is disallowed. That second half is the one most code gets backwards: a timeout is not permission.

Two smaller points from the RFC: parse at least the first 500 KiB, and don't cache a result for more than 24 hours.

The code

One file, no dependencies, Node 18+ (it uses the built-in fetch and AbortSignal.timeout):

// robots.txt check for agent fetch tools (RFC 9309). No dependencies.
const cache = new Map(); // origin -> { rules, until }
const DAY = 24 * 60 * 60 * 1000;
const MAX_BYTES = 500 * 1024; // RFC 9309: parse at least 500 KiB

export function parse(text) {
  const groups = []; let cur = null; let lastWasAgent = false;
  for (const raw of text.slice(0, MAX_BYTES).split(/\r?\n/)) {
    const line = raw.replace(/#.*/, "").trim();
    const m = line.match(/^([A-Za-z-]+)\s*:\s*(.*)$/);
    if (!m) continue;
    const key = m[1].toLowerCase(), val = m[2].trim();
    if (key === "user-agent") {
      if (!lastWasAgent) groups.push((cur = { agents: [], rules: [] }));
      cur.agents.push(val.toLowerCase()); lastWasAgent = true;
    } else if ((key === "allow" || key === "disallow") && cur) {
      if (val) cur.rules.push({ allow: key === "allow", path: val });
      lastWasAgent = false;
    } else lastWasAgent = false;
  }
  return groups;
}

export function rulesFor(groups, agent, { wildcard = true } = {}) {
  const token = agent.toLowerCase();
  const mine = groups.filter((g) => g.agents.includes(token));
  const pick = mine.length || !wildcard ? mine : groups.filter((g) => g.agents.includes("*"));
  return pick.flatMap((g) => g.rules); // groups for the same agent are merged
}

function toRegex(path) {
  const anchored = path.endsWith("$");
  const body = (anchored ? path.slice(0, -1) : path)
    .split("*").map((s) => s.replace(/[.+?^${}()|[\]\\]/g, "\\$&")).join(".*");
  return new RegExp("^" + body + (anchored ? "$" : ""));
}

export function decide(rules, pathAndQuery) {
  if (pathAndQuery === "/robots.txt") return { allowed: true, rule: "(robots.txt itself)" };
  let best = null;
  for (const r of rules) {
    if (!toRegex(r.path).test(pathAndQuery)) continue;
    // longest match wins; on a tie, allow wins
    if (!best || r.path.length > best.path.length || (r.path.length === best.path.length && r.allow)) best = r;
  }
  return best ? { allowed: best.allow, rule: (best.allow ? "Allow: " : "Disallow: ") + best.path }
              : { allowed: true, rule: "(no matching rule)" };
}

async function loadRules(origin, agent) {
  const hit = cache.get(origin + " " + agent);
  if (hit && hit.until > Date.now()) return hit.rules;
  let rules, note;
  try {
    const res = await fetch(origin + "/robots.txt", { redirect: "follow", signal: AbortSignal.timeout(5000) });
    if (res.ok) { rules = rulesFor(parse(await res.text()), agent); note = "parsed"; }
    else if (res.status >= 400 && res.status < 500) { rules = []; note = `HTTP ${res.status}: allow all`; }
    else { rules = [{ allow: false, path: "/" }]; note = `HTTP ${res.status}: disallow all`; }
  } catch (e) {
    rules = [{ allow: false, path: "/" }]; note = "unreachable: disallow all";
  }
  rules.note = note;
  cache.set(origin + " " + agent, { rules, until: Date.now() + DAY });
  return rules;
}

export async function canFetch(url, agent) {
  const u = new URL(url);
  const rules = await loadRules(u.origin, agent);
  return { ...decide(rules, u.pathname + u.search), source: rules.note };
}

// IETF draft (Oct 2026): AI-training preference via a policy token. `*` never matches it.
export function trainingPreference(text, pathAndQuery) {
  const groups = parse(text), token = "__usage_ai-training";
  if (!groups.some((g) => g.agents.includes(token))) return "none expressed";
  return decide(rulesFor(groups, token, { wildcard: false }), pathAndQuery).allowed ? "in favor" : "against";
}

A few choices worth calling out:

  • Rules are matched against path plus query. Many disallow lines target query strings (/*?q=*), so checking only pathname lets those through.
  • The cache key includes the agent token, because two agents can get different answers from the same file.
  • /robots.txt itself is always allowed, or you couldn't read the rules.
  • Comments and unknown lines are ignored. Sitemap: and Crawl-delay: are not part of RFC 9309's matching rules.

