heygen-com
diff --git a/‎docs/concepts/data-attributes.mdx‎
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diff --git a/‎docs/packages/cli.mdx‎
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diff --git a/‎packages/cli/src/commands/render.test.ts‎
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diff --git a/‎packages/cli/src/commands/render.ts‎
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diff --git a/‎packages/cli/src/utils/dockerRunArgs.test.ts‎
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@@ -30,6 +30,7 @@ Hyperframes uses HTML data attributes to control timing, media playback, and [co
 | `data-height` | `"1080"` | Composition height in pixels |
 | `data-composition-src` | `"./intro.html"` | Path to external [composition](/concepts/compositions) HTML file |
 | `data-variable-values` | `'{"title":"Hello"}'` | JSON object of values passed to a nested composition. HyperFrames carries these values through, but your composition script must read and apply them manually. |
+| `data-composition-variables` | `'[{"id":"title","type":"string","label":"Title","default":"Hello"}]'` | JSON array of declared variables (`id`, `type`, `label`, `default`). Drives Studio editing UI and provides defaults read by `window.__hyperframes.getVariables()`. The CLI flag `hyperframes render --variables '<json>'` overrides these defaults at render time. |
 
 ## Element Visibility
 
 
@@ -606,9 +606,49 @@ This is suppressed in CI environments, non-TTY shells, and when `HYPERFRAMES_NO_
     | `--browser-gpu` / `--no-browser-gpu` | — | on locally, off in Docker | Use or opt out of host GPU acceleration for local Chrome/WebGL capture |
     | `--docker` | — | off | Use Docker for [deterministic rendering](/concepts/determinism) |
     | `--quiet` | — | off | Suppress verbose output |
+    | `--variables` | JSON object | — | Variable overrides merged over `data-composition-variables` defaults. Read via `window.__hyperframes.getVariables()` |
+    | `--variables-file` | path | — | Path to a JSON file with variable overrides (alternative to `--variables`) |
 
     CRF and target bitrate default to the `--quality` preset. Use `--crf` or `--video-bitrate` for fine-grained overrides; `RenderConfig.crf` and `RenderConfig.videoBitrate` accept the same overrides programmatically.
 
+    #### Parametrized renders
+
+    Render the same composition with different content by declaring variables on the composition root and overriding them at render time:
+
+    ```html index.html
+    <html
+      data-composition-id="root"
+      data-composition-variables='[
+        {"id":"title","label":"Title","type":"string","default":"Hello"},
+        {"id":"theme","label":"Theme","type":"enum","options":[
+          {"value":"light","label":"Light"},
+          {"value":"dark","label":"Dark"}
+        ],"default":"light"}
+      ]'>
+      <body>
+        <h1 id="hero" class="clip" data-start="0" data-duration="3"></h1>
+        <script>
+          const vars = window.__hyperframes.getVariables();
+          document.getElementById("hero").textContent = vars.title;
+          document.body.dataset.theme = vars.theme;
+        </script>
+      </body>
+    </html>
+    ```
+
+    ```bash
+    # Render with declared defaults (preview also uses the defaults)
+    npx hyperframes render --output default.mp4
+
+    # Override at render time — missing keys fall through to declared defaults
+    npx hyperframes render --variables '{"title":"Q4 Report","theme":"dark"}' --output q4.mp4
+
+    # Pass values from a JSON file
+    npx hyperframes render --variables-file ./vars.json --output out.mp4
+    ```
+
+    `getVariables()` returns the merged result of declared defaults and any `--variables` overrides, so the same composition runs unchanged in dev preview and in production renders.
+
     #### WebM with Transparency
 
     Use `--format webm` to render compositions with a transparent background. This produces VP9 video with alpha channel in a WebM container — the standard format for overlayable video.
 
@@ -1,4 +1,4 @@
-import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
+import { afterEach, beforeAll, beforeEach, describe, expect, it, vi } from "vitest";
 
