MotionSpec — Web Animation Accessibility MCP

MotionSpec — web animation accessibility MCP server

MotionSpec is an MCP server and CLI for web animation accessibility. It helps AI coding agents and frontend developers compile validated animation specs to deterministic GSAP + CSS and find motion-accessibility candidates in a page's loaded CSS: missing prefers-reduced-motion guards and missing pause paths for continuous animation.

The MIT compiler includes reduced-motion handling and loop pause controls by default. Its static audit supports review of WCAG 2.2.2 (Pause, Stop, Hide — Level A) and WCAG 2.3.3 (Animation from Interactions — Level AAA). Findings need contextual review; a clean scan is not a full accessibility or WCAG-conformance assessment. Runtime JavaScript animation, flashing, video and Canvas are outside the audit's scope.

• Create web animation: choose a catalog primitive, validate a JSON spec, then compile vanilla-GSAP JavaScript and CSS. The internal WAAPI lowering is not a public build target.
• Check CSS motion: audit a URL for reduced-motion and pause-path candidates, then review the reported selectors and suggested fixes.
• Prevent regressions: use the CI example to compare findings against an explicitly accepted baseline.

Local stdio MCP exposes all five tools without an API key (npx -y motionspec). The hosted endpoint at https://api.motionspec.dev/mcp provides keyless motion_catalog and motion_validate; hosted compile, audit and stats require a key. See the installation guide and motion accessibility guide. Docs and product: https://motionspec.dev/docs.

The thesis: capability lives in the catalog, not the model. A bigger model can write more elaborate specs, but it can never emit a primitive, parameter, or selector the Trust Boundary hasn't approved. The compiler trusts only what passes.

request ──> Routing (small model, Stage A) ──> MotionSpec (JSON)
                 │ cache · 1 repair-retry · escalation     │
                 ▼                                          ▼
            telemetry                          TRUST BOUNDARY (fail-closed)
                                                            │
                                              ┌─────────────┴─────────────┐
                                              ▼                           ▼
                                   Compiler (no model, Stage B)   WAAPI lowering (internal — no CLI/MCP path yet)
                                              │                           │
                                   out/*.motion.js + .css       Element.animate / IO / @keyframes

Not to be confused with

Not to be confused with: the Android Material Components MotionSpec class, the iOS material-motion MotionSpec, Motion.dev / Framer Motion, the usemotion.com calendar app, the Motion Specialties mobility brand, or text-to-video generators (Runway/Sora/Kling/Viggle). MotionSpec checks the UI animation inside web apps — it does not generate video.

60-second start

npx motionspec                               # stdio MCP server — no install needed
claude mcp add motionspec -- npx motionspec  # register in Claude Code / any MCP host

npm install -g motionspec                    # or take the CLI:
motion compile spec.json                     # deterministic build → ./out in your cwd

The host LLM authors the spec; the Trust Boundary stays enforced either way. Listed on the MCP Registry as io.github.MasterPlayspots/motionspec. A hosted MCP endpoint is live: keyless motion_catalog + motion_validate at https://api.motionspec.dev/mcp (streamable-http; keyed tiers cover compile/audit/stats) — setup: https://motionspec.dev/docs.

Claude Code plugin

This repo is also a Claude Code plugin: it bundles the MCP server (npx motionspec, all five tools, local, keyless) with two skills — /motionspec:motion (the author→validate→compile workflow) and /motionspec:audit <url> (motion-accessibility check, WCAG 2.2.2/2.3.3). Try it directly from a clone with claude --plugin-dir ., or install it from the community marketplace once listed:

/plugin marketplace add anthropics/claude-plugins-community
/plugin install motionspec@claude-community

Status

Schema v1 is frozen: specVersion "1.0" is the stable public contract; "0.1" is deprecated and accepted until v1.2 (a tripwire test enforces the revisit). The [MS-XXX] error-code registry is public API — codes are never reused or redefined.

What the compiler guarantees

1. Allow-list — a primitive not in the catalog never reaches the compiler.
2. Injection-proof — ids, selectors, string params and triggers are charset-validated; every interpolation is a JS literal (JSON.stringify) or a CSS-screened raw value through one shared safety gate (safety.js). Malicious model output is rejected fail-closed — tested and fuzzed over 6000 random specs.
3. a11y by construction (motion) — safe defaults, enforced gates, and proof per build. respectReducedMotion is default-on at the compiler level (fail-safe): omitting it still yields a prefers-reduced-motion guard. Opting out is possible but emits MS-GLOBALS-RRM-OFF; a prompt-side instruction alone can never disable the guard.
4. Pause/Stop for loops (WCAG 2.2.2) — every continuous loop primitive is tagged a11y.persistent, and the compiler emits a pause path by construction: an animation-play-state: paused rule keyed on html[data-ms-paused] (outside the reduced-motion guard, so it is always live) plus, under pauseControls: "auto" (the fail-safe default), one accessible pause/stop toggle (type="button", aria-pressed in sync, ≥24 px target, visible focus ring, not rendered under reduced motion). pauseControls: "api" keeps the CSS contract and leaves the control to the integrator; "off" opts out but emits MS-GLOBALS-PAUSE-OFF when a persistent motion is present. The promote-gate refuses any infinite/repeat:-1 primitive that is not a11y.persistent. A spec with no loops adds zero extra bytes.
5. Determinism — same spec ⇒ byte-identical code (golden-file tests for the GSAP output and for the internal WAAPI lowering).
6. Versioned — schema frozen v1; catalog SemVer enforced by a diff-gate (a tightened bound shipped as a "patch" fails CI); specs may pin catalogVersion for reproducibility (MS-CATALOG-PIN-MISMATCH fail-closed).
7. Observability — every request logs model | model-repaired | cache-hit | escalate-* (local: JSONL sink · hosted: Cloudflare Analytics Engine, PII-scrubbed). Escalation clusters are the growth signal for new primitives.

