diff --git a/packages/core/src/index.ts b/packages/core/src/index.ts
index 5f8789e7fc..499eb6597c 100644
--- a/packages/core/src/index.ts
+++ b/packages/core/src/index.ts
@@ -268,11 +268,20 @@ export {
   type ToneMapSourceColour,
 } from "./hdrToneMap";
 export {
+  HEAVY_BITRATE,
+  HEAVY_PIXEL_SHARE,
   PREVIEW_PROXY_BOX_PARAM,
+  PREVIEW_PROXY_LIGHT_PARAM,
+  PREVIEW_QUALITY_LEVELS,
   formatPreviewProxyBox,
+  isHeavyForPreview,
+  isPreviewQuality,
   parsePreviewProxyBox,
   quantizePreviewProxyBox,
+  scalePreviewProxyBox,
   type PreviewProxyBox,
+  type PreviewQuality,
+  type PreviewSourceFacts,
 } from "./previewProxyBox";
 
 // Inline scripts
diff --git a/packages/core/src/previewProxyBox.test.ts b/packages/core/src/previewProxyBox.test.ts
index 3e1a2bab14..183adaab73 100644
--- a/packages/core/src/previewProxyBox.test.ts
+++ b/packages/core/src/previewProxyBox.test.ts
@@ -1,5 +1,12 @@
 import { describe, expect, it } from "vitest";
-import { parsePreviewProxyBox, quantizePreviewProxyBox } from "./previewProxyBox";
+import {
+  isHeavyForPreview,
+  isPreviewQuality,
+  parsePreviewProxyBox,
+  quantizePreviewProxyBox,
+  scalePreviewProxyBox,
+  type PreviewSourceFacts,
+} from "./previewProxyBox";
 
 describe("preview proxy box", () => {
   it("rounds each side up to the next rung", () => {
@@ -28,3 +35,34 @@ describe("preview proxy box", () => {
     expect(parsePreviewProxyBox("")).toBeNull();
   });
 });
+
+describe("lighter copies of files Chrome can play", () => {
+  const full: PreviewSourceFacts = { width: 1920, height: 1080, bitDepth: 8, bitrate: 10_000_000 };
+  const box = { width: 1218, height: 724 };
+  const shown = { width: 1448, height: 862 };
+
+  it("multiplies the shown box by the level, on the ladder", () => {
+    expect(scalePreviewProxyBox(box, 1)).toEqual(box);
+    expect(scalePreviewProxyBox(box, 0.5)).toEqual({ width: 724, height: 362 });
+    expect(scalePreviewProxyBox(box, 0.125)).toEqual({ width: 256, height: 256 });
+  });
+
+  it("calls a file heavy for a much larger source, 10-bit or deeper, or 40 Mbit/s and up", () => {
+    expect(isHeavyForPreview(full, shown)).toBe(false);
+    expect(isHeavyForPreview({ ...full, width: 3840, height: 2160 }, shown)).toBe(true);
+    expect(isHeavyForPreview({ ...full, bitDepth: 10 }, shown)).toBe(true);
+    expect(isHeavyForPreview({ ...full, bitrate: 39_999_999 }, shown)).toBe(false);
+    expect(isHeavyForPreview({ ...full, bitrate: 40_000_000 }, shown)).toBe(true);
+  });
+
+  it("calls a source heavy when the copy box has half its pixels or fewer", () => {
+    const half = { width: 1448, height: 724 };
+    expect(isHeavyForPreview({ ...full, width: 2896, height: 1448 }, half)).toBe(true);
+    expect(isHeavyForPreview({ ...full, width: 1448, height: 1000 }, half)).toBe(false);
+  });
+
+  it("knows the four levels and nothing else", () => {
+    expect([1, 0.5, 0.25, 0.125].every(isPreviewQuality)).toBe(true);
+    expect([0, 0.75, 2, "1", null].some(isPreviewQuality)).toBe(false);
+  });
+});
diff --git a/packages/core/src/previewProxyBox.ts b/packages/core/src/previewProxyBox.ts
index 46016422a1..bb6d4d4f85 100644
--- a/packages/core/src/previewProxyBox.ts
+++ b/packages/core/src/previewProxyBox.ts
@@ -1,6 +1,8 @@
 /** Query parameter carrying the largest box, in device pixels, a preview shows a proxied video at. */
 export const PREVIEW_PROXY_BOX_PARAM = "hf-proxy-box";
 
+export const PREVIEW_PROXY_LIGHT_PARAM = "hf-proxy-light";
+
 export type PreviewProxyBox = { width: number; height: number };
 
 // 256 * 2^(k/4), even: nearby boxes share one cached copy, and a rung overshoots by at most ~19%.
@@ -33,3 +35,32 @@ export function parsePreviewProxyBox(raw: string): PreviewProxyBox | null {
   const height = Number(match[2]);
   return RUNGS.includes(width) && RUNGS.includes(height) ? { width, height } : null;
 }
+
+export const PREVIEW_QUALITY_LEVELS = [1, 0.5, 0.25, 0.125] as const;
+export type PreviewQuality = (typeof PREVIEW_QUALITY_LEVELS)[number];
+
+export function isPreviewQuality(value: unknown): value is PreviewQuality {
+  return PREVIEW_QUALITY_LEVELS.some((level) => level === value);
+}
+
+export type PreviewSourceFacts = {
+  width: number;
+  height: number;
+  bitDepth: number;
+  bitrate: number;
+};
+
+export const HEAVY_PIXEL_SHARE = 0.5;
+export const HEAVY_BITRATE = 40_000_000;
+
+export function scalePreviewProxyBox(box: PreviewProxyBox, level: PreviewQuality): PreviewProxyBox {
+  return quantizePreviewProxyBox(box.width * level, box.height * level) ?? box;
+}
+
+export function isHeavyForPreview(facts: PreviewSourceFacts, copyBox: PreviewProxyBox): boolean {
+  return (
+    copyBox.width * copyBox.height <= HEAVY_PIXEL_SHARE * facts.width * facts.height ||
+    facts.bitDepth > 8 ||
+    facts.bitrate >= HEAVY_BITRATE
+  );
+}
diff --git a/packages/core/src/runtime/bridge.ts b/packages/core/src/runtime/bridge.ts
index a5a5e89bbd..f665d07d23 100644
--- a/packages/core/src/runtime/bridge.ts
+++ b/packages/core/src/runtime/bridge.ts
@@ -33,6 +33,7 @@ type BridgeDeps = {
   onSetRuntimeData?: (channel: string, payload: unknown, requestId?: number) => void;
   onClearRuntimeData?: (channel: string, requestId?: number) => void;
   onSetDisplayScale?: (scale: number) => void;
+  onSetPreviewQuality?: (level: number) => void;
   getCanonicalFps: () => number;
 };
 
