From e1925d4317fcd13c0e5799cd4767a3e2dd4c9238 Mon Sep 17 00:00:00 2001
From: Jerrai <jerrai@heygen.com>
Date: Sun, 11 Oct 2026 19:07:00 +0000
Subject: [PATCH] feat(audio): composition master bus (data-fx-chain on the
 root) (#5420)
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* feat(audio): composition master bus (data-fx-chain on the root)

A clip can belong to only one audio group, so there was no way to put one
effect, such as a limiter, across the whole mix. The data-fx-chain on the
top-level composition root now runs once over the sum of every clip and every
group, after each group's own chain and fader. No new attribute: a sub-composition
root's data-fx-chain was never applied to anything and still is not.

Render: when a master chain exists, mixAudioTracks writes a float WAV instead
of AAC, applyAudioFxChain runs the chain over it, and ffmpeg encodes AAC last,
so the chain reads unclipped samples. An unreadable chain is a mix failure, as
for a group. Preview: the transport splices the chain between the clips' sum and
the monitor, re-anchored per play generation and watched for edits.

Only the top-level root is read. No master automation: the root's data-automation
is not read as the bus's. No lint rule: data-fx-chain is valid on clips, groups
and roots, and the one place it does nothing (a sub-composition root) has always
been silent.

— Jerrai

* fix(core): preview routes every audible video through the master bus

A root data-fx-chain is attached only inside the Web Audio graph, but a plainly
audible <video> joined that graph only for its own chain, group, automation or a
volume over 1. So preview limited the music and left the talking head dry, and a
composition whose only audio is video got no master chain at all, while export
sums every audible element into the master. A master chain on the top-level
composition root is now a reason to join, the same as a video's own chain.
Elements under a root with no chain keep native output. Sub-composition roots
are still ignored.

— Jerrai

* fix(audio): master bus review round 2 (temp dir, tests, docs, Studio meter)

- audioMixer: a fatal FX error in the master pass or a group bus left the
  work directory (the full float sum, about 1.4 GB an hour) on disk. The
  final mix now runs in try/finally and the group rethrow removes it first.
- Tests: the "once over the sum" case now uses a limiter then a gain, so a
  per-clip chain (-10.4) and a doubled one (-18) both fail it; a new case
  proves the sum stays float until the chain reads it; the fader case uses
  a group volume of 0.5 so the fader's position is exercised; the work-dir
  removal is covered for both paths.
- Docs: "Three rules exist" (the master-bus lint rule is gone), a bad master
  chain stops the render with audio_processing_failed, a sub-composition
  file opened on its own has its root chain as the master bus, and a seek
  clears a group or master limiter.
- Studio: TimelineElement.masterChain carries the root's data-fx-chain, so
  the program meter shows for a video-only composition whose only audio
  routing is the master chain, as preview now plays it.
- Comment blocks over four lines cut; webAudioTransport.setRate split so
  its CRAP score is under the Fallow threshold.

— Jerrai

* fix(audio): master chain reaches tracks the render adds; honest plain-limiter copy

- Master chain: it was carried on each parsed AudioElement, so a track the
  producer adds itself (appendAutoDetectedVideoAudio, the browser-discovered
  audio in probeStage) never had it, and a composition whose only audio was
  such a track exported with no master chain, no ceiling and no warning. It is
  now a composition property: parseMasterFxChain(html) at compile, carried on
  CompiledComposition and CompositionMetadata, passed to processCompositionAudio
  by runAudioStage (in-process and distributed plan). AudioElement.masterFxChain
  is removed so there is one source. Each forward (compile stage, local
  orchestrator, distributed plan) has a test that fails when it is dropped.
- Plain limiter: it follows the level with no lookahead and Level after is
  applied past the ceiling, so it cannot promise "nothing gets past" or "never
  clips". Studio copy (description, panel text, knob label, summary) and the
  hyperframes-audio skill now say a fast or inter-sample peak can pass it and
  point to the true-peak limiter for a delivery limit. A copy test guards it.

— Jerrai

* fix(producer): keep master bus comments within the ratchet; run the plan test with the ffmpeg tests

Drop three one-line field comments that raised their files' comment share,
and list plan.masterBus.test.ts with the integration tests because plan()
reads the ffmpeg version.

— Jerrai

* chore(ci): repin the producer test-classification source in the reachability map

The reachability map pins the hash of test-classification.mjs, which now
lists one more integration test. The map's producer lane command already
covers every test under src, in both the unit and integration matrix lanes.

— Jerrai
---
 docs/reference/audio-effects.mdx              |  37 +++-
 packages/core/src/audibleVideo.test.ts        |  17 +-
 packages/core/src/audibleVideo.ts             |   4 +-
 packages/core/src/audioFx.ts                  |   3 +-
 packages/core/src/audioFxCopy.test.ts         |  24 ++-
 packages/core/src/audioFxCopy.ts              |   8 +-
 packages/core/src/audioGroups.test.ts         |  25 +++
 packages/core/src/audioGroups.ts              |  16 ++
 packages/core/src/runtime/audioFx.test.ts     |  13 +-
 packages/core/src/runtime/audioFx.ts          |  21 +-
 packages/core/src/runtime/init.test.ts        |  63 ++++++
 packages/core/src/runtime/init.ts             |   4 +-
 .../src/runtime/webAudioTransport.test.ts     |  86 ++++++++
 .../core/src/runtime/webAudioTransport.ts     |  81 +++++--
 packages/engine/src/index.ts                  |   1 +
 .../engine/src/services/audioFxRender.test.ts | 200 ++++++++++++++++++
 .../engine/src/services/audioMixer.test.ts    |  43 +++-
 packages/engine/src/services/audioMixer.ts    | 130 ++++++++++--
 .../producer/scripts/test-classification.mjs  |   1 +
 .../distributed/plan.masterBus.test.ts        |  50 +++++
 .../producer/src/services/distributed/plan.ts |   1 +
 .../src/services/htmlCompiler.test.ts         |  19 ++
 .../producer/src/services/htmlCompiler.ts     |   4 +
 .../producer/src/services/render/shared.ts    |   1 +
 .../services/render/stages/audioStage.test.ts |  52 ++++-
 .../src/services/render/stages/audioStage.ts  |   4 +-
 .../render/stages/compileStage.test.ts        |  34 +++
 .../services/render/stages/compileStage.ts    |   1 +
 .../renderOrchestrator.masterBus.test.ts      |  87 ++++++++
 .../src/services/renderOrchestrator.ts        |   1 +
 .../studio/src/player/lib/timelineDOM.test.ts |  55 +++++
 packages/studio/src/player/lib/timelineDOM.ts |   5 +
 .../src/player/store/timelineElement.ts       |   3 +-
 .../studio/src/utils/audioMeterMath.test.ts   |   9 +
 scripts/test-reachability.json                |   2 +-
 skills-manifest.json                          |   2 +-
 skills/hyperframes-audio/SKILL.md             |  13 +-
 .../references/attributes.md                  |   9 +
 38 files changed, 1072 insertions(+), 57 deletions(-)
 create mode 100644 packages/producer/src/services/distributed/plan.masterBus.test.ts
 create mode 100644 packages/producer/src/services/renderOrchestrator.masterBus.test.ts

diff --git a/docs/reference/audio-effects.mdx b/docs/reference/audio-effects.mdx
index 910724a745..c1f9b8ddd4 100644
--- a/docs/reference/audio-effects.mdx
+++ b/docs/reference/audio-effects.mdx
@@ -180,6 +180,41 @@ dangles.
   preview never lays it out.
 - Group ids and element ids share one namespace.
 
+### The master bus: `data-fx-chain` on the composition root
+
+```html
+<div id="root" data-composition-id="main" data-width="1920" data-height="1080"
+  data-fx-chain="{&quot;version&quot;:1,&quot;nodes&quot;:[
+    {&quot;type&quot;:&quot;truepeak&quot;,&quot;id&quot;:&quot;m1&quot;,&quot;label&quot;:&quot;Delivery ceiling&quot;,
+     &quot;params&quot;:{&quot;ceiling&quot;:-1.5,&quot;lookahead&quot;:3,&quot;release&quot;:80}}
+  ]}"
+>
+```
+
+The master bus is the `data-fx-chain` on the top-level composition root. It runs
+once over the sum of every clip and every group, after each group's own chain and
+fader, so a limiter here sees the peak of the whole mix. A clip can belong to
+only one group, so this is the way to put one effect across everything.
+
+- Only the **top-level** composition root is read as the master bus. On a clip or
+  a group `data-fx-chain` keeps its own meaning, and on a sub-composition's root
+  it is not applied while a parent composition contains it. A sub-composition file
+  previewed or rendered on its own is the top-level root, so its chain is the
+  master bus there.
+- There is no master automation: `data-automation` on the root is not read as
+  the master's.
+- The render sums the tracks to a float WAV, runs the chain, and only then
+  encodes AAC, so the chain reads unclipped samples. Preview inserts the chain
+  between the clips' sum and the monitor.
+- With a master chain on the root, preview plays every audible `<video>` through
+  that bus too, as the render does. Without one, a video at volume 1 with no
+  chain, group or automation of its own stays on native output.
+- An unreadable master chain **fails the whole mix**: the render stops with an
+  `audio_processing_failed` error under every strictness setting, and exports
+  nothing. Preview plays dry with no message, and no lint rule checks a chain's
+  contents, so one typo on the root blocks every export of that composition.
+  Check a master chain in preview before rendering.
+
 ## Effect registry
 
 Sixteen effects. Values outside a range are clamped on read. **AUTO** marks a
@@ -223,7 +258,7 @@ the peak arrives, and holds that estimate at `ceiling`. Channels share one
 gain. It delays its output by `lookahead` plus about 0.3 ms (3.3 to 10.3 ms across
 the range): the offline render trims that delay, live preview plays it, so the
 sound is that much behind the picture. A seek clears the limiter's delay line and
-held gain on a group bus. Changing `lookahead` rebuilds the node.
+held gain on a group or master bus. Changing `lookahead` rebuilds the node.
 