One build target, one internal lowering

What you can get out of the compiler today, through the CLI or the MCP tools, is exactly one target:

• vanilla-gsap — GSAP + ScrollTrigger. meta.target accepts nothing else (schema frozen at v1).

A second lowering exists in the codebase and is kept green by the test suite, but it is not reachable through any interface:

• WAAPI/CSS lowering (src/compiler/lower-waapi.js) — zero-GSAP output on Element.animate, IntersectionObserver, and @keyframes/position: sticky. Full catalog coverage, byte-identical golden per primitive, same accessibility guard, same CSS safety gate. This is the framework-decoupling hedge: the IR outlives any animation library. Internal — referenced only by the tests and bin/promote-gate.js; there is no CLI flag, no MCP tool and no schema target for it (ADR-0001 freezes meta.target to vanilla-gsap; engine wiring is out of scope, ADR-0002). Do not plan a build on it until a release note says otherwise.

The catalog grows itself — humans keep the taste

The Catalog Forge (CI workflow, manual dispatch) picks the top telemetry-ranked gap, generates one candidate primitive, drives it through a multi-stage gauntlet — meta-schema, mandatory reduced-motion fallback, performance budget, output determinism (entropy tokens like Math.random/Date.now fail the gate), catalog-SemVer legality, golden creation — and opens a PR. It cannot merge, publish, or deploy: structurally (workflow permissions carry no packages/id-token, PR-only) and by regression test (forge-workflow-guard fails CI if anyone smuggles a publish step in). Gate 1 is always a human taste review.

MCP server

Input is size-capped (MS-INPUT-TOO-LARGE, 64 KB). The stdio server exposes one tool factory as the single source of truth, contract-tested in test/mcp.test.mjs. A hosted MCP endpoint is live: keyless motion_catalog + motion_validate at https://api.motionspec.dev/mcp; keyed tiers cover compile/audit/stats.

Motion-a11y checker

motion audit <url> (CLI, --json for the machine payload) and the motion_audit MCP tool run a static scan of a page's HTML and linked stylesheets — no headless browser, no new dependency. It reports four motion problems: CSS animation/transition without an effective prefers-reduced-motion guard (WCAG 2.3.3 — the guard must win the cascade), animated properties other than transform/opacity, infinite animations with no pause path (animation-play-state/data-*), and <marquee>/autoplay motion over 5 s (WCAG 2.2.2). Colour/opacity-only transitions are not motion under 2.3.3 and are not reported; loading indicators (spinners, skeletons) are listed as review items with no score impact. Each finding carries a selector, the WCAG reference, and a copy-paste fix. The CLI, the motion_audit tool and the free check at motionspec.dev/motion-check run the same engine with the same limits (12 stylesheets, 2 MB, 8 s) and the same score. It is honest about its limits: runtime motion (WAAPI/GSAP/JS, WebGL libraries) is reported as not audited (V2) rather than silently passed, and a page with no CSS motion is status: "not-measurable" with score: null instead of a perfect score. The legacy reduced-motion-safe badge string is returned only for a measurable page with zero findings and no runtime motion library; it means only that these checks found no candidates in the loaded CSS, not that the page or its runtime motion is certified.

Use in CI

motion audit --json is stable enough to gate a pull request. examples/ci/motion-audit.yml is a copy-and-adapt GitHub Actions workflow that builds your site, serves the build directory on localhost, audits the paths you list with npx -y -p [email protected] motion audit <url> --json (local, MIT, no key, no hosted call), and compares each page with a checked-in baseline .motionspec/baseline.json.

The gate fails when a page got worse — the same rule MotionSpec's weekly re-scan uses: the score fell, or the number of Level-A findings (WCAG 2.2.2 Pause, Stop, Hide) rose. Scores are compared only when both runs are measurable and use the same scoring version; a baseline written by 1.2.7 or earlier (scoring v1) needs one re-baseline. A page without a baseline entry never fails; that run is the baseline. Re-baselining is a deliberate manual run (workflow_dispatch with update_baseline: true) that uploads the new file as an artifact for you to commit — the workflow never commits on its own. Fixed findings are listed as - fixed: lines, new ones as + new finding:.

