feat(sdk): ws-b variables/brand — object-valued font/image + B1 JSON model (#1569)

## WS-B — variables / brand, object-valued (end-to-end)

Part of the AI Studio (Pacific) SDK integration. **Base of the SDK-hotspot stack** (`main → ws-b → ws-c → ws-d → ws-3c → ws-3f`).

### Problem
The variable system was split-brained: SDK `setVariableValue` wrote a `--{id}` CSS custom prop, while the runtime `getVariables()` read a separate JSON model (`data-composition-variables` / `__hfVariables`). The two never connected, and there was no `--brand-*` convention. Variables were scalar-only.

### What this does
- **B1 — one source of truth.** `setVariableValue` now drives the runtime variable model (`data-composition-variables` / `__hfVariables`), with CSS compatibility emitted as explicit `stylePath`-based patches alongside the model patch. A brand kit is a variables JSON; a batch of `setVariableValue` re-skins in one frame.
- **B2 — object-valued variables.** The `CompositionVariable` union extends from scalar-only to typed objects: `font` (`{name, source}`) and `image` (`{url, …}`), end-to-end (core union → SDK op → runtime merge). Colors stay scalar (per §7 LOCKED decision).

### Implementation notes
CSS compatibility was moved out of `apply-patches.ts` (where it was incorrectly writing CSS props as a side-effect of model patches, breaking inverse/undo) and into explicit patches emitted in `mutate.ts`. Forward emits `[modelPatch, cssPatch]` for scalars; inverse correctly generates `patchRemove` for the CSS prop when there was no prior CSS prop. Font/image variables never become CSS props.

### Files (12 changed, +441 −32)
- `packages/core`: `core.types.ts`, `lint/rules/composition.ts`, `parsers/htmlParser.ts` (+test), `runtime/validateVariables.ts`
- `packages/sdk`: `engine/mutate.ts` (+test), `engine/apply-patches.ts`, `engine/patches.ts`, `index.ts`, `types.ts`

### Gates
- `bun run build` ✅
- `bun test` SDK 304/0 ✅ · `validateVariables.test.ts` 13/0 ✅
- `bunx oxlint` 0/0 ✅ · `bunx oxfmt --check` ✅
- `fallow audit --gate new-only` ✅ (complexity inherited only)

> The +8 new `htmlParser.test.ts` font/image tests fail under the pre-existing `DOMParser is not defined` happy-dom limitation (main already carries 425 such failures) — not a logic bug; the pure runtime logic is covered by `validateVariables.test.ts`.

### Deferred
Brand-kit picker UI and `batch(setVariableValue × N)` wiring are Pacific-side; per-composition variable scoping beyond `__hfVariablesByComp`.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

packages/sdk/src/engine/apply-patches.ts

@@ -76,6 +76,38 @@ function parsePath(path: string): ParsedPath | null {
   return null;
 }
 
+// ─── Variable JSON model helper ───────────────────────────────────────────────
+
+type VariableDecl = { id: string; default: unknown; [key: string]: unknown };
+
+/**
+ * Apply a variable value to `data-composition-variables` on
+ * `document.documentElement`. When `newDefault` is null (remove op),
+ * the variable's `default` is left unchanged (we never erase the schema;
+ * only the override is removed). When `newDefault` is a value, the matching
+ * declaration's `default` is updated in-place. No-ops gracefully when the
+ * attribute or declaration is absent.
+ */
+function applyVariableDefault(document: Document, id: string, newDefault: unknown): void {
+  const htmlEl = (document as Document & { documentElement?: Element }).documentElement;
+  if (!htmlEl) return;
+  const raw = htmlEl.getAttribute("data-composition-variables");
+  if (!raw) return;
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(raw);
+  } catch {
+    return;
+  }
+  if (!Array.isArray(parsed)) return;
+  const arr = parsed as VariableDecl[];
+  const idx = arr.findIndex((v) => typeof v === "object" && v !== null && v.id === id);
+  if (idx < 0) return;
+  if (newDefault === null) return; // remove op: leave schema default unchanged
+  arr[idx] = { ...arr[idx]!, default: newDefault };
+  htmlEl.setAttribute("data-composition-variables", JSON.stringify(arr));
+}
+
 // ─── Patch application ───────────────────────────────────────────────────────
 
