Why XML tags are so fundamental to Claude

The article even references English's built-in delimiter, the quotation mark, which is reprented as a token for Claude, part of its training data.

So are we sure the lesson isn't simply to leverage delimiters, such as quotation marks, in prompts, period? The article doesn't identify any way in which XML is superior to quotation marks in scenarios requiring the type of disambiguation quotation marks provide.

Rather, the example XML tags shown seem to be serving as a shorthand for notating sections of the prompt ("treat this part of the prompt in this particular way"). That's useful, but seems to be addressing concerns that are separate from those contemplated by the author.

It'd not clear why within any section XML markers would do better than something like markdown, other than claude being explicitly post-trained with XML prompts as opposed to markdown. One hypothesis could be that since a large portion of the training corpus is websites, XML is more natural to use since it's "learned" the structure of XML better than markdown. Another could be that explicit start/end tags make identifying matching delimiters easier than JSON (which requires counting matching brackets) or markdown (where the end of a section is implicitly defined by the presence of a new header element).

To me it seems like handling symbols that start and end sequences that could contain further start and end symbols is a difficult case.

Humans can't do this very well either, we use visual aids such as indentation, synax hilighting or resort to just plain counting of levels.

Obviously it's easy to throw parameters and training at the problem, you can easily synthetically generate all the XML training data you want.

I can't help but think that training data should have a metadata token per content token. A way to encode the known information about each token that is not represented in the literal text.

Especially tagging tokens explicitly as fiction, code, code from a known working project, something generated by itself, something provided by the user.

While it might be fighting the bitter lesson, I think for explicitly structured data there should be benefits. I'd even go as far to suggest the metadata could handle nesting if it contained dimensions that performed rope operations to keep track of the depth.

If you had such a metadata stream per token there's also the possibility of fine tuning instruction models to only follow instructions with a 'said by user' metadata, and then at inference time filter out that particular metadata signal from all other inputs.

It seems like that would make prompt injection much harder.

Are we really at the point where some people see XML as a spooky old technology? The phrasing dotted around this article makes me feel that way. I find this quite strange.

And then do we end up over indexing on Claude and maybe this ends up hurting other models for those using multiple tools.

I just dislike how much of AI is people saying "do this thing for better results" with no definitive proof but alas it comes with the non determinism.

At least this one has the stamp of approval by Claude codes team itself.

https://i.imgur.com/HGa0i3m.png

E.g. instead of

    <examples>
      <ex1>
        <input>....</input>
        <output>.....</output>
      </ex1>
      <ex2>....</ex2>
      ...
    </examples>
    <instructions>....</instructions>
    <input>{actual input}</input>

    ...instructions...
    input: ....
    output: {..json here}
    ...maybe further instructions...
    input: {actual input}

N of 1 anecdote anyway for one use case.

And while we're at it, instead of wall-of-text, I also feel like outputs could be structured at least into thinking and content, maybe other sections.

One thing I've found: even with XML tags, you still need to validate and parse defensively. Models will occasionally nest tags wrong, omit closing tags, or hallucinate new tag names. Having a fallback parser that extracts content even from malformed XML has saved me more than once.

The real win is that XML tags give you a natural way to do few-shot prompting with structure. You can show the model exactly what shape the output should take, and it follows remarkably well.

I'd post a link, but unfortunately many are highly NSFW. Just search for "Claude jailbreak" on reddit or something.

You'll start to see how Claude really thinks. They'll put things in <ethic_reminders>, <cyber_warning> or <ip_reminder>. You could actually even snip these off in an API, overwrite them, or if your prompt-fu is good, convince Claude that these tags are prompt injections. It's also interesting noting how jailbreaking is easier on thinking mode because the jailbreaking prompts will gaslight Claude into thinking that these tags are attacks.

There's a lot of speculation in this thread, but go and have a spar with Claude instead.

Ex: <message>...</message> helps keep track. Even better? <message78>...</message78>. That's ugly xml, but great for LLMs. Likewise, using standard ontologies for identifiers (ex: we'll do OCSF, AT&CK, & CIM for splunk/kusto in louie.ai), even if they're not formally XML.

For all these things... these intuitions need backing by evals in practice, and part of why I begrudgingly flipped from JSON to XML

[1] well of course XML is still heavily used in stuff like interfacing with automated wire transfers with big banks (at least in Europe) and all the digital payments directives etc. But XML is not widely used by the "cool" stuff.

HTML also descended from SGML, and it’s hard to imagine a more deeply grooved structure in these models, given their training data.

So if you want to annotate text with semantics in a way models will understand…

> In other words, XML tags have not only a special place at inference level but also during training

Their cited source has 0 proof of that. It's just like python/C/html in training. Doesn't mean it's special. And no, you don't need to format your prompts as python code just because of that.

> In truth, it does not matter that these tags are XML. Other models use ad hoc delimiters (as explained in a previous article; example: <|begin_of_text|> and <|end_of_text|>) and Claude could have done the same. What matters is what these tags represent.

Those strings are just representations of special tokens in models for EOS. What does it have to do with anything this article pretends to know about?

Please don't post such intellectual trash on here :')

Claude analysis of the article:

The author is making an interesting philosophical argument — that XML tags in Claude function as metalinguistic delimiters analogous to quotation marks in natural language, formulaic speech markers in Homer, or recognition sequences in DNA.

The core thesis is about first-order vs. second-order expression boundaries, which is a legitimate linguistic/information-theory concept. But to your actual question — do they understand what tokens are?

No, not in the technical sense you're pointing at. The article conflates two very different things:

1. Tokenizer-level special tokens — things like <|begin_of_text|>, <|end_of_text|>, <|start_header_id|> etc. These are literal entries in the vocabulary with dedicated token IDs. They're not "learned" through training in the same way — they're hardcoded into the tokenizer and have special roles in the attention mechanism during training. They exist at a fundamentally different layer than XML tags in prompt text.

2. XML tags as structured text within the input — these are just regular tokens (<, instructions, >) that Claude learned to attend to during RLHF/training because Anthropic's training data and system prompts heavily use them. They're effective because of training distribution, not because they occupy some special place in the tokenizer.

The author notices that other models use <|begin_of_text|> style delimiters and says Claude "could have done the same" but chose XML instead. That's a category error. Claude also has special tokens at the tokenizer level — XML tags in prompts are a completely separate mechanism operating at a different abstraction layer.

The philosophical observation about delimiter necessity in communication systems is fine on its own. But grafting it onto a misunderstanding of how tokenization and model architecture actually work weakens the argument. They're essentially pattern-matching on surface-level similarities (both use angle brackets!) without understanding the underlying mechanics.

To be realistic, this design needs more weirdly sexual etsy garbage, “one weird tip,” and “punch the monkey”