TL;DR : When your primary consumer is an LLM, XML and JSON waste tokens by repeating structure in every element. A compact positional format reduces consumption by 50%. Turns out this idea already had a name: TOON (Token-Oriented Object Notation). Same selective pressure — expensive tokens and repeated keys — same solution.

Anthropic uses XML for everything. Their system prompts are wrapped in <instructions> , their examples in <example> , their tools in <function> . If you work with Claude, you live surrounded by tags.

So when I asked Claude to design the output for a CLI meant to be consumed by LLMs, the obvious temptation was: XML. If the model gets information in XML, shouldn't the CLI return it in XML?

Turns out, no.

The problem: compulsive repetition

Imagine your CLI lists issues from a project tracker. You have 50 issues.…