 /**
@@ -195,14 +227,12 @@ function applyOne(parsed: ParsedDocument, patch: JsonPatchOp, p: ParsedPath): vo
     }
 
     case "variable": {
-      const root = findRoot(parsed.document);
-      if (!root || !p.id) return;
-      const cssVar = `--${p.id}`;
-      if (patch.op === "remove") {
-        setElementStyles(root, { [cssVar]: null });
-      } else {
-        setElementStyles(root, { [cssVar]: String(patch.value) });
-      }
+      if (!p.id) return;
+      // B1: update the JSON model (data-composition-variables) so
+      // getVariables() returns the correct value in both preview and render.
+      // CSS compat is handled by explicit style-path patches emitted by mutate.ts,
+      // so we do NOT write CSS here — the style case above handles those patches.
+      applyVariableDefault(parsed.document, p.id, patch.op === "remove" ? null : patch.value);
       break;
     }
 

packages/sdk/src/engine/mutate.test.ts

@@ -31,6 +31,40 @@ function fresh() {
   return parseMutable(BASE_HTML);
 }
 
+/** Full HTML fixture with data-composition-variables for B1/B2 tests. */
+const VARIABLES_HTML = `<!DOCTYPE html>
+<html data-composition-id="c1" data-composition-duration="5" data-composition-variables='${JSON.stringify(
+  [
+    { id: "brand-color-primary", type: "color", label: "Primary color", default: "#0066cc" },
+    {
+      id: "brand-font",
+      type: "font",
+      label: "Brand font",
+      default: "Inter",
+      source: "https://fonts.googleapis.com/css2?family=Inter",
+      default_name: "sans-serif",
+      default_source: "",
+    },
+    { id: "brand-logo", type: "image", label: "Brand logo", default: "/logo.png" },
+  ],
+)}'>
+<body>
+<div data-hf-id="hf-stage" data-hf-root style="width: 1280px; height: 720px" data-duration="5">
+</div>
+</body></html>`;
+
+function freshWithVars() {
+  return parseMutable(VARIABLES_HTML);
+}
+
+/** Read the default value for a variable id from the parsed document. */
+function readVarDefault(parsed: ReturnType<typeof parseMutable>, id: string): unknown {
+  const raw = parsed.document.documentElement?.getAttribute("data-composition-variables");
+  if (!raw) return undefined;
+  const arr = JSON.parse(raw) as Array<{ id: string; default: unknown }>;
+  return arr.find((v) => v.id === id)?.default;
+}
+
 // ─── setStyle ────────────────────────────────────────────────────────────────
 
 describe("setStyle", () => {
@@ -369,7 +403,7 @@ describe("setElementStyles key normalization", () => {
 // ─── setVariableValue ─────────────────────────────────────────────────────────
 
