Anthropic's Most-Used Internal Skill Is a 321-Byte Prompt
A 321-byte Markdown file that installs like an npm package shows that the unit of AI productization is shifting from code to precise constraint design. The skill's viral reception also exposes how badly current models default to verbosity—half its value is compensating for the model's own output tendencies.
ELI5 generates a self-contained HTML artifact that explains any topic—from DNS to a legacy code module—using big pictures and few words, as if to someone who knows nothing. The skill's source code is a 321-byte Markdown file whose core instruction is one sentence: "Explain like I'm someone who knows nothing about this topic, using a HTML artifact with big pictures and few words."
Its effectiveness comes from three constraints packed into that sentence: an absolute beginner audience anchor, a visual-first output format that prevents AI from defaulting to verbose essays, and an HTML carrier that makes diagrams actually renderable. A former Stripe engineer noted the same concept was their most-used internal LLM tool in 2023, though without HTML artifact support at the time.
Commentary on the skill's popularity also surfaced a blunt critique: the model's default verbosity is so bad that a 26-word prompt to force simplicity became a viral product. The skill was committed to the community repository just 40 minutes before the announcement tweet, after five rapid revisions to the prompt wording.
The skill's value is split: half comes from well-designed constraints, half from compensating for a model that defaults to overwrought prose. The most-liked comment—"not more intelligence, more intellegible"—frames this as a product debt the model itself carries.
A 321-byte Markdown file installable via a CLI marketplace mirrors the npm/Homebrew packaging model, but the package contents are instructions for AI behavior rather than executable code. This suggests a new product category where the asset is a tested prompt, not a software library.
The five pre-release revisions to a 26-word sentence indicate that prompt engineering at this level is editing, not architecture. The work is removing words until only the constraints that actually prevent a specific failure mode remain.