Are the ai2 Underline accordions free?

Yes. The whole styled layer is free and MIT licensed, like the component core.

Can I change the size?

Yes. Every variation takes a size prop of sm, md, lg or xl, aligned to the base Button height scale.

Underline accordions

Five minimal accordions where an animated underline sweeps in under the title on open: a left sweep, a center reveal, a thick grow, a double line and a dashed line. Each is self-contained, sized, token-driven and animates its height on open.

AccordionUnderline

Installation

The styled layer is free and installs like any other ai2 component.

Run the following command

npx shadcn@latest add @ai2/accordion-underline

Dependencies, the @ai2/tokens theme and the component file are installed together.

Install dependencies

npm install motion lucide-react

Copy the source

components/ui/accordion-underline.tsx

"use client"

import * as React from "react"
import { ChevronDown } from "lucide-react"
import { AnimatePresence, motion, useReducedMotion } from "motion/react"

import { cn } from "@/lib/utils"

/* Underline accordion family: 5 minimal accordions. When a row opens an
   animated underline sweeps in below the header; the variants change how the
   line enters and how it looks. Every export is a complete accordion: internal
   single-open state, a button header with aria-expanded, framer height auto
   animation. Colour comes ONLY from tokens. Alpha via color-mix. Under
   reduced-motion the line appears instantly instead of transforming and the
   panel opens instantly. Size = padding + text scale. Renders without props
   too. */

export type StyledSize = "sm" | "md" | "lg" | "xl"

export interface StyledAccordionItem {
  value: string
  title: React.ReactNode
  content: React.ReactNode
}

interface StyledAccordionProps {
  className?: string
  size?: StyledSize
  items?: StyledAccordionItem[]
  defaultOpen?: string
}

const headerPad: Record<StyledSize, string> = {
  sm: "px-1 py-2.5 text-sm",
  md: "px-1 py-3 text-sm",
  lg: "px-1 py-4 text-base",
  xl: "px-1 py-5 text-lg",
}

const bodyPad: Record<StyledSize, string> = {
  sm: "px-1 pb-2.5 text-sm",
  md: "px-1 pb-3 text-sm",
  lg: "px-1 pb-4 text-base",
  xl: "px-1 pb-5 text-base",
}

const iconSize: Record<StyledSize, string> = {
  sm: "size-4",
  md: "size-4",
  lg: "size-5",
  xl: "size-5",
}

const defaultItems: StyledAccordionItem[] = [
  {
    value: "item-1",
    title: "What is included?",
    content:
      "Every plan ships the full component library, semantic tokens, and lifetime updates for the tier you own.",
  },
  {
    value: "item-2",
    title: "How does installation work?",
    content:
      "Add any component with a single CLI command. Files land in your project and stay fully editable.",
  },
  {
    value: "item-3",
    title: "Can I use it in production?",
    content:
      "Yes. The system is framework agnostic, self contained, and safe to ship to any number of projects.",
  },
]

function useSingleOpen(defaultOpen?: string) {
  const [open, setOpen] = React.useState<string | null>(defaultOpen ?? null)
  const toggle = React.useCallback(
    (value: string) => setOpen((cur) => (cur === value ? null : value)),
    []
  )
  return { open, toggle }
}

const headerBase =
  "relative flex w-full items-center justify-between gap-3 text-left font-medium outline-none transition-colors focus-visible:ring-[3px] focus-visible:ring-ring/50"

function Panel({
  isOpen,
  size,
  children,
}: {
  isOpen: boolean
  size: StyledSize
  children: React.ReactNode
}) {
  const reduce = useReducedMotion()
  return (
    <AnimatePresence initial={false}>
      {isOpen && (
        <motion.div
          key="panel"
          initial={reduce ? false : { height: 0, opacity: 0 }}
          animate={{ height: "auto", opacity: 1 }}
          exit={{ height: 0, opacity: 0 }}
          transition={
            reduce ? { duration: 0 } : { duration: 0.25, ease: [0.4, 0, 0.2, 1] }
          }
          className="overflow-hidden"
        >
          <div className={cn(bodyPad[size], "text-muted-foreground")}>
            {children}
          </div>
        </motion.div>
      )}
    </AnimatePresence>
  )
}

function Chevron({ isOpen, size }: { isOpen: boolean; size: StyledSize }) {
  const reduce = useReducedMotion()
  return (
    <motion.span
      aria-hidden
      className="shrink-0 text-muted-foreground"
      animate={{ rotate: isOpen ? 180 : 0 }}
      transition={reduce ? { duration: 0 } : { duration: 0.2 }}
    >
      <ChevronDown className={iconSize[size]} />
    </motion.span>
  )
}

interface LineStyle {
  /* cizginin origin/giris yonu */
  mode: "left" | "center" | "grow"
  /* line thickness plus extra styling (e.g. double, dashed) */
  lineClass: string
}