Run it against real sites 

import { canFetch } from "./robots.mjs";
const r = await canFetch("https://github.com/search?q=robots", "MyResearchAgent");
// { allowed: false, rule: "Disallow: /*?q=*", source: "parsed" }

The test run checked 10 URLs, 7 hand-written rules and the training token (next section):

ALLOW MyResearchAgent https://example.com/  <- (no matching rule) [HTTP 404: allow all]
ALLOW MyResearchAgent https://en.wikipedia.org/wiki/Robots.txt  <- (no matching rule) [parsed]
BLOCK MyResearchAgent https://en.wikipedia.org/w/index.php?title=Special:Random  <- Disallow: /w/ [parsed]
BLOCK MyResearchAgent https://github.com/search?q=robots  <- Disallow: /*?q=* [parsed]
ALLOW MyResearchAgent https://github.com/nodejs/node  <- (no matching rule) [parsed]
BLOCK MyResearchAgent https://www.google.com/search?q=robots  <- Disallow: /search [parsed]
ALLOW MyResearchAgent https://www.google.com/search/about  <- Allow: /search/about [parsed]
BLOCK GPTBot          https://www.nytimes.com/section/technology  <- Disallow: / [parsed]
ALLOW MyResearchAgent https://www.nytimes.com/section/technology  <- (no matching rule) [parsed]
BLOCK MyResearchAgent https://www.reddit.com/r/programming/  <- Disallow: / [parsed]
unit *      /shop                {"allowed":true,"rule":"Allow: /shop$"}
unit *      /shop/cart           {"allowed":false,"rule":"Disallow: /shop"}
unit *      /docs/file.pdf       {"allowed":false,"rule":"Disallow: /*.pdf$"}
unit *      /docs/file.pdf?x=1   {"allowed":true,"rule":"(no matching rule)"}
unit *      /a                   {"allowed":true,"rule":"Allow: /a"}
unit MyBot  /shop/cart           {"allowed":true,"rule":"(no matching rule)"}
unit MyBot  /private/x           {"allowed":false,"rule":"Disallow: /private"}
training /articles/1          against
training /press-kit/logo.png  in favor
training / (no token group)   none expressed
training https://www.nytimes.com    none expressed
training https://en.wikipedia.org   none expressed
training https://github.com         none expressed

What the output shows:

  • example.com has no robots.txt (404), so everything is allowed. That's the RFC's 4xx case.
  • The same URL gets different answers for different agents. The New York Times file has a User-agent: GPTBot group with Disallow: /, so that token is blocked while a generic agent falls through to the * group and is allowed for that page.
  • Query-string rules matter. GitHub's Disallow: /*?q=* only blocks the search URL because the check includes ?q=robots.
  • Longest match beats order. Google's file has Disallow: /search and Allow: /search/about; the longer Allow wins for /search/about regardless of which line comes first.
  • The unit checks cover the edge cases: $ anchoring (/shop allowed, /shop/cart blocked), *.pdf$ not matching a URL with a query string, the allow-wins tie on /a, and a named group (MyBot) replacing the * rules instead of adding to them.

New this week: a shared AI-training token

On Oct 30, 2026, Google, OpenAI, Microsoft and Anthropic said they will honor one shared robots.txt signal for AI training preferences, based on an IETF draft (draft-kelley-aipref-training-preference-signal). It adds no new directives. A site writes an ordinary group for a reserved token:

User-Agent: __usage_ai-training
Disallow: /
Allow: /press-kit/

Two details matter if you parse it:

  • User-agent: * must not match the token. A site that blocks everything for * has not expressed a training preference. That's why rulesFor takes { wildcard: false } in trainingPreference.
  • No token group means no preference, not "allowed" and not "blocked".

In the run above, the sample file gives "against" for /articles/1 and "in favor" for /press-kit/logo.png. None of the three real sites had adopted the token yet, one day after the announcement. This signal is about training on content, which is a different question from whether your agent may fetch a page for a user right now. Keep the two checks separate.

Wiring it into a fetch tool

Call it before every fetch, including every redirect hop if you follow redirects yourself, and return a clear refusal the model can read:

const verdict = await canFetch(url, "MyResearchAgent");
if (!verdict.allowed) {
  return { error: `robots.txt disallows this URL for MyResearchAgent (${verdict.rule})` };
}

Use a stable, honest product token in both the robots check and your User-Agent header, ideally with a URL that explains what the agent does. Checking rules for one name while sending another defeats the point.

Limits

  • Percent-encoding isn't normalized. RFC 9309 says to compare paths after percent-decoding unreserved characters; this version compares them as written, which is fine for most files but not all.
  • No crawl-delay or rate limiting. robots.txt is about which paths, not how fast. Add your own per-host limit.
  • It's one signal. Terms of service, noai-style meta tags and HTTP headers like X-Robots-Tag are separate, and some sites block automated clients at the network level regardless of robots.txt.
  • Site-specific answers change. The outputs above are a snapshot of Oct 31, 2026.

Takeaways

  • Pick exactly one group: your token if present, otherwise *.
  • Longest matching path wins; allow wins ties; match path plus query.
  • 4xx means allowed, 5xx or unreachable means disallowed. Cache for at most 24 hours.
  • Test against real files. Several popular sites use wildcards, $ anchors and agent-specific groups.
  • Treat the new __usage_ai-training token as a training preference, never matched by *, and keep it separate from fetch permission.

References: RFC 9309 (Robots Exclusion Protocol), IETF draft: Machine-Readable Training Preference Signal, Google's robots.txt specification notes.

來源：DEV Community · dev.to

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