diff --git a/.fallowrc.jsonc b/.fallowrc.jsonc
index 4f5f8f6b28..7c85694fc3 100644
--- a/.fallowrc.jsonc
+++ b/.fallowrc.jsonc
@@ -90,6 +90,12 @@
     "packages/cli/src/cloud/_gen/**",
   ],
   "ignoreExports": [
+    // Public player-state types are consumed by package users outside this
+    // workspace. Keep the barrel stable even when no in-repo import needs it.
+    {
+      "file": "packages/studio/src/player/index.ts",
+      "exports": ["ZoomMode"],
+    },
     // Part of useTimelineEditing's inferred public return type; consumers invoke
     // handleTimelineGroupResize without importing the change type directly.
     {
diff --git a/packages/core/package.json b/packages/core/package.json
index fe4659d933..85ac59c276 100644
--- a/packages/core/package.json
+++ b/packages/core/package.json
@@ -107,6 +107,12 @@
       "import": "./src/runtime/startExpression.ts",
       "types": "./src/runtime/startExpression.ts"
     },
+    "./runtime/protocol": {
+      "bun": "./src/runtime/protocol.ts",
+      "node": "./dist/runtime/protocol.js",
+      "import": "./src/runtime/protocol.ts",
+      "types": "./src/runtime/protocol.ts"
+    },
     "./compiler/html-document": {
       "bun": "./src/compiler/htmlDocument.ts",
       "node": "./dist/compiler/htmlDocument.js",
@@ -307,6 +313,10 @@
         "import": "./dist/runtime/startExpression.js",
         "types": "./dist/runtime/startExpression.d.ts"
       },
+      "./runtime/protocol": {
+        "import": "./dist/runtime/protocol.js",
+        "types": "./dist/runtime/protocol.d.ts"
+      },
       "./compiler/html-document": {
         "import": "./dist/compiler/htmlDocument.js",
         "types": "./dist/compiler/htmlDocument.d.ts"
diff --git a/packages/core/src/runtime/bridge.test.ts b/packages/core/src/runtime/bridge.test.ts
index 7e58cdfcec..e5d432dbc4 100644
--- a/packages/core/src/runtime/bridge.test.ts
+++ b/packages/core/src/runtime/bridge.test.ts
@@ -19,6 +19,7 @@ function createMockDeps() {
     onSetRootDuration: vi.fn(),
     onEnablePickMode: vi.fn(),
     onDisablePickMode: vi.fn(),
+    getCanonicalFps: vi.fn(() => 30),
   };
 }
 
@@ -54,7 +55,22 @@ describe("installRuntimeControlBridge", () => {
     const deps = createMockDeps();
     const handler = installRuntimeControlBridge(deps);
     handler(makeControlMessage("seek", { frame: 150, seekMode: "drag" }));
-    expect(deps.onSeek).toHaveBeenCalledWith(150, "drag");
+    expect(deps.onSeek).toHaveBeenCalledWith(5, "drag");
+  });
+
+  it("prefers canonical seconds in protocol v1 seek messages", () => {
+    const deps = createMockDeps();
+    const handler = installRuntimeControlBridge(deps);
+    handler(
+      makeControlMessage("seek", {
+        protocolVersion: 1,
+        capabilities: ["seconds-time", "rational-fps", "seek-keep-playing"],
+        fps: { numerator: 60, denominator: 1 },
+        timeSeconds: 2.5,
+        frame: 999,
+      }),
+    );
+    expect(deps.onSeek).toHaveBeenCalledWith(2.5, "commit");
   });
 
   it("seek defaults frame to 0 and seekMode to commit", () => {
@@ -245,7 +261,32 @@ describe("installRuntimeControlBridge", () => {
     const postSpy = vi.spyOn(window.parent, "postMessage");
     const deps = createMockDeps();
     installRuntimeControlBridge(deps);
-    expect(postSpy).toHaveBeenCalledWith({ source: "hf-preview", type: "ready" }, "*");
+    expect(postSpy).toHaveBeenCalledWith(
+      expect.objectContaining({
+        source: "hf-preview",
+        type: "ready",
+        protocolVersion: 1,
+        fps: { numerator: 30, denominator: 1 },
+      }),
+      "*",
+    );
+    postSpy.mockRestore();
+  });
+
+  it("rejects an unknown protocol major with a diagnostic", () => {
+    const postSpy = vi.spyOn(window.parent, "postMessage");
+    const deps = createMockDeps();
+    const handler = installRuntimeControlBridge(deps);
+    handler(makeControlMessage("play", { protocolVersion: 2 }));
+
+    expect(deps.onPlay).not.toHaveBeenCalled();
+    expect(postSpy).toHaveBeenCalledWith(
+      expect.objectContaining({
+        type: "diagnostic",
+        code: "runtime.protocol.unsupported_protocol_version",
+      }),
+      "*",
+    );
     postSpy.mockRestore();
   });
 });
diff --git a/packages/core/src/runtime/bridge.ts b/packages/core/src/runtime/bridge.ts
index 44597243a5..cb718647ac 100644
--- a/packages/core/src/runtime/bridge.ts
+++ b/packages/core/src/runtime/bridge.ts
@@ -1,12 +1,18 @@
 import { swallow } from "./diagnostics";
 import type { HfColorGradingTarget } from "../colorGrading";
 import type { RuntimeBridgeControlMessage, RuntimeOutboundMessage } from "./types";
+import {
+  inspectRuntimeProtocol,
+  runtimeProtocolFpsToNumber,
+  runtimeProtocolMetadata,
+  type RuntimeProtocolV1,
+} from "./protocol";
 
 type BridgeDeps = {
   onPlay: () => void;
   onPause: () => void;
   onStopMedia: () => void;
-  onSeek: (frame: number, seekMode: "drag" | "commit") => void;
+  onSeek: (timeSeconds: number, seekMode: "drag" | "commit") => void;
   onTick: () => void;
   onSetMuted: (muted: boolean) => void;
   onSetVolume: (volume: number) => void;
@@ -22,18 +28,25 @@ type BridgeDeps = {
   ) => void;
   onEnablePickMode: () => void;
   onDisablePickMode: () => void;
+  getCanonicalFps: () => number;
 };
 
+let runtimeProtocolFps = 30;
+
+export function setRuntimeProtocolFps(fps: number): void {
+  runtimeProtocolFps = Number.isFinite(fps) && fps > 0 ? fps : 30;
+}
+
 export function postRuntimeMessage(payload: RuntimeOutboundMessage): void {
   try {
-    window.parent.postMessage(payload, "*");
+    window.parent.postMessage({ ...payload, ...runtimeProtocolMetadata(runtimeProtocolFps) }, "*");
   } catch (err) {
     // Cross-frame posting can throw if the parent is gone or origin-isolated.
     swallow("bridge.postMessage", err);
   }
 }
 
-type BridgeControlData = Partial<RuntimeBridgeControlMessage>;
+type BridgeControlData = Partial<RuntimeBridgeControlMessage & RuntimeProtocolV1>;
 type ControlHandler = (data: BridgeControlData, deps: BridgeDeps) => void;
 
 // Per-action dispatchers. Splitting the handler into a lookup table keeps the
@@ -43,7 +56,7 @@ const CONTROL_HANDLERS: Record<string, ControlHandler> = {
   play: (_d, deps) => deps.onPlay(),
   pause: (_d, deps) => deps.onPause(),
   "stop-media": (_d, deps) => deps.onStopMedia(),
-  seek: (data, deps) => deps.onSeek(Number(data.frame ?? 0), data.seekMode ?? "commit"),
+  seek: (data, deps) => deps.onSeek(resolveSeekTimeSeconds(data, deps), data.seekMode ?? "commit"),
   tick: (_d, deps) => deps.onTick(),
   "set-muted": (data, deps) => deps.onSetMuted(Boolean(data.muted)),
   "set-volume": (data, deps) =>
@@ -64,6 +77,31 @@ const CONTROL_HANDLERS: Record<string, ControlHandler> = {
   "flash-elements": (data) => handleFlashElements(data),
 };
 