 The ceiling is held against that 4x estimate, not against the waveform itself. A
 4x interpolator cannot see the peaks of content close to Nyquist, so the true peak
diff --git a/packages/core/src/audibleVideo.test.ts b/packages/core/src/audibleVideo.test.ts
index 1d0ea261a5..2e2ee3fca2 100644
--- a/packages/core/src/audibleVideo.test.ts
+++ b/packages/core/src/audibleVideo.test.ts
@@ -50,14 +50,19 @@ describe("AUDIBLE_MEDIA_SELECTOR", () => {
 describe("audibleVideoNeedsWebAudio", () => {
   it("keeps a unity, unprocessed video on native output", () => {
     expect(audibleVideoNeedsWebAudio({})).toBe(false);
+    expect(audibleVideoNeedsWebAudio({ volume: 1, masterChain: null })).toBe(false);
+    expect(audibleVideoNeedsWebAudio({ masterChain: "" })).toBe(false);
     expect(audibleVideoNeedsWebAudio({ volume: 1, fxChain: "", automation: null })).toBe(false);
     expect(audibleVideoNeedsWebAudio({ volume: Number.NaN })).toBe(false);
   });
 
-  it.each([{ volume: 1.5 }, { fxChain: "[]" }, { automation: "{}" }, { audioGroup: "music" }])(
-    "routes a video carrying %o",
-    (fields) => {
-      expect(audibleVideoNeedsWebAudio(fields)).toBe(true);
-    },
-  );
+  it.each([
+    { volume: 1.5 },
+    { fxChain: "[]" },
+    { automation: "{}" },
+    { audioGroup: "music" },
+    { volume: 1, masterChain: '{"version":1,"nodes":[]}' },
+  ])("routes a video carrying %o", (fields) => {
+    expect(audibleVideoNeedsWebAudio(fields)).toBe(true);
+  });
 });
diff --git a/packages/core/src/audibleVideo.ts b/packages/core/src/audibleVideo.ts
index 30c9aa5fd2..6c3b9aead3 100644
--- a/packages/core/src/audibleVideo.ts
+++ b/packages/core/src/audibleVideo.ts
@@ -18,8 +18,10 @@ export function audibleVideoNeedsWebAudio(fields: {
   fxChain?: string | null;
   automation?: string | null;
   audioGroup?: string | null;
+  masterChain?: string | null;
 }): boolean {
   return (
-    (fields.volume ?? 1) > 1 || Boolean(fields.fxChain || fields.automation || fields.audioGroup)
+    (fields.volume ?? 1) > 1 ||
+    Boolean(fields.fxChain || fields.automation || fields.audioGroup || fields.masterChain)
   );
 }
diff --git a/packages/core/src/audioFx.ts b/packages/core/src/audioFx.ts
index 7a75c2d947..8718390ad9 100644
--- a/packages/core/src/audioFx.ts
+++ b/packages/core/src/audioFx.ts
@@ -288,7 +288,8 @@ export const HF_AUDIO_FX: readonly HfAudioFxDef[] = [
     id: "limiter",
     label: "Limiter",
     group: "dynamics",
-    description: "Hard ceiling: nothing gets past the limit.",
+    description:
+      "Peak limiter that follows the signal's level. It has no lookahead, so a fast peak or an inter-sample peak can pass the ceiling; use the true-peak limiter for a delivery limit.",
     params: [
       {
         kind: "number",
diff --git a/packages/core/src/audioFxCopy.test.ts b/packages/core/src/audioFxCopy.test.ts
index ad974b505b..72c69bfde3 100644
--- a/packages/core/src/audioFxCopy.test.ts
+++ b/packages/core/src/audioFxCopy.test.ts
@@ -1,5 +1,5 @@
 import { describe, expect, it } from "vitest";
-import { defaultAudioFxParams, HF_AUDIO_FX } from "./audioFx.js";
+import { defaultAudioFxParams, getAudioFxDef, HF_AUDIO_FX } from "./audioFx.js";
 import { HF_AUDIO_FX_PRESETS } from "./audioFxPresets.js";
 import { audioBandAt, BANDS, EFFECT_COPY, PRESET_PROBLEM, SUMMARY } from "./audioFxCopy.js";
 
@@ -197,3 +197,25 @@ describe("the true-peak copy does not promise a hard ceiling", () => {
     expect(hint).toMatch(/headroom/);
   });
 });
+
+describe("the plain limiter copy does not promise a hard ceiling", () => {
+  // The limiter follows the signal's level with no lookahead, and Level after is applied past
+  // the ceiling, so a fast or inter-sample peak can end above it.
+  it("says nothing about never clipping or nothing getting past", () => {
+    const copy = EFFECT_COPY.limiter;
+    const summary = SUMMARY.limiter;
+    const description = getAudioFxDef("limiter")?.description;
+    if (!copy || !summary || !description) throw new Error("limiter copy is missing");
+    const shown = [
+      description,
+      copy.does,
+      copy.reachFor,
+      copy.params.limit?.label,
+      summary({}),
+    ].join("\n");
+    expect(shown).not.toMatch(
+      /never|nothing|guarantee|hard ceiling|no louder|must not|cannot exceed/i,
+    );
+    expect(shown).toMatch(/can (still )?(get )?pass|get past/i);
+  });
+});
diff --git a/packages/core/src/audioFxCopy.ts b/packages/core/src/audioFxCopy.ts
index c6e2ac7aa0..d3c83d50ee 100644
--- a/packages/core/src/audioFxCopy.ts
+++ b/packages/core/src/audioFxCopy.ts
@@ -152,13 +152,13 @@ export const EFFECT_COPY: Record<string, EffectCopy> = {
   },
   limiter: {
     title: "Peak Ceiling",
-    does: "Nothing gets louder than this, ever. A safety net at the end of the chain.",
-    reachFor: "You want to be sure it never clips or spikes.",
+    does: "Pulls the loudest moments down toward this ceiling. It reacts to what it hears, so a fast peak can still get past, and Level after is applied on top of it.",
+    reachFor: "You want peaks held down and can accept a small overshoot.",
     primary: "limit",
     primaryEnds: { low: "A lot of headroom", high: "Right up to the edge" },
     params: {
       limit: {
-        label: "Never exceed",
+        label: "Ceiling",
         ends: { low: "A lot of headroom", high: "Right up to the edge" },
       },
       attack: { label: "How fast it catches" },
@@ -411,7 +411,7 @@ export const SUMMARY: Record<string, (p: P) => string> = {
       : `${n(p.gain) > 0 ? "More" : "Less"} sparkle above ${hz(p.frequency)}`,
   compressor: (p) =>
     `Evening out, ${strength(Math.min(1, (n(p.ratio, 3) - 1) / 7), ["gentle", "moderate", "firm"])}`,
-  limiter: (p) => `Nothing louder than ${n(p.limit, -1)} dB`,
+  limiter: (p) => `Peaks pulled down toward ${n(p.limit, -1)} dB`,
   truepeak: (p) => `Peaks pulled down toward ${n(p.ceiling, -1)} dBTP`,
   gate: (p) => `Closing gaps quieter than ${n(p.threshold, -45)} dB`,
   saturate: (p) =>
diff --git a/packages/core/src/audioGroups.test.ts b/packages/core/src/audioGroups.test.ts
index 722da0a225..7b2821319a 100644
--- a/packages/core/src/audioGroups.test.ts
+++ b/packages/core/src/audioGroups.test.ts
@@ -4,6 +4,7 @@ import {
   ensureAudioGroupInertStyle,
   HF_AUDIO_GROUP_ATTR,
   isMemberGroupHidden,
+  readMasterFxChain,
   resolveAudioGroups,
   resolveCarveSourceIds,
   resolveGroupElement,
@@ -305,3 +306,27 @@ describe("ensureAudioGroupInertStyle", () => {
     expect(document.querySelectorAll("#__hf-audio-group-inert")).toHaveLength(1);
   });
 });
+
+describe("readMasterFxChain", () => {
+  const CHAIN = `{&quot;version&quot;:1,&quot;nodes&quot;:[]}`;
+
+  it("reads the chain off the top-level composition root", () => {
+    document.body.innerHTML = `<div data-composition-id="main" data-fx-chain="${CHAIN}"></div>`;
+    expect(readMasterFxChain(document)).toBe('{"version":1,"nodes":[]}');
+  });
+
+  it("is undefined when the root carries none", () => {
+    document.body.innerHTML = `<div data-composition-id="main"><audio id="a"></audio></div>`;
+    expect(readMasterFxChain(document)).toBeUndefined();
+  });
+
+  it("ignores a chain on a nested composition root", () => {
+    document.body.innerHTML = `<div data-composition-id="main"><div data-composition-id="inner" data-fx-chain="${CHAIN}"></div></div>`;
+    expect(readMasterFxChain(document)).toBeUndefined();
+  });
+
+  it("ignores a chain on a clip", () => {
+    document.body.innerHTML = `<div data-composition-id="main"><audio id="a" data-fx-chain="${CHAIN}"></audio></div>`;
+    expect(readMasterFxChain(document)).toBeUndefined();
+  });
+});
diff --git a/packages/core/src/audioGroups.ts b/packages/core/src/audioGroups.ts
index b0cc736d11..1482eba48a 100644
--- a/packages/core/src/audioGroups.ts
+++ b/packages/core/src/audioGroups.ts
@@ -128,6 +128,22 @@ export function isMemberGroupHidden(
   return resolveGroupElement(doc, groupId)?.hasAttribute("data-hidden") ?? false;
 }
 
+/**
+ * The top-level composition root, the only element the master bus is read from;
+ * a sub-composition's root is skipped, so repeating one cannot stack chains.
+ */
+export function resolveMasterBusElement(root: ParentNode): Element | null {
+  for (const el of root.querySelectorAll("[data-composition-id]")) {
+    if (!el.parentElement?.closest("[data-composition-id]")) return el;
+  }
+  return null;
+}
+
+/** The master bus chain: the top-level root's `data-fx-chain`, if it carries one. */
+export function readMasterFxChain(root: ParentNode): string | undefined {
+  return resolveMasterBusElement(root)?.getAttribute(HF_AUDIO_FX_ATTR) || undefined;
+}
+
 export function resolveAudioGroups(root: ParentNode): HfAudioGroup[] {
   const membersByGroup = new Map<string, string[]>();
   const candidates = root.querySelectorAll(
diff --git a/packages/core/src/runtime/audioFx.test.ts b/packages/core/src/runtime/audioFx.test.ts
index 6820a4bdc3..b1d110083c 100644
--- a/packages/core/src/runtime/audioFx.test.ts
+++ b/packages/core/src/runtime/audioFx.test.ts
@@ -1,6 +1,6 @@
 // @vitest-environment happy-dom
 import { describe, expect, it } from "vitest";
-import { attachElementFxChain } from "./audioFx.js";
+import { attachElementFxChain, type FxAttributes } from "./audioFx.js";
 import { defaultAudioFxParams, HF_AUDIO_FX, HF_AUDIO_FX_ATTR } from "../audioFx.js";
 
 /**
@@ -435,7 +435,7 @@ describe("attachElementFxChain", () => {
       ],
     });
 
-    const build = () => {
+    const build = (attrs?: FxAttributes) => {
       const clock = { currentTime: 0 };
       const made: { frequency: TimedParam }[] = [];
       class TimedNode extends Node {
@@ -461,6 +461,7 @@ describe("attachElementFxChain", () => {
         new Node() as never,
         new Node() as never,
         { scheduledAt: 0, elapsed: 0, rate: 1 },
+        attrs,
       );
       return { clock, node, handle, param: () => made[0]?.frequency as unknown as TimedParam };
     };
@@ -495,6 +496,14 @@ describe("attachElementFxChain", () => {
       expect(param().last()).toEqual({ time: 4, duration: 1 });
     });
 
+    it("books no lane for a chain told its element carries none (the master bus)", () => {
+      // The root's `data-automation` belongs to the composition, not to the bus.
+      const { param } = build({ automation: null });
+      expect(param().last()).toBeUndefined();
+      const { param: ownParam } = build();
+      expect(ownParam().last()).toEqual({ time: 0, duration: 8 });
+    });
+
     it("ignores a rate that is not a rate", () => {
       const { clock, handle, param } = build();
       clock.currentTime = 2;
diff --git a/packages/core/src/runtime/audioFx.ts b/packages/core/src/runtime/audioFx.ts
index 085f10a152..2a4376ecca 100644
--- a/packages/core/src/runtime/audioFx.ts
+++ b/packages/core/src/runtime/audioFx.ts
@@ -38,10 +38,9 @@ const NO_AUTOMATION: HfAutomation = { version: 1, lanes: [] };
 function readAutomation(
   el: { getAttribute?(name: string): string | null },
   chain: HfAudioFxChain,
+  attr: string | null = HF_AUDIO_AUTOMATION_ATTR,
 ): HfAutomation {
-  const raw =
-    (typeof el.getAttribute === "function" ? el.getAttribute(HF_AUDIO_AUTOMATION_ATTR) : null) ??
-    "";
+  const raw = (attr && typeof el.getAttribute === "function" ? el.getAttribute(attr) : null) ?? "";
   if (!raw) return NO_AUTOMATION;
   try {
     return resolveAutomation(parseAutomation(raw), chain);
@@ -119,12 +118,18 @@ export interface ElementFxHandle {
   setRate(rate: number): void;
 }
 