 describe("setVariableValue", () => {
-  it("sets CSS custom property on root element", () => {
+  it("sets CSS custom property on root element (fragment doc — compat)", () => {
     const parsed = fresh();
     const result = applyOp(parsed, {
       type: "setVariableValue",
@@ -385,6 +419,122 @@ describe("setVariableValue", () => {
   it("override-set key maps correctly", () => {
     expect(pathToKey("/variables/brand-color-primary")).toBe("var.brand-color-primary");
   });
+
+  // B1 — drives the JSON model (data-composition-variables)
+
+  it("B1: scalar color round-trips through override-set and runtime JSON model", () => {
+    const parsed = freshWithVars();
+    const before = serializeDocument(parsed);
+    const result = applyOp(parsed, {
+      type: "setVariableValue",
+      id: "brand-color-primary",
+      value: "#ff0000",
+    });
+    expect(result.forward[0]?.path).toBe("/variables/brand-color-primary");
+    expect(result.forward[0]?.value).toBe("#ff0000");
+    // JSON model updated
+    expect(readVarDefault(parsed, "brand-color-primary")).toBe("#ff0000");
+    // CSS compat prop also written
+    const root = parsed.document.querySelector("[data-hf-root]");
+    expect(root?.getAttribute("style")).toContain("--brand-color-primary: #ff0000");
+    // inverse restores
+    applyPatchesToDocument(parsed, result.inverse);
+    expect(serializeDocument(parsed)).toBe(before);
+  });
+
+  it("B1: scalar inverse patch restores prior value (replace → old value)", () => {
+    const parsed = freshWithVars();
+    // Set once
+    applyOp(parsed, { type: "setVariableValue", id: "brand-color-primary", value: "#ff0000" });
+    const snap = serializeDocument(parsed);
+    // Set again
+    const result2 = applyOp(parsed, {
+      type: "setVariableValue",
+      id: "brand-color-primary",
+      value: "#00ff00",
+    });
+    expect(readVarDefault(parsed, "brand-color-primary")).toBe("#00ff00");
+    applyPatchesToDocument(parsed, result2.inverse);
+    expect(serializeDocument(parsed)).toBe(snap);
+  });
+
+  // B2 — object-valued font variable
+
+  it("B2: font {name,source} object round-trips through JSON model (no CSS prop)", () => {
+    const parsed = freshWithVars();
+    const fontValue = { name: "Roboto", source: "https://fonts.googleapis.com/css2?family=Roboto" };
+    const result = applyOp(parsed, {
+      type: "setVariableValue",
+      id: "brand-font",
+      value: fontValue,
+    });
+    expect(result.forward[0]?.path).toBe("/variables/brand-font");
+    expect(result.forward[0]?.value).toEqual(fontValue);
+    // JSON model updated
+    expect(readVarDefault(parsed, "brand-font")).toEqual(fontValue);
+    // NO CSS custom prop for object values
+    const root = parsed.document.querySelector("[data-hf-root]");
+    const style = root?.getAttribute("style") ?? "";
+    expect(style).not.toContain("--brand-font");
+    // override-set key holds the object (one var.{id} key, no sub-key explosion)
+    expect(pathToKey("/variables/brand-font")).toBe("var.brand-font");
+  });
+
+  it("B2: font inverse restores prior default (object → object)", () => {
+    const parsed = freshWithVars();
+    const before = serializeDocument(parsed);
+    const fontValue = { name: "Roboto", source: "https://fonts.googleapis.com/css2?family=Roboto" };
+    const result = applyOp(parsed, {
+      type: "setVariableValue",
+      id: "brand-font",
+      value: fontValue,
+    });
+    expect(readVarDefault(parsed, "brand-font")).toEqual(fontValue);
+    applyPatchesToDocument(parsed, result.inverse);
+    expect(serializeDocument(parsed)).toBe(before);
+  });
+
+  it("B2: image {url} object round-trips through JSON model (no CSS prop)", () => {
+    const parsed = freshWithVars();
+    const imgValue = { url: "https://example.com/brand-logo.png" };
+    const result = applyOp(parsed, {
+      type: "setVariableValue",
+      id: "brand-logo",
+      value: imgValue,
+    });
+    expect(result.forward[0]?.path).toBe("/variables/brand-logo");
+    expect(result.forward[0]?.value).toEqual(imgValue);
+    expect(readVarDefault(parsed, "brand-logo")).toEqual(imgValue);
+    const root = parsed.document.querySelector("[data-hf-root]");
+    expect(root?.getAttribute("style") ?? "").not.toContain("--brand-logo");
+  });
+
+  it("B2: image inverse restores prior default", () => {
+    const parsed = freshWithVars();
+    const before = serializeDocument(parsed);
+    const result = applyOp(parsed, {
+      type: "setVariableValue",
+      id: "brand-logo",
+      value: { url: "https://example.com/new-logo.png" },
+    });
+    applyPatchesToDocument(parsed, result.inverse);
+    expect(serializeDocument(parsed)).toBe(before);
+  });
+
+  it("B1/batch: multiple setVariableValue calls fold to independent overrides", () => {
+    const parsed = freshWithVars();
+    applyOp(parsed, { type: "setVariableValue", id: "brand-color-primary", value: "#ff0000" });
+    applyOp(parsed, {
+      type: "setVariableValue",
+      id: "brand-font",
+      value: { name: "Roboto", source: "https://fonts.googleapis.com/css2?family=Roboto" },
+    });
+    expect(readVarDefault(parsed, "brand-color-primary")).toBe("#ff0000");
+    expect(readVarDefault(parsed, "brand-font")).toEqual({
+      name: "Roboto",
+      source: "https://fonts.googleapis.com/css2?family=Roboto",
+    });
+  });
 });
 