+function resolveSeekTimeSeconds(data: BridgeControlData, deps: BridgeDeps): number {
+  const explicitSeconds = Number(data.timeSeconds);
+  if (Number.isFinite(explicitSeconds)) return Math.max(0, explicitSeconds);
+  const messageFps = runtimeProtocolFpsToNumber(data.fps);
+  const fps = messageFps ?? deps.getCanonicalFps();
+  return Math.max(0, Number(data.frame ?? 0)) / fps;
+}
+
+function rejectUnsupportedProtocol(data: BridgeControlData): boolean {
+  const protocol = inspectRuntimeProtocol(data);
+  if (protocol.status !== "unsupported") return false;
+  postRuntimeMessage({
+    source: "hf-preview",
+    type: "diagnostic",
+    code: `runtime.protocol.${protocol.code}`,
+    details: {
+      receivedVersion:
+        typeof protocol.receivedVersion === "string" || typeof protocol.receivedVersion === "number"
+          ? protocol.receivedVersion
+          : null,
+    },
+  });
+  return true;
+}
+
 function handleFlashElements(data: BridgeControlData): void {
   // Briefly highlight elements — used by the chat-canvas bridge
   // to show what changed after an agent edit
@@ -78,6 +116,7 @@ export function installRuntimeControlBridge(deps: BridgeDeps): (event: MessageEv
   const handler = (event: MessageEvent) => {
     const data = event.data as BridgeControlData | null;
     if (!data || data.source !== "hf-parent" || data.type !== "control") return;
+    if (rejectUnsupportedProtocol(data)) return;
     const action = data.action;
     if (typeof action !== "string") return;
     const fn = CONTROL_HANDLERS[action];
diff --git a/packages/core/src/runtime/init.ts b/packages/core/src/runtime/init.ts
index 67ace5d152..e311c19300 100644
--- a/packages/core/src/runtime/init.ts
+++ b/packages/core/src/runtime/init.ts
@@ -1,5 +1,5 @@
 // fallow-ignore-file code-duplication complexity
-import { installRuntimeControlBridge, postRuntimeMessage } from "./bridge";
+import { installRuntimeControlBridge, postRuntimeMessage, setRuntimeProtocolFps } from "./bridge";
 import { initRuntimeAnalytics, emitAnalyticsEvent } from "./analytics";
 import { injectCompositionCssVariables } from "./getVariables";
 import { createCssAdapter } from "./adapters/css";
@@ -93,6 +93,7 @@ export function initSandboxRuntimeModular(): void {
   applyVariableBindings(document);
   const exportRenderFps = resolveExportRenderFps();
   state.canonicalFps = exportRenderFps.fps ?? state.canonicalFps;
+  setRuntimeProtocolFps(state.canonicalFps);
   if (window.__HF_EXPORT_RENDER_SEEK_CONFIG) {
     console.info("[hyperframes] render runtime fps", {
       canonicalFps: state.canonicalFps,
@@ -2172,10 +2173,9 @@ export function initSandboxRuntimeModular(): void {
         if (el instanceof HTMLMediaElement && !el.paused) el.pause();
       }
     },
-    onSeek: (frame, _seekMode) => {
-      const time = Math.max(0, frame) / state.canonicalFps;
-      player.seek(time);
-      emitAnalyticsEvent("composition_seeked", { time });
+    onSeek: (timeSeconds, _seekMode) => {
+      player.seek(timeSeconds);
+      emitAnalyticsEvent("composition_seeked", { time: timeSeconds });
     },
     onSetMuted: (muted) => {
       state.bridgeMuted = muted;
@@ -2266,6 +2266,7 @@ export function initSandboxRuntimeModular(): void {
     },
     onEnablePickMode: () => picker.enablePickMode(),
     onDisablePickMode: () => picker.disablePickMode(),
+    getCanonicalFps: () => state.canonicalFps,
   });
 