@@ -76,6 +77,7 @@ const CONTROL_HANDLERS = new Map<string, ControlHandler>(
     "set-playback-rate": (data, deps) => deps.onSetPlaybackRate(Number(data.playbackRate ?? 1)),
     "set-idle-heartbeat": (data, deps) => deps.onSetIdleHeartbeat(Boolean(data.slow)),
     "set-display-scale": (data, deps) => deps.onSetDisplayScale?.(Number(data.scale)),
+    "set-preview-quality": (data, deps) => deps.onSetPreviewQuality?.(Number(data.level)),
     "set-root-duration": (data, deps) => deps.onSetRootDuration(Number(data.durationSeconds ?? 0)),
     "set-play-range": (data, deps) =>
       deps.onSetPlayRange(
diff --git a/packages/core/src/runtime/init.ts b/packages/core/src/runtime/init.ts
index 7550fe35ed..b015a34af9 100644
--- a/packages/core/src/runtime/init.ts
+++ b/packages/core/src/runtime/init.ts
@@ -95,6 +95,7 @@ import {
   handleMetadataForProxy,
   isRenderMode,
   maybeProxyProactively,
+  setPreviewQuality,
   setProxyDisplayScale,
 } from "./mediaProxy";
 import {
@@ -5973,6 +5974,7 @@ export function initSandboxRuntimeModular(): void {
       setProxyDisplayScale(scale);
       setPreviewRasterScale(scale);
     },
+    onSetPreviewQuality: setPreviewQuality,
     onSetIdleHeartbeat: (slow) => {
       slowIdleHeartbeat = slow;
       wakeTransport();
diff --git a/packages/core/src/runtime/mediaProxy.test.ts b/packages/core/src/runtime/mediaProxy.test.ts
index 3a74daef82..e17816da34 100644
--- a/packages/core/src/runtime/mediaProxy.test.ts
+++ b/packages/core/src/runtime/mediaProxy.test.ts
@@ -869,3 +869,133 @@ describe("a copy sized to how large the video shows", () => {
     expect(boxOf(el)).toBe("2436x4096");
   });
 });
+
+describe("preview quality: lighter copies of heavy files", () => {
+  const HEAVY = { width: 3840, height: 2160, bitDepth: 8, bitrate: 10_000_000 };
+  const LIGHT = { width: 1280, height: 720, bitDepth: 8, bitrate: 10_000_000 };
+
+  // A framed page showing a 1920x1080 stage at full size on a 1x screen: a 2048x1218 box.
+  async function stage(facts: Record<string, typeof HEAVY>, codecMap?: Record<string, unknown>) {
+    vi.resetModules();
+    vi.spyOn(window, "parent", "get").mockReturnValue({} as Window);
+    const mod = await import("./mediaProxy");
+    window.__HF_MEDIA_SOURCE_FACTS__ = facts;
+    if (codecMap) window.__HF_MEDIA_CODEC_MAP__ = codecMap as never;
+    Object.defineProperty(document.documentElement, "clientWidth", {
+      configurable: true,
+      value: 1920,
+    });
+    Object.defineProperty(document.documentElement, "clientHeight", {
+      configurable: true,
+      value: 1080,
+    });
+    vi.spyOn(window, "devicePixelRatio", "get").mockReturnValue(1);
+    const video = (src = "/video.mp4") => {
+      const el = createVideo(src);
+      stubCanPlayType(el, "probably");
+      el.getBoundingClientRect = () => ({ width: 1920, height: 1080 }) as DOMRect;
+      return el;
+    };
+    mod.setProxyDisplayScale(1);
+    return { ...mod, video };
+  }
+
+  const params = (el: HTMLMediaElement) => new URL(el.src, document.baseURI).searchParams;
+
+  afterEach(() => {
+    delete (window as { __HF_MEDIA_SOURCE_FACTS__?: unknown }).__HF_MEDIA_SOURCE_FACTS__;
+  });
+
+  it("changes nothing at full quality", async () => {
+    const { video, setPreviewQuality, maybeProxyProactively } = await stage({
+      "/video.mp4": HEAVY,
+    });
+    const el = video();
+    maybeProxyProactively(el);
+    setPreviewQuality(1);
+    await settle();
+
+    expect(isProxied(el)).toBe(false);
+    expect(fetch).not.toHaveBeenCalled();
+  });
+
+  it("swaps a heavy file Chrome can play to a copy at the shown box times the level", async () => {
+    const { video, setPreviewQuality } = await stage({ "/video.mp4": HEAVY });
+    const el = video();
+
+    setPreviewQuality(0.5);
+    await settle();
+
+    expect(params(el).get("hf-proxy")).toBe("auto");
+    expect(params(el).get("hf-proxy-box")).toBe("1024x724");
+    expect(params(el).get("hf-proxy-light")).toBe("1");
+  });
+
+  it("leaves a file alone that is not heavy at that box", async () => {
+    const { video, setPreviewQuality } = await stage({ "/video.mp4": LIGHT });
+    const el = video();
+
+    setPreviewQuality(0.5);
+    await settle();
+
+    expect(isProxied(el)).toBe(false);
+  });
+
+  it("puts the original back at full quality", async () => {
+    const { video, setPreviewQuality } = await stage({ "/video.mp4": HEAVY });
+    const el = video();
+    setPreviewQuality(0.5);
+    await settle();
+    expect(isProxied(el)).toBe(true);
+
+    setPreviewQuality(1);
+
+    expect(el.getAttribute("src")).toBe("/video.mp4");
+    expect(el.load).toHaveBeenCalledTimes(2);
+  });
+
+  it("lightens a video that appears after the level was lowered", async () => {
+    const { video, setPreviewQuality, maybeProxyProactively } = await stage({
+      "/video.mp4": HEAVY,
+    });
+    setPreviewQuality(0.25);
+    const el = video();
+
+    maybeProxyProactively(el);
+    await settle();
+
+    expect(params(el).get("hf-proxy-box")).toBe("512x362");
+  });
+
+  it("sizes the copy a file cannot play without by the level too, and resizes it as the level moves", async () => {
+    const { video, setPreviewQuality, maybeProxyProactively } = await stage(
+      { "/video.mp4": HEAVY },
+      { "/video.mp4": HEVC_ENTRY },
+    );
+    const el = video();
+    stubCanPlayType(el, "");
+    maybeProxyProactively(el);
+    await settle();
+    expect(params(el).get("hf-proxy-box")).toBe("2048x1218");
+    expect(params(el).has("hf-proxy-light")).toBe(false);
+
+    setPreviewQuality(0.5);
+    await settle();
+    expect(params(el).get("hf-proxy-box")).toBe("1024x724");
+    expect(params(el).has("hf-proxy-light")).toBe(false);
+
+    setPreviewQuality(1);
+    await settle();
+    expect(params(el).get("hf-proxy-box")).toBe("2048x1218");
+  });
+
+  it("ignores a level that is not one of the four", async () => {
+    const { video, setPreviewQuality } = await stage({ "/video.mp4": HEAVY });
+    const el = video();
+
+    setPreviewQuality(0.7);
+    await settle();
+
+    expect(isProxied(el)).toBe(false);
+  });
+});
diff --git a/packages/core/src/runtime/mediaProxy.ts b/packages/core/src/runtime/mediaProxy.ts
index 33aed24341..0b99fe50e6 100644
--- a/packages/core/src/runtime/mediaProxy.ts
+++ b/packages/core/src/runtime/mediaProxy.ts
@@ -9,9 +9,15 @@ import { waitForServedProxy } from "./proxyWait";
 import { registerSeekCompletion } from "./adapters/seek-dispatch";
 import {
   formatPreviewProxyBox,
+  isHeavyForPreview,
+  isPreviewQuality,
   PREVIEW_PROXY_BOX_PARAM,
+  PREVIEW_PROXY_LIGHT_PARAM,
   quantizePreviewProxyBox,
+  scalePreviewProxyBox,
   type PreviewProxyBox,
+  type PreviewQuality,
+  type PreviewSourceFacts,
 } from "../previewProxyBox";
 
 /**
@@ -34,6 +40,7 @@ export type MediaCodecMapEntry = {
 declare global {
   interface Window {
     __HF_MEDIA_CODEC_MAP__?: Record<string, MediaCodecMapEntry>;
+    __HF_MEDIA_SOURCE_FACTS__?: Record<string, PreviewSourceFacts>;
   }
 }
 
@@ -45,7 +52,7 @@ const DIAGNOSTIC_FALLBACK_CODE = "runtime_media_proxy_fallback";
  *  did) proxy it — a remote asset, or the proxy URL itself failing. */
 const DIAGNOSTIC_UNAVAILABLE_CODE = "runtime_media_proxy_unavailable";
 
-type ProxyTrigger = "proactive" | "reactive" | "tertiary";
+type ProxyTrigger = "proactive" | "reactive" | "tertiary" | "light";
 
 // `swappedElements` gates every trigger, so a second undecodable-video signal never re-swaps.
 