 // ─── setCompositionMetadata ───────────────────────────────────────────────────

packages/sdk/src/engine/mutate.ts

@@ -7,7 +7,15 @@
  * Phase 3b (parser-backed) will add setClassStyle + 7 GSAP ops as additional handlers.
  */
 
-import type { CanResult, EditOp, GsapTweenSpec, HfId, JsonPatchOp } from "../types.js";
+import type {
+  CanResult,
+  EditOp,
+  FontValue,
+  GsapTweenSpec,
+  HfId,
+  ImageValue,
+  JsonPatchOp,
+} from "../types.js";
 import type { ParsedDocument } from "./model.js";
 import {
   resolveScoped,
@@ -37,6 +45,7 @@ import {
   styleSheetPath,
   scalarChange,
   scalarDelete,
+  valueChange,
   patchAdd,
   patchRemove,
 } from "./patches.js";
@@ -680,23 +689,116 @@ function handleSetCompositionMetadata(
   return result;
 }
 
+// ─── Variable JSON model helpers ─────────────────────────────────────────────
+
+type VariableDecl = { id: string; default: unknown; [key: string]: unknown };
+
+/**
+ * Read the current `default` value for a variable id from
+ * `document.documentElement`'s `data-composition-variables` attribute.
+ * Returns undefined when the attribute is absent, the JSON is invalid,
+ * or no entry matches the given id.
+ */
+function readVariableDefault(document: Document, id: string): unknown {
+  const htmlEl = (document as Document & { documentElement?: Element }).documentElement;
+  if (!htmlEl) return undefined;
+  const raw = htmlEl.getAttribute("data-composition-variables");
+  if (!raw) return undefined;
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(raw);
+  } catch {
+    return undefined;
+  }
+  if (!Array.isArray(parsed)) return undefined;
+  const entry = (parsed as unknown[]).find(
+    (v): v is VariableDecl => typeof v === "object" && v !== null && (v as VariableDecl).id === id,
+  );
+  return entry?.default;
+}
+
+/**
+ * Upsert a variable's `default` in `data-composition-variables` on
+ * `document.documentElement`. No-ops when the attribute is absent or
+ * contains no declaration for the given id (we never auto-add declarations
+ * for undeclared variables — keep the schema authoritative).
+ * Returns true when the attribute was updated.
+ */
+function writeVariableDefault(document: Document, id: string, newDefault: unknown): boolean {
+  const htmlEl = (document as Document & { documentElement?: Element }).documentElement;
+  if (!htmlEl) return false;
+  const raw = htmlEl.getAttribute("data-composition-variables");
+  if (!raw) return false;
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(raw);
+  } catch {
+    return false;
+  }
+  if (!Array.isArray(parsed)) return false;
+  const arr = parsed as VariableDecl[];
+  const idx = arr.findIndex((v) => typeof v === "object" && v !== null && v.id === id);
+  if (idx < 0) return false; // variable not declared — don't auto-add
+  arr[idx] = { ...arr[idx]!, default: newDefault };
+  htmlEl.setAttribute("data-composition-variables", JSON.stringify(arr));
+  return true;
+}
+
+/**
+ * True when the value is a FontValue or ImageValue object
+ * (object-valued; must NOT be written as a CSS custom property).
+ */
+function isObjectVariableValue(
+  value: string | number | boolean | FontValue | ImageValue,
+): value is FontValue | ImageValue {
+  return typeof value === "object" && value !== null;
+}
+
 function handleSetVariableValue(
   parsed: ParsedDocument,
   id: string,
-  value: string | number | boolean,
+  value: string | number | boolean | FontValue | ImageValue,
 ): MutationResult {
   const root = findRoot(parsed.document);
   if (!root) return EMPTY;
 
