Show HN: Choreo – an animation DSL that verifies its own rendered output

Choreo, a new DSL for web animation that verifies its own rendered output, has been released as a research artifact. The tool compiles animation descriptions into self-contained SVG+CSS and includes a verifier that checks spatio-temporal assertions against the actual rendered trace, enabling AI agents to reliably generate animations with a 94% success rate when using a verify-and-correct loop, according to the project's documentation.

An animation format an AI can author reliably — and that proves it matches the spec. Choreo is a small, semantic DSL for web animation. You describe what should happen "the badge pops as the checkmark draws on, then the label slides up" , and Choreo compiles it to self-contained SVG + CSS. The catch that makes it different: every Choreo program carries its own spatio-temporal checks, and a verifier proves the rendered animation actually satisfies them. Existing tools GSAP, Motion, Lottie, Rive are excellent at letting humans author animation. Choreo targets a different problem: Authorable by an agent, not a person. A bounded grammar an LLM hits reliably from a spec — no timeline scrubbing, no coordinate math. It checks itself. The animation ships with assertions like during 0..500 : fadesIn card and at end : settled all , andconfirms them against the @choreo-oss/verify rendered trace — the actual pixels over time, not the source. That verification is the part nothing else does. Visual-regression tools Chromatic, Percy, Applitools diff screenshots ; they can tell you a frame looks different, but not that "the toast still slides in before it fades and settles by 1.2s." Choreo's verifier asserts motion contracts . Why this matters for AI.LLM animation synthesis is right ~59% of the time unaided; a formal verify-and-correct loop lifts it to ~94% MoVer, SIGGRAPH 2025 . Choreo is that loop, packaged: a substrate an agent can generateanda checker that catches when it's wrong. scene "signup-success" size 800x450 duration 2600ms background f1f5f9 entities card = rect 320x220 fill ffffff radius 12 ring = circle 30 stroke 16a34a stroke-width 4 tick = path "M8 16 L14 22 L24 10" size 32x32 stroke 16a34a stroke-width 4 label = text "Account created" size 18 weight 600 color 0f172a layout center card inside ring card top 36 center tick in ring below label ring gap 18 motion seq fadeIn card 500ms easeOut par pop ring 400ms drawOn tick 450ms easeOut slideIn label from bottom 10 400ms easeOut checks during 0..500 : fadesIn card during 500..900 : grows ring and fadesIn ring at 1800 : below label, ring always : inside ring, card and inside label, card at end : settled all cd lang node src/cli.mjs examples/signup-success.choreo -o out --verify → out/signup-success.html a self-contained animation → out/signup-success.spec.mjs the checks block, as a verifier spec → runs @choreo-oss/verify: 19/19 checks passed | Package | What it is | |---|---| lang/ | checks → verifier spec verify/ @choreo-oss/verify — samples a rendered animation trace Playwright and asserts spatio-temporal predicates. Works on any web animation — CSS, GSAP, Motion, Remotion renders — because it verifies traces, not source. bench/ @choreo-oss/verify as a neutral referee. @choreo-oss/verify is useful on its own, against animations you didn't write in Choreo: export default { name: 'signup-flow', target: 'file:///…/signup-flow.html', // any URL, file, or inline HTML duration: 2600, entities: { toast: ' toast', msg: ' msg' }, checks: { during: 0, 400 , assert: 'movesLeft toast ', 'fadesIn toast ' }, { at: 1200, assert: 'visible toast ', 'inside msg, toast ' }, { atEnd: true, assert: 'fadedOut toast ' }, , }; cd verify && npm install npm run demo 19/19 checks pass on examples/signup-flow.html npm run demo:broken 12/19 — catches 3 planted regressions, with frames, exit 1 Choreo is a research artifact, released as-is — it began as an internal probe into whether a verifiable DSL is the right substrate for AI-generated animation. It is not a supported product; there is no roadmap commitment and no support SLA. Issues and PRs are welcome, but treat it as a foundation to build on, not a maintained service. MIT /JieGouAI/choreo/blob/main/LICENSE . Note the benchmark vendors GSAP under GreenSock's own license — see NOTICE /JieGouAI/choreo/blob/main/NOTICE .