+/** The attribute a chain's lanes are read from; the master bus has none. */
+export interface FxAttributes {
+  automation: string | null;
+}
+
 export function attachElementFxChain(
   ctx: BaseAudioContext,
   el: { getAttribute?(name: string): string | null },
   source: AudioNode,
   destination: AudioNode,
   timing?: AutomationTiming,
+  attrs: FxAttributes = { automation: HF_AUDIO_AUTOMATION_ATTR },
 ): ElementFxHandle | null {
   const { chain } = readChain(el);
 
@@ -202,7 +207,13 @@ export function attachElementFxChain(
   const scheduleFor = (next: HfAudioFxChain, at: AutomationTiming | null): void => {
     automated =
       at && handle
-        ? scheduleChainAutomation(readAutomation(el, next), next, handle.nodes, at, handle.presets)
+        ? scheduleChainAutomation(
+            readAutomation(el, next, attrs.automation),
+            next,
+            handle.nodes,
+            at,
+            handle.presets,
+          )
         : [];
   };
 
@@ -282,7 +293,7 @@ export function attachElementFxChain(
     });
     observer.observe(target, {
       attributes: true,
-      attributeFilter: [HF_AUDIO_FX_ATTR, HF_AUDIO_AUTOMATION_ATTR],
+      attributeFilter: attrs.automation ? [HF_AUDIO_FX_ATTR, attrs.automation] : [HF_AUDIO_FX_ATTR],
     });
   }
 
diff --git a/packages/core/src/runtime/init.test.ts b/packages/core/src/runtime/init.test.ts
index c0b68ca4d7..abac71baef 100644
--- a/packages/core/src/runtime/init.test.ts
+++ b/packages/core/src/runtime/init.test.ts
@@ -5883,6 +5883,69 @@ describe("initSandboxRuntimeModular", () => {
       expect(ctx.mediaElementSources).toBe(0);
     });
 
+    describe("under a composition root with a master chain", () => {
+      const MASTER_CHAIN = '{"version":1,"nodes":[]}';
+      const setRootChain = () =>
+        document.querySelector("[data-root]")?.setAttribute("data-fx-chain", MASTER_CHAIN);
+
+      it("joins the graph for a plain audible video at unity, so the master bus limits it", async () => {
+        const video = mountMedia("video", { "data-has-audio": "true", "data-volume": "1" });
+        setRootChain();
+        const captureSpy = spyCapture();
+
+        await startPlayback();
+
+        expect(captureSpy).toHaveBeenCalledTimes(1);
+        expect(captureSpy.mock.calls[0]?.[0]).toBe(video);
+        expect(captureSpy.mock.calls[0]?.[4]).toBe(1);
+      });
+
+      it("acquires a media element source for it", async () => {
+        mountMedia("video", { "data-has-audio": "true" });
+        setRootChain();
+
+        await startPlayback();
+        await Promise.resolve();
+
+        expect(ctx.mediaElementSources).toBe(1);
+      });
+
+      it("joins a video whose only audio is the composition, and every audio clip beside it", async () => {
+        const video = mountMedia("video", { "data-has-audio": "true" });
+        const audio = mountMedia("audio");
+        setRootChain();
+        const captureSpy = spyCapture();
+
+        await startPlayback();
+
+        expect(captureSpy.mock.calls.map((call) => call[0])).toEqual([video, audio]);
+      });
+
+      it("still keeps a muted or data-has-audio=false video out of the graph", async () => {
+        mountMedia("video", { "data-has-audio": "true", muted: "" });
+        mountMedia("video", { "data-has-audio": "false" });
+        setRootChain();
+        const captureSpy = spyCapture();
+
+        await startPlayback();
+
+        expect(captureSpy).not.toHaveBeenCalled();
+      });
+
+      it("does not count a chain on a sub-composition root", async () => {
+        const video = mountMedia("video", { "data-has-audio": "true" });
+        const inner = document.createElement("div");
+        inner.setAttribute("data-composition-id", "inner");
+        inner.setAttribute("data-fx-chain", MASTER_CHAIN);
+        video.before(inner);
+        const captureSpy = spyCapture();
+
+        await startPlayback();
+
+        expect(captureSpy).not.toHaveBeenCalled();
+      });
+    });
+
     it.each([
       ["data-fx-chain", "[]"],
       ["data-automation", "[]"],
diff --git a/packages/core/src/runtime/init.ts b/packages/core/src/runtime/init.ts
index 295c0a3548..d85e18a6a5 100644
--- a/packages/core/src/runtime/init.ts
+++ b/packages/core/src/runtime/init.ts
@@ -114,6 +114,7 @@ import {
   ensureAudioGroupInertStyle,
   HF_AUDIO_GROUP_TAG,
   isMemberGroupHidden,
+  readMasterFxChain,
 } from "../audioGroups";
 import { clampNativeMediaVolume } from "../audioGain";
 import { quantizeTimeToFrame } from "../inline-scripts/parityContract";
@@ -383,7 +384,7 @@ function pageAnimationsForOnePass(): () => Animation[] {
   return () => (list ??= document.getAnimations());
 }
 
-// A `<video>` joins only for a gain `el.volume` cannot express: capture is a one-way door.
+// A `<video>` joins only for a gain `el.volume` cannot express or a root master chain: capture is a one-way door.
 const joinsWebAudio = (el: Element): el is HTMLMediaElement =>
   isAudioElement(el) ||
   (isVideoElement(el) &&
@@ -393,6 +394,7 @@ const joinsWebAudio = (el: Element): el is HTMLMediaElement =>
       fxChain: el.getAttribute("data-fx-chain"),
       automation: el.getAttribute("data-automation"),
       audioGroup: el.getAttribute("data-audio-group"),
+      masterChain: readMasterFxChain(el.ownerDocument),
     }));
 const WEB_AUDIO_MEDIA = "audio[data-start], video[data-start]";
 const webAudioMediaIn = (root: ParentNode): HTMLMediaElement[] =>
diff --git a/packages/core/src/runtime/webAudioTransport.test.ts b/packages/core/src/runtime/webAudioTransport.test.ts
index dcb384c6c6..79217cebe7 100644
--- a/packages/core/src/runtime/webAudioTransport.test.ts
+++ b/packages/core/src/runtime/webAudioTransport.test.ts
@@ -1113,6 +1113,92 @@ describe("WebAudioTransport", () => {
       expect(fader.gain.value).toBeCloseTo(0.5, 6);
     });
 
+    describe("master bus", () => {
+      const GAIN_CHAIN = JSON.stringify({
+        version: 1,
+        nodes: [{ type: "gain", id: "g", params: { gain: -6 } }],
+      });
+
+      function withBusOut(setup: ReturnType<typeof setupGroupTransport>) {
+        const busOut = { gain: { value: 1 }, connect: vi.fn(), disconnect: vi.fn() };
+        (setup.transport as unknown as { _busOut: unknown })._busOut = busOut;
+        return busOut;
+      }
+
+      it("splices the root's chain between the sum and the monitor", async () => {
+        document.body.innerHTML = `<div id="root" data-composition-id="main"></div>`;
+        document.getElementById("root")!.setAttribute("data-fx-chain", GAIN_CHAIN);
+        const setup = setupGroupTransport();
+        const busOut = withBusOut(setup);
+
+        await scheduleGrouped(setup.transport, setup.gen, "a");
+
+        const { masterGain } = setup.mock;
+        expect(masterGain.disconnect).toHaveBeenCalledWith(busOut);
+        // Wired into the chain's input, not straight to the bus output.
+        expect(masterGain.connect).toHaveBeenCalledTimes(1);
+        expect(masterGain.connect).not.toHaveBeenCalledWith(busOut);
+      });
+
+      it("feeds grouped members through the same master bus", async () => {
+        document.body.innerHTML = `<div id="root" data-composition-id="main" data-fx-chain='${GAIN_CHAIN}'></div>`;
+        const setup = setupGroupTransport();
+        const busOut = withBusOut(setup);
+
+        await scheduleGrouped(setup.transport, setup.gen, "a", "vo");
+
+        expect(setup.mock.masterGain.disconnect).toHaveBeenCalledWith(busOut);
+        expect(setup.mock.masterGain.connect).not.toHaveBeenCalledWith(busOut);
+      });
+
+      it("limits a plain video at unity played from its media element", async () => {
+        document.body.innerHTML = `<div id="root" data-composition-id="main" data-fx-chain='${GAIN_CHAIN}'><video id="talk" data-has-audio="true"></video></div>`;
+        const setup = setupGroupTransport();
+        const busOut = withBusOut(setup);
+        const video = document.getElementById("talk") as unknown as HTMLMediaElement;
+
+        await setup.transport.scheduleMediaElementPlayback(video, 0, 0, 0, 1, setup.gen, 1);
+
+        expect(setup.mock.gainNodes[0]!.connect).toHaveBeenCalledWith(setup.mock.masterGain);
+        expect(setup.mock.masterGain.disconnect).toHaveBeenCalledWith(busOut);
+        expect(setup.mock.masterGain.connect).not.toHaveBeenCalledWith(busOut);
+      });
+
+      it("leaves the sum wired straight through when the root has no chain", async () => {
+        document.body.innerHTML = `<div id="root" data-composition-id="main"></div>`;
+        const setup = setupGroupTransport();
+        const busOut = withBusOut(setup);
+
+        await scheduleGrouped(setup.transport, setup.gen, "a");
+
+        expect(setup.mock.masterGain.connect).toHaveBeenLastCalledWith(busOut);
+      });
+
+      it("ignores a chain on a sub-composition's root", async () => {
+        document.body.innerHTML = `<div id="root" data-composition-id="main"><div data-composition-id="inner" data-fx-chain='${GAIN_CHAIN}'></div></div>`;
+        const setup = setupGroupTransport();
+        const busOut = withBusOut(setup);
+
+        await scheduleGrouped(setup.transport, setup.gen, "a");
+
+        expect(setup.mock.masterGain.connect).toHaveBeenLastCalledWith(busOut);
+      });
+
+      it("is built once for the session, not once per member", async () => {
+        document.body.innerHTML = `<div id="root" data-composition-id="main" data-fx-chain='${GAIN_CHAIN}'></div>`;
+        const setup = setupGroupTransport();
+        withBusOut(setup);
+
+        await scheduleGrouped(setup.transport, setup.gen, "a");
+        const nodesAfterFirst = setup.mock.gainNodes.length;
+        await scheduleGrouped(setup.transport, setup.gen, "b");
+
+        // The second member adds its own element gain and nothing else.
+        expect(setup.mock.gainNodes.length).toBe(nodesAfterFirst + 1);
+        expect(setup.mock.masterGain.disconnect).toHaveBeenCalledTimes(1);
+      });
+    });
+
     describe("group mute (B5)", () => {
       it("a group created with data-hidden already set starts muted (mute gain at 0)", async () => {
         document.body.innerHTML = `<hf-audio-group id="vo" data-hidden></hf-audio-group>`;
diff --git a/packages/core/src/runtime/webAudioTransport.ts b/packages/core/src/runtime/webAudioTransport.ts
index d02770764f..8d90eb4e0d 100644
--- a/packages/core/src/runtime/webAudioTransport.ts
+++ b/packages/core/src/runtime/webAudioTransport.ts
@@ -1,4 +1,9 @@
-import { attachElementFxChain, readElementAutomation, type ElementFxHandle } from "./audioFx.js";
+import {
+  attachElementFxChain,
+  readElementAutomation,
+  type ElementFxHandle,
+  type FxAttributes,
+} from "./audioFx.js";
 import {
   clearParamLane,
   scheduleParamLane,
@@ -6,7 +11,12 @@ import {
   type AutomationTiming,
 } from "../audio/audioFxAutomation.js";
 import { VOLUME_RANGE } from "../audioAutomation.js";
-import { audioGroupOf, readAudioGroupVolume, resolveGroupElement } from "../audioGroups.js";
+import {
+  audioGroupOf,
+  readAudioGroupVolume,
+  resolveGroupElement,
+  resolveMasterBusElement,
+} from "../audioGroups.js";
 import { swallow } from "./diagnostics";
 import { createLevelTap, type LevelTap, type StereoLevel } from "./levelTap.js";
 import { clampAudioGain } from "../audioGain.js";
@@ -14,6 +24,8 @@ import { getDebugSurface } from "./globals.js";
 import { readElementPlaybackRate } from "./media.js";
 import { classifyWebAudioMediaRoute, reportWebAudioMediaRoute } from "./webAudioRoute.js";
 
