Content Signals in robots.txt

What it is

Content Signals is a proposed extension to robots.txt that adds new directives expressing how a site wants its content treated downstream — specifically by AI systems. The directives live in normal robots.txt groups and declare boolean preferences such as "you may use this for search" or "you may not train on this".

User-agent: *
Allow: /
Content-Signal: search=yes, ai-input=yes, ai-train=no

The three values that are emerging as canonical:

• search — whether the content may be indexed by search engines.
• ai-input — whether the content may be fetched as live input to an AI system (retrieval-augmented generation, summarisation at query time).
• ai-train — whether the content may be included in a training corpus.

Each takes yes or no. Multiple values are comma-separated on a single Content-Signal: line.

Status of the proposal

This is not yet a settled standard. The work is split across two ongoing efforts:

• The IETF AI Preferences working group (aipref) is working on a vocabulary and protocol-level definition.
• The IAB Tech Lab Content Signals project has published a parallel specification aimed at industry adoption.

Drafts have been circulating since 2024 and the vocabulary is converging. Treat Content Signals as recommended-to-experiment-with, not as a finalised standard. The directive will be ignored by every crawler that does not yet parse it — which today is most of them.

Why it matters

• A declarative opt-in / opt-out at the right layer. Existing User-agent: GPTBot / Disallow: directives are coarse — they block a specific bot from fetching anything. Content Signals separates what the bot may do from who the bot is.
• Validators already check for it. isitagentready.com explicitly looks for Content-Signal: lines in robots.txt. Sites that want a clean agent-readiness scorecard should add them.
• Some major model providers have signalled future support. Cloudflare, Google, and others have published positions on the IETF drafts.

How to implement

Per-group, in robots.txt. Place the Content-Signal: line inside the same group as User-agent: and Allow: / Disallow:.

User-agent: *
Allow: /
Content-Signal: search=yes, ai-input=yes, ai-train=yes

The example above says: "any crawler may use this content for search, for AI input, and for AI training." That is the right declaration for a public spec that wants to be readable.

Different signals per crawler if your policy varies. Use a targeted group:

User-agent: GPTBot
Allow: /
Content-Signal: search=yes, ai-input=yes, ai-train=no

User-agent: *
Allow: /
Content-Signal: search=yes, ai-input=yes, ai-train=yes

Pair with crawler-specific blocks where the answer is "no". Content Signals is a hint; many crawlers still only obey Disallow:. A Content-Signal: ai-train=no paired with User-agent: GPTBot \n Disallow: / is stronger than either alone.

Don't treat it as legal force. It is a declaration. Compliance is voluntary, and the legal status of "you used my content for training despite my Content-Signal" is still developing.

Common mistakes

• Spelling: Content-Signal: (singular). Not Content-Signals:.
• Putting the directive outside a group (no preceding User-agent: line). Some parsers will silently ignore it.
• Conflicting with Disallow:. If you Disallow: / for a bot, that bot was never going to fetch the page to read your Content-Signal:. They contradict; pick one.
• Inventing values beyond yes / no. The vocabulary is small on purpose.
• Treating it as a substitute for Disallow:. It is complementary.

Verification

• curl -s https://example.com/robots.txt | grep -i content-signal lists every directive.
• Is It Agent Ready? flips botAccessControl.contentSignals to pass.
• Track the IETF aipref WG for the final published vocabulary — names may yet shift before the RFC is published.