 const producerState = vi.hoisted(() => ({
   createdJobs: [] as Array<Record<string, unknown>>,
@@ -99,4 +99,109 @@ describe("renderLocal browser GPU config", () => {
     expect(resolveBrowserGpuForCli(false, false, "hardware")).toBe(false);
     expect(resolveBrowserGpuForCli(true, undefined, "hardware")).toBe(false);
   });
+
+  it("forwards parsed --variables payload to createRenderJob", async () => {
+    const { renderLocal } = await import("./render.js");
+    await renderLocal("/tmp/project", "/tmp/out.mp4", {
+      fps: 30,
+      quality: "standard",
+      format: "mp4",
+      gpu: false,
+      browserGpu: false,
+      hdrMode: "auto",
+      quiet: true,
+      variables: { title: "Hello", count: 3 },
+    });
+
+    expect(producerState.createdJobs[0]?.variables).toEqual({ title: "Hello", count: 3 });
+  });
+
+  it("omits variables from createRenderJob when not provided", async () => {
+    const { renderLocal } = await import("./render.js");
+    await renderLocal("/tmp/project", "/tmp/out.mp4", {
+      fps: 30,
+      quality: "standard",
+      format: "mp4",
+      gpu: false,
+      browserGpu: false,
+      hdrMode: "auto",
+      quiet: true,
+    });
+
+    expect(producerState.createdJobs[0]?.variables).toBeUndefined();
+  });
+});
+
+describe("parseVariablesArg", () => {
+  let parseVariablesArg: typeof import("./render.js").parseVariablesArg;
+
+  beforeAll(async () => {
+    ({ parseVariablesArg } = await import("./render.js"));
+  });
+
+  function expectErr<T extends { kind: string }>(
+    result: import("./render.js").VariablesParseResult,
+  ): T {
+    if (result.ok) throw new Error(`expected error, got ${JSON.stringify(result.value)}`);
+    return result.error as T;
+  }
+
+  it("returns undefined when neither flag is set", () => {
+    expect(parseVariablesArg(undefined, undefined)).toEqual({ ok: true, value: undefined });
+  });
+
+  it("parses inline JSON object", () => {
+    expect(parseVariablesArg('{"title":"Hello","n":3}', undefined)).toEqual({
+      ok: true,
+      value: { title: "Hello", n: 3 },
+    });
+  });
+
+  it("parses file JSON via injected reader", () => {
+    const fakeReader = (path: string) => {
+      if (path === "vars.json") return '{"theme":"dark"}';
+      throw new Error("unexpected path");
+    };
+    expect(parseVariablesArg(undefined, "vars.json", fakeReader)).toEqual({
+      ok: true,
+      value: { theme: "dark" },
+    });
+  });
+
+  it("rejects when both flags are set", () => {
+    const err = expectErr(parseVariablesArg('{"a":1}', "vars.json"));
+    expect(err).toEqual({ kind: "conflict" });
+  });
+
+  it("rejects unparseable JSON with a source-aware kind", () => {
+    expect(expectErr(parseVariablesArg("{not json", undefined))).toMatchObject({
+      kind: "parse-error",
+      source: "inline",
+    });
+    expect(expectErr(parseVariablesArg(undefined, "x", () => "{not json"))).toMatchObject({
+      kind: "parse-error",
+      source: "file",
+    });
+  });
+
+  it("rejects non-object payloads (array, string, null, number)", () => {
+    for (const payload of ["[1,2]", '"hello"', "null", "42"]) {
+      expect(expectErr(parseVariablesArg(payload, undefined))).toEqual({ kind: "shape-error" });
+    }
+  });
+
+  it("surfaces filesystem errors from --variables-file", () => {
+    const err = expectErr<{
+      kind: "read-error";
+      path: string;
+      cause: string;
+    }>(
+      parseVariablesArg(undefined, "missing.json", () => {
+        throw new Error("ENOENT: no such file");
+      }),
+    );
+    expect(err.kind).toBe("read-error");
+    expect(err.path).toBe("missing.json");
+    expect(err.cause).toMatch(/ENOENT/);
+  });
 });
@@ -11,6 +11,14 @@ export const examples: Example[] = [
   ["Parallel rendering with 6 workers", "hyperframes render --workers 6 --output fast.mp4"],
   ["Opt out of browser GPU render", "hyperframes render --no-browser-gpu --output cpu.mp4"],
   ["HDR output (auto-detected)", "hyperframes render --output hdr-output.mp4"],
+  [
+    "Override composition variables (parametrized render)",
+    'hyperframes render --variables \'{"title":"Q4 Report","theme":"dark"}\' --output q4.mp4',
+  ],
+  [
+    "Variables from a JSON file",
+    "hyperframes render --variables-file ./vars.json --output out.mp4",
+  ],
 ];
 import { cpus, freemem, tmpdir } from "node:os";
 import { resolve, dirname, join, basename } from "node:path";
@@ -124,6 +132,16 @@ export default defineCommand({
       type: "string",
       description: "Max concurrent renders when using the producer server (1-10). Default: 2.",
     },
+    variables: {
+      type: "string",
+      description:
+        'JSON object of variable values, merged over the composition\'s data-composition-variables defaults. Example: --variables \'{"title":"Hello"}\'. Read inside the composition via window.__hyperframes.getVariables().',
+    },
+    "variables-file": {
+      type: "string",
+      description:
+        "Path to a JSON file with variable values (alternative to --variables). The file must contain a single JSON object.",
+    },
   },
   async run({ args }) {
     // ── Resolve project ────────────────────────────────────────────────────
@@ -328,6 +346,9 @@ export default defineCommand({
       process.exit(1);
     }
 