+const MASTER_FX_ATTRS: FxAttributes = { automation: null };
+
 function normalizeRate(rate: number): number {
   if (!Number.isFinite(rate) || rate <= 0) return 1;
   return rate;
@@ -139,6 +151,10 @@ export class WebAudioTransport {
   private _mediaElementSources = new WeakMap<HTMLMediaElement, MediaElementAudioSourceNode>();
   private _activeSources: ScheduledSource[] = [];
   private _masterGain: GainNode | null = null;
+  /** Where the master bus chain lands; the program meter taps here. */
+  private _busOut: GainNode | null = null;
+  /** The chain on the composition root, built on first schedule and kept for the session. */
+  private _master: { fx: ElementFxHandle | null; generation: number } | null = null;
   /** Preview volume and mute. Downstream of `_masterGain`; meters tap the program. */
   private _monitorGain: GainNode | null = null;
   private _masterVolume = 1;
@@ -190,8 +206,10 @@ export class WebAudioTransport {
       this._ctx = new AudioContext();
       this._ctx.onstatechange = () => this.rest();
       this._masterGain = this._ctx.createGain();
+      this._busOut = this._ctx.createGain();
       this._monitorGain = this._ctx.createGain();
-      this._masterGain.connect(this._monitorGain);
+      this._masterGain.connect(this._busOut);
+      this._busOut.connect(this._monitorGain);
       this._monitorGain.connect(this._ctx.destination);
       this.applyMasterGain();
       if (this._metering) this.attachMasterTap();
@@ -573,7 +591,7 @@ export class WebAudioTransport {
 
   private attachMasterTap(): void {
     if (this._ctx && this._masterGain && !this._masterTap) {
-      this._masterTap = createLevelTap(this._ctx, this._masterGain);
+      this._masterTap = createLevelTap(this._ctx, this._busOut ?? this._masterGain);
     }
   }
 
@@ -601,6 +619,36 @@ export class WebAudioTransport {
     }
   }
 
+  /**
+   * The master bus chain, between the sum of every clip and group and the monitor.
+   * A seek re-anchors it once per play generation; no root yet is retried next schedule.
+   */
+  private ensureMasterBus(doc: Document, timing: AutomationTiming): void {
+    if (!this._ctx || !this._masterGain || !this._busOut) return;
+    if (this._master) {
+      if (this._master.generation === this._playGeneration) return;
+      try {
+        this._master.fx?.reanchor(timing);
+        this._master.generation = this._playGeneration;
+      } catch (err) {
+        swallow("webAudioTransport.masterReanchor", err);
+      }
+      return;
+    }
+    const root = resolveMasterBusElement(doc);
+    if (!root) return;
+    this._masterGain.disconnect(this._busOut);
+    const fx = attachElementFxChain(
+      this._ctx,
+      root,
+      this._masterGain,
+      this._busOut,
+      timing,
+      MASTER_FX_ATTRS,
+    );
+    this._master = { fx, generation: this._playGeneration };
+  }
+
   /** Master, unless `el` belongs to a group — then that group's bus (built on
    *  first use, per `groupInput`). */
   private resolveDestination(
@@ -610,9 +658,10 @@ export class WebAudioTransport {
     safeRate: number,
   ): GainNode | null {
     if (!this._masterGain) return null;
+    const groupTiming: AutomationTiming = { scheduledAt, elapsed: compositionTime, rate: safeRate };
+    this.ensureMasterBus(el.ownerDocument, groupTiming);
     const groupId = audioGroupOf(el);
     if (!groupId) return this._masterGain;
-    const groupTiming: AutomationTiming = { scheduledAt, elapsed: compositionTime, rate: safeRate };
     return this.groupInput(groupId, el.ownerDocument, groupTiming) ?? this._masterGain;
   }
 
@@ -785,17 +834,22 @@ export class WebAudioTransport {
         swallow("webAudioTransport.setRate", err);
       }
     }
-    // Group buses are not in `_activeSources` — they outlive it — so their FX
-    // automation needs re-aiming here too, or a rate change leaves a group's
-    // envelopes running the old plan over audio at the new speed.
-    for (const group of this._groups.values()) {
+    this.setBusRate(safeRate);
+    return true;
+  }
+
+  // Group buses and the master bus are not in `_activeSources` — they outlive it — so
+  // their FX automation needs re-aiming too, or a rate change leaves their envelopes
+  // running the old plan over audio at the new speed.
+  private setBusRate(rate: number): void {
+    const buses = [...this._groups.values(), ...(this._master ? [this._master] : [])];
+    for (const bus of buses) {
       try {
-        group.fx?.setRate(safeRate);
+        bus.fx?.setRate(rate);
       } catch (err) {
-        swallow("webAudioTransport.setRate.group", err);
+        swallow("webAudioTransport.setRate.bus", err);
       }
     }
-    return true;
   }
 
   // A bounded source's wall-clock duration was baked into start()'s duration
@@ -887,6 +941,8 @@ export class WebAudioTransport {
     this.stopMetering();
     for (const group of this._groups.values()) group.dispose();
     this._groups.clear();
+    this._master?.fx?.dispose();
+    this._master = null;
     this._bufferCache.clear();
     this._failedSrcs.clear();
     this._mediaElementSources = new WeakMap();
@@ -903,6 +959,7 @@ export class WebAudioTransport {
     }
     this._ctx = null;
     this._masterGain = null;
+    this._busOut = null;
     this._monitorGain = null;
     this._masterVolume = 1;
     this._masterMuted = false;
diff --git a/packages/engine/src/index.ts b/packages/engine/src/index.ts
index 8a69146f7a..9a0c01280d 100644
--- a/packages/engine/src/index.ts
+++ b/packages/engine/src/index.ts
@@ -256,6 +256,7 @@ export { createVideoFrameInjector } from "./services/videoFrameInjector.js";
 export {
   MIXED_AUDIO_FILENAME,
   parseAudioElements,
+  parseMasterFxChain,
   processCompositionAudio,
 } from "./services/audioMixer.js";
 export { cloneCaptureWarning, cloneCaptureWarnings } from "./services/captureWarning.js";
diff --git a/packages/engine/src/services/audioFxRender.test.ts b/packages/engine/src/services/audioFxRender.test.ts
index be5b6ca67c..6d2c944f97 100644
--- a/packages/engine/src/services/audioFxRender.test.ts
+++ b/packages/engine/src/services/audioFxRender.test.ts
@@ -6,6 +6,12 @@ import { afterEach, beforeEach, describe, expect, it } from "vitest";
 import { applyVolumeEnvelopeToWav } from "./audioVolumeEnvelope.js";
 import { defaultAudioFxParams, type HfAudioFxChain } from "@hyperframes/core/audio-fx";
 import { applyAudioFxChain, AudioFxRenderError, readWav, writeWav } from "./audioFxRender.js";
+import {
+  MIXED_AUDIO_FILENAME,
+  parseAudioElements,
+  parseMasterFxChain,
+  processCompositionAudio,
+} from "./audioMixer.js";
 import { resolveHeadlessShellPath } from "./browserManager.js";
 import { getFfmpegBinary } from "../utils/ffmpegBinaries.js";
 