@@ -63,6 +70,10 @@ function currentSrcValue(el: HTMLMediaElement): string {
   return el.currentSrc || el.src;
 }
 
+// Below 1, heavy files get lighter copies at the shown box times this level; 1 is the preview as it always was.
+let previewQuality: PreviewQuality = 1;
+const lightElements = new WeakSet<HTMLMediaElement>();
+
 // How large the host shows this document, in its CSS pixels per ours (player fit x page zoom).
 // A top-level page shows itself at 1; a framed one waits for its host to say.
 let displayScale: number | null = window.parent === window ? 1 : null;
@@ -72,7 +83,13 @@ const displayScaleKnown = new Promise<void>((arrived) => (displayScaleArrived =
 const DISPLAY_SCALE_WAIT_MS = 1000;
 
 // Elements on a size-bounded copy: the source, the copy's URL and the box it was made for.
-type BoxedCopy = { originalSrc: string; variant: string; href: string; box: PreviewProxyBox };
+type BoxedCopy = {
+  originalSrc: string;
+  variant: string;
+  href: string;
+  box: PreviewProxyBox;
+  light: boolean;
+};
 const boxedElements = new Map<HTMLMediaElement, BoxedCopy>();
 // The larger copy each element waits for; a newer ask replaces it and the older wait stops.
 const pendingUpgrades = new WeakMap<HTMLMediaElement, string>();
@@ -106,6 +123,19 @@ export function setProxyDisplayScale(scale: number): void {
   for (const el of boxedElements.keys()) keepSized(el, scale);
 }
 
+function codecEntryOf(el: HTMLMediaElement): MediaCodecMapEntry | null {
+  const map = window.__HF_MEDIA_CODEC_MAP__;
+  const key = deriveCodecMapKey(el);
+  return map && key !== null ? lookupCodecMapEntry(key, map) : null;
+}
+
+function sourceFactsOf(el: HTMLMediaElement): PreviewSourceFacts | null {
+  const facts = window.__HF_MEDIA_SOURCE_FACTS__;
+  const key = deriveCodecMapKey(el);
+  return facts && key !== null ? lookupCodecMapEntry(key, facts) : null;
+}
+
+// fallow-ignore-next-line complexity
 function keepSized(el: HTMLMediaElement, scale: number): void {
   const copy = boxedElements.get(el);
   if (!copy) return;
@@ -113,32 +143,68 @@ function keepSized(el: HTMLMediaElement, scale: number): void {
     boxedElements.delete(el);
     return;
   }
-  const needed = shownBox(el, scale);
-  if (needed && needed.width <= copy.box.width && needed.height <= copy.box.height) return;
-  // Never smaller on either side than the copy it replaces.
-  const box = needed && {
-    width: Math.max(needed.width, copy.box.width),
-    height: Math.max(needed.height, copy.box.height),
+  const shown = shownBox(el, scale);
+  const lowered = previewQuality < 1;
+  const needed = shown && lowered ? scalePreviewProxyBox(shown, previewQuality) : shown;
+  if (lowered && !needed) return;
+  if (needed && lowered && copy.light && lightCopyNoLongerNeeded(el, needed)) {
+    return restoreOriginal(el);
+  }
+  if (boxCovers(copy.box, needed, lowered)) return;
+  requestBox(el, copy, lowered || !needed ? needed : growOnly(needed, copy.box));
+}
+
+function lightCopyNoLongerNeeded(el: HTMLMediaElement, needed: PreviewProxyBox): boolean {
+  const facts = sourceFactsOf(el);
+  return facts !== null && !isHeavyForPreview(facts, needed);
+}
+
+function boxCovers(current: PreviewProxyBox, needed: PreviewProxyBox | null, exact: boolean) {
+  if (exact) return needed?.width === current.width && needed.height === current.height;
+  return needed !== null && needed.width <= current.width && needed.height <= current.height;
+}
+
+function growOnly(needed: PreviewProxyBox, current: PreviewProxyBox): PreviewProxyBox {
+  return {
+    width: Math.max(needed.width, current.width),
+    height: Math.max(needed.height, current.height),
   };
-  const href = appendProxyParam(copy.originalSrc, copy.variant, box);
+}
+
+function requestBox(el: HTMLMediaElement, copy: BoxedCopy, box: PreviewProxyBox | null): void {
+  const href = appendProxyParam(copy.originalSrc, copy.variant, box, copy.light);
   if (pendingUpgrades.get(el) === href) return;
   pendingUpgrades.set(el, href);
   const wanted = () =>
     pendingUpgrades.get(el) === href && el.isConnected && el.getAttribute("src") === copy.href;
   void waitForServedProxy(href, wanted).then((served) => {
-    if (served && wanted()) loadProxy(el, href, copy.originalSrc, copy.variant, box);
+    if (served && wanted()) loadProxy(el, href, copy.originalSrc, copy.variant, box, copy.light);
     if (pendingUpgrades.get(el) === href) pendingUpgrades.delete(el);
   });
 }
 
+/** Puts a video that only got a lighter copy back on its original. */
+function restoreOriginal(el: HTMLMediaElement): void {
+  const original = swappedElements.get(el) ?? null;
+  for (const state of [boxedElements, pendingUpgrades, proxyRequested, swappedElements]) {
+    state.delete(el);
+  }
+  lightElements.delete(el);
+  evictMediaSyncState(el);
+  if (original === null) el.removeAttribute("src");
+  else el.setAttribute("src", original);
+  el.load();
+}
+
 function loadProxy(
   el: HTMLMediaElement,
   href: string,
   originalSrc: string,
   variant: string,
   box: PreviewProxyBox | null,
+  light = false,
 ): void {
-  if (box) boxedElements.set(el, { originalSrc, variant, href, box });
+  if (box) boxedElements.set(el, { originalSrc, variant, href, box, light });
   else boxedElements.delete(el);
   // Sync state measured on the previous file would read the new file's buffering as drift.
   evictMediaSyncState(el);
@@ -202,10 +268,7 @@ export function deriveCodecMapKey(el: HTMLMediaElement): string | null {
  * ending in `"/notfoo.mp4"`, since that would require the literal substring
  * `"/foo.mp4"` to appear where the last `/` already fell a segment later).
  */
-function lookupLongestExactSuffix(
-  pathname: string,
-  map: Record<string, MediaCodecMapEntry>,
-): MediaCodecMapEntry | null {
+function lookupLongestExactSuffix<T>(pathname: string, map: Record<string, T>): T | null {
   let bestKey: string | null = null;
   for (const key of Object.keys(map)) {
     if (!pathname.endsWith(key)) continue;
@@ -214,10 +277,7 @@ function lookupLongestExactSuffix(
   return bestKey ? (map[bestKey] ?? null) : null;
 }
 
-function lookupNormalizedSuffix(
-  pathname: string,
-  map: Record<string, MediaCodecMapEntry>,
-): MediaCodecMapEntry | null {
+function lookupNormalizedSuffix<T>(pathname: string, map: Record<string, T>): T | null {
   // Case-insensitive filesystems and Unicode-normalizing filesystems can
   // serve an authored URL whose spelling differs from the canonical path
   // ffprobe returned (Clip.mp4 vs clip.mp4, NFC vs NFD). Exact matching stays
@@ -226,7 +286,7 @@ function lookupNormalizedSuffix(
   // spellings never select the wrong asset.
   const normalizedPathname = pathname.normalize("NFC").toLowerCase();
   let normalizedBestLength = -1;
-  let normalizedBest: MediaCodecMapEntry | null = null;
+  let normalizedBest: T | null = null;
   let ambiguous = false;
   for (const [key, entry] of Object.entries(map)) {
     const normalizedKey = key.normalize("NFC").toLowerCase();
@@ -242,10 +302,7 @@ function lookupNormalizedSuffix(
   return ambiguous ? null : normalizedBest;
 }
 
-function lookupCodecMapEntry(
-  pathname: string,
-  map: Record<string, MediaCodecMapEntry>,
-): MediaCodecMapEntry | null {
+function lookupCodecMapEntry<T>(pathname: string, map: Record<string, T>): T | null {
   return (
     map[pathname] ??
     lookupLongestExactSuffix(pathname, map) ??
@@ -253,10 +310,16 @@ function lookupCodecMapEntry(
   );
 }
 
-function appendProxyParam(src: string, variant: string, box: PreviewProxyBox | null): string {
+function appendProxyParam(
+  src: string,
+  variant: string,
+  box: PreviewProxyBox | null,
+  light = false,
+): string {
   const url = new URL(src, document.baseURI);
   url.searchParams.set(PROXY_QUERY_PARAM, variant);
   if (box) url.searchParams.set(PREVIEW_PROXY_BOX_PARAM, formatPreviewProxyBox(box));
+  if (light && box) url.searchParams.set(PREVIEW_PROXY_LIGHT_PARAM, "1");
   return url.href;
 }
 
@@ -300,6 +363,49 @@ function emitUnavailableDiagnostic(
   console.info(`[hyperframes] ${DIAGNOSTIC_UNAVAILABLE_CODE}: "${asset}" (${reason}): ${note}`);
 }
 