   state.deterministicAdapters = [
diff --git a/packages/core/src/runtime/protocol.test.ts b/packages/core/src/runtime/protocol.test.ts
new file mode 100644
index 0000000000..35790b9765
--- /dev/null
+++ b/packages/core/src/runtime/protocol.test.ts
@@ -0,0 +1,50 @@
+import { describe, expect, it } from "vitest";
+import {
+  inspectRuntimeProtocol,
+  runtimeProtocolFpsFromNumber,
+  runtimeProtocolFpsToNumber,
+  runtimeProtocolMetadata,
+} from "./protocol";
+
+describe("runtime protocol", () => {
+  it.each([24, 30, 60, 24_000 / 1_001, 30_000 / 1_001])(
+    "round-trips %s fps as a rational",
+    (fps) => {
+      expect(runtimeProtocolFpsToNumber(runtimeProtocolFpsFromNumber(fps))).toBeCloseTo(fps, 6);
+    },
+  );
+
+  it("accepts protocol v1 metadata and exposes its fps", () => {
+    expect(inspectRuntimeProtocol(runtimeProtocolMetadata(60))).toMatchObject({
+      status: "supported",
+      fps: 60,
+    });
+  });
+
+  it("tolerates additive capabilities within protocol v1", () => {
+    const metadata = runtimeProtocolMetadata(30);
+    expect(
+      inspectRuntimeProtocol({
+        ...metadata,
+        capabilities: [...metadata.capabilities, "future-cap"],
+      }),
+    ).toMatchObject({ status: "supported", fps: 30 });
+  });
+
+  it("keeps an explicit legacy fallback", () => {
+    expect(inspectRuntimeProtocol({ source: "hf-preview" }, 24)).toEqual({
+      status: "legacy",
+      fps: 24,
+    });
+  });
+
+  it("rejects unknown protocol versions and malformed v1 metadata", () => {
+    expect(inspectRuntimeProtocol({ protocolVersion: 2 })).toMatchObject({
+      status: "unsupported",
+      code: "unsupported_protocol_version",
+    });
+    expect(
+      inspectRuntimeProtocol({ protocolVersion: 1, fps: { numerator: 30, denominator: 1 } }),
+    ).toMatchObject({ status: "unsupported", code: "invalid_protocol_metadata" });
+  });
+});
diff --git a/packages/core/src/runtime/protocol.ts b/packages/core/src/runtime/protocol.ts
new file mode 100644
index 0000000000..c138b236f1
--- /dev/null
+++ b/packages/core/src/runtime/protocol.ts
@@ -0,0 +1,92 @@
+export const RUNTIME_PROTOCOL_VERSION = 1 as const;
+
+export const RUNTIME_PROTOCOL_CAPABILITIES = [
+  "seconds-time",
+  "rational-fps",
+  "seek-keep-playing",
+] as const;
+
+export type RuntimeProtocolFps = {
+  numerator: number;
+  denominator: number;
+};
+
+export type RuntimeProtocolV1 = {
+  protocolVersion: typeof RUNTIME_PROTOCOL_VERSION;
+  capabilities: readonly string[];
+  fps: RuntimeProtocolFps;
+};
+
+export type RuntimeProtocolInspection =
+  | { status: "legacy"; fps: number }
+  | { status: "supported"; fps: number; metadata: RuntimeProtocolV1 }
+  | {
+      status: "unsupported";
+      code: "unsupported_protocol_version" | "invalid_protocol_metadata";
+      receivedVersion: unknown;
+    };
+
+function greatestCommonDivisor(a: number, b: number): number {
+  let left = Math.abs(a);
+  let right = Math.abs(b);
+  while (right !== 0) {
+    const remainder = left % right;
+    left = right;
+    right = remainder;
+  }
+  return left || 1;
+}
+
+export function runtimeProtocolFpsFromNumber(value: number): RuntimeProtocolFps {
+  const safe = Number.isFinite(value) && value > 0 ? value : 30;
+  const denominator = Number.isInteger(safe) ? 1 : 1_000_000;
+  const numerator = Math.round(safe * denominator);
+  const divisor = greatestCommonDivisor(numerator, denominator);
+  return { numerator: numerator / divisor, denominator: denominator / divisor };
+}
+
+export function runtimeProtocolFpsToNumber(value: unknown): number | null {
+  if (typeof value !== "object" || value === null) return null;
+  const fps = value as Partial<RuntimeProtocolFps>;
+  if (!Number.isFinite(fps.numerator) || !Number.isFinite(fps.denominator)) return null;
+  if ((fps.numerator ?? 0) <= 0 || (fps.denominator ?? 0) <= 0) return null;
+  return Number(fps.numerator) / Number(fps.denominator);
+}
+
+export function runtimeProtocolMetadata(fps: number): RuntimeProtocolV1 {
+  return {
+    protocolVersion: RUNTIME_PROTOCOL_VERSION,
+    capabilities: RUNTIME_PROTOCOL_CAPABILITIES,
+    fps: runtimeProtocolFpsFromNumber(fps),
+  };
+}
+
+function hasDeclaredCapabilities(value: unknown): boolean {
+  return Array.isArray(value) && value.every((capability) => typeof capability === "string");
+}
+
+export function inspectRuntimeProtocol(value: unknown, legacyFps = 30): RuntimeProtocolInspection {
+  if (typeof value !== "object" || value === null) return { status: "legacy", fps: legacyFps };
+  const message = value as Record<string, unknown>;
+  if (message.protocolVersion === undefined) return { status: "legacy", fps: legacyFps };
+  if (message.protocolVersion !== RUNTIME_PROTOCOL_VERSION) {
+    return {
+      status: "unsupported",
+      code: "unsupported_protocol_version",
+      receivedVersion: message.protocolVersion,
+    };
+  }
+  const fps = runtimeProtocolFpsToNumber(message.fps);
+  if (fps === null || !hasDeclaredCapabilities(message.capabilities)) {
+    return {
+      status: "unsupported",
+      code: "invalid_protocol_metadata",
+      receivedVersion: message.protocolVersion,
+    };
+  }
+  return {
+    status: "supported",
+    fps,
+    metadata: message as RuntimeProtocolV1,
+  };
+}
diff --git a/packages/core/src/runtime/types.ts b/packages/core/src/runtime/types.ts
index 4f54a1a2f7..f53a80534d 100644
--- a/packages/core/src/runtime/types.ts
+++ b/packages/core/src/runtime/types.ts
@@ -27,6 +27,7 @@ export type RuntimeBridgeControlMessage = {
   type: "control";
   action: RuntimeBridgeControlAction;
   frame?: number;
+  timeSeconds?: number;
   muted?: boolean;
   volume?: number;
   durationSeconds?: number;
diff --git a/packages/core/tsconfig.json b/packages/core/tsconfig.json
index bf6eb101e7..1e4146234b 100644
--- a/packages/core/tsconfig.json
+++ b/packages/core/tsconfig.json
@@ -16,7 +16,8 @@
   "files": [
     "src/runtime/clipTree.ts",
     "src/runtime/mediaVolumeEnvelope.ts",
-    "src/runtime/positionEdits.ts"
+    "src/runtime/positionEdits.ts",
+    "src/runtime/protocol.ts"
   ],
   "include": ["src/**/*"],
   "exclude": [
diff --git a/packages/player/package.json b/packages/player/package.json
index d80d48bae3..b6c93e2440 100644
--- a/packages/player/package.json
+++ b/packages/player/package.json
@@ -28,7 +28,7 @@
     }
   },
   "scripts": {
-    "build": "tsup",
+    "build": "tsup && node scripts/verify-runtime-pin.mjs",
     "typecheck": "tsc --noEmit && tsc --noEmit -p tests/perf/tsconfig.json",
     "test": "vitest run",
     "perf": "bun run tests/perf/index.ts"
diff --git a/packages/player/scripts/verify-runtime-pin.mjs b/packages/player/scripts/verify-runtime-pin.mjs
new file mode 100644
index 0000000000..d19de8fa96
--- /dev/null
+++ b/packages/player/scripts/verify-runtime-pin.mjs
@@ -0,0 +1,25 @@
+#!/usr/bin/env node
+
+import { readFileSync } from "node:fs";
+import { join } from "node:path";
+
+const root = join(import.meta.dirname, "..");
+const { version } = JSON.parse(readFileSync(join(root, "package.json"), "utf8"));
+const expected = `@hyperframes/core@${version}/dist/hyperframe.runtime.iife.js`;
+const bundles = [
+  "dist/hyperframes-player.js",
+  "dist/hyperframes-player.cjs",
+  "dist/hyperframes-player.global.js",
+];
+
+for (const bundle of bundles) {
+  const source = readFileSync(join(root, bundle), "utf8");
+  if (!source.includes(expected)) {
+    throw new Error(`${bundle} does not pin the injected runtime to ${version}`);
+  }
+  if (source.includes("@hyperframes/core/dist/hyperframe.runtime.iife.js")) {
+    throw new Error(`${bundle} still contains an unversioned runtime URL`);
+  }
+}
+
+console.log(`Verified Player runtime injection is pinned to @hyperframes/core@${version}.`);
diff --git a/packages/player/src/composition-probe.test.ts b/packages/player/src/composition-probe.test.ts
index fee97a7bfe..2b4a749ac4 100644
--- a/packages/player/src/composition-probe.test.ts
+++ b/packages/player/src/composition-probe.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { readCompositionSizeFromDocument } from "./composition-probe.js";
+import { readCompositionSizeFromDocument, runtimeCdnUrlForVersion } from "./composition-probe.js";
 
 describe("readCompositionSizeFromDocument", () => {
   it("reads dimensions from the composition root", () => {
@@ -24,3 +24,15 @@ describe("readCompositionSizeFromDocument", () => {
     expect(readCompositionSizeFromDocument(doc)).toBeNull();
   });
 });
+
+describe("runtimeCdnUrlForVersion", () => {
+  it("pins the injected core runtime to the Player-compatible version", () => {
+    expect(runtimeCdnUrlForVersion("1.2.3")).toBe(
+      "https://cdn.jsdelivr.net/npm/@hyperframes/core@1.2.3/dist/hyperframe.runtime.iife.js",
+    );
+  });
+
+  it("rejects values that could create an unversioned or malformed URL", () => {
+    expect(() => runtimeCdnUrlForVersion("latest")).toThrow("Invalid HyperFrames runtime version");
+  });
+});
diff --git a/packages/player/src/composition-probe.ts b/packages/player/src/composition-probe.ts
index 5ac4c18ddf..b7fdfc0fdd 100644
--- a/packages/player/src/composition-probe.ts
+++ b/packages/player/src/composition-probe.ts
@@ -19,8 +19,19 @@ import {
   isRuntimeDurationAdapter,
 } from "./timeline-adapters.js";
 