@@ -530,3 +536,197 @@ describe("an empty track", () => {
     expect(existsSync(output)).toBe(false);
   });
 });
+
+/** Highest sample level of a file in dBFS, as ffmpeg's volumedetect reports it. */
+function maxVolumeDb(path: string): number {
+  const result = spawnSync(
+    getFfmpegBinary(),
+    ["-nostdin", "-hide_banner", "-i", path, "-af", "volumedetect", "-f", "null", "-"],
+    { encoding: "utf-8" },
+  );
+  const match = result.stderr.match(/max_volume:\s*(-?[\d.]+) dB/);
+  if (!match?.[1]) throw new Error(`Could not measure ${path}: ${result.stderr}`);
+  return Number(match[1]);
+}
+
+const chainJson = (...nodes: Record<string, unknown>[]): string =>
+  JSON.stringify({
+    version: 1,
+    nodes: nodes.map((node, i) => ({ id: `n${i + 1}`, enabled: true, ...node })),
+  });
+
+describe("master bus: reading it", () => {
+  const html = (root: string, inner = ""): string =>
+    `<div ${root}>${inner}<audio id="a" src="a.wav" data-start="0" data-duration="1"></audio></div>`;
+  const CHAIN = '{"version":1,"nodes":[]}';
+  const attr = `data-fx-chain='${CHAIN}'`;
+
+  it("reads the root's chain as the master, not as a clip's own", () => {
+    const doc = html(`data-composition-id="main" ${attr}`);
+    expect(parseMasterFxChain(doc)).toBe(CHAIN);
+    const [a] = parseAudioElements(doc);
+    expect(a?.fxChain).toBeUndefined();
+    expect(a?.groupFxChain).toBeUndefined();
+  });
+
+  it("has none when the root carries none", () => {
+    expect(parseMasterFxChain(html(`data-composition-id="main"`))).toBeUndefined();
+  });
+
+  it("leaves a sub-composition root's chain as it was: not the master, not its clips' chain", () => {
+    const inner = `<div data-composition-id="inner" ${attr}><audio id="b" src="b.wav" data-start="0" data-duration="1"></audio></div>`;
+    const doc = html(`data-composition-id="main"`, inner);
+    expect(parseMasterFxChain(doc)).toBeUndefined();
+    const [a, b] = parseAudioElements(doc);
+    for (const el of [a, b]) {
+      expect(el?.fxChain).toBeUndefined();
+      expect(el?.groupFxChain).toBeUndefined();
+    }
+  });
+
+  it("takes the master from the top-level root when a sub-composition root carries one too", () => {
+    const MASTER = '{"version":1,"nodes":[{"type":"gain","id":"m","params":{"gain":-3}}]}';
+    const inner = `<div data-composition-id="inner" ${attr}></div>`;
+    expect(
+      parseMasterFxChain(html(`data-composition-id="main" data-fx-chain='${MASTER}'`, inner)),
+    ).toBe(MASTER);
+  });
+});
+
+describe.skipIf(!HAS_FFMPEG)("master bus: reporting an unreadable chain", () => {
+  it("fails the mix with a failure instead of throwing", async () => {
+    writeWav(join(dir, "a.wav"), new Float32Array(SR).fill(0.1), SR);
+    const result = await processCompositionAudio(
+      [
+        {
+          id: "a",
+          src: "a.wav",
+          start: 0,
+          end: 1,
+          mediaStart: 0,
+          layer: 0,
+          volume: 1,
+          type: "audio",
+        },
+      ],
+      dir,
+      join(dir, "work"),
+      join(dir, `bad-${MIXED_AUDIO_FILENAME}`),
+      1,
+      undefined,
+      { masterFxChain: "{not json" },
+    );
+    expect(result.success).toBe(false);
+    expect(result.failures?.[0]?.elementId).toBe("master");
+  });
+});
+
+describe.skipIf(!HAS_BROWSER || !HAS_FFMPEG)("master bus: rendering it", () => {
+  const element = (id: string, extra: Record<string, unknown> = {}) => ({
+    id,
+    src: `${id}.wav`,
+    start: 0,
+    end: 1,
+    mediaStart: 0,
+    layer: 0,
+    volume: 1,
+    type: "audio" as const,
+    ...extra,
+  });
+  const steady = (peak: number, freq: number): Float32Array =>
+    Float32Array.from({ length: SR }, (_, i) => peak * Math.sin((2 * Math.PI * freq * i) / SR));
+
+  it("limits the sum of everything that feeds it", async () => {
+    // Two tones at -1 dBFS sum to about +5 dBFS: no clip's own chain can see that.
+    writeWav(join(dir, "a.wav"), steady(0.9, 300), SR, 1, true);
+    writeWav(join(dir, "b.wav"), steady(0.9, 300), SR, 1, true);
+    const master = chainJson({
+      type: "truepeak",
+      params: { ceiling: -6, lookahead: 3, release: 80 },
+    });
+    const out = join(dir, `limited-${MIXED_AUDIO_FILENAME}`);
+    const flat = join(dir, `flat-${MIXED_AUDIO_FILENAME}`);
+    const mix = (path: string, masterFxChain?: string) =>
+      processCompositionAudio(
+        [element("a"), element("b")],
+        dir,
+        join(dir, "work"),
+        path,
+        1,
+        undefined,
+        { masterFxChain },
+      );
+    expect((await mix(flat)).success).toBe(true);
+    expect((await mix(out, master)).success).toBe(true);
+    // The unprocessed sum has nowhere to go but full scale.
+    expect(maxVolumeDb(flat)).toBeGreaterThan(-1);
+    expect(maxVolumeDb(out)).toBeLessThanOrEqual(-5.5);
+  }, 180_000);
+
+  /** Mix `elements` for one second and read the file's highest level in dBFS. */
+  const mixedPeakDb = async (
+    name: string,
+    elements: Parameters<typeof processCompositionAudio>[0],
+    masterFxChain?: string,
+  ): Promise<number> => {
+    const out = join(dir, `${name}-${MIXED_AUDIO_FILENAME}`);
+    const result = await processCompositionAudio(
+      elements,
+      dir,
+      join(dir, "work"),
+      out,
+      1,
+      undefined,
+      { masterFxChain },
+    );
+    expect(result.success).toBe(true);
+    return maxVolumeDb(out);
+  };
+  const writeTone = (id: string, peak: number): void =>
+    writeWav(join(dir, `${id}.wav`), steady(peak, 300), SR, 1, true);
+
+  it("applies the root's data-fx-chain once over the sum, from authored HTML", async () => {
+    // Two in-phase 0.3 tones sum to -4.4 dBFS. A -6 dBTP limiter then a -6 dB gain
+    // land that at -12. Per clip, neither clip reaches the ceiling and the result is
+    // -10.4; applied twice it is -18. A linear chain alone could not tell these apart.
+    writeTone("a", 0.3);
+    writeTone("b", 0.3);
+    const master = chainJson(
+      { type: "truepeak", params: { ceiling: -6, lookahead: 3, release: 80 } },
+      { type: "gain", params: { gain: -6 } },
+    );
+    const clip = (id: string) =>
+      `<audio id="${id}" src="${id}.wav" data-start="0" data-end="1" data-duration="1"></audio>`;
+    const html = `<div data-composition-id="main" data-fx-chain='${master}'>${clip("a")}${clip("b")}</div>`;
+    const elements = parseAudioElements(html);
+    expect(elements).toHaveLength(2);
+    expect(await mixedPeakDb("once", elements, parseMasterFxChain(html))).toBeCloseTo(-12, 0);
+  }, 180_000);
+
+  it("keeps the sum in float until the chain has read it", async () => {
+    // The sum is +5 dBFS. A -6 dB master gain lands it at -1 if the sum is float; a sum
+    // clipped to 16 bits at full scale first would land at -6.
+    writeTone("a", 0.9);
+    writeTone("b", 0.9);
+    const masterFxChain = chainJson({ type: "gain", params: { gain: -6 } });
+    const peak = await mixedPeakDb("float", [element("a"), element("b")], masterFxChain);
+    expect(peak).toBeGreaterThan(-2);
+  }, 180_000);
+
+  it("runs after each group's own chain and fader", async () => {
+    // The group limits its 0.5 tone to -20 dBFS, then its fader takes 6 dB off: -26. A
+    // +12 dB master gain after both lands at -14. A fader ahead of the limiter would
+    // leave -20 and end at -8; a master ahead of the group would be limited away at -26.
+    writeTone("g", 0.5);
+    const group = {
+      groupId: "bus",
+      groupFxChain: chainJson({
+        type: "truepeak",
+        params: { ceiling: -20, lookahead: 3, release: 80 },
+      }),
+      groupVolume: 0.5,
+    };
+    const masterFxChain = chainJson({ type: "gain", params: { gain: 12 } });
+    expect(await mixedPeakDb("ordered", [element("g", group)], masterFxChain)).toBeCloseTo(-14, 0);
+  }, 180_000);
+});
diff --git a/packages/engine/src/services/audioMixer.test.ts b/packages/engine/src/services/audioMixer.test.ts
index af39c210e2..612e69ebe0 100644
--- a/packages/engine/src/services/audioMixer.test.ts
+++ b/packages/engine/src/services/audioMixer.test.ts
@@ -1,6 +1,6 @@
 // fallow-ignore-file code-duplication
 import { afterEach, describe, expect, it, vi } from "vitest";
-import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { existsSync, mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
 
@@ -66,6 +66,7 @@ vi.mock("../utils/ffprobe.js", async (importOriginal) => {
 });
 
 import { RATE_RANGE } from "@hyperframes/core/audio-automation";
+import { AudioFxRenderError } from "./audioFxRender.js";
 import { parseAudioElements, processCompositionAudio } from "./audioMixer.js";
 
 describe("parseAudioElements strict literal timing", () => {
@@ -105,6 +106,10 @@ describe("parseAudioElements — <source> children", () => {
 });
 describe("processCompositionAudio", () => {
   const tempDirs: string[] = [];
+  const gainChain = JSON.stringify({
+    version: 1,
+    nodes: [{ type: "gain", id: "g", params: { gain: -3 } }],
+  });
 
   afterEach(() => {
     vi.unstubAllGlobals();
@@ -580,6 +585,42 @@ describe("processCompositionAudio", () => {
     });
   });
 
+  it.each([
+    ["the master pass", {}, { masterFxChain: gainChain }],
+    ["a group bus", { groupId: "bus", groupFxChain: gainChain }, {}],
+  ])("removes its work directory when %s fails fatally", async (_label, extra, config) => {
+    const baseDir = mkdtempSync(join(tmpdir(), "hf-audio-base-"));
+    const workDir = mkdtempSync(join(tmpdir(), "hf-audio-work-"));
+    tempDirs.push(baseDir, workDir);
+    writeFileSync(join(baseDir, "bed.wav"), "stub");
+    applyAudioFxChainMock.mockRejectedValueOnce(new AudioFxRenderError("browser would not start"));
+
+    await expect(
+      processCompositionAudio(
+        [
+          {
+            id: "bed",
+            src: "bed.wav",
+            start: 0,
+            end: 2,
+            mediaStart: 0,
+            layer: 0,
+            volume: 1,
+            type: "audio",
+            ...extra,
+          },
+        ],
+        baseDir,
+        workDir,
+        join(baseDir, "out.m4a"),
+        2,
+        undefined,
+        config,
+      ),
+    ).rejects.toThrow(AudioFxRenderError);
+    expect(existsSync(workDir)).toBe(false);
+  });
+
   it("lets an FX tail run past the clip, still bounded by the composition", async () => {
     // A reverb is still decaying when the clip's own audio stops. Trimming at
     // the clip boundary is what cut every tail short in the render.
diff --git a/packages/engine/src/services/audioMixer.ts b/packages/engine/src/services/audioMixer.ts
index cdbdaf63e4..7937bf51c7 100644
--- a/packages/engine/src/services/audioMixer.ts
+++ b/packages/engine/src/services/audioMixer.ts
@@ -27,7 +27,11 @@ import type {
   MixResult,
 } from "./audioMixer.types.js";
 import { applyVolumeEnvelopeToWav } from "./audioVolumeEnvelope.js";
-import { HF_AUDIO_FX_ATTR, parseAudioFxChain } from "@hyperframes/core/audio-fx";
+import {
+  HF_AUDIO_FX_ATTR,
+  parseAudioFxChain,
+  type HfAudioFxChain,
+} from "@hyperframes/core/audio-fx";
 import {
   HF_AUDIO_AUTOMATION_ATTR,
   parseAutomation,
@@ -49,7 +53,7 @@ import {
   timeAtSourceTime,
   type RateSpec,
 } from "@hyperframes/core";
-import { resolveAudioGroups } from "@hyperframes/core/audio-groups";
+import { readMasterFxChain, resolveAudioGroups } from "@hyperframes/core/audio-groups";
 import { applyAudioFxChain, AudioFxRenderError } from "./audioFxRender.js";
 import type { AudioVolumeKeyframe } from "./audioMixer.types.js";
 
@@ -518,6 +522,15 @@ export function volumeLaneKeyframes(
   return out;
 }
 
+/**
+ * The top-level root's `data-fx-chain`. It belongs to the composition, not to a clip,
+ * so `processCompositionAudio` takes it beside the tracks: a track the render adds
+ * itself has no parsed element to carry it.
+ */
+export function parseMasterFxChain(html: string): string | undefined {
+  return readMasterFxChain(parseHTML(unwrapTemplate(html)).document);
+}
+
 export function parseAudioElements(html: string): AudioElement[] {
   const elements: AudioElement[] = [];
   const { document } = parseHTML(unwrapTemplate(html));
@@ -744,6 +757,7 @@ async function mixAudioTracks(
   totalDuration: number,
   signal?: AbortSignal,
   config?: Partial<Pick<EngineConfig, "ffmpegProcessTimeout" | "audioGain">>,
+  floatWav = false,
 ): Promise<MixResult> {
   const ffmpegProcessTimeout = config?.ffmpegProcessTimeout ?? DEFAULT_CONFIG.ffmpegProcessTimeout;
   const masterOutputGain = config?.audioGain ?? DEFAULT_CONFIG.audioGain;
@@ -807,10 +821,7 @@ async function mixAudioTracks(
       scriptPath,
       "-map",
       "[out]",
-      "-acodec",
-      "aac",
-      "-b:a",
-      "192k",
+      ...(floatWav ? ["-acodec", "pcm_f32le", "-ar", "48000"] : ["-acodec", "aac", "-b:a", "192k"]),
       "-t",
       String(totalDuration),
       "-y",
@@ -1027,6 +1038,56 @@ async function mixGroupMembers(
   return { success: true, degradedAutomation };
 }
 
