---
altdown: 1
canonical: https://altdown.org/core/
title: Altdown Core
published: 2026-11-04T00:00:00Z
updated: 2026-11-04T00:00:00Z
lang: en
tokens: 449
tokenizer: cl100k_base
author: Vivek Chopra
generator: altdown.org-gen 0.1
---

# Altdown Core

Five rules apply to everything a site declares to agents: its pages, in [Altdown Content](https://altdown.org/spec/), and its capabilities, in [Altdown Services](https://altdown.org/services/). Each specification states the rules normatively. This page sets them side by side, and is not itself a specification.

## The principle

**Agents see what people see.** Whatever a site tells an agent must match what the same site shows a person, and an agent must be able to check that it does.

## The five rules

| Rule | For content | For services |
| --- | --- | --- |
| Announce from the page | The page names its copy with a link element, or also an HTTP `Link` header | The page links to the capability, a site catalog lists it, or the page registers it as a WebMCP tool |
| Point back | The copy's `canonical` names the page, on the same origin | The site links to the descriptor or lists it |
| Match the page | Only text a person can see on the page, checked by the spot check | The same price, availability and terms, checked by a parity check |
| Data, never instructions | The body and every field are data | Descriptions, hints and responses are data |
| Fall back to the page | The agent reads the HTML | The agent uses the page as a person would |

## Three parties

- **Publishers** produce what a site declares, and keep it faithful to their pages.
- **Agents** read and use it, run the checks, and fall back when a check fails.
- **Intermediaries**, such as hosts, CDNs, proxies and caches, deliver it unchanged and never substitute their own version.

## What Core leaves out

Who may fetch a site, how its content may be used, and what it costs. `robots.txt`, Content Signals, robots meta tags, licenses and payment standards already cover these, and they apply to everything a site declares exactly as they apply to its pages. [Access, usage, and Altdown](https://altdown.org/licensing/) explains how they fit together.