+declare const __HYPERFRAMES_RUNTIME_CDN_URL__: string;
+
+export function runtimeCdnUrlForVersion(version: string): string {
+  if (!/^\d+\.\d+\.\d+(?:-[0-9A-Za-z.-]+)?(?:\+[0-9A-Za-z.-]+)?$/.test(version)) {
+    throw new Error(`Invalid HyperFrames runtime version: ${version}`);
+  }
+  return `https://cdn.jsdelivr.net/npm/@hyperframes/core@${version}/dist/hyperframe.runtime.iife.js`;
+}
+
 const RUNTIME_CDN_URL =
-  "https://cdn.jsdelivr.net/npm/@hyperframes/core/dist/hyperframe.runtime.iife.js";
+  typeof __HYPERFRAMES_RUNTIME_CDN_URL__ === "string"
+    ? __HYPERFRAMES_RUNTIME_CDN_URL__
+    : runtimeCdnUrlForVersion("0.0.0-dev");
 
 export interface ProbeResult {
   duration: number;
diff --git a/packages/player/src/hyperframes-player.test.ts b/packages/player/src/hyperframes-player.test.ts
index 76b9358fab..9b0af53da9 100644
--- a/packages/player/src/hyperframes-player.test.ts
+++ b/packages/player/src/hyperframes-player.test.ts
@@ -1008,7 +1008,27 @@ describe("HyperframesPlayer seek() sync path", () => {
         source: "hf-parent",
         type: "control",
         action: "seek",
-        frame: Math.round(12.5 * 30),
+        timeSeconds: 12.5,
+        frame: 375,
+        protocolVersion: 1,
+      }),
+      "*",
+    );
+  });
+
+  it("carries a frame fallback for accepted legacy cross-origin runtimes", () => {
+    // An older runtime ignores protocol-v1 metadata and reads only `frame`.
+    // Omitting it makes that bridge default every seek to frame zero.
+    const post = vi.fn();
+    stubContentWindow({ postMessage: post });
+
+    player.seek(7.5);
+
+    expect(post).toHaveBeenCalledWith(
+      expect.objectContaining({
+        action: "seek",
+        timeSeconds: 7.5,
+        frame: 225,
       }),
       "*",
     );
@@ -1162,7 +1182,15 @@ describe("HyperframesPlayer seek() sync path", () => {
 
     player.seek(7);
 
-    expect(post).toHaveBeenCalledWith(expect.objectContaining({ action: "seek", frame: 210 }), "*");
+    expect(post).toHaveBeenCalledWith(
+      expect.objectContaining({
+        action: "seek",
+        timeSeconds: 7,
+        frame: 210,
+        protocolVersion: 1,
+      }),
+      "*",
+    );
   });
 
   it("does not throw when contentWindow access raises (cross-origin embed)", () => {
diff --git a/packages/player/src/hyperframes-player.ts b/packages/player/src/hyperframes-player.ts
index 8cb607b3d7..dd59a3f166 100644
--- a/packages/player/src/hyperframes-player.ts
+++ b/packages/player/src/hyperframes-player.ts
@@ -18,6 +18,7 @@ import { createShaderLoader } from "./shader-loader-element.js";
 import { ShaderLoaderState } from "./shader-loader-state.js";
 import { PLAYER_STYLES } from "./styles.js";
 import { type DirectTimelineAdapter } from "./timeline-adapters.js";
+import { runtimeProtocolMetadata } from "@hyperframes/core/runtime/protocol";
 
 // Playback-rate bounds mirror the runtime clamp in
 // packages/core/src/runtime/init.ts (applyPlaybackRate) and media.ts so the
@@ -95,6 +96,7 @@ class HyperframesPlayer extends HTMLElement {
   private _parentTickRaf: number | null = null;
   private _media: ParentMediaManager;
   private _scenes: { id: string; start: number; duration: number }[] = [];
+  private _runtimeFps = 30;
 
   constructor() {
     super();
@@ -322,7 +324,13 @@ class HyperframesPlayer extends HTMLElement {
 
   seek(timeInSeconds: number) {
     if (!this._trySyncSeek(timeInSeconds) && !this._tryDirectTimelineSeek(timeInSeconds)) {
-      this._sendControl("seek", { frame: Math.round(timeInSeconds * 30) });
+      this._sendControl("seek", {
+        timeSeconds: timeInSeconds,
+        // Legacy runtimes read only `frame`. Protocol-v1 runtimes prefer
+        // `timeSeconds`, so carrying both keeps cross-origin embeds seekable
+        // while preserving seconds-first precision between current peers.
+        frame: Math.round(timeInSeconds * this._runtimeFps),
+      });
     }
     this._directTimelineClock.stop();
     this._stopParentTickClock();
@@ -491,7 +499,13 @@ class HyperframesPlayer extends HTMLElement {
   private _sendControl(action: string, extra: Record<string, unknown> = {}) {
     try {
       this.iframe.contentWindow?.postMessage(
-        { source: "hf-parent", type: "control", action, ...extra },
+        {
+          ...extra,
+          source: "hf-parent",
+          type: "control",
+          action,
+          ...runtimeProtocolMetadata(this._runtimeFps),
+        },
         "*",
       );
     } catch {
@@ -655,6 +669,9 @@ class HyperframesPlayer extends HTMLElement {
       getIframeDoc: () => this.iframe.contentDocument,
       onRuntimeReady: () => this._replayBridgeState(),
       onRuntimeTimelineReady: (duration) => this._onRuntimeTimelineReady(duration),
+      setRuntimeFps: (fps) => {
+        this._runtimeFps = fps;
+      },
       shouldPromoteMediaAutoplayFallback: () => !this._isSlideshowPlayer(),
       setScenes: (scenes) => {
         this._scenes = scenes;
diff --git a/packages/player/src/runtime-message-handler.test.ts b/packages/player/src/runtime-message-handler.test.ts
index 6a1beba13a..569bf9f8b3 100644
--- a/packages/player/src/runtime-message-handler.test.ts
+++ b/packages/player/src/runtime-message-handler.test.ts
@@ -3,6 +3,7 @@ import { describe, expect, it, vi } from "vitest";
 import { handleRuntimeMessage, type MessageHandlerCallbacks } from "./runtime-message-handler.js";
 import type { ParentMediaManager } from "./parent-media.js";
 import type { ShaderLoaderState } from "./shader-loader-state.js";
+import { runtimeProtocolMetadata } from "@hyperframes/core/runtime/protocol";
 
 // Only the stage-size branch is exercised here; the rest of the callback
 // surface is satisfied with inert spies so the handler's type contract
@@ -13,6 +14,7 @@ const makeCallbacks = (): MessageHandlerCallbacks => ({
   dispatchEvent: vi.fn(),
   onRuntimeReady: vi.fn(),
   onRuntimeTimelineReady: vi.fn(),
+  setRuntimeFps: vi.fn(),
   seek: vi.fn(),
   play: vi.fn(),
   getLoop: vi.fn(() => false),
@@ -104,10 +106,20 @@ describe("handleRuntimeMessage media autoplay fallback", () => {
 });
 
 describe("handleRuntimeMessage timeline ready", () => {
-  const timelineEvent = (durationInFrames: unknown, source: object): MessageEvent =>
+  const timelineEvent = (
+    durationInFrames: unknown,
+    source: object,
+    metadata: Record<string, unknown> = {},
+  ): MessageEvent =>
     ({
       source,
-      data: { source: "hf-preview", type: "timeline", durationInFrames, scenes: [] },
+      data: {
+        source: "hf-preview",
+        type: "timeline",
+        durationInFrames,
+        scenes: [],
+        ...metadata,
+      },
     }) as unknown as MessageEvent;
 
   it("reports a finite positive timeline duration in seconds", () => {
@@ -119,6 +131,45 @@ describe("handleRuntimeMessage timeline ready", () => {
     expect(callbacks.onRuntimeTimelineReady).toHaveBeenCalledWith(4);
   });
 