+/**
+ * Sum the tracks to a float WAV, run the master chain over the sum, then AAC-encode.
+ * Groups are already folded into `tracks`, and the sum stays float until the encoder.
+ */
+async function mixThroughMasterBus(
+  tracks: AudioTrack[],
+  masterChain: HfAudioFxChain,
+  outputPath: string,
+  workDir: string,
+  totalDuration: number,
+  signal?: AbortSignal,
+  config?: Partial<Pick<EngineConfig, "ffmpegProcessTimeout" | "audioGain">>,
+): Promise<MixResult> {
+  const sumPath = join(workDir, "master-sum.wav");
+  const sum = await mixAudioTracks(tracks, sumPath, totalDuration, signal, config, true);
+  if (!sum.success) return { ...sum, outputPath };
+
+  const fx = await applyAudioFxChain(sumPath, masterChain, join(workDir, "master-fx.wav"), {
+    trackId: "master",
+    signal,
+  });
+  const encoded = await runFfmpeg(
+    [
+      "-i",
+      fx.path,
+      "-t",
+      String(totalDuration),
+      "-acodec",
+      "aac",
+      "-b:a",
+      "192k",
+      "-y",
+      outputPath,
+    ],
+    { signal, timeout: config?.ffmpegProcessTimeout ?? DEFAULT_CONFIG.ffmpegProcessTimeout },
+  );
+  if (!encoded.success) {
+    const failure = ffmpegFailure("mix", encoded);
+    return {
+      success: false,
+      outputPath,
+      durationMs: sum.durationMs + encoded.durationMs,
+      tracksProcessed: 0,
+      error: failure.detail,
+      failures: [failure],
+    };
+  }
+  return { ...sum, outputPath, durationMs: sum.durationMs + encoded.durationMs };
+}
+
 export async function processCompositionAudio(
   elements: AudioElement[],
   baseDir: string,
@@ -1034,7 +1095,10 @@ export async function processCompositionAudio(
   outputPath: string,
   totalDuration: number,
   signal?: AbortSignal,
-  config?: Partial<Pick<EngineConfig, "ffmpegProcessTimeout" | "audioGain">>,
+  config?: Partial<Pick<EngineConfig, "ffmpegProcessTimeout" | "audioGain">> & {
+    /** Serialised master bus chain, from `parseMasterFxChain`. */
+    masterFxChain?: string;
+  },
   compiledDir?: string,
 ): Promise<MixResult> {
   const startMs = Date.now();
@@ -1356,12 +1420,15 @@ export async function processCompositionAudio(
   // The producer only surfaces audio failures when `success` is false; mixing
   // the remaining tracks made the omitted cue indistinguishable from a valid
   // render unless someone manually audited that exact audio window.
-  const bail = (): MixResult => {
+  const removeWorkDir = (): void => {
     try {
       rmSync(workDir, { recursive: true, force: true });
     } catch {
       /* ignore */
     }
+  };
+  const bail = (): MixResult => {
+    removeWorkDir();
     return {
       success: false,
       outputPath,
@@ -1375,6 +1442,28 @@ export async function processCompositionAudio(
   };
   if (failures.length > 0) return bail();
 
+  // Parsed before any group is mixed, so a malformed master chain fails fast and
+  // as a failure, the way an unreadable group chain does.
+  let masterChain: HfAudioFxChain | undefined;
+  const masterFxJson = config?.masterFxChain;
+  if (masterFxJson) {
+    try {
+      masterChain = parseAudioFxChain(masterFxJson);
+    } catch (err: unknown) {
+      failures.push({
+        stage: "mix",
+        reason: "internal",
+        owner: "user",
+        retryable: false,
+        elementId: "master",
+        detail: boundedDetail(
+          `Master bus chain is unreadable: ${err instanceof Error ? err.message : String(err)}`,
+        ),
+      });
+      return bail();
+    }
+  }
+
   // Sub-mix each group into one processed bus (design doc §1.3), then fold it
   // into the flat track list as a single AudioTrack — everything downstream
   // (the final mixAudioTracks call) never has to know groups exist. Group
@@ -1479,7 +1568,10 @@ export async function processCompositionAudio(
         ...(bakedEnvelope || !laneKeyframes?.length ? {} : { volumeKeyframes: laneKeyframes }),
       });
     } catch (err: unknown) {
-      if (err instanceof AudioFxRenderError) throw err;
+      if (err instanceof AudioFxRenderError) {
+        removeWorkDir();
+        throw err;
+      }
       failures.push({
         stage: "mix",
         reason: "internal",
@@ -1496,12 +1588,22 @@ export async function processCompositionAudio(
   }
   if (failures.length > 0) return bail();
 
-  const mixResult = await mixAudioTracks(tracks, outputPath, totalDuration, signal, config);
-
+  let mixResult: MixResult;
   try {
-    rmSync(workDir, { recursive: true, force: true });
-  } catch {
-    /* ignore */
+    mixResult =
+      masterChain && tracks.length > 0
+        ? await mixThroughMasterBus(
+            tracks,
+            masterChain,
+            outputPath,
+            workDir,
+            totalDuration,
+            signal,
+            config,
+          )
+        : await mixAudioTracks(tracks, outputPath, totalDuration, signal, config);
+  } finally {
+    removeWorkDir();
   }
 
   // A group whose sub-mix had to drop member automation reports it the same
diff --git a/packages/producer/scripts/test-classification.mjs b/packages/producer/scripts/test-classification.mjs
index 3c3969bca9..2aa4f1673b 100644
--- a/packages/producer/scripts/test-classification.mjs
+++ b/packages/producer/scripts/test-classification.mjs
@@ -18,6 +18,7 @@ const INTEGRATION_TEST_FILES = new Set([
   "src/services/distributed/assemble.test.ts",
   "src/services/distributed/chunkBoundary.test.ts",
   "src/services/distributed/crossWorkerIdempotency.test.ts",
+  "src/services/distributed/plan.masterBus.test.ts",
   "src/services/distributed/plan.test.ts",
   "src/services/distributed/planSizeCap.test.ts",
   "src/services/distributed/renderChunk.test.ts",
diff --git a/packages/producer/src/services/distributed/plan.masterBus.test.ts b/packages/producer/src/services/distributed/plan.masterBus.test.ts
new file mode 100644
index 0000000000..1fe95cda02
--- /dev/null
+++ b/packages/producer/src/services/distributed/plan.masterBus.test.ts
@@ -0,0 +1,50 @@
+import { afterEach, beforeEach, expect, it, vi } from "vitest";
+import { mkdirSync, mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { plan } from "./plan.js";
+
+const MASTER_CHAIN = '{"version":1,"nodes":[]}';
+const seen = vi.hoisted((): { audioInput?: { masterFxChain?: string } } => ({}));
+
+vi.mock("../render/stages/audioStage.js", () => ({
+  runAudioStage: async (input: { masterFxChain?: string; workDir: string }) => {
+    seen.audioInput = input;
+    return {
+      audioOutputPath: `${input.workDir}/audio.m4a`,
+      hasAudio: false,
+      audioProcessMs: 0,
+    };
+  },
+}));
+
+let runRoot: string;
+
+beforeEach(() => {
+  runRoot = mkdtempSync(join(tmpdir(), "hf-plan-master-bus-"));
+});
+
+afterEach(() => {
+  rmSync(runRoot, { recursive: true, force: true });
+});
+
+it("plan() gives the audio stage the composition's master chain", async () => {
+  const projectDir = join(runRoot, "project");
+  mkdirSync(projectDir, { recursive: true });
+  writeFileSync(
+    join(projectDir, "index.html"),
+    `<!doctype html>
+<html><head><meta charset="utf-8"></head><body>
+  <div data-composition-id="root" data-width="320" data-height="240" data-duration="1" data-fx-chain='${MASTER_CHAIN}'>
+    <p>master bus plan fixture</p>
+  </div>
+</body></html>`,
+    "utf-8",
+  );
+  const planDir = join(runRoot, "plan");
+  mkdirSync(planDir, { recursive: true });
+
+  await plan(projectDir, { fps: 30, width: 320, height: 240, format: "mp4" }, planDir);
+
+  expect(seen.audioInput?.masterFxChain).toBe(MASTER_CHAIN);
+});
diff --git a/packages/producer/src/services/distributed/plan.ts b/packages/producer/src/services/distributed/plan.ts
index e776e269e7..d6c5e1408c 100644
--- a/packages/producer/src/services/distributed/plan.ts
+++ b/packages/producer/src/services/distributed/plan.ts
@@ -1130,6 +1130,7 @@ export async function buildLocalExecutionPlan(
     ffmpegProcessTimeout: cfg.ffmpegProcessTimeout,
     audioGain: cfg.audioGain,
     audios: composition.audios,
+    masterFxChain: composition.masterFxChain,
     abortSignal,
     assertNotAborted,
   });
diff --git a/packages/producer/src/services/htmlCompiler.test.ts b/packages/producer/src/services/htmlCompiler.test.ts
index 0e39d2aeeb..85050835b8 100644
--- a/packages/producer/src/services/htmlCompiler.test.ts
+++ b/packages/producer/src/services/htmlCompiler.test.ts
@@ -744,6 +744,25 @@ describe("detectRenderModeHints", () => {
     expect(document.querySelector("#root")?.getAttribute("src")).toBe("root.png");
   });
 
+  it("reads the root's data-fx-chain as the composition's master chain, apart from any clip", async () => {
+    const projectDir = mkdtempSync(join(tmpdir(), "hf-master-chain-"));
+    const chain = '{"version":1,"nodes":[]}';
+    writeFileSync(
+      join(projectDir, "index.html"),
+      `<!doctype html><html><body>
+  <div data-composition-id="main" data-width="100" data-height="100" data-duration="1" data-fx-chain='${chain}'>
+    <video id="talk" src="talk.mp4" data-start="0" data-duration="1"></video>
+  </div>
+</body></html>`,
+    );
+
+    const compiled = await compileForRender(projectDir, join(projectDir, "index.html"), projectDir);
+
+    expect(compiled.masterFxChain).toBe(chain);
+    expect(compiled.audios.map((audio) => audio.id)).toEqual(["talk-audio"]);
+    expect(compiled.audios[0]).not.toHaveProperty("fxChain");
+  });
+
   // Shared fixture builder for the assertSubCompositionsUsable / EmptyCompositionError
   // pre-flight tests below. `subCompFiles` is a map of compositions/-relative
   // filename to raw file content (empty string / malformed text / valid HTML).
diff --git a/packages/producer/src/services/htmlCompiler.ts b/packages/producer/src/services/htmlCompiler.ts
index f03f109743..1711954b43 100644
--- a/packages/producer/src/services/htmlCompiler.ts
+++ b/packages/producer/src/services/htmlCompiler.ts
@@ -79,6 +79,7 @@ import {
   analyzeKeyframeIntervals,
   assertMediaPayload,
   NotMediaPayloadError,
+  parseMasterFxChain,
   probeMediaProfile,
 } from "@hyperframes/engine";
 import {
@@ -110,6 +111,7 @@ export interface CompiledComposition {
   videos: VideoElement[];
   audios: AudioElement[];
   images: ImageElement[];
+  masterFxChain?: string;
   unresolvedCompositions: UnresolvedElement[];
   /** Assets that resolve outside projectDir. Keys are the path used in HTML, values are absolute filesystem paths. */
   externalAssets: Map<string, string>;
@@ -2146,6 +2148,7 @@ export async function compileForRender(
   // two bare `<video>`s, both auto-numbered `hf-video-0` — collapsed into one
   // entry and injected frames onto whichever element came first. See #3340.
   const { videos, audios, images } = collectRenderMedia(html);
+  const masterFxChain = parseMasterFxChain(html);
 