+  const modelPath = variablePath(id);
+  const oldVarDefault = readVariableDefault(parsed.document, id);
+
+  if (isObjectVariableValue(value)) {
+    // Object values (font / image): write to JSON model only — objects are not
+    // valid CSS custom property values (LOCKED §7).
+    writeVariableDefault(parsed.document, id, value);
+    const p = valueChange(modelPath, oldVarDefault ?? null, value);
+    return { forward: [p.forward], inverse: [p.inverse] };
+  }
+
+  // Scalar values: update the JSON model (B1 — drives the runtime) and also
+  // keep the CSS custom prop as secondary / compat for compositions that
+  // CSS-bind directly to --{id}.
   const cssVar = `--${id}`;
+  const rootId = root.getAttribute("data-hf-id");
   const oldStyles = getElementStyles(root);
-  const oldValue = oldStyles[cssVar] ?? null;
+  const oldCssValue = oldStyles[cssVar] ?? null;
   const newVal = String(value);
   setElementStyles(root, { [cssVar]: newVal });
+  writeVariableDefault(parsed.document, id, value);
 
-  const path = variablePath(id);
-  const p = scalarChange(path, oldValue, newVal);
-  return { forward: [p.forward], inverse: [p.inverse] };
+  // Emit explicit patches for both the JSON model (canonical) and the CSS compat
+  // prop. Keeping them separate means apply-patches.ts can handle each path type
+  // purely (variable path → model only; style path → CSS only), so inverse patches
+  // correctly restore the exact pre-call state without CSS-side-effect ambiguity.
+  const modelP = valueChange(modelPath, oldVarDefault ?? null, value);
+  const forward: JsonPatchOp[] = [modelP.forward];
+  const inverse: JsonPatchOp[] = [modelP.inverse];
+
+  if (rootId) {
+    const cssPatch = scalarChange(stylePath(rootId, cssVar), oldCssValue, newVal);
+    forward.push(cssPatch.forward);
+    inverse.push(cssPatch.inverse);
+  }
+
+  return { forward, inverse };
 }
 
 // ─── GSAP selector helpers ───────────────────────────────────────────────────

packages/sdk/src/engine/patches.ts

@@ -212,6 +212,21 @@ export function scalarChange(
   return { forward, inverse };
 }
 
+/**
+ * Emit forward (replace or add) + inverse (replace or remove) for any JSON-serializable value.
+ * Use instead of scalarChange when the value may be an object (e.g. font/image variable).
+ * The old value is captured whole — no sub-key diffing.
+ */
+export function valueChange(
+  path: string,
+  oldValue: unknown,
+  newValue: unknown,
+): { forward: JsonPatchOp; inverse: JsonPatchOp } {
+  const forward = oldValue == null ? patchAdd(path, newValue) : patchReplace(path, newValue);
+  const inverse = oldValue == null ? patchRemove(path) : patchReplace(path, oldValue);
+  return { forward, inverse };
+}
+
 /** Emit forward remove + inverse add for a deletion. */
 export function scalarDelete(
   path: string,