+  it.each([
+    [24, 48, 2],
+    [60, 120, 2],
+    [24_000 / 1_001, 24, 1.001],
+  ])("uses explicit %s fps metadata", (fps, durationInFrames, expectedSeconds) => {
+    const frameWindow = {} as Window;
+    const callbacks = makeCallbacks();
+
+    handleRuntimeMessage(
+      timelineEvent(durationInFrames, frameWindow, runtimeProtocolMetadata(fps)),
+      frameWindow,
+      callbacks,
+    );
+
+    expect(callbacks.setRuntimeFps).toHaveBeenCalledWith(expect.closeTo(fps, 6));
+    expect(callbacks.onRuntimeTimelineReady).toHaveBeenCalledWith(
+      expect.closeTo(expectedSeconds, 6),
+    );
+  });
+
+  it("rejects unknown protocol majors with a public diagnostic event", () => {
+    const frameWindow = {} as Window;
+    const callbacks = makeCallbacks();
+
+    handleRuntimeMessage(
+      timelineEvent(120, frameWindow, { protocolVersion: 2 }),
+      frameWindow,
+      callbacks,
+    );
+
+    expect(callbacks.onRuntimeTimelineReady).not.toHaveBeenCalled();
+    const event = vi.mocked(callbacks.dispatchEvent).mock.calls[0]?.[0] as CustomEvent;
+    expect(event.type).toBe("runtimeprotocolerror");
+    expect(event.detail).toMatchObject({
+      code: "unsupported_protocol_version",
+      receivedVersion: 2,
+    });
+  });
+
   it("does not report invalid timeline durations as ready", () => {
     const frameWindow = {} as Window;
     const callbacks = makeCallbacks();
diff --git a/packages/player/src/runtime-message-handler.ts b/packages/player/src/runtime-message-handler.ts
index a28a30d58e..d927e53e73 100644
--- a/packages/player/src/runtime-message-handler.ts
+++ b/packages/player/src/runtime-message-handler.ts
@@ -12,8 +12,7 @@ import {
 } from "./playback-state.js";
 import type { ShaderLoaderState } from "./shader-loader-state.js";
 import type { ShaderTransitionState } from "./shader-options.js";
-
-const FPS = 30;
+import { inspectRuntimeProtocol } from "@hyperframes/core/runtime/protocol";
 
 type SceneRecord = { id: string; start: number; duration: number };
 
@@ -45,6 +44,7 @@ export interface MessageHandlerCallbacks extends PlaybackStateCallbacks {
    *  player uses this as the cross-origin readiness signal because the
    *  same-origin composition probe cannot inspect CDN iframes. */
   onRuntimeTimelineReady: (duration: number) => void;
+  setRuntimeFps?: (fps: number) => void;
   /** Called with the scene list whenever a "timeline" message is received. */
   setScenes: (scenes: SceneRecord[]) => void;
   /** Return false to ignore the iframe runtime's audible-media autoplay fallback.
@@ -62,6 +62,16 @@ export function handleRuntimeMessage(
   if (event.source !== frameWindow) return;
   const data = event.data as Record<string, unknown> | undefined;
   if (!data || data["source"] !== "hf-preview") return;
+  const protocol = inspectRuntimeProtocol(data);
+  if (protocol.status === "unsupported") {
+    callbacks.dispatchEvent(
+      new CustomEvent("runtimeprotocolerror", {
+        detail: { code: protocol.code, receivedVersion: protocol.receivedVersion },
+      }),
+    );
+    return;
+  }
+  callbacks.setRuntimeFps?.(protocol.fps);
 
   if (data["type"] === "shader-transition-state") {
     const state: ShaderTransitionState =
@@ -86,7 +96,7 @@ export function handleRuntimeMessage(
     callbacks.setPlaybackState(
       applyRuntimeStateMessage(
         { frame: (data["frame"] as number) ?? 0, isPlaying: !!data["isPlaying"] },
-        FPS,
+        protocol.fps,
         callbacks.getPlaybackState(),
         callbacks,
       ),
@@ -112,7 +122,7 @@ export function handleRuntimeMessage(
   if (data["type"] === "timeline" && (data["durationInFrames"] as number) > 0) {
     if (Number.isFinite(data["durationInFrames"])) {
       const pb = callbacks.getPlaybackState();
-      const duration = (data["durationInFrames"] as number) / FPS;
+      const duration = (data["durationInFrames"] as number) / protocol.fps;
       callbacks.setPlaybackState({ ...pb, duration });
       callbacks.updateControlsTime(pb.currentTime, duration);
       callbacks.onRuntimeTimelineReady(duration);
diff --git a/packages/player/tsup.config.ts b/packages/player/tsup.config.ts
index 87ae66a4aa..8b6f443556 100644
--- a/packages/player/tsup.config.ts
+++ b/packages/player/tsup.config.ts
@@ -1,4 +1,8 @@
 import { defineConfig } from "tsup";
+import { readFileSync } from "node:fs";
+
+const packageVersion = JSON.parse(readFileSync(new URL("./package.json", import.meta.url), "utf8"))
+  .version as string;
 
 export default defineConfig({
   entry: ["src/hyperframes-player.ts", "src/slideshow/hyperframes-slideshow.ts"],
@@ -9,4 +13,9 @@ export default defineConfig({
   clean: true,
   minify: true,
   sourcemap: true,
+  define: {
+    __HYPERFRAMES_RUNTIME_CDN_URL__: JSON.stringify(
+      `https://cdn.jsdelivr.net/npm/@hyperframes/core@${packageVersion}/dist/hyperframe.runtime.iife.js`,
+    ),
+  },
 });
diff --git a/packages/player/vitest.config.ts b/packages/player/vitest.config.ts
index 5f8f7a426a..98c4399003 100644
--- a/packages/player/vitest.config.ts
+++ b/packages/player/vitest.config.ts
@@ -10,6 +10,7 @@ export default defineConfig({
   resolve: {
     alias: {
       "@hyperframes/core/slideshow": resolve(coreRoot, "slideshow/index.ts"),
+      "@hyperframes/core/runtime/protocol": resolve(coreRoot, "runtime/protocol.ts"),
     },
   },
   test: {
diff --git a/packages/studio/src/components/editor/propertyPanelColorGradingSection.tsx b/packages/studio/src/components/editor/propertyPanelColorGradingSection.tsx
index 14a6cb1808..72553582e1 100644
--- a/packages/studio/src/components/editor/propertyPanelColorGradingSection.tsx
+++ b/packages/studio/src/components/editor/propertyPanelColorGradingSection.tsx
@@ -24,6 +24,10 @@ import type { DomEditSelection } from "./domEditing";
 import { ColorGradingControls } from "./propertyPanelColorGradingControls";
 import { stripQueryAndHash } from "./propertyPanelHelpers";
 import { Section } from "./propertyPanelPrimitives";
+import {
+  acceptStudioRuntimeMessage,
+  postRuntimeControlMessage,
+} from "../../player/lib/runtimeProtocol";
 
 const COLOR_GRADING_DATA_KEY = HF_COLOR_GRADING_ATTR.replace(/^data-/, "");
 const RUNTIME_STATUS_REFRESH_DELAYS = [50, 250, 1000, 2500] as const;
@@ -393,35 +397,23 @@ export function ColorGradingSection({
 