+type CopyJob = {
+  originalSrc: string;
+  originalAttr: string | null;
+  variant: string;
+  lightFacts?: PreviewSourceFacts;
+  level: PreviewQuality;
+  live: () => boolean;
+};
+
+// fallow-ignore-next-line complexity
+async function makeCopy(el: HTMLMediaElement, job: CopyJob, scale: number | null) {
+  const { originalSrc, originalAttr, variant, lightFacts, level, live } = job;
+  // Asked once the shown size is known, so the copy is made at that size and only once.
+  const shown = scale === null ? null : shownBox(el, scale);
+  const box = shown && level < 1 ? scalePreviewProxyBox(shown, level) : shown;
+  if (lightFacts && !(box && isHeavyForPreview(lightFacts, box))) {
+    if (proxyRequested.get(el) === originalAttr) proxyRequested.delete(el);
+    return;
+  }
+  const proxiedSrc = appendProxyParam(originalSrc, variant, box, lightFacts !== undefined);
+  const served = await waitForServedProxy(proxiedSrc, live);
+  if (!live()) {
+    if (proxyRequested.get(el) === originalAttr) proxyRequested.delete(el);
+    if (lightFacts && el.isConnected && el.getAttribute("src") === originalAttr) {
+      applyPreviewQuality(el);
+    }
+    return;
+  }
+  if (!served) {
+    emitUnavailableDiagnostic(el, "proxy_playback_failed", originalSrc);
+    return;
+  }
+  swappedElements.set(el, originalAttr);
+  if (lightFacts) lightElements.add(el);
+  loadProxy(el, proxiedSrc, originalSrc, variant, box, lightFacts !== undefined);
+  // The shown size may have grown while this copy was being made.
+  if (displayScale !== null) keepSized(el, displayScale);
+  return new Promise((landed) => {
+    el.addEventListener("loadeddata", landed, { once: true });
+    el.addEventListener("error", landed, { once: true });
+  });
+}
+
 /**
  * Swap `el` to its alpha-aware proxy URL, evict stale per-source sync state, and
  * emit the one-time diagnostic + console line. Safe to call from any of the
@@ -309,6 +415,7 @@ export function swapToProxy(
   el: HTMLMediaElement,
   entry: MediaCodecMapEntry | null = null,
   trigger: ProxyTrigger = "reactive",
+  lightFacts?: PreviewSourceFacts,
 ): void {
   if (swappedElements.has(el)) return;
   if (proxyRequested.has(el) && proxyRequested.get(el) === el.getAttribute("src")) return;
@@ -323,29 +430,15 @@ export function swapToProxy(
   }
   const originalAttr = el.getAttribute("src");
   proxyRequested.set(el, originalAttr);
-  const live = () => el.isConnected && el.getAttribute("src") === originalAttr;
-  const swap = shownScale().then(async (scale) => {
-    // Asked once the shown size is known, so the copy is made at that size and only once.
-    const box = scale === null ? null : shownBox(el, scale);
-    const proxiedSrc = appendProxyParam(originalSrc, variant, box);
-    const served = await waitForServedProxy(proxiedSrc, live);
-    if (!live()) {
-      if (proxyRequested.get(el) === originalAttr) proxyRequested.delete(el);
-      return;
-    }
-    if (!served) {
-      emitUnavailableDiagnostic(el, "proxy_playback_failed", originalSrc);
-      return;
-    }
-    swappedElements.set(el, originalAttr);
-    loadProxy(el, proxiedSrc, originalSrc, variant, box);
-    // The shown size may have grown while this copy was being made.
-    if (displayScale !== null) keepSized(el, displayScale);
-    return new Promise((landed) => {
-      el.addEventListener("loadeddata", landed, { once: true });
-      el.addEventListener("error", landed, { once: true });
-    });
-  });
+  const level = previewQuality;
+  // A lighter copy waits for no level but its own; a copy the file cannot play without finishes at any level.
+  const live = () =>
+    el.isConnected &&
+    el.getAttribute("src") === originalAttr &&
+    (!lightFacts || previewQuality === level);
+  const job: CopyJob = { originalSrc, originalAttr, variant, lightFacts, level, live };
+  const swap = shownScale().then((scale) => makeCopy(el, job, scale));
+  if (lightFacts) return;
   // A frame capture holds for the copy as it held for a loading video before, up to the same cap.
   registerSeekCompletion(
     Promise.race([swap, new Promise((cap) => setTimeout(cap, HOLD_CAP_MS))]),
@@ -383,15 +476,31 @@ export function maybeProxyProactively(el: HTMLMediaElement): void {
   if (isRenderMode(el)) return;
   if (!isVideoElement(el)) return;
   if (swappedElements.has(el)) return;
-  const map = window.__HF_MEDIA_CODEC_MAP__;
-  if (!map) return;
-  const key = deriveCodecMapKey(el);
-  if (key === null) return;
-  const entry = lookupCodecMapEntry(key, map);
-  if (!entry || !entry.browserHostile) return;
-  const canPlay = entry.representativeMime ? el.canPlayType(entry.representativeMime) : "";
-  if (canPlay === "probably" || canPlay === "maybe") return;
-  swapToProxy(el, entry, "proactive");
+  const entry = codecEntryOf(el);
+  const canPlay = entry?.representativeMime ? el.canPlayType(entry.representativeMime) : "";
+  const playable = canPlay === "probably" || canPlay === "maybe";
+  if (entry?.browserHostile && !playable) swapToProxy(el, entry, "proactive");
+  else if (previewQuality < 1) lightenIfHeavy(el, entry);
+}
+
+function lightenIfHeavy(el: HTMLMediaElement, entry: MediaCodecMapEntry | null): void {
+  if (swappedElements.has(el) || proxyRequested.has(el)) return;
+  const facts = sourceFactsOf(el);
+  if (facts) swapToProxy(el, entry, "light", facts);
+}
+
+function applyPreviewQuality(el: HTMLMediaElement): void {
+  if (isRenderMode(el) || !isVideoElement(el)) return;
+  if (previewQuality === 1 && lightElements.has(el)) return restoreOriginal(el);
+  if (boxedElements.has(el) && displayScale !== null) return keepSized(el, displayScale);
+  if (previewQuality < 1) lightenIfHeavy(el, codecEntryOf(el));
+}
+
+/** The host's preview quality: below 1, heavy videos play lighter copies sized to the shown box times the level. */
+export function setPreviewQuality(level: number): void {
+  if (!isPreviewQuality(level) || level === previewQuality) return;
+  previewQuality = level;
+  for (const el of document.querySelectorAll("video")) applyPreviewQuality(el);
 }
 
 /**
diff --git a/packages/core/src/runtime/types.ts b/packages/core/src/runtime/types.ts
index 35ed38aea7..d1f4e27f76 100644
--- a/packages/core/src/runtime/types.ts
+++ b/packages/core/src/runtime/types.ts
@@ -22,6 +22,7 @@ type RuntimeBridgeControlActionBase =
   | "set-web-audio-media-disabled"
   | "set-idle-heartbeat"
   | "set-display-scale"
+  | "set-preview-quality"
   | "set-root-duration"
   | "set-play-range"
   | "stop-media"
@@ -41,6 +42,7 @@ type RuntimeBridgeControlMessageBase = {
   disabled?: boolean;
   slow?: boolean;
   scale?: number;
+  level?: number;
   playbackRate?: number;
   target?: HfColorGradingTarget | string | null;
   grading?: RuntimeJson;
diff --git a/packages/studio-server/src/helpers/mediaCodecMap.test.ts b/packages/studio-server/src/helpers/mediaCodecMap.test.ts
index 3ab8ab549e..60caaaa8e0 100644
--- a/packages/studio-server/src/helpers/mediaCodecMap.test.ts
+++ b/packages/studio-server/src/helpers/mediaCodecMap.test.ts
@@ -10,6 +10,7 @@ import {
   probeAssetCodec,
   proxyVariantFor,
   resolveProxyVariantRequest,
+  scanProjectMedia,
   scanProjectMediaCodecMap,
   shouldPrewarmProxy,
   type AssetCodecFacts,
@@ -24,7 +25,14 @@ const FAKE_FFPROBE_PATH = process.execPath;
 function makeRunner(
   codecByPath: Record<
     string,
-    string | { codecName: string; pixFmt?: string; tags?: Record<string, string> }
+    | string
+    | {
+        codecName: string;
+        pixFmt?: string;
+        tags?: Record<string, string>;
+        size?: [number, number];
+        bitRate?: string;
+      }
   >,
 ): FfprobeRunner {
   return (_command, args) => {
@@ -38,6 +46,9 @@ function makeRunner(
             codec_name: normalized.codecName,
             pix_fmt: normalized.pixFmt,
             tags: normalized.tags,
+            width: normalized.size?.[0],
+            height: normalized.size?.[1],
+            bit_rate: normalized.bitRate,
           },
         ]
       : [];
@@ -328,6 +339,68 @@ describe("decideMediaProxyEligibility", () => {
   });
 });
 