The machine payload is { ok, url, status, score, scoring, scoring_doc, summary, badge, findings, groups, disclosures, coverage }; read status before using score (it is null when the page is not measurable). Note that audit takes a URL, not a directory — hence the local server step. And the scope caveat travels with it: this is a static CSS scan (no inline style="", @import, CSS-in-JS, external JS bundles, video/GIF/Canvas, or flashing checks); a green gate means "no regression in the loaded CSS", not "accessible".

Specification & conformance

MotionSpec is a governed format, not just a tool. The normative spec is SPEC.md (versioned 1.0, RFC-2119 MUST/SHOULD/MAY over the JSON Schema, with a documented ADR-based change process). CONFORMANCE.md defines the five checks (schema, diagnostics, output, determinism, accessibility) an implementation passes to call itself MotionSpec 1.0 compatible, run against the published test/golden corpus. Multiple implementations passing the same corpus is what makes it a standard.

Standards mapping

MotionSpec checks a limited set of motion signals. These are candidates for review, not a conformance verdict or a substitute for testing the delivered interface.

Animating only transform and opacity is a performance recommendation, not by itself a WCAG success criterion. The audit does not measure flashing (2.3.1), runtime GSAP/WAAPI, video, Canvas, or the usability of a pause control. It does not establish compliance with EN 301 549, Section 508, EAA, BFSG, or any other legal framework.

Quickstart (from a clone)

npm ci                                      # install (0 runtime deps beyond MCP SDK + zod)
npm test                                    # 374 tests: validator, goldens, router, fuzz, parity
node bin/motion.js catalog                  # primitives + catalog version
node bin/motion.js compile examples/hero.motionspec.json
node bin/motion.js pipeline "Hero headline fades in, cards staggered" --mock
node bin/motion.js stats                    # telemetry (model / repaired / cache-hit / escalate)

Live model instead of --mock: set MOTION_API_KEY (or OPENROUTER_API_KEY); optional MOTION_MODEL (default anthropic/claude-haiku-4.5) and MOTION_BASE_URL (any OpenAI-compatible endpoint). See .env.example.

Gates (run these — they are the contract)

npm test                   # full suite, fail-closed trust boundary + golden determinism
npm run coverage           # FAILS under 90/90/75 (lines/functions/branches)
npm run catalog-lock:check # ADR-0001 D2: a tightened bound shipped as a "patch" fails here
npm run sbom && npm run sbom:check && node bin/license-check.js
npm run e2e                # real-browser Playwright (CI x86 runner)

Releases run the whole chain plus a canonical-clone guard and finish with a registry truth check — a version is "live" when the npm dist-tag says so, not when a local run went green.

Security

Defense in depth on the hosted path: constant-time admin-secret comparison (no timing side channel on position or length) · customer keys stored hashed (SHA-256) in KV, fail-closed on any lookup error · pre-auth per-IP rate limiting closes the key-enumeration gap before auth work starts, per-key limiting after · throttled abuse alerts with zero PII · telemetry scrubbed before storage · strict CSP/X-Frame-Options/nosniff on the only ungated page (a data-free dashboard shell). Full posture incl. reporting: SECURITY.md. Last audit (2026-07-03): no critical findings, no secret ever committed across 197 commits of history.

Layout

schema/            MotionSpec JSON schema (static contract, parity-tested against the validator)
primitives/        catalog: 40 verified primitives (safe templates)
catalog.lock.json  released catalog baseline (SemVer diff-gate)
src/compiler/      validate.js (Trust Boundary) · compile.js (GSAP) · lower-waapi.js (WAAPI/CSS)
                   safety.js (one shared CSS gate) · keyword-map.js · catalog.js · catalog-semver.js
src/router/        prompt.js · clients.js (openai-compat + mock) · route.js · cache.js · telemetry
src/mcp/           server.mjs (stdio) · register-tools.js (shared tool factory)
src/forge/         generate.js · prioritize.js — the gauntlet-verified catalog forge
src/discover/      gap analysis: request intents ↔ catalog coverage
src/demo/          device-verification demo pages (`?rm=1` simulates reduced motion)
bin/               motion.js (CLI) · promote-gate.js — dev/CI gate scripts stay repo-only
test/              374 tests incl. injection, fuzz, goldens (GSAP + internal WAAPI lowering), parity; test/e2e (Playwright)
docs/              ADR records (docs/adr/) and per-primitive reference (docs/primitives/)

Docs

• Web animation accessibility — reduced motion, pause controls, GSAP output, review workflow and audit limits.
• Discovery and search measurement — separate registry, GitHub, npm and web search surfaces, with a repeatable query log.
• Usage measurement plan — current local telemetry, proposed aggregate hosted counters, and limits of npm download statistics.
• AGENTS.md — what a coding agent should know: when to use MotionSpec, the commands, the three motion rules (reduced motion · pause path · no flashing), and what the audit does not check. The same rules in editor form: .cursor/rules/motionspec.mdc and .github/copilot-instructions.md.
• SECURITY.md — security posture of the npm package and hosted endpoint.
• docs/adr/0001-schema-freeze-v1.md — the frozen v1 contract and why.

Contributing

CONTRIBUTING.md covers setup, the gate-driven PR checklist, commit conventions, golden-file regeneration, and a short architecture tour. Issue templates live under .github/ISSUE_TEMPLATE/.

License

MIT.