   const postColorGrading = useCallback(
     (nextGrading: NormalizedHfColorGrading) => {
-      previewIframeRef?.current?.contentWindow?.postMessage(
-        {
-          source: "hf-parent",
-          type: "control",
-          action: "set-color-grading",
-          target,
-          grading: toBridgeColorGrading(nextGrading),
-        },
-        "*",
-      );
+      postRuntimeControlMessage(previewIframeRef?.current?.contentWindow, "set-color-grading", {
+        target,
+        grading: toBridgeColorGrading(nextGrading),
+      });
     },
     [previewIframeRef, target],
   );
 
   const postCompare = useCallback(
     (enabled: boolean) => {
-      previewIframeRef?.current?.contentWindow?.postMessage(
+      postRuntimeControlMessage(
+        previewIframeRef?.current?.contentWindow,
+        "set-color-grading-compare",
         {
-          source: "hf-parent",
-          type: "control",
-          action: "set-color-grading-compare",
           target,
-          compare: {
-            enabled,
-            position: 1,
-            lineWidth: 0,
-          },
+          compare: { enabled, position: 1, lineWidth: 0 },
         },
-        "*",
       );
     },
     [previewIframeRef, target],
@@ -440,7 +432,9 @@ export function ColorGradingSection({
     const onMessage = (event: MessageEvent) => {
       if (event.source !== iframe.contentWindow) return;
       const data = event.data as { source?: unknown; type?: unknown } | null;
-      if (data?.source === "hf-preview" && data.type === "ready") refreshAndReplay();
+      if (data?.source !== "hf-preview" || data.type !== "ready") return;
+      if (!acceptStudioRuntimeMessage(data)) return;
+      refreshAndReplay();
     };
     iframe.addEventListener("load", refreshAndReplay);
     window.addEventListener("message", onMessage);
diff --git a/packages/studio/src/hooks/timelineEditingHelpers.ts b/packages/studio/src/hooks/timelineEditingHelpers.ts
index ebd9f5e949..7609d8a9f5 100644
--- a/packages/studio/src/hooks/timelineEditingHelpers.ts
+++ b/packages/studio/src/hooks/timelineEditingHelpers.ts
@@ -9,6 +9,8 @@ import { getTimelineElementIdentity } from "../player/lib/timelineElementHelpers
 import { saveProjectFilesWithHistory, type RecordEditInput } from "../utils/studioFileHistory";
 import type { TimelineZIndexReorderCommit } from "./useTimelineEditingTypes";
 import { extendRootDurationInSource } from "../utils/rootDuration";
+import { postRuntimeControlMessage } from "../player/lib/runtimeProtocol";
+
 export { deleteSelectedKeyframes } from "./deleteSelectedKeyframes";
 function isHTMLElement(element: Element | null): element is HTMLElement {
   if (!element) return false;
@@ -153,15 +155,9 @@ function postRootDurationToPreview(
 ): void {
   const duration = Number(durationSeconds);
   if (!Number.isFinite(duration) || duration <= 0) return;
-  iframe?.contentWindow?.postMessage(
-    {
-      source: "hf-parent",
-      type: "control",
-      action: "set-root-duration",
-      durationSeconds: duration,
-    },
-    "*",
-  );
+  postRuntimeControlMessage(iframe?.contentWindow, "set-root-duration", {
+    durationSeconds: duration,
+  });
 }
 // fallow-ignore-next-line complexity
 function resolveResizePlaybackStart(
diff --git a/packages/studio/src/hooks/useCaptionDetection.ts b/packages/studio/src/hooks/useCaptionDetection.ts
index fcf83e5501..c16a18a861 100644
--- a/packages/studio/src/hooks/useCaptionDetection.ts
+++ b/packages/studio/src/hooks/useCaptionDetection.ts
@@ -1,5 +1,6 @@
 import { useEffect } from "react";
 import { useCaptionStore } from "../captions/store";
+import { acceptStudioRuntimeMessage } from "../player/lib/runtimeProtocol";
 import { useCaptionSync } from "../captions/hooks/useCaptionSync";
 import { parseCaptionComposition } from "../captions/parser";
 
@@ -111,6 +112,7 @@ export function useCaptionDetection({
     const handleMessage = (e: MessageEvent) => {
       const data = e.data;
       if (data?.source === "hf-preview" && (data?.type === "state" || data?.type === "timeline")) {
+        if (!acceptStudioRuntimeMessage(data)) return;
         tryActivateCaptions();
       }
     };
diff --git a/packages/studio/src/hooks/useCompositionDimensions.ts b/packages/studio/src/hooks/useCompositionDimensions.ts
index 16c63d51df..49a5371496 100644
--- a/packages/studio/src/hooks/useCompositionDimensions.ts
+++ b/packages/studio/src/hooks/useCompositionDimensions.ts
@@ -1,6 +1,33 @@
 import { useState } from "react";
 import { useMountEffect } from "./useMountEffect";
 import type { CompositionDimensions } from "../components/renders/RenderQueue";
+import { acceptStudioRuntimeMessage } from "../player/lib/runtimeProtocol";
+
+function readCompositionSizeMessage(data: unknown): CompositionDimensions | null {
+  if (!isStageSizeMessage(data)) return null;
+  const message = data;
+  if (!acceptStudioRuntimeMessage(message)) return null;
+  return readPositiveDimensions(message.width, message.height);
+}
+
+function isStageSizeMessage(value: unknown): value is Record<string, unknown> {
+  if (typeof value !== "object") return false;
+  if (value === null) return false;
+  const message = value as Record<string, unknown>;
+  return message.source === "hf-preview" && message.type === "stage-size";
+}
+
+function readPositiveNumber(value: unknown): number | null {
+  if (typeof value !== "number") return null;
+  return Number.isFinite(value) && value > 0 ? value : null;
+}
+
+function readPositiveDimensions(width: unknown, height: unknown): CompositionDimensions | null {
+  const parsedWidth = readPositiveNumber(width);
+  const parsedHeight = readPositiveNumber(height);
+  if (parsedWidth === null || parsedHeight === null) return null;
+  return { width: parsedWidth, height: parsedHeight };
+}
 
 export function useCompositionDimensions() {
   const [compositionDimensions, setCompositionDimensions] = useState<CompositionDimensions | null>(
@@ -9,12 +36,12 @@ export function useCompositionDimensions() {
 
   useMountEffect(() => {
     const handleMessage = (e: MessageEvent) => {
-      const data = e.data;
-      if (data?.source !== "hf-preview" || data?.type !== "stage-size") return;
-      const { width, height } = data as { width: number; height: number };
-      if (!(width > 0) || !(height > 0)) return;
+      const dimensions = readCompositionSizeMessage(e.data);
+      if (!dimensions) return;
       setCompositionDimensions((prev) =>
-        prev && prev.width === width && prev.height === height ? prev : { width, height },
+        prev && prev.width === dimensions.width && prev.height === dimensions.height
+          ? prev
+          : dimensions,
       );
     };
     window.addEventListener("message", handleMessage);
diff --git a/packages/studio/src/hooks/useElementPicker.ts b/packages/studio/src/hooks/useElementPicker.ts
index 6ba1dfb614..b04848377d 100644
--- a/packages/studio/src/hooks/useElementPicker.ts
+++ b/packages/studio/src/hooks/useElementPicker.ts
@@ -1,6 +1,10 @@
 import { useState, useCallback, useRef } from "react";
 import { useMountEffect } from "./useMountEffect";
 import { resolveSourceFile, applyPatch } from "../utils/sourcePatcher";
+import {
+  acceptStudioRuntimeMessage,
+  postRuntimeControlMessage,
+} from "../player/lib/runtimeProtocol";
 
 export interface PickedElement {
   id: string | null;
@@ -65,10 +69,7 @@ export function useElementPicker(
 