+describe("a lighter copy of a file Chrome can play", () => {
+  const playable = {
+    codecName: "h264",
+    browserHostile: false,
+    representativeMime: null,
+    hasAlpha: false,
+    source: { width: 3840, height: 2160, bitDepth: 8, bitrate: 10_000_000 },
+  };
+
+  it("is allowed only for the box it is asked at, and only when the file is heavy there", () => {
+    expect(decideMediaProxyEligibility(playable)).toEqual({
+      eligible: false,
+      reason: "browser_safe_codec",
+    });
+    expect(decideMediaProxyEligibility(playable, { width: 724, height: 362 })).toEqual({
+      eligible: true,
+    });
+    expect(
+      decideMediaProxyEligibility(
+        { ...playable, source: { ...playable.source, width: 1920, height: 1080 } },
+        { width: 2048, height: 1218 },
+      ),
+    ).toEqual({ eligible: false, reason: "browser_safe_codec" });
+  });
+
+  it("reads the size, bit depth and bitrate off the probe, and keeps them with the codec facts", async () => {
+    const project = tmpProject();
+    const videoPath = join(project, "cam.mov");
+    writeFileSync(videoPath, "fake video bytes");
+
+    const facts = await probeAssetCodec(
+      videoPath,
+      makeRunner({
+        [videoPath]: {
+          codecName: "hevc",
+          pixFmt: "yuv420p10le",
+          size: [3840, 2160],
+          bitRate: "60000000",
+        },
+      }),
+    );
+
+    expect(facts?.source).toEqual({ width: 3840, height: 2160, bitDepth: 10, bitrate: 60_000_000 });
+  });
+
+  it("scans the sizes of the playable videos too, while the codec map keeps only the hostile ones", async () => {
+    const project = tmpProject();
+    writeFileSync(join(project, "cam.mp4"), "fake video bytes");
+    writeFileSync(join(project, "broll.mov"), "fake video bytes");
+
+    const scan = await scanProjectMedia(project, [{ html: videoHtml("cam.mp4", "broll.mov") }], {
+      runner: makeRunner({
+        [join(project, "cam.mp4")]: { codecName: "h264", size: [1920, 1080] },
+        [join(project, "broll.mov")]: { codecName: "hevc", size: [3840, 2160] },
+      }),
+    });
+
+    expect(Object.keys(scan.codecMap)).toEqual(["/broll.mov"]);
+    expect(Object.keys(scan.sourceFacts).sort()).toEqual(["/broll.mov", "/cam.mp4"]);
+  });
+});
+
 describe("proxyVariantFor", () => {
   it("uses VP8 for alpha and H.264 otherwise", () => {
     const facts = {
diff --git a/packages/studio-server/src/helpers/mediaCodecMap.ts b/packages/studio-server/src/helpers/mediaCodecMap.ts
index 8966e22422..c14e43335c 100644
--- a/packages/studio-server/src/helpers/mediaCodecMap.ts
+++ b/packages/studio-server/src/helpers/mediaCodecMap.ts
@@ -9,6 +9,11 @@ import {
   isUnresolvedAssetPlaceholder,
   resolveExistingLocalAsset as resolveAsset,
 } from "@hyperframes/parsers/asset-resolution";
+import {
+  isHeavyForPreview,
+  type PreviewProxyBox,
+  type PreviewSourceFacts,
+} from "@hyperframes/core";
 import { probeMediaMetadata, type FfprobeRunner } from "./mediaMetadata.js";
 
 /**
@@ -27,6 +32,8 @@ export interface AssetCodecFacts {
   /** Source carries an alpha channel (`MediaMetadata.hasAlpha`). Alpha sources use a
    * VP8/WebM proxy so their transparency is preserved across Chromium builds. */
   hasAlpha: boolean;
+  /** Size, bit depth and bitrate, for deciding whether a playable file deserves a lighter copy. */
+  source?: PreviewSourceFacts;
 }
 
 /** Server-root-relative URL pathname -> that asset's codec facts. */
@@ -176,20 +183,34 @@ export type MediaProxyEligibility =
   | { eligible: true }
   | { eligible: false; reason: MediaProxyIneligibilityReason };
 
-/** Single policy gate shared by proactive scans and on-demand proxy routes. */
-export function decideMediaProxyEligibility(facts: AssetCodecFacts | null): MediaProxyEligibility {
+/**
+ * Single policy gate shared by proactive scans and on-demand proxy routes. A file Chrome can play is
+ * eligible only for a lighter copy (`lightBox`, the box that request asks for) and only when it is heavy there.
+ */
+export function decideMediaProxyEligibility(
+  facts: AssetCodecFacts | null,
+  lightBox?: PreviewProxyBox,
+): MediaProxyEligibility {
   if (!facts) return { eligible: false, reason: "unknown_codec" };
-  if (!facts.browserHostile) return { eligible: false, reason: "browser_safe_codec" };
-  return { eligible: true };
+  if (facts.browserHostile) return { eligible: true };
+  if (lightBox && facts.source && isHeavyForPreview(facts.source, lightBox)) {
+    return { eligible: true };
+  }
+  return { eligible: false, reason: "browser_safe_codec" };
 }
 
-function codecFactsFor(codecName: string, hasAlpha: boolean): AssetCodecFacts {
+function codecFactsFor(
+  codecName: string,
+  hasAlpha: boolean,
+  source?: PreviewSourceFacts,
+): AssetCodecFacts {
   const hostile = hostileCodecEntry(codecName);
   return {
     codecName,
     browserHostile: hostile !== undefined,
     representativeMime: hostile?.representativeMime ?? null,
     hasAlpha,
+    ...(source && { source }),
   };
 }
 
@@ -205,7 +226,7 @@ async function probeCodecFacts(
   if (metadata.kind !== "video") return null;
   const codecName = metadata.color.codecName;
   if (!codecName) return null;
-  return codecFactsFor(codecName, metadata.hasAlpha === true);
+  return codecFactsFor(codecName, metadata.hasAlpha === true, metadata.source);
 }
 
 interface CachedAssetProbe {
@@ -347,20 +368,12 @@ export interface ScanProjectMediaCodecMapOptions {
   runner?: FfprobeRunner;
 }
 
-/**
- * Scans a project's composition HTML for local `<video src>` references and
- * returns the injection map: root-relative URL pathname -> codec facts.
- * Best-effort throughout — a video whose codec can't be determined (missing
- * ffprobe, probe error, no video stream) is simply omitted, never thrown.
- */
-export async function scanProjectMediaCodecMap(
+async function probeProjectAssets(
   projectDir: string,
   htmlSources: HtmlSourceLike[],
-  options: ScanProjectMediaCodecMapOptions = {},
-): Promise<MediaCodecMap> {
+  options: ScanProjectMediaCodecMapOptions,
+): Promise<Array<[string, AssetCodecFacts | null]>> {
   const candidates = collectLocalVideoAssets(projectDir, htmlSources);
-  if (candidates.size === 0) return {};
-
   const cache = options.cache ?? defaultProbeCache;
   const entries = [...candidates.entries()]; // [resolvedPath, rootRelativePathname]
   const facts = new Array<AssetCodecFacts | null>(entries.length).fill(null);
@@ -376,11 +389,35 @@ export async function scanProjectMediaCodecMap(
       }
     }),
   );
+  return entries.map(([, pathname], index) => [pathname, facts[index] ?? null]);
+}
 