   // Advisory video checks (sparse keyframes, VFR). Fire-and-forget — these spawn
   // ffprobe subprocesses and should not block compilation since they only produce warnings.
@@ -2198,6 +2201,7 @@ export async function compileForRender(
     videos,
     audios,
     images,
+    ...(masterFxChain ? { masterFxChain } : {}),
     unresolvedCompositions,
     externalAssets,
     width,
diff --git a/packages/producer/src/services/render/shared.ts b/packages/producer/src/services/render/shared.ts
index 7beb08a99f..9e07ff286e 100644
--- a/packages/producer/src/services/render/shared.ts
+++ b/packages/producer/src/services/render/shared.ts
@@ -47,6 +47,7 @@ export interface CompositionMetadata {
   videos: VideoElement[];
   audios: AudioElement[];
   images: ImageElement[];
+  masterFxChain?: string;
   width: number;
   height: number;
 }
diff --git a/packages/producer/src/services/render/stages/audioStage.test.ts b/packages/producer/src/services/render/stages/audioStage.test.ts
index 14d7f7a35f..a04db699cf 100644
--- a/packages/producer/src/services/render/stages/audioStage.test.ts
+++ b/packages/producer/src/services/render/stages/audioStage.test.ts
@@ -2,7 +2,7 @@ import { afterEach, describe, expect, it, vi } from "vitest";
 import { mkdtempSync, rmSync } from "node:fs";
 import { join } from "node:path";
 import { tmpdir } from "node:os";
-import type { AudioElement } from "@hyperframes/engine";
+import { parseMasterFxChain, type AudioElement, type VideoElement } from "@hyperframes/engine";
 
 const { processCompositionAudioMock } = vi.hoisted(() => ({
   processCompositionAudioMock: vi.fn(),
@@ -14,6 +14,7 @@ vi.mock("@hyperframes/engine", async (importOriginal) => {
 });
 
 import { runAudioStage } from "./audioStage.js";
+import { appendAutoDetectedVideoAudio } from "./extractVideosStage.js";
 import { EncoderInterruptedError } from "../encoderInterruption.js";
 
 // Regression: hasAudio flipping to false used to be indistinguishable from
@@ -87,6 +88,55 @@ describe("runAudioStage", () => {
     );
   });
 
+  it("hands the master chain to the mixer when the only track is the one the render added for a plain video", async () => {
+    // A plain <video> has no audio entry at compile time; the render adds one after probing
+    // its file. That track is not parsed from HTML, so the chain cannot ride on it.
+    const chain = '{"version":1,"nodes":[]}';
+    const html = `<div data-composition-id="main" data-fx-chain='${chain}'><video id="talk" src="talk.mp4" data-start="0" data-duration="5"></video></div>`;
+    const video: VideoElement = {
+      id: "talk",
+      src: "talk.mp4",
+      start: 0,
+      end: 5,
+      mediaStart: 0,
+      loop: false,
+      hasAudio: true,
+    };
+    const composition = {
+      videos: [video],
+      audios: [] as AudioElement[],
+      masterFxChain: parseMasterFxChain(html),
+    };
+    appendAutoDetectedVideoAudio(composition, [
+      {
+        videoId: "talk",
+        metadata: { hasAudio: true },
+      } as Parameters<typeof appendAutoDetectedVideoAudio>[1][number],
+    ]);
+    expect(composition.audios.map((audio) => audio.id)).toEqual(["talk-audio"]);
+    processCompositionAudioMock.mockResolvedValue({
+      success: true,
+      outputPath: "audio.m4a",
+      durationMs: 1,
+      tracksProcessed: 1,
+    });
+
+    await runAudioStage(
+      makeInput({ audios: composition.audios, masterFxChain: composition.masterFxChain }),
+    );
+
+    expect(processCompositionAudioMock).toHaveBeenCalledWith(
+      composition.audios,
+      expect.any(String),
+      expect.any(String),
+      expect.any(String),
+      5,
+      undefined,
+      { ffmpegProcessTimeout: 3_600_000, audioGain: 1, masterFxChain: chain },
+      expect.any(String),
+    );
+  });
+
   it("falls back to a generic message when the mixer fails without an error string", async () => {
     processCompositionAudioMock.mockResolvedValue({
       success: false,
diff --git a/packages/producer/src/services/render/stages/audioStage.ts b/packages/producer/src/services/render/stages/audioStage.ts
index ff9ea7d4b8..dae085046d 100644
--- a/packages/producer/src/services/render/stages/audioStage.ts
+++ b/packages/producer/src/services/render/stages/audioStage.ts
@@ -40,6 +40,7 @@ export interface AudioStageInput {
   audioGain: number;
   /** Read-only view of `composition.audios`. */
   audios: CompositionMetadata["audios"];
+  masterFxChain?: string;
   abortSignal: AbortSignal | undefined;
   assertNotAborted: () => void;
   /** Where a per-track failure's detail goes. Optional so tests need not pass one. */
@@ -72,6 +73,7 @@ export async function runAudioStage(input: AudioStageInput): Promise<AudioStageR
     ffmpegProcessTimeout,
     audioGain,
     audios,
+    masterFxChain,
     abortSignal,
     assertNotAborted,
     log,
@@ -99,7 +101,7 @@ export async function runAudioStage(input: AudioStageInput): Promise<AudioStageR
         audioOutputPath,
         duration,
         abortSignal,
-        { ffmpegProcessTimeout, audioGain },
+        { ffmpegProcessTimeout, audioGain, masterFxChain },
         compiledDir,
       );
     } catch (err) {
diff --git a/packages/producer/src/services/render/stages/compileStage.test.ts b/packages/producer/src/services/render/stages/compileStage.test.ts
index 08cef195fd..2132a27405 100644
--- a/packages/producer/src/services/render/stages/compileStage.test.ts
+++ b/packages/producer/src/services/render/stages/compileStage.test.ts
@@ -242,6 +242,40 @@ describe("runCompileStage — forceScreenshot snapshot", () => {
   });
 });
 
+const MASTER_CHAIN = '{"version":1,"nodes":[]}';
+const MASTER_BUS_HTML = `<!doctype html>
+<html>
+<head><meta charset="utf-8"></head>
+<body>
+  <div data-composition-id="root" data-width="1920" data-height="1080" data-duration="1" data-fx-chain='${MASTER_CHAIN}'>
+    <p>master bus composition</p>
+  </div>
+</body>
+</html>`;
+
+describe("runCompileStage — master bus", () => {
+  it("hands the root's chain to the composition metadata the audio stage reads", async () => {
+    const fixture = setupFixture(MASTER_BUS_HTML);
+    try {
+      const result = await runCompileStage({
+        projectDir: join(fixture.workDir, "project"),
+        workDir: fixture.workDir,
+        htmlPath: fixture.htmlPath,
+        entryFile: "index.html",
+        job: createJob(),
+        cfg: createCfg(),
+        needsAlpha: false,
+        log: noopLog,
+        assertNotAborted: () => {},
+      });
+      expect(result.compiled.masterFxChain).toBe(MASTER_CHAIN);
+      expect(result.composition.masterFxChain).toBe(MASTER_CHAIN);
+    } finally {
+      fixture.cleanup();
+    }
+  });
+});
+
 describe("runCompileStage — font fetch cancellation", () => {
   let fixture: CompileFixture | null = null;
 
diff --git a/packages/producer/src/services/render/stages/compileStage.ts b/packages/producer/src/services/render/stages/compileStage.ts
index 30a2eab850..1ab3bd55ae 100644
--- a/packages/producer/src/services/render/stages/compileStage.ts
+++ b/packages/producer/src/services/render/stages/compileStage.ts
@@ -310,6 +310,7 @@ export async function runCompileStage(input: CompileStageInput): Promise<Compile
     videos: compiled.videos,
     audios: compiled.audios,
     images: compiled.images,
+    masterFxChain: compiled.masterFxChain,
     width: compiled.width,
     height: compiled.height,
   };
diff --git a/packages/producer/src/services/renderOrchestrator.masterBus.test.ts b/packages/producer/src/services/renderOrchestrator.masterBus.test.ts
new file mode 100644
index 0000000000..039c107c47
--- /dev/null
+++ b/packages/producer/src/services/renderOrchestrator.masterBus.test.ts
@@ -0,0 +1,87 @@
+import { afterEach, expect, it, vi } from "vitest";
+import { mkdtempSync, rmSync, writeFileSync } from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import { createRenderJob, executeRenderJob } from "./renderOrchestrator.js";
+
+const MASTER_CHAIN = '{"version":1,"nodes":[]}';
+const seen = vi.hoisted((): { audioInput?: { masterFxChain?: string } } => ({}));
+
+vi.mock("@hyperframes/engine", async (importOriginal) => ({
+  ...(await importOriginal<typeof import("@hyperframes/engine")>()),
+  assertConfiguredFfmpegBinariesExist: () => {},
+}));
+vi.mock("./render/stages/compileStage.js", () => ({
+  runCompileStage: async () => ({
+    compiled: { html: "<div></div>" },
+    composition: {
+      width: 320,
+      height: 180,
+      duration: 1,
+      videos: [],
+      images: [],
+      audios: [],
+      masterFxChain: '{"version":1,"nodes":[]}',
+    },
+    deviceScaleFactor: 1,
+    outputWidth: 320,
+    outputHeight: 180,
+    compileOnlyMs: 0,
+    forceScreenshot: true,
+  }),
+}));
+vi.mock("./render/stages/probeStage.js", () => ({
+  runProbeStage: async ({ compiled }: { compiled: object }) => ({
+    compiled,
+    fileServer: null,
+    probeSession: null,
+    lastBrowserConsole: [],
+    duration: 1,
+    totalFrames: 30,
+    browserProbeMs: 0,
+  }),
+}));
+vi.mock("./render/stages/extractVideosStage.js", async (importOriginal) => ({
+  ...(await importOriginal<typeof import("./render/stages/extractVideosStage.js")>()),
+  runExtractVideosStage: async () => ({
+    extractionResult: null,
+    frameLookup: null,
+    videoReadinessSkipIds: new Set<string>(),
+    videoMetadataHints: new Map(),
+    nativeHdrVideoIds: new Set<string>(),
+    videoTransfers: new Map(),
+    nativeHdrImageIds: new Set<string>(),
+    imageTransfers: new Map(),
+    hdrImageSrcPaths: new Map(),
+    imageColorSpaces: [],
+    videoExtractMs: 0,
+    failureToEnforce: undefined,
+  }),
+}));
+vi.mock("./render/stages/audioStage.js", () => ({
+  runAudioStage: async (input: { masterFxChain?: string }) => {
+    seen.audioInput = input;
+    throw new Error("audio stage reached");
+  },
+}));
+
+const dirs: string[] = [];
+afterEach(() => {
+  for (const dir of dirs.splice(0)) rmSync(dir, { recursive: true, force: true });
+});
+
+it("the local renderer gives the audio stage the composition's master chain", async () => {
+  const root = mkdtempSync(join(tmpdir(), "hf-master-bus-orchestrator-"));
+  dirs.push(root);
+  writeFileSync(join(root, "index.html"), "<div></div>");
+  const job = createRenderJob({
+    fps: 30,
+    quality: "standard",
+    hdrMode: "force-sdr",
+    logger: { info() {}, warn() {}, error() {}, debug() {} },
+  });
+  await expect(executeRenderJob(job, root, join(root, "output.mp4"))).rejects.toThrow(
+    "audio stage reached",
+  );
+  expect(seen.audioInput?.masterFxChain).toBe(MASTER_CHAIN);
+});
diff --git a/packages/producer/src/services/renderOrchestrator.ts b/packages/producer/src/services/renderOrchestrator.ts
index 5c9f49307e..fce6e022ab 100644
--- a/packages/producer/src/services/renderOrchestrator.ts
+++ b/packages/producer/src/services/renderOrchestrator.ts
@@ -3356,6 +3356,7 @@ async function executeRenderPipeline(input: {
           ffmpegProcessTimeout: cfg.ffmpegProcessTimeout,
           audioGain: cfg.audioGain,
           audios: composition.audios,
+          masterFxChain: composition.masterFxChain,
           abortSignal: executionSignal,
           assertNotAborted,
           log,
diff --git a/packages/studio/src/player/lib/timelineDOM.test.ts b/packages/studio/src/player/lib/timelineDOM.test.ts
index 3b5790b1e0..6e3af94ce3 100644
--- a/packages/studio/src/player/lib/timelineDOM.test.ts
+++ b/packages/studio/src/player/lib/timelineDOM.test.ts
@@ -632,6 +632,61 @@ describe("audio FX attributes on parsed elements", () => {
   });
 });
 
