MotionSpec

A formal intermediate language for scroll-driven web motion. An LLM translates a request into a schema-validated JSON spec; a deterministic compiler turns that spec into GSAP or native WAAPI/CSS — injection-proof and catalog-validated by construction, with an enforced prefers-reduced-motion fallback and a performance budget.

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 (no GSAP)
                                              │                           │
                                   out/*.motion.js + .css       Element.animate / IO / @keyframes

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, per-key-gated MCP endpoint exists as a private beta (access by arrangement).

Source access: the npm package and the hosted MCP are public-facing; the source repository is private (access by arrangement). Sections marked (source access) assume a granted clone.

Related MCP server: GSAP-Animation-Generate

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 on both targets).

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.

Two build targets, one boundary

Every catalog primitive compiles through the same validated spec to:

• vanilla-gsap — GSAP + ScrollTrigger, the production default.

• WAAPI/CSS lowering — 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.

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, per-key-gated MCP endpoint is available as a private beta (access by arrangement).

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 a prefers-reduced-motion guard (WCAG 2.3.3), 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). Each finding carries a selector, the WCAG reference, and a copy-paste fix. It is honest about its limits: runtime motion (WAAPI/GSAP/JS) is reported as not audited (V2) rather than silently passed. A page that clears every check earns the reduced-motion-safe badge — the exact output MotionSpec itself produces.

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 turns specific legal and normative accessibility requirements into compiler-enforced defaults. Each check maps to the frameworks that mandate it:

Notes: EN 301 549 is the EU harmonised standard whose clause 9 adopts the WCAG success criteria by number. U.S. Section 508 (Revised) incorporates WCAG 2.0 Level A and AA — SC 2.2.2 is Level A and therefore in scope; SC 2.3.3 is Level AAA and is provided as a stronger guarantee than the baseline requires. The European Accessibility Act (EAA) and its German transposition (BFSG, applicable from 28 June 2025) require covered digital products and services to be accessible, with conformance commonly demonstrated against EN 301 549. MotionSpec enforces the motion subset of these obligations by construction; it does not by itself make an entire product conformant.

Quickstart (source access)

npm ci                                      # install (0 runtime deps beyond MCP SDK + zod)
npm test                                    # 292 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 (source access)

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/              292 tests incl. injection, fuzz, goldens (both targets), parity; test/e2e (Playwright)
docs/              ADR records (docs/adr/) and per-primitive reference (docs/primitives/)

Docs

• 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

The source repository is currently private (access by arrangement). Once granted: 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.