-  const map: MediaCodecMap = {};
-  entries.forEach(([, pathname], index) => {
-    const entryFacts = facts[index];
-    if (entryFacts?.browserHostile) map[pathname] = entryFacts;
-  });
-  return map;
+/** What a scan of a project's local videos finds: the hostile ones for the proxy map, and every readable one's size. */
+export interface ProjectMediaScan {
+  codecMap: MediaCodecMap;
+  /** Root-relative URL pathname -> size, bit depth and bitrate. */
+  sourceFacts: Record<string, PreviewSourceFacts>;
+}
+
+/** Scans composition HTML for local `<video src>` references; a video that can't be probed is omitted. */
+export async function scanProjectMedia(
+  projectDir: string,
+  htmlSources: HtmlSourceLike[],
+  options: ScanProjectMediaCodecMapOptions = {},
+): Promise<ProjectMediaScan> {
+  const scan: ProjectMediaScan = { codecMap: {}, sourceFacts: {} };
+  for (const [pathname, facts] of await probeProjectAssets(projectDir, htmlSources, options)) {
+    if (facts?.browserHostile) scan.codecMap[pathname] = facts;
+    if (facts?.source) scan.sourceFacts[pathname] = facts.source;
+  }
+  return scan;
+}
+
+/** The injection map alone: root-relative URL pathname -> codec facts of the hostile videos. */
+export async function scanProjectMediaCodecMap(
+  projectDir: string,
+  htmlSources: HtmlSourceLike[],
+  options: ScanProjectMediaCodecMapOptions = {},
+): Promise<MediaCodecMap> {
+  return (await scanProjectMedia(projectDir, htmlSources, options)).codecMap;
 }
diff --git a/packages/studio-server/src/helpers/mediaMetadata.test.ts b/packages/studio-server/src/helpers/mediaMetadata.test.ts
index 73006c0e4c..74c968a7ae 100644
--- a/packages/studio-server/src/helpers/mediaMetadata.test.ts
+++ b/packages/studio-server/src/helpers/mediaMetadata.test.ts
@@ -120,6 +120,33 @@ describe("probeMediaMetadata", () => {
     expect(flac.kind).toBe("audio");
   });
 
+  it("reads the size, bit depth and bitrate a lighter copy is judged by", async () => {
+    const probe = (stream: object, format: object = {}) =>
+      probeMediaMetadata("/tmp/clip.mp4", () => ({
+        status: 0,
+        stdout: JSON.stringify({ streams: [{ codec_type: "video", ...stream }], format }),
+        stderr: "",
+      }));
+
+    expect(
+      (await probe({ width: 3840, height: 2160, pix_fmt: "yuv420p10le", bit_rate: "9" })).source,
+    ).toEqual({
+      width: 3840,
+      height: 2160,
+      bitDepth: 10,
+      bitrate: 9,
+    });
+    expect(
+      (await probe({ width: 1920, height: 1080, pix_fmt: "yuv420p" }, { bit_rate: "55000000" }))
+        .source,
+    ).toEqual({ width: 1920, height: 1080, bitDepth: 8, bitrate: 55_000_000 });
+    expect(
+      (await probe({ width: 1920, height: 1080, pix_fmt: "yuv420p", bits_per_raw_sample: "12" }))
+        .source?.bitDepth,
+    ).toBe(12);
+    expect((await probe({ pix_fmt: "yuv420p" })).source).toBeUndefined();
+  });
+
   it("ignores attached cover art and reads the real video stream", async () => {
     const metadata = await probeMediaMetadata("/tmp/clip.mp4", () => ({
       status: 0,
diff --git a/packages/studio-server/src/helpers/mediaMetadata.ts b/packages/studio-server/src/helpers/mediaMetadata.ts
index 78fff6cb5c..8489ec0165 100644
--- a/packages/studio-server/src/helpers/mediaMetadata.ts
+++ b/packages/studio-server/src/helpers/mediaMetadata.ts
@@ -3,6 +3,7 @@ import { extname } from "node:path";
 import {
   firstFrameColourArgs,
   parseFirstFrameColour,
+  type PreviewSourceFacts,
   type ToneMapSourceColour,
 } from "@hyperframes/core";
 import { findFfBinary } from "@hyperframes/parsers/ff-binaries";
@@ -77,6 +78,7 @@ export interface MediaMetadata {
   /** Video stream carries alpha (pix_fmt or the VP8/VP9 alpha_mode tag). */
   hasAlpha?: boolean;
   durationSeconds?: number;
+  source?: PreviewSourceFacts;
   probeError?: string;
 }
 
@@ -89,6 +91,9 @@ interface FfprobeStream {
   color_transfer?: string;
   color_primaries?: string;
   bits_per_raw_sample?: string;
+  width?: number;
+  height?: number;
+  bit_rate?: string;
   disposition?: { attached_pic?: number };
   tags?: Record<string, string | undefined>;
 }
@@ -192,6 +197,27 @@ export async function probeFirstFrameColour(
   return result.status === 0 ? parseFirstFrameColour(String(result.stderr)) : {};
 }
 
+function bitDepthOf(stream: FfprobeStream): number {
+  const raw = Number(stream.bits_per_raw_sample);
+  if (Number.isFinite(raw) && raw > 0) return raw;
+  const suffixed = /(\d{1,3})(?:le|be)$/.exec(stream.pix_fmt ?? "");
+  return suffixed ? Number(suffixed[1]) : 8;
+}
+
+function sourceFactsOf(
+  stream: FfprobeStream | undefined,
+  formatBitrate: string | undefined,
+): PreviewSourceFacts | undefined {
+  if (!stream?.width || !stream.height) return undefined;
+  const bitrate = Number(stream.bit_rate ?? formatBitrate);
+  return {
+    width: stream.width,
+    height: stream.height,
+    bitDepth: bitDepthOf(stream),
+    bitrate: Number.isFinite(bitrate) ? bitrate : 0,
+  };
+}
+
 export async function probeMediaMetadata(
   filePath: string,
   runner: FfprobeRunner = execFileRunner,
@@ -216,7 +242,7 @@ export async function probeMediaMetadata(
       "-v",
       "error",
       "-show_entries",
-      "stream=codec_type,codec_name,profile,pix_fmt,color_space,color_transfer,color_primaries,bits_per_raw_sample:stream_disposition=attached_pic:stream_tags=alpha_mode:format=duration",
+      "stream=codec_type,codec_name,profile,pix_fmt,color_space,color_transfer,color_primaries,bits_per_raw_sample,width,height,bit_rate:stream_disposition=attached_pic:stream_tags=alpha_mode:format=duration,bit_rate",
       "-of",
       "json",
       "--",
@@ -235,7 +261,7 @@ export async function probeMediaMetadata(
   try {
     const parsed = JSON.parse(String(result.stdout || "{}")) as {
       streams?: FfprobeStream[];
-      format?: { duration?: string };
+      format?: { duration?: string; bit_rate?: string };
     };
     const stream = parsed.streams?.find((item) => {
       if (kind === "image") return item.codec_type === "video";
@@ -247,6 +273,7 @@ export async function probeMediaMetadata(
       metadata.hasAlpha = stream !== undefined && videoStreamHasAlpha(stream);
       const duration = Number(parsed.format?.duration);
       if (Number.isFinite(duration) && duration > 0) metadata.durationSeconds = duration;
+      metadata.source = sourceFactsOf(stream, parsed.format?.bit_rate);
     }
     return metadata;
   } catch {
diff --git a/packages/studio-server/src/helpers/mediaProxyPreview.test.ts b/packages/studio-server/src/helpers/mediaProxyPreview.test.ts
index febb29967d..bb12771fb3 100644
--- a/packages/studio-server/src/helpers/mediaProxyPreview.test.ts
+++ b/packages/studio-server/src/helpers/mediaProxyPreview.test.ts
@@ -23,7 +23,10 @@ function hostile(codecName: string, representativeMime: string | null): AssetCod
   return { codecName, browserHostile: true, representativeMime, hasAlpha: false };
 }
 
