---
title: Introduction
description: Agentside serves markdown twins of your pages to AI agents and lets you A/B test what each agent reads.
sidebar:
  order: 0
---

Agentside is a small SDK that answers AI agents with a markdown version of your pages. People keep getting your HTML from the same URL, while agents that ask for markdown or identify themselves by user agent get a clean twin that is cheaper to read and easier to cite.

It is built on [Effect](https://effect.website) and only depends on the fetch-standard `Request` and `Response`, so one config runs in TanStack Start, Astro, Next.js, Hono, Cloudflare Workers, Bun and anything else that speaks fetch.

```ts
import { agentside } from "@usenotra/agentside";

const twins = agentside({
  routes: {
    "/": "# Acme\n\nAcme sends invoices your customers actually pay.",
    "/pricing": () => renderPricingMarkdown(),
  },
});

const response = await twins.handle(request, () => app.fetch(request));
```

## What you get

- **Content negotiation** for `.md` URLs, `Accept: text/markdown` and known AI agent user agents, with a `Link` header on HTML pages that have a twin.
- **A/B variants per agent** so you can serve a different twin to a vendor, a purpose such as search or a single agent and measure what happens.
- **Error handling that protects agents** from broken twins. A failing variant falls back to control and a failing twin falls back to your HTML page.
- **Caching** with a TTL, request deduplication and no caching of failures.
- **Typed errors** and the raw Effect program through `@usenotra/agentside/effect`.

**[Quickstart](/docs/quickstart)**

Install Agentside and serve your first twin.

**[Migrating from Dualmark](/docs/migrating-from-dualmark)**

Map your Dualmark config to Agentside routes.

## Benchmarks

We are benchmarking Agentside against Dualmark on negotiation, cached reads and cold renders. The results and the harness will be published in the [benchmarks section of the README](https://github.com/usenotra/agentside#benchmarks).