+    // ── Resolve --variables / --variables-file ──────────────────────────
+    const variables = resolveVariablesArg(args.variables, args["variables-file"]);
+
     // ── Render ────────────────────────────────────────────────────────────
     if (useDocker) {
       await renderDocker(project.dir, outputPath, {
@@ -341,6 +362,7 @@ export default defineCommand({
         crf,
         videoBitrate,
         quiet,
+        variables,
       });
     } else {
       await renderLocal(project.dir, outputPath, {
@@ -355,6 +377,7 @@ export default defineCommand({
         videoBitrate,
         quiet,
         browserPath,
+        variables,
       });
     }
   },
@@ -372,6 +395,118 @@ interface RenderOptions {
   videoBitrate?: string;
   quiet: boolean;
   browserPath?: string;
+  variables?: Record<string, unknown>;
+}
+
+export type VariablesParseError =
+  | { kind: "conflict" }
+  | { kind: "read-error"; path: string; cause: string }
+  | { kind: "parse-error"; source: "inline" | "file"; cause: string }
+  | { kind: "shape-error" };
+
+export type VariablesParseResult =
+  | { ok: true; value: Record<string, unknown> | undefined }
+  | { ok: false; error: VariablesParseError };
+
+/**
+ * Pure parser for `--variables` / `--variables-file` flag pair. Splits out
+ * from `resolveVariablesArg` so validation paths are unit-testable without
+ * triggering `process.exit`. Reports failures via a structured `kind`
+ * discriminant so the side-effecting wrapper owns all UI strings.
+ */
+export function parseVariablesArg(
+  inline: string | undefined,
+  filePath: string | undefined,
+  readFile: (path: string) => string = (p) => readFileSync(resolve(p), "utf8"),
+): VariablesParseResult {
+  if (inline != null && filePath != null) {
+    return { ok: false, error: { kind: "conflict" } };
+  }
+  let raw: string | undefined;
+  let source: "inline" | "file" | undefined;
+  if (inline != null) {
+    raw = inline;
+    source = "inline";
+  } else if (filePath != null) {
+    try {
+      raw = readFile(filePath);
+      source = "file";
+    } catch (error: unknown) {
+      return {
+        ok: false,
+        error: {
+          kind: "read-error",
+          path: filePath,
+          cause: error instanceof Error ? error.message : String(error),
+        },
+      };
+    }
+  }
+  if (raw == null) return { ok: true, value: undefined };
+
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(raw);
+  } catch (error: unknown) {
+    return {
+      ok: false,
+      error: {
+        kind: "parse-error",
+        source: source ?? "inline",
+        cause: error instanceof Error ? error.message : String(error),
+      },
+    };
+  }
+  if (parsed == null || typeof parsed !== "object" || Array.isArray(parsed)) {
+    return { ok: false, error: { kind: "shape-error" } };
+  }
+  return { ok: true, value: parsed as Record<string, unknown> };
+}
+
+function variablesErrorMessage(error: VariablesParseError): { title: string; message: string } {
+  switch (error.kind) {
+    case "conflict":
+      return {
+        title: "Conflicting variables flags",
+        message: "Use either --variables or --variables-file, not both.",
+      };
+    case "read-error":
+      return {
+        title: "Could not read --variables-file",
+        message: `${error.path}: ${error.cause}`,
+      };
+    case "parse-error":
+      return {
+        title:
+          error.source === "file"
+            ? "Invalid JSON in --variables-file"
+            : "Invalid JSON in --variables",
+        message: error.cause,
+      };
+    case "shape-error":
+      return {
+        title: "Invalid variables payload",
+        message: 'Variables must be a JSON object (e.g. {"title":"Hello"}).',
+      };
+  }
+}
+
+/**
+ * Resolve `--variables` / `--variables-file` into a plain object, or
+ * `undefined` when neither flag is set. Exits the process with a friendly
+ * error box on any validation failure.
+ */
+export function resolveVariablesArg(
+  inline: string | undefined,
+  filePath: string | undefined,
+): Record<string, unknown> | undefined {
+  const result = parseVariablesArg(inline, filePath);
+  if (!result.ok) {
+    const { title, message } = variablesErrorMessage(result.error);
+    errorBox(title, message);
+    process.exit(1);
+  }
+  return result.value;
 }
 
 export function resolveBrowserGpuForCli(
@@ -507,6 +642,7 @@ async function renderDocker(
       crf: options.crf,
       videoBitrate: options.videoBitrate,
       quiet: options.quiet,
+      variables: options.variables,
     },
   });
 
@@ -575,6 +711,7 @@ export async function renderLocal(
     hdrMode: options.hdrMode,
     crf: options.crf,
     videoBitrate: options.videoBitrate,
+    variables: options.variables,
   });
 
   const onProgress = options.quiet
 
@@ -187,4 +187,27 @@ describe("buildDockerRunArgs", () => {
     expect(args).toContain("10M");
     expect(args).not.toContain("--crf");
   });
+
+  it("forwards --variables JSON to the container when set", () => {
+    const args = buildDockerRunArgs({
+      ...FIXED_INPUT,
+      options: { ...BASE, variables: { title: "Hello", n: 3 } },
+    });
+    const idx = args.indexOf("--variables");
+    expect(idx).toBeGreaterThan(-1);
+    expect(args[idx + 1]).toBe('{"title":"Hello","n":3}');
+  });
+
+  it("omits --variables when none provided", () => {
+    const args = buildDockerRunArgs({ ...FIXED_INPUT, options: BASE });
+    expect(args).not.toContain("--variables");
+  });
+
+  it("omits --variables when payload is empty", () => {
+    const args = buildDockerRunArgs({
+      ...FIXED_INPUT,
+      options: { ...BASE, variables: {} },
+    });
+    expect(args).not.toContain("--variables");
+  });
 });