packages/sdk/src/index.ts

@@ -4,6 +4,8 @@ export type {
   OverrideSet,
   EditOp,
   ElasticHold,
+  FontValue,
+  ImageValue,
   GsapTweenSpec,
   HfId,
   JsonPatchOp,

packages/sdk/src/session.ts

@@ -14,8 +14,10 @@ import type {
   EditOp,
   ElementSnapshot,
   FindQuery,
+  FontValue,
   GsapTweenSpec,
   HfId,
+  ImageValue,
   JsonPatchOp,
   OverrideSet,
   PatchEvent,
@@ -133,7 +135,7 @@ class CompositionImpl implements Composition {
     this.dispatch({ type: "removeElement", target: id });
   }
 
-  setVariableValue(id: string, value: string | number | boolean): void {
+  setVariableValue(id: string, value: string | number | boolean | FontValue | ImageValue): void {
     this.dispatch({ type: "setVariableValue", id, value });
   }
 
@@ -269,7 +271,9 @@ class CompositionImpl implements Composition {
       const key = pathToKey(p.path);
       if (key !== null) {
         this.overrides[key] =
-          p.op === "remove" ? null : (p.value as string | number | boolean | null);
+          p.op === "remove"
+            ? null
+            : (p.value as string | number | boolean | Record<string, unknown> | null);
       }
     }
 
@@ -453,7 +457,9 @@ class CompositionImpl implements Composition {
       const key = pathToKey(p.path);
       if (key !== null) {
         this.overrides[key] =
-          p.op === "remove" ? null : (p.value as string | number | boolean | null);
+          p.op === "remove"
+            ? null
+            : (p.value as string | number | boolean | Record<string, unknown> | null);
       }
     }
 

packages/sdk/src/types.ts

@@ -49,8 +49,14 @@ export interface SdkDocument {
  * Sparse map of `hfId.prop.path → value` overrides layered on top of the base template.
  * null value = removal marker (element or property deleted by user).
  * Examples: { "hf-x7k2.style.fontSize": "96px", "hf-y3a1.text": "Hello", "hf-z5k2": null }
+ *
+ * Font and image variable overrides store their object values under the var.{id} key:
+ * { "var.brand-font": { name: "Roboto", source: "https://fonts.googleapis.com/…" } }
  */
-export type OverrideSet = Record<string, string | number | boolean | null>;
+export type OverrideSet = Record<
+  string,
+  string | number | boolean | Record<string, unknown> | null
+>;
 
 // ─── can() result ─────────────────────────────────────────────────────────────
 
@@ -89,7 +95,11 @@ export type EditOp =
     }
   | { type: "setClassStyle"; selector: string; styles: Record<string, string | null> }
   | { type: "setCompositionMetadata"; width?: number; height?: number; duration?: number }
-  | { type: "setVariableValue"; id: string; value: string | number | boolean }
+  | {
+      type: "setVariableValue";
+      id: string;
+      value: string | number | boolean | FontValue | ImageValue;
+    }
   | { type: "addGsapTween"; target: HfId; tween: GsapTweenSpec }
   | { type: "setGsapTween"; animationId: string; properties: Partial<GsapTweenSpec> }
   | {
@@ -178,6 +188,24 @@ export interface ElasticHold {
   fill: "freeze" | "loop";
 }
 
+/**
+ * Object value for a `font` variable (LOCKED §7 — object-valued, never a CSS string).
+ * `name` is the CSS font-family value; `source` is the stylesheet URL to load.
+ */
+export interface FontValue {
+  name: string;
+  source: string;
+}
+
+/**
+ * Object value for an `image` variable (LOCKED §7 — object-valued, never a CSS string).
+ * `url` is the image src; additional fields (alt, fit, etc.) are forward-compatible.
+ */
+export interface ImageValue {
+  url: string;
+  [key: string]: unknown;
+}
+
 export interface GsapTweenSpec {
   method: "from" | "to" | "fromTo" | "set";
   position?: number | string;