   const enablePick = useCallback(() => {
     try {
-      getActiveIframe()?.contentWindow?.postMessage(
-        { source: "hf-parent", type: "control", action: "enable-pick-mode" },
-        "*",
-      );
+      postRuntimeControlMessage(getActiveIframe()?.contentWindow, "enable-pick-mode");
       setIsPickMode(true);
     } catch {
       /* cross-origin */
@@ -77,10 +78,7 @@ export function useElementPicker(
 
   const disablePick = useCallback(() => {
     try {
-      getActiveIframe()?.contentWindow?.postMessage(
-        { source: "hf-parent", type: "control", action: "disable-pick-mode" },
-        "*",
-      );
+      postRuntimeControlMessage(getActiveIframe()?.contentWindow, "disable-pick-mode");
     } catch {
       /* cross-origin */
     }
@@ -96,6 +94,7 @@ export function useElementPicker(
     const handleMessage = (e: MessageEvent) => {
       const data = e.data;
       if (data?.source !== "hf-preview") return;
+      if (!acceptStudioRuntimeMessage(data)) return;
       // Accept events from either the primary iframe or the active override
       const activeIframe = getActiveIframe();
       if (!activeIframe) return;
diff --git a/packages/studio/src/hooks/useTimelineEditing.test.tsx b/packages/studio/src/hooks/useTimelineEditing.test.tsx
index cb4466f88a..3cc74fbfd7 100644
--- a/packages/studio/src/hooks/useTimelineEditing.test.tsx
+++ b/packages/studio/src/hooks/useTimelineEditing.test.tsx
@@ -288,12 +288,13 @@ describe("useTimelineEditing timeline z-index reorder", () => {
     expect(forceReloadSdkSession).toHaveBeenCalledTimes(1);
     expect(reloadPreview).not.toHaveBeenCalled();
     expect(postMessageSpy).toHaveBeenCalledWith(
-      {
+      expect.objectContaining({
         source: "hf-parent",
         type: "control",
         action: "set-root-duration",
         durationSeconds: 5,
-      },
+        protocolVersion: 1,
+      }),
       "*",
     );
 
@@ -354,12 +355,13 @@ describe("useTimelineEditing timeline z-index reorder", () => {
     expect(forceReloadSdkSession).toHaveBeenCalledTimes(1);
     expect(reloadPreview).not.toHaveBeenCalled();
     expect(postMessageSpy).toHaveBeenCalledWith(
-      {
+      expect.objectContaining({
         source: "hf-parent",
         type: "control",
         action: "set-root-duration",
         durationSeconds: 5,
-      },
+        protocolVersion: 1,
+      }),
       "*",
     );
 
diff --git a/packages/studio/src/player/hooks/useTimelinePlayer.seek.test.ts b/packages/studio/src/player/hooks/useTimelinePlayer.seek.test.ts
index 3036380532..45db5c9fcc 100644
--- a/packages/studio/src/player/hooks/useTimelinePlayer.seek.test.ts
+++ b/packages/studio/src/player/hooks/useTimelinePlayer.seek.test.ts
@@ -153,6 +153,61 @@ describe("useTimelinePlayer seek hydration", () => {
     unmountWithAct(root);
     unsubscribe();
   });
+
+  it("does not settle from an unsupported runtime protocol message", () => {
+    const { api, root } = renderTimelinePlayerHarness();
+    const iframe = document.createElement("iframe");
+    const iframeWindow = {
+      postMessage: vi.fn(),
+      scrollTo: vi.fn(),
+      addEventListener: vi.fn(),
+      removeEventListener: vi.fn(),
+    } as Record<string, unknown>;
+    Object.defineProperty(iframe, "contentWindow", {
+      value: iframeWindow,
+      configurable: true,
+    });
+    Object.defineProperty(iframe, "contentDocument", {
+      value: document.implementation.createHTMLDocument("preview"),
+      configurable: true,
+    });
+
+    act(() => {
+      api.iframeRef.current = iframe;
+      api.onIframeLoad();
+    });
+    expect(usePlayerStore.getState().timelineReady).toBe(false);
+
+    iframeWindow.__player = {
+      play: vi.fn(),
+      pause: vi.fn(),
+      seek: vi.fn(),
+      getTime: () => 0,
+      getDuration: () => 30,
+      isPlaying: () => false,
+    };
+    act(() => {
+      window.dispatchEvent(
+        new MessageEvent("message", {
+          source: iframeWindow as unknown as Window,
+          data: { source: "hf-preview", type: "state", protocolVersion: 999 },
+        }),
+      );
+    });
+    expect(usePlayerStore.getState().timelineReady).toBe(false);
+
+    act(() => {
+      window.dispatchEvent(
+        new MessageEvent("message", {
+          source: iframeWindow as unknown as Window,
+          data: { source: "hf-preview", type: "state" },
+        }),
+      );
+    });
+    expect(usePlayerStore.getState().timelineReady).toBe(true);
+
+    unmountWithAct(root);
+  });
 });
 
 describe("useTimelinePlayer audio controls (#835)", () => {
diff --git a/packages/studio/src/player/hooks/useTimelinePlayer.ts b/packages/studio/src/player/hooks/useTimelinePlayer.ts
index 4ca1b452c3..44bb17717d 100644
--- a/packages/studio/src/player/hooks/useTimelinePlayer.ts
+++ b/packages/studio/src/player/hooks/useTimelinePlayer.ts
@@ -46,6 +46,7 @@ import { scrubMusicAtSeek, stopScrubPreviewAudio } from "../lib/playbackScrub";
 import { applyCachedSourceDurations, probeMissingSourceDurations } from "../lib/mediaProbe";
 import { shouldResumeForwardPlaybackAfterSeek, shouldStopAfterSeek } from "../lib/playbackSeek";
 import { applyPreviewVariablesToUrl } from "../../hooks/previewVariablesStore";
+import { acceptStudioRuntimeMessage } from "../lib/runtimeProtocol";
 
 /**
  * Whether the derived elements differ from the current ones in any field that
@@ -492,6 +493,9 @@ export function useTimelinePlayer() {
       if (e.source && ourIframe && e.source !== ourIframe.contentWindow) {
         return;
       }
+      if (data?.source === "hf-preview") {
+        if (!acceptStudioRuntimeMessage(data)) return;
+      }
       if (data?.source === "hf-preview" && data?.type === "state") {
         try {
           if (usePlayerStore.getState().elements.length === 0) {
diff --git a/packages/studio/src/player/hooks/useTimelineSyncCallbacks.ts b/packages/studio/src/player/hooks/useTimelineSyncCallbacks.ts
index 4992efb5c0..f7c222be70 100644
--- a/packages/studio/src/player/hooks/useTimelineSyncCallbacks.ts
+++ b/packages/studio/src/player/hooks/useTimelineSyncCallbacks.ts
@@ -26,6 +26,7 @@ import {
   autoHealMissingCompositionIds,
   buildMissingCompositionElements,
 } from "../lib/timelineIframeHelpers";
+import { acceptedRuntimeMessageFps, inspectStudioRuntimeMessage } from "../lib/runtimeProtocol";
 
