diff --git a/packages/core/scripts/test-hyperframe-runtime-duration-guards.ts b/packages/core/scripts/test-hyperframe-runtime-duration-guards.ts
index 9d68a5d35b..a1555aec97 100644
--- a/packages/core/scripts/test-hyperframe-runtime-duration-guards.ts
+++ b/packages/core/scripts/test-hyperframe-runtime-duration-guards.ts
@@ -25,7 +25,7 @@ assert(
   "init.ts should not hard-clamp safe duration to media floor",
 );
 assert(
-  initSource.includes("resolveMediaWindowDurationSeconds"),
+  initSource.includes("longestMediaWindow"),
   "init.ts should compute media window duration across timed media",
 );
 
diff --git a/packages/core/src/runtime/compositionLength.test.ts b/packages/core/src/runtime/compositionLength.test.ts
index 57ec301c3a..1a53370ff6 100644
--- a/packages/core/src/runtime/compositionLength.test.ts
+++ b/packages/core/src/runtime/compositionLength.test.ts
@@ -4,7 +4,9 @@ import {
   findRootCompositionElement,
   LOOP_INFLATED_TIMELINE_SECONDS,
   MIN_VALID_TIMELINE_DURATION_SECONDS