/* Shared underline shell: an animated underline below the header. Rows are
   divided by a token line; the open row gets a primary line sweeping in.
   lineStyle is each variant. */
function UnderlineShell({
  props,
  lineStyle,
}: {
  props: StyledAccordionProps
  lineStyle: LineStyle
}) {
  const { className, size = "md", items = defaultItems, defaultOpen } = props
  const { open, toggle } = useSingleOpen(defaultOpen)
  const reduce = useReducedMotion()
  return (
    <div
      data-slot="styled-accordion"
      className={cn("flex flex-col", className)}
    >
      {items.map((item, i) => {
        const isOpen = open === item.value
        const originClass =
          lineStyle.mode === "left"
            ? "origin-left"
            : lineStyle.mode === "center"
              ? "origin-center"
              : "origin-left"
        return (
          <div
            key={item.value}
            className={cn(i > 0 && "border-t border-border")}
          >
            <button
              type="button"
              aria-expanded={isOpen}
              onClick={() => toggle(item.value)}
              className={cn(headerBase, headerPad[size])}
            >
              <span className="min-w-0">{item.title}</span>
              <Chevron isOpen={isOpen} size={size} />
              <motion.span
                aria-hidden
                className={cn(
                  "pointer-events-none absolute inset-x-0 bottom-0",
                  originClass,
                  lineStyle.lineClass
                )}
                initial={false}
                animate={
                  reduce
                    ? { opacity: isOpen ? 1 : 0 }
                    : { scaleX: isOpen ? 1 : 0, opacity: isOpen ? 1 : 0 }
                }
                transition={
                  reduce
                    ? { duration: 0 }
                    : { duration: 0.28, ease: [0.4, 0, 0.2, 1] }
                }
              />
            </button>
            <Panel isOpen={isOpen} size={size}>
              {item.content}
            </Panel>
          </div>
        )
      })}
    </div>
  )
}

/* Sweep: a single primary line sweeping in from the left. */
export function SweepAccordion(props: StyledAccordionProps) {
  return (
    <UnderlineShell
      props={props}
      lineStyle={{ mode: "left", lineClass: "h-0.5 bg-primary" }}
    />
  )
}

/* Center: a primary line opening outwards from the centre. */
export function CenterAccordion(props: StyledAccordionProps) {
  return (
    <UnderlineShell
      props={props}
      lineStyle={{ mode: "center", lineClass: "h-0.5 bg-primary" }}
    />
  )
}

/* Grow: a thicker line growing from the left. */
export function GrowAccordion(props: StyledAccordionProps) {
  return (
    <UnderlineShell
      props={props}
      lineStyle={{ mode: "grow", lineClass: "h-1 rounded-full bg-primary" }}
    />
  )
}

/* Double: a two-layer double line (primary plus a soft one below). */
export function DoubleAccordion(props: StyledAccordionProps) {
  return (
    <UnderlineShell
      props={props}
      lineStyle={{
        mode: "left",
        lineClass:
          "h-0.5 bg-primary shadow-[0_3px_0_0_color-mix(in_oklab,var(--color-primary)_35%,transparent)]",
      }}
    />
  )
}

/* Dashed: a dashed primary underline sweeping in from the left. */
export function DashedAccordion(props: StyledAccordionProps) {
  return (
    <UnderlineShell
      props={props}
      lineStyle={{
        mode: "left",
        lineClass:
          "h-0 border-b-2 border-dashed border-primary",
      }}
    />
  )
}

Manual installs skip the @ai2/tokens theme, so add the token CSS from the theming guide or the tone colors will be missing.

Variations

5 takes on the same idea. Each is its own export, and every one accepts a size prop (sm, md, lg, xl) aligned to the base Button scale.

Sweep

A single primary underline that sweeps in from the left.

Center

An underline that opens outward from the center.

Grow

A thicker rounded underline that grows from the left.

Double

A layered double underline with a soft second line.

Dashed

A dashed primary underline that sweeps in from the left.

ai2 Underline accordions: 5 styled variations on the token system

The ai2 Underline accordions are a set of 5 decorative button variations from the styled layer of the @ai2 design system, built around minimal collapsible sections with an animated title underline. They are free and MIT licensed, and every color comes from a semantic token, so they theme with the rest of ai2 in light and dark.

Motion runs on framer-motion: framer-motion sweeps the underline and animates the panel height on open. The styled layer is opt-in, so the dependency only lands if you use it; the base components stay lean. Under reduced motion, the underline appears instantly and the panels expand and collapse with no animation.

What is in the ai2 Underline accordions?

5 exports in one file: Sweep, Center, Grow, Double and Dashed. Each renders a native button and takes a size prop (sm, md, lg, xl) aligned to the base Button. They are separate from the base Button on purpose: the base keeps its clean variant, tone and size axes, while the styled layer carries the effects.

You own the file. Copy the one category file and you have all 5 variations, with no runtime dependency on ai2 itself.

Why use it

On-system by construction: Every color resolves to an ai2 semantic token, so the buttons follow your theme in light and dark with no extra work.
Effect without the sprawl: The decorations live in a dedicated styled file, so the base Button keeps its clean, predictable API.
Accessible and honest: Each renders a real button element, keeps a visible focus ring, and respects prefers-reduced-motion.

Features

Token-driven color: No hardcoded hex or oklch; the look recolors with your theme tokens.
framer-motion: framer-motion sweeps the underline and animates the panel height on open.
Reduced-motion aware: Under prefers-reduced-motion, the underline appears instantly and the panels expand and collapse with no animation.
Size aligned to the base: Every variation takes sm, md, lg and xl matching the base Button height scale, so styled and base buttons line up in a row.

Production tips

Use it for emphasis, not everywhere: Styled buttons draw the eye. Reserve them for the one action you want people to take on a screen, and use the base Button for the rest.
Keep labels as verbs: The decoration adds weight, so a clear action label keeps the button scannable.
Pick one variation per surface: The variations share a family; using two different ones in the same view competes for attention.

Works with the rest of ai2

The Underline accordions sit alongside the base Button and the rest of the @ai2 registry. They share the same token file, so a styled action next to a base button or a badge stays visually consistent in both modes.