 interface UseTimelineSyncCallbacksParams {
   iframeRef: React.RefObject<HTMLIFrameElement | null>;
@@ -132,6 +133,9 @@ export function useTimelineSyncCallbacks({
       clips: ClipManifestClip[];
       durationInFrames: number;
       scenes?: Array<{ id: string; label: string; start: number; duration: number }>;
+      protocolVersion?: unknown;
+      capabilities?: unknown;
+      fps?: unknown;
     }) => {
       if (!data.clips || data.clips.length === 0) {
         return;
@@ -222,7 +226,7 @@ export function useTimelineSyncCallbacks({
           hostEl,
         });
       });
-      const rawDuration = data.durationInFrames / 30;
+      const rawDuration = data.durationInFrames / acceptedRuntimeMessageFps(data);
       // Clamp non-finite or absurdly large durations — the runtime can emit
       // Infinity when it detects a loop-inflated GSAP timeline without an
       // explicit data-duration on the root composition. Floor the manifest total
@@ -418,6 +422,10 @@ export function useTimelineSyncCallbacks({
       if (e.source && iframe && e.source !== iframe.contentWindow) return;
       const data = e.data;
       if (data?.source === "hf-preview" && (data?.type === "state" || data?.type === "timeline")) {
+        // The main message handler owns protocol-error diagnostics. This readiness-only
+        // listener mirrors its acceptance gate without dispatching a duplicate event:
+        // an unsupported runtime must not make the iframe appear successfully settled.
+        if (inspectStudioRuntimeMessage(data).status === "unsupported") return;
         trySettle();
       }
     };
diff --git a/packages/studio/src/player/index.ts b/packages/studio/src/player/index.ts
index 72e50a2bcd..db2bc1f423 100644
--- a/packages/studio/src/player/index.ts
+++ b/packages/studio/src/player/index.ts
@@ -11,7 +11,9 @@ export { resolveIframe } from "./lib/timelineDOM";
 
 // Store
 export { usePlayerStore, liveTime } from "./store/playerStore";
-export type { SelectElementOptions, TimelineElement } from "./store/playerStore";
+// Public library surface; external consumers are invisible to the workspace analyzer.
+// fallow-ignore-next-line unused-exports
+export type { SelectElementOptions, TimelineElement, ZoomMode } from "./store/playerStore";
 
 // Utils
 export { formatTime } from "./lib/time";
diff --git a/packages/studio/src/player/lib/runtimeProtocol.test.ts b/packages/studio/src/player/lib/runtimeProtocol.test.ts
new file mode 100644
index 0000000000..3747266af0
--- /dev/null
+++ b/packages/studio/src/player/lib/runtimeProtocol.test.ts
@@ -0,0 +1,48 @@
+import { describe, expect, it, vi } from "vitest";
+import {
+  acceptStudioRuntimeMessage,
+  acceptedRuntimeMessageFps,
+  createRuntimeControlMessage,
+  inspectStudioRuntimeMessage,
+  postRuntimeControlMessage,
+} from "./runtimeProtocol";
+
+describe("Studio runtime protocol", () => {
+  it("versions every control message and declares rational fps", () => {
+    expect(createRuntimeControlMessage("seek", { timeSeconds: 1.25 }, 60)).toEqual({
+      source: "hf-parent",
+      type: "control",
+      action: "seek",
+      protocolVersion: 1,
+      capabilities: ["seconds-time", "rational-fps", "seek-keep-playing"],
+      fps: { numerator: 60, denominator: 1 },
+      timeSeconds: 1.25,
+    });
+  });
+
+  it("posts the typed message to the target window", () => {
+    const target = { postMessage: vi.fn() };
+    postRuntimeControlMessage(target as unknown as Window, "pause");
+    expect(target.postMessage).toHaveBeenCalledWith(
+      expect.objectContaining({ action: "pause", protocolVersion: 1 }),
+      "*",
+    );
+  });
+
+  it("preserves legacy 30fps messages and rejects unknown majors", () => {
+    expect(inspectStudioRuntimeMessage({ source: "hf-preview" })).toEqual({
+      status: "legacy",
+      fps: 30,
+    });
+    expect(inspectStudioRuntimeMessage({ protocolVersion: 2 })).toMatchObject({
+      status: "unsupported",
+      code: "unsupported_protocol_version",
+    });
+  });
+
+  it("reads explicit fps for accepted timeline messages", () => {
+    const message = createRuntimeControlMessage("pause", {}, 60);
+    expect(acceptedRuntimeMessageFps(message)).toBe(60);
+    expect(acceptStudioRuntimeMessage(message)).toMatchObject({ status: "supported", fps: 60 });
+  });
+});
diff --git a/packages/studio/src/player/lib/runtimeProtocol.ts b/packages/studio/src/player/lib/runtimeProtocol.ts
new file mode 100644
index 0000000000..7aeccb582a
--- /dev/null
+++ b/packages/studio/src/player/lib/runtimeProtocol.ts
@@ -0,0 +1,65 @@
+import {
+  inspectRuntimeProtocol,
+  runtimeProtocolMetadata,
+  type RuntimeProtocolInspection,
+} from "@hyperframes/core/runtime/protocol";
+
+export type RuntimeControlMessage = {
+  source: "hf-parent";
+  type: "control";
+  action: string;
+} & ReturnType<typeof runtimeProtocolMetadata> &
+  Record<string, unknown>;
+
+export function createRuntimeControlMessage(
+  action: string,
+  payload: Record<string, unknown> = {},
+  fps = 30,
+): RuntimeControlMessage {
+  return {
+    ...payload,
+    source: "hf-parent",
+    type: "control",
+    action,
+    ...runtimeProtocolMetadata(fps),
+  };
+}
+
+export function postRuntimeControlMessage(
+  target: Pick<Window, "postMessage"> | null | undefined,
+  action: string,
+  payload: Record<string, unknown> = {},
+  fps = 30,
+): void {
+  target?.postMessage(createRuntimeControlMessage(action, payload, fps), "*");
+}
+
+export function inspectStudioRuntimeMessage(value: unknown): RuntimeProtocolInspection {
+  return inspectRuntimeProtocol(value, 30);
+}
+
+function dispatchRuntimeProtocolError(inspection: RuntimeProtocolInspection): void {
+  if (inspection.status !== "unsupported") return;
+  window.dispatchEvent(
+    new CustomEvent("runtimeprotocolerror", {
+      detail: {
+        code: inspection.code,
+        receivedVersion: inspection.receivedVersion,
+      },
+    }),
+  );
+}
+
+export function acceptStudioRuntimeMessage(
+  value: unknown,
+): Exclude<RuntimeProtocolInspection, { status: "unsupported" }> | null {
+  const inspection = inspectStudioRuntimeMessage(value);
+  if (inspection.status !== "unsupported") return inspection;
+  dispatchRuntimeProtocolError(inspection);
+  return null;
+}
+
+export function acceptedRuntimeMessageFps(value: unknown): number {
+  const inspection = inspectStudioRuntimeMessage(value);
+  return inspection.status === "supported" ? inspection.fps : 30;
+}
diff --git a/packages/studio/src/player/lib/timelineIframeHelpers.ts b/packages/studio/src/player/lib/timelineIframeHelpers.ts
index 6fa45bf2c4..f00700f458 100644
--- a/packages/studio/src/player/lib/timelineIframeHelpers.ts
+++ b/packages/studio/src/player/lib/timelineIframeHelpers.ts
@@ -20,6 +20,7 @@ import {
   buildTimelineElementIdentity,
   readTimelineElementZIndex,
 } from "./timelineElementHelpers";
+import { postRuntimeControlMessage } from "./runtimeProtocol";
 
 // ---------------------------------------------------------------------------
 // Viewport / DOM normalisation
@@ -103,10 +104,7 @@ function postPreviewControl(
   action: string,
   payload: Record<string, unknown>,
 ): void {
-  iframe.contentWindow?.postMessage(
-    { source: "hf-parent", type: "control", action, ...payload },
-    "*",
-  );
+  postRuntimeControlMessage(iframe.contentWindow, action, payload);
 }
 
 export function shouldMutePreviewAudio(audioMuted: boolean, playbackRate: number): boolean {