+describe("the master chain on parsed elements", () => {
+  const CHAIN = '{"version":1,"nodes":[{"type":"truepeak","id":"m1","params":{}}]}';
+  const parseVideo = (rootAttrs: string, inner = ""): TimelineElement | undefined => {
+    const doc = new DOMParser().parseFromString(
+      `<div data-composition-id="main" data-start="0" data-duration="10" ${rootAttrs}>
+         <video id="talk" data-start="0" data-duration="10" data-has-audio="true"></video>${inner}
+       </div>`,
+      "text/html",
+    );
+    return parseTimelineFromDOM(doc, 10).find((e) => e.domId === "talk");
+  };
+
+  it("copies the root's data-fx-chain onto an element that has none of its own", () => {
+    const talk = parseVideo(`data-fx-chain='${CHAIN}'`);
+    expect(talk?.masterChain).toBe(CHAIN);
+    expect(talk?.fxChain).toBeUndefined();
+  });
+
+  it("leaves it unset when the root carries none", () => {
+    expect(parseVideo("")?.masterChain).toBeUndefined();
+  });
+
+  it("ignores a sub-composition root's chain", () => {
+    const inner = `<div data-composition-id="inner" data-fx-chain='${CHAIN}'></div>`;
+    expect(parseVideo("", inner)?.masterChain).toBeUndefined();
+  });
+
+  it("carries it on the manifest path too", () => {
+    const doc = makeDoc(`
+      <div data-composition-id="main" data-fx-chain='${CHAIN}'>
+        <video id="talk" data-has-audio="true"></video>
+      </div>
+    `);
+    const element = createTimelineElementFromManifestClip({
+      clip: {
+        id: "talk",
+        label: "Talk",
+        kind: "video",
+        tagName: "video",
+        start: 0,
+        duration: 10,
+        track: 0,
+        compositionId: null,
+        parentCompositionId: "main",
+        compositionSrc: null,
+        assetUrl: null,
+      },
+      fallbackIndex: 0,
+      doc,
+      hostEl: doc.getElementById("talk"),
+    });
+    expect(element.masterChain).toBe(CHAIN);
+  });
+});
+
 // A composition clip trimmed by 30 px collapsed to its first nested video's 1.77 s: that length capped the trim.
 describe("a composition clip's source length", () => {
   const scene = `
diff --git a/packages/studio/src/player/lib/timelineDOM.ts b/packages/studio/src/player/lib/timelineDOM.ts
index 23b8b68ff9..1aa1ae8dbf 100644
--- a/packages/studio/src/player/lib/timelineDOM.ts
+++ b/packages/studio/src/player/lib/timelineDOM.ts
@@ -12,6 +12,7 @@ import type { TimelineElement } from "../store/playerStore";
 import type { ClipManifestClip, IframeWindow, TimelineLike } from "./playbackTypes";
 import { resolveCssStackingContextId } from "@hyperframes/core/runtime/stacking-context";
 import { readClipTiming } from "@hyperframes/core/composition-contract";
+import { readMasterFxChain } from "@hyperframes/core/audio-groups";
 import { linkScopeOf } from "@hyperframes/core/media-link";
 import { createRuntimeStartTimeResolver } from "@hyperframes/core/runtime/start-resolver";
 import { groupInfoFor } from "./timelineGroupInfo";
@@ -167,6 +168,8 @@ export function createTimelineElementFromManifestClip(params: {
       if (info.fxChain) entry.audioGroupFxChain = info.fxChain;
       if (info.automation) entry.audioGroupAutomation = info.automation;
     }
+    const masterChain = readMasterFxChain(doc ?? hostEl.ownerDocument);
+    if (masterChain) entry.masterChain = masterChain;
     const fxChain = hostEl.getAttribute("data-fx-chain");
     if (fxChain) entry.fxChain = fxChain;
     const automation = hostEl.getAttribute("data-automation");
@@ -330,6 +333,8 @@ export function parseTimelineFromDOM(
     // Read from the element, like the manifest path does: without these an audio
     // clip parsed straight from the DOM reserved no automation height and drew no
     // lanes, while the property panel still showed its chain.
+    const domMasterChain = readMasterFxChain(doc);
+    if (domMasterChain) entry.masterChain = domMasterChain;
     const domFxChain = el.getAttribute("data-fx-chain");
     if (domFxChain) entry.fxChain = domFxChain;
     const domAutomation = el.getAttribute("data-automation");
diff --git a/packages/studio/src/player/store/timelineElement.ts b/packages/studio/src/player/store/timelineElement.ts
index 459dd08025..68f7d1940f 100644
--- a/packages/studio/src/player/store/timelineElement.ts
+++ b/packages/studio/src/player/store/timelineElement.ts
@@ -62,9 +62,10 @@ export interface TimelineElement {
   /** Clip-edge fades from `data-fade-in` / `data-fade-out`, seconds; absent means none. */
   fadeIn?: number;
   fadeOut?: number;
-  /** Verbatim `data-fx-chain` / `data-automation`; see automationLaneData. */
+  /** Verbatim `data-fx-chain` / `data-automation` (`masterChain`: the root's); see automationLaneData. */
   fxChain?: string;
   automation?: string;
+  masterChain?: string;
   /** Path from data-composition-src — identifies sub-composition elements */
   compositionSrc?: string;
   /** Set by data-timeline-locked on the host element — disables move and trim in Studio. */
diff --git a/packages/studio/src/utils/audioMeterMath.test.ts b/packages/studio/src/utils/audioMeterMath.test.ts
index b5fbfb2518..530efda348 100644
--- a/packages/studio/src/utils/audioMeterMath.test.ts
+++ b/packages/studio/src/utils/audioMeterMath.test.ts
@@ -69,6 +69,15 @@ describe("hasProjectAudio", () => {
     expect(hasProjectAudio([clip({ tag: "video", hasAudio: true, ...fields })])).toBe(true);
   });
 
+  it("counts an audible video when the composition root carries a master chain", () => {
+    const masterChain = '{"version":1,"nodes":[]}';
+    expect(hasProjectAudio([clip({ tag: "video", hasAudio: true, masterChain })])).toBe(true);
+    expect(
+      hasProjectAudio([clip({ tag: "video", muted: true, hasAudio: true, masterChain })]),
+    ).toBe(false);
+    expect(hasProjectAudio([clip({ tag: "video", masterChain })])).toBe(false);
+  });
+
   it("ignores a muted video even when it carries processing", () => {
     expect(hasProjectAudio([clip({ tag: "video", hasAudio: true, muted: true, volume: 2 })])).toBe(
       false,
diff --git a/scripts/test-reachability.json b/scripts/test-reachability.json
index 9ac2e6c265..86c7aae277 100644
--- a/scripts/test-reachability.json
+++ b/scripts/test-reachability.json
@@ -61,7 +61,7 @@
       ],
       "sources": {
         "packages/producer/scripts/run-test-lane.mjs": "638885e14480d03290803389325a655f3749442167d784e07bb3af3404598af7",
-        "packages/producer/scripts/test-classification.mjs": "5cf0848da08f0fd2c40521d1556cfdb00b5a072cbd0ea0bc54945990abb709e4",
+        "packages/producer/scripts/test-classification.mjs": "cdfa4035d0e2c0b1fea4d4c5b4697e0287d48c4474ab105f5e3d1ce839cca500",
         ".github/workflows/ci.yml#job#producer-source-tests": "45da72aa6cf08cdfd5bcf5311e89f12af4842f9eebc6735df19da8c02f4d4864",
         "packages/producer/package.json#script#test:unit": "ae6beda8319d166774be958ea5593cd27dbf7f6c9bce1d787a3fe28dfc3477b1",
         "packages/producer/package.json#script#test:integration": "008589f37d026877595a13501cbb66abd5cbe50e21fdcac6904f45466c3f475d",
diff --git a/skills-manifest.json b/skills-manifest.json
index 228c680a78..b6ab8e2aac 100644
--- a/skills-manifest.json
+++ b/skills-manifest.json
@@ -26,7 +26,7 @@
       "files": 122
     },
     "hyperframes-audio": {
-      "hash": "5dce20c2722a7af2",
+      "hash": "780c9d4d8d23ab40",
       "files": 8
     },
     "hyperframes-cli": {
diff --git a/skills/hyperframes-audio/SKILL.md b/skills/hyperframes-audio/SKILL.md
index a560e7baa6..1f01987a13 100644
--- a/skills/hyperframes-audio/SKILL.md
+++ b/skills/hyperframes-audio/SKILL.md
@@ -197,9 +197,9 @@ for rumble; a low-pass darkens or muffles deliberately.
 
 **Dynamics** (`gain`, `compressor`, `limiter`, `truepeak`, `gate`) decide how a track's level
 behaves over time. Compression narrows the distance between loud and quiet so the
-quiet parts can come up. A limiter is a ceiling — it does not shape anything, it
-guarantees nothing gets past. `limiter` follows the level, so for a delivery
-ceiling use `truepeak`, which looks ahead and measures the inter-sample peak. It
+quiet parts can come up. A limiter pulls peaks down toward a ceiling and does not
+shape anything else. `limiter` follows the level and has no lookahead, so a fast
+or inter-sample peak can pass its ceiling; for a delivery ceiling use `truepeak`, which looks ahead and measures the inter-sample peak. It
 holds a 4x estimate at the ceiling, not the waveform itself, so full-band noise can
 end up to 1.7 dB over: under a hard delivery limit set the ceiling at least 2 dB
 below it. A gate removes what is below a threshold, which is
@@ -344,6 +344,13 @@ of, so only the filters survive — and a bus and its members are one signal pat
 so the bed then runs through the bus's filters AND its own. The
 `audio_group_carve_attr` lint rule catches it.
 
+**One effect across the whole mix: the master bus.** A clip names exactly one
+group, so a limiter or EQ over everything cannot be a group. Put it in
+`data-fx-chain` on the top-level composition root: it runs once over the sum of all clips and groups, after each
+group's chain and fader. A delivery ceiling is the usual use — `truepeak` last in
+the master chain. Only the top-level root is read as the master bus; a
+sub-composition root's `data-fx-chain` is not applied inside a parent composition. No master automation.
+
 **One clip is not a bus.** A group exists to give several tracks one chain, one
 fader and one clock. Wrapping a single clip in a bus buys nothing the clip's own
 `data-fx-chain` does not already do, and it doubles the places a later edit has
diff --git a/skills/hyperframes-audio/references/attributes.md b/skills/hyperframes-audio/references/attributes.md
index 8be1d40f21..c2056ab23e 100644
--- a/skills/hyperframes-audio/references/attributes.md
+++ b/skills/hyperframes-audio/references/attributes.md
@@ -110,6 +110,15 @@ cannot move: worklet processor options, a WaveShaper curve and a convolution
 impulse are all set wholesale. `fx-registry.md` marks each parameter; the
 worklet effects (`compressor`, `limiter`, `truepeak`, `gate`, `bitcrush`) have none at all.
 
+## `data-fx-chain` on the root — the master bus
+
+The `data-fx-chain` JSON on the top-level composition root (the element with
+`data-composition-id` that no other composition contains) is the master bus. It
+runs once over the sum of every clip and group, after each group's own chain and
+fader. On a clip or a group `data-fx-chain` keeps its own meaning, and on a
+sub-composition root inside a parent composition it is not applied. The master bus has no
+automation lanes.
+
 ## `data-fx-carve` — the carve's settings
 
 ```json