-async function loadHelper(map: MediaCodecMap): Promise<{
+async function loadHelper(
+  map: MediaCodecMap,
+  sourceFacts: Record<string, unknown> = {},
+): Promise<{
   injectMediaCodecMapIntoHtml: typeof import("./mediaProxyPreview.js").injectMediaCodecMapIntoHtml;
   mediaProxyDemand: typeof import("./mediaCodecMap.js").mediaProxyDemand;
   resolveProxy: ReturnType<typeof vi.fn>;
@@ -35,7 +38,7 @@ async function loadHelper(map: MediaCodecMap): Promise<{
   // replaced, so this asserts the shipped policy rather than a copy of it.
   vi.doMock("./mediaCodecMap.js", async () => ({
     ...(await vi.importActual<typeof import("./mediaCodecMap.js")>("./mediaCodecMap.js")),
-    scanProjectMediaCodecMap: async () => map,
+    scanProjectMedia: async () => ({ codecMap: map, sourceFacts }),
   }));
   const { injectMediaCodecMapIntoHtml } = await import("./mediaProxyPreview.js");
   const { mediaProxyDemand } = await import("./mediaCodecMap.js");
@@ -55,6 +58,33 @@ afterEach(() => {
   for (const dir of dirs.splice(0)) rmSync(dir, { recursive: true, force: true });
 });
 
+describe("injectMediaCodecMapIntoHtml source facts", () => {
+  const facts = { "/videos/cam.mp4": { width: 3840, height: 2160, bitDepth: 10, bitrate: 0 } };
+
+  it("hands the page the sizes of its videos, and no codec map when none is hostile", async () => {
+    const { injectMediaCodecMapIntoHtml } = await loadHelper({}, facts);
+
+    const html = await injectMediaCodecMapIntoHtml(
+      "<html><head></head><body></body></html>",
+      tmpProject(),
+      [{ html: "<html></html>" }],
+    );
+
+    expect(html).toContain("window.__HF_MEDIA_SOURCE_FACTS__=");
+    expect(html).toContain('"/videos/cam.mp4"');
+    expect(html).not.toContain("__HF_MEDIA_CODEC_MAP__");
+  });
+
+  it("leaves the page untouched when there is nothing to say about its videos", async () => {
+    const { injectMediaCodecMapIntoHtml } = await loadHelper({}, {});
+    const page = "<html><head></head><body></body></html>";
+
+    expect(await injectMediaCodecMapIntoHtml(page, tmpProject(), [{ html: "<html></html>" }])).toBe(
+      page,
+    );
+  });
+});
+
 describe("injectMediaCodecMapIntoHtml pre-warm", () => {
   it("pre-warms the codec with no cross-platform decode and leaves the widely-decoded one lazy", async () => {
     const projectDir = tmpProject();
diff --git a/packages/studio-server/src/helpers/mediaProxyPreview.ts b/packages/studio-server/src/helpers/mediaProxyPreview.ts
index 3ef8c41e08..7487ad1f0a 100644
--- a/packages/studio-server/src/helpers/mediaProxyPreview.ts
+++ b/packages/studio-server/src/helpers/mediaProxyPreview.ts
@@ -6,10 +6,10 @@ import {
   createMediaCodecProbeCache,
   proxyVariantFor,
   recordProxyPrewarm,
-  scanProjectMediaCodecMap,
+  scanProjectMedia,
   shouldPrewarmProxy,
   type HtmlSourceLike,
-  type MediaCodecMap,
+  type ProjectMediaScan,
   type MediaCodecProbeCache,
 } from "./mediaCodecMap.js";
 import { resolveProxy, PROXY_PARAMS_VERSION } from "./proxyTranscoder.js";
@@ -97,18 +97,16 @@ export async function injectMediaCodecMapIntoHtml(
   signal?: AbortSignal,
   box?: PreviewProxyBox,
 ): Promise<string> {
-  let map: MediaCodecMap;
+  let scan: ProjectMediaScan;
   try {
-    map = await scanProjectMediaCodecMap(
-      projectDir,
-      htmlSources,
-      probeCache ? { cache: probeCache } : {},
-    );
+    scan = await scanProjectMedia(projectDir, htmlSources, probeCache ? { cache: probeCache } : {});
   } catch {
     // Best-effort: a scan failure must never block serving the page.
     return html;
   }
-  if (Object.keys(map).length === 0) return html;
+  const { codecMap: map, sourceFacts } = scan;
+  const hasMap = Object.keys(map).length > 0;
+  if (!hasMap && Object.keys(sourceFacts).length === 0) return html;
   for (const [rootRelativePathname, facts] of Object.entries(map)) {
     if (!prewarm || !shouldPrewarmProxy(facts)) continue;
     recordProxyPrewarm();
@@ -125,11 +123,17 @@ export async function injectMediaCodecMapIntoHtml(
   }
   // <-escape prevents a src path containing "</script>" from breaking out of
   // the injected tag, mirroring injectPreviewVariables in helpers/previewVariables.ts.
-  const json = JSON.stringify(map)
-    .replace(/</g, "\\u003c")
-    .replace(/\u2028/g, "\\u2028")
-    .replace(/\u2029/g, "\\u2029");
-  const tag = `<script data-hf-media-codec-map>window.__HF_MEDIA_CODEC_MAP__=${json};</script>`;
+  const safeJson = (value: unknown): string =>
+    JSON.stringify(value)
+      .replace(/</g, "\\u003c")
+      .replace(/\u2028/g, "\\u2028")
+      .replace(/\u2029/g, "\\u2029");
+  // An empty map stays out: the runtime reads a present map as leave to rescue unlisted files.
+  const assignments = [
+    ...(hasMap ? [`window.__HF_MEDIA_CODEC_MAP__=${safeJson(map)};`] : []),
+    `window.__HF_MEDIA_SOURCE_FACTS__=${safeJson(sourceFacts)};`,
+  ];
+  const tag = `<script data-hf-media-codec-map>${assignments.join("")}</script>`;
   return insertBeforeCloseTag(html, "head", `${tag}\n`) ?? `${tag}\n${html}`;
 }
 
diff --git a/packages/studio-server/src/helpers/proxyTranscoder.test.ts b/packages/studio-server/src/helpers/proxyTranscoder.test.ts
index daea63571b..09fc50ec2f 100644
--- a/packages/studio-server/src/helpers/proxyTranscoder.test.ts
+++ b/packages/studio-server/src/helpers/proxyTranscoder.test.ts
@@ -451,6 +451,22 @@ describe("resolveProxy", () => {
     await expect(resultPromise).resolves.toBe(vp8Path);
   });
 
+  it("puts a keyframe every second into a copy, so scrubbing lands close", async () => {
+    const { spawn, calls } = createSpawnSpy();
+    const { resolveProxy } = await loadModule(spawn, FFMPEG_PATH);
+    const projectDir = tmpProject();
+    const sourcePath = join(projectDir, "clip.mov");
+    writeFileSync(sourcePath, "source-bytes");
+
+    const result = resolveProxy(projectDir, sourcePath);
+    await flush();
+
+    const args = calls[0]!.args;
+    expect(args[args.indexOf("-force_key_frames") + 1]).toBe("expr:gte(t,n_forced*1)");
+    succeed(calls[0]!);
+    await result;
+  });
+
   it("decodes a VP9 alpha source with libvpx-vp9 so the VP8 proxy keeps its alpha", async () => {
     const { spawn, calls } = createSpawnSpy();
     const { resolveProxy } = await loadModule(spawn, FFMPEG_PATH, false, null, "vp9");
diff --git a/packages/studio-server/src/helpers/proxyTranscoder.ts b/packages/studio-server/src/helpers/proxyTranscoder.ts
index f71143973d..4c20649a1d 100644
--- a/packages/studio-server/src/helpers/proxyTranscoder.ts
+++ b/packages/studio-server/src/helpers/proxyTranscoder.ts
@@ -35,7 +35,7 @@ import { mkdirWithinProject, realpath, realProjectRoot } from "./safePath.js";
  * dropped from the queue or its ffmpeg is killed.
  */
 
-export const PROXY_PARAMS_VERSION = "v6";
+export const PROXY_PARAMS_VERSION = "v7";
 
 export const CACHE_DIR_NAME = ".transcode-cache";
 
@@ -595,6 +595,7 @@ async function encodeInChunks(
   for (const piece of pieces) if (existsSync(piece)) unlinkSync(piece);
 }
 
