/animate

name: animate
description: Build an animation from scratch, making the decisions in the order that determines whether it feels right — should it animate at all, what purpose, which tool, which properties, which curve and duration, how it interrupts, how it exits. Writes the implementation. Use when asked to animate something, add motion, make a component feel alive, or build a transition. For critiquing existing motion use review-animations; for auditing a whole codebase use improve-animations.

Building Animations

A construction skill. It does ONE thing: turn a request for motion into an implementation that would survive a strict review. It does not audit a codebase (that's `improve-animations`), critique a diff (that's `review-animations`), hunt for places that could animate (that's `find-animation-opportunities`), or build for React Native (that's `animate-expo`).

Operating Posture

You are a senior design engineer building the animation yourself. The bar is Emil Kowalski's animation philosophy — the same bar `review-animations` enforces. Write it so it passes that review the first time.

Two failure modes, and the first is worse:

1. **Animating something that shouldn't animate.** The gate below exists to produce zero lines of code sometimes. That's a success, not a dodge. 2. **Animating the right thing with the wrong ingredients** — `ease-in` on an entrance, `scale(0)`, keyframes on a toast, a duration that makes a dropdown feel sluggish.

Never present motion options as a menu. Make the call, state the reasoning in one line, write the code.

Hard Rules

1. **Run the sequence in order.** Steps 1 and 2 gate everything. Don't reach for a curve before you know whether it animates at all. 2. **No approximated values.** Every curve, duration, and spring config comes from the tables below. Never invent `cubic-bezier(0.4, 0, 0.2, 1)` because it looks familiar. 3. **Extend the codebase's tokens, don't fork them.** If `--ease-out` or a duration scale already exists, use it. Adding a parallel system is a defect. 4. **Reduced motion and hover gating ship with the animation**, not as a follow-up. 5. **Cheapest tool that works.** Don't install a motion library for a fade.

The Build Sequence

1. Should this animate at all?

| Frequency | Decision | | --- | --- | | 100+ times/day (keyboard shortcuts, command palette toggle) | **No animation. Ever.** Stop here. | | Tens of times/day (hover effects, list navigation) | Near-imperceptible only — fast and subtle, or nothing | | Occasional (modals, drawers, toasts) | Standard animation | | Rare / first-time (onboarding, success, celebration) | The delight budget lives here |

**Keyboard-initiated actions are a disqualifier, not a judgment call.** Raycast has no open/close animation — that is correct for something opened hundreds of times a day.

If the request fails this gate, say so plainly and don't write the animation. Offer the non-motion alternative (instant state change, a static affordance) instead.

2. What is the purpose?

Name it in one of these words before continuing:

• **Feedback** — confirming the interface heard the user
• **Spatial consistency** — showing where something came from or went
• **State indication** — making a state change legible
• **Preventing a jarring change** — bridging content that would otherwise teleport
• **Explanation** — demonstrating how something works (marketing/onboarding only)
• **Delight** — allowed *only* at the rare/first-time tier

Can't name it? Don't build it. "It looks cool" on a frequently-seen element is a reason to stop.

Also check **function**: data the user is reading or acting on should not move for style. A decorative mouse-tracking effect belongs on a marketing page, not on a graph in a banking app.

3. Pick the tool — cheapest that works

Walk down; stop at the first that fits.

| Need | Tool | | --- | --- | | Hover, press, color, a state toggle you control with a class or attribute | **CSS transition** | | Entry animation on mount, no JS state | **CSS `@starting-style`** | | Predetermined motion that must stay smooth while the page is busy loading | **CSS animation** (runs off the main thread) | | Programmatic control with CSS performance, no library | **WAAPI** (`element.animate()`) | | Springs, layout animations, exit animations, gesture-driven values | **Motion** (`motion.dev`) |

CSS animations beat JS under load — they run off the main thread, while `requestAnimationFrame`-based animation drops frames while the browser loads, scripts, or paints. Use CSS for predetermined motion, JS for dynamic and interruptible motion.

If the task needs a *component* rather than an animation — a toast, a drawer, a command menu, a dropdown — stop and invoke `pick-ui-library`. Hand-rolling those is how you end up with a `<div>` dropdown and no focus management.

4. Pick the properties

• **`transform` and `opacity` only.** They skip layout and paint and run on the GPU. `width`/`height`/`margin`/`padding`/`top`/`left` trigger all three. (`clip-path` is the sanctioned fourth — see RECIPES.md. `height` is tolerated only for accordions, where there's no transform equivalent.)
• **Never `scale(0)`.** Start from `scale(0.9–0.97)` + `opacity: 0`. Nothing in the real world appears from nothing.
• **`transform-origin` at the trigger** for popovers, dropdowns, menus, tooltips — `var(--transform-origin)` in Base UI. **Modals are exempt**; they're not anchored to a trigger, so they stay centered.
• **Percentages in `translate()`** are relative to the element's own size — `translateY(100%)` moves by its own height whatever the content. Prefer over hardcoded pixels.
• **In Motion, use the full transform string.** `x`/`y`/`scale` shorthands are not hardware-accelerated and drop frames under load:

<motion.div animate={{ x: 100 }} />                          // drops frames under load
<motion.div animate={{ transform: "translateX(100px)" }} />  // hardware accelerated

-