+const KEYFRAME_EVERY_SECOND_ARGS = ["-force_key_frames", "expr:gte(t,n_forced*1)"];
 // Fixed recipe; changing either array means bumping PROXY_PARAMS_VERSION.
 const H264_ARGS = [
   "-c:v",
@@ -717,6 +718,7 @@ async function runFfmpeg(
         ...(range ? ["-t", String(range.lengthSeconds)] : []),
         "-vf",
         videoFilter,
+        ...KEYFRAME_EVERY_SECOND_ARGS,
         ...(keepsAlpha ? [...VP8_ARGS, ...vp8Threads(range ? gate.size : 1)] : H264_ARGS),
         target,
       ],
diff --git a/packages/studio-server/src/routes/preview.test.ts b/packages/studio-server/src/routes/preview.test.ts
index 71ca3b2e3d..627d87eb4c 100644
--- a/packages/studio-server/src/routes/preview.test.ts
+++ b/packages/studio-server/src/routes/preview.test.ts
@@ -1390,6 +1390,7 @@ describe("hf-proxy negotiation and media codec map injection (U3)", () => {
       browserHostile: boolean;
       representativeMime: string | null;
       hasAlpha: boolean;
+      source?: { width: number; height: number; bitDepth: number; bitrate: number };
     } | null>;
   }): Promise<typeof import("./preview.js")> {
     vi.resetModules();
@@ -1424,7 +1425,10 @@ describe("hf-proxy negotiation and media codec map injection (U3)", () => {
       ...(await vi.importActual<typeof import("../helpers/mediaCodecMap.js")>(
         "../helpers/mediaCodecMap.js",
       )),
-      scanProjectMediaCodecMap: opts.scanMapImpl ?? (async () => ({})),
+      scanProjectMedia: async (...args: Parameters<ScanMapImpl>) => ({
+        codecMap: await (opts.scanMapImpl ?? (async () => ({})))(...args),
+        sourceFacts: {},
+      }),
       createMediaCodecProbeCache: () => new Map(),
       probeAssetCodec:
         opts.probeAssetCodecImpl ??
@@ -1434,18 +1438,6 @@ describe("hf-proxy negotiation and media codec map injection (U3)", () => {
           representativeMime: null,
           hasAlpha: false,
         })),
-      decideMediaProxyEligibility: (
-        facts: {
-          browserHostile: boolean;
-          hasAlpha: boolean;
-        } | null,
-      ) => {
-        if (!facts) return { eligible: false, reason: "unknown_codec" };
-        if (!facts.browserHostile) {
-          return { eligible: false, reason: "browser_safe_codec" };
-        }
-        return { eligible: true };
-      },
       isProxyVariant: (value: string) => value === "h264" || value === "vp8",
       isProxyVariantRequest: (value: string) =>
         value === "auto" || value === "h264" || value === "vp8",
@@ -1578,9 +1570,72 @@ describe("hf-proxy negotiation and media codec map injection (U3)", () => {
       expect(sized.headers.get("ETag")).not.toBe(full.headers.get("ETag"));
     });
 
+    describe("a lighter copy of a file Chrome can play", () => {
+      const playable = (width: number, height: number) => async () => ({
+        codecName: "h264",
+        browserHostile: false,
+        representativeMime: null,
+        hasAlpha: false,
+        source: { width, height, bitDepth: 8, bitrate: 10_000_000 },
+      });
+
+      it("is made at the asked box when the file is heavy there", async () => {
+        const projectDir = createProjectDir();
+        writeFileSync(join(projectDir, "clip.mp4"), "original-bytes");
+        const resolveProxyMock = vi.fn(async () => {
+          const proxyPath = join(projectDir, "light.mp4");
+          writeFileSync(proxyPath, "light-bytes");
+          return proxyPath;
+        });
+        const { registerPreviewRoutes: register } = await loadPreviewModule({
+          resolveProxyImpl: resolveProxyMock,
+          probeAssetCodecImpl: playable(3840, 2160),
+        });
+        const app = new Hono();
+        register(app, createAdapter(projectDir));
+
+        const res = await app.request(
+          "http://localhost/projects/demo/preview/clip.mp4?hf-proxy=h264&hf-proxy-light=1&hf-proxy-box=724x362",
+        );
+
+        expect(res.status).toBe(200);
+        expect(await res.text()).toBe("light-bytes");
+        expect(resolveProxyMock).toHaveBeenCalledWith(
+          projectDir,
+          join(projectDir, "clip.mp4"),
+          "h264",
+          { width: 724, height: 362 },
+        );
+      });
+
+      it("is refused when the file is not heavy at that box, and without the ask", async () => {
+        const projectDir = createProjectDir();
+        writeFileSync(join(projectDir, "clip.mp4"), "original-bytes");
+        const resolveProxyMock = vi.fn(async () => "should-not-be-called");
+        const { registerPreviewRoutes: register } = await loadPreviewModule({
+          resolveProxyImpl: resolveProxyMock,
+          probeAssetCodecImpl: playable(1920, 1080),
+        });
+        const app = new Hono();
+        register(app, createAdapter(projectDir));
+
+        const small = await app.request(
+          "http://localhost/projects/demo/preview/clip.mp4?hf-proxy=h264&hf-proxy-light=1&hf-proxy-box=2048x1218",
+        );
+        const unasked = await app.request(
+          "http://localhost/projects/demo/preview/clip.mp4?hf-proxy=h264&hf-proxy-box=724x362",
+        );
+
+        expect(small.status).toBe(422);
+        expect(unasked.status).toBe(422);
+        expect(resolveProxyMock).not.toHaveBeenCalled();
+      });
+    });
+
     it.each([
       ["a size off the ladder", "?hf-proxy=h264&hf-proxy-box=1000x2000"],
       ["a size without a copy", "?hf-proxy-box=1448x2048"],
+      ["a lighter copy without a size", "?hf-proxy=h264&hf-proxy-light=1"],
     ])("refuses %s before making anything", async (_name, query) => {
       const projectDir = createProjectDir();
       writeFileSync(join(projectDir, "clip.mp4"), "original-hevc-bytes");
diff --git a/packages/studio-server/src/routes/preview.ts b/packages/studio-server/src/routes/preview.ts
index 396f73ffd2..30c061941d 100644
--- a/packages/studio-server/src/routes/preview.ts
+++ b/packages/studio-server/src/routes/preview.ts
@@ -18,7 +18,11 @@ import {
   STUDIO_PREVIEW_MARK_META,
   STUDIO_PREVIEW_ERRORS,
 } from "@hyperframes/core/studio-preview-mark";
-import { PREVIEW_PROXY_BOX_PARAM, parsePreviewProxyBox } from "@hyperframes/core";
+import {
+  PREVIEW_PROXY_BOX_PARAM,
+  PREVIEW_PROXY_LIGHT_PARAM,
+  parsePreviewProxyBox,
+} from "@hyperframes/core";
 import {
   GSAP_BASE_URL_META,
   gsapCdnDist,
@@ -697,7 +701,12 @@ export function registerPreviewRoutes(api: Hono, adapter: PreviewApiAdapter): vo
     const proxyParam = c.req.query("hf-proxy");
     const boxParam = c.req.query(PREVIEW_PROXY_BOX_PARAM);
     const proxyBox = boxParam === undefined ? undefined : parsePreviewProxyBox(boxParam);
-    if (proxyBox === null || (proxyBox && proxyParam === undefined)) {
+    const lightRequested = c.req.query(PREVIEW_PROXY_LIGHT_PARAM) !== undefined;
+    if (
+      proxyBox === null ||
+      (proxyBox && proxyParam === undefined) ||
+      (lightRequested && !proxyBox)
+    ) {
       return c.text("not found", 404);
     }
     let proxyVariant: ProxyVariant | undefined;
@@ -710,7 +719,7 @@ export function registerPreviewRoutes(api: Hono, adapter: PreviewApiAdapter): vo
         return c.text("not found", 404);
       }
       const facts = await probeAssetCodec(file, undefined, mediaCodecProbeCache);
-      const eligibility = decideMediaProxyEligibility(facts);
+      const eligibility = decideMediaProxyEligibility(facts, lightRequested ? proxyBox : undefined);
       if (!eligibility.eligible) {
         return c.text(`media proxy unavailable: ${eligibility.reason}`, 422);
       }
diff --git a/scripts/package-size-budgets.json b/scripts/package-size-budgets.json
index 57dc3835ad..1e54a521ab 100644
--- a/scripts/package-size-budgets.json
+++ b/scripts/package-size-budgets.json
@@ -36,7 +36,7 @@
     "unpacked": 541601
   },
   "@hyperframes/producer": {
-    "packed": 781312,
+    "packed": 828932,
     "unpacked": 2840135
   },
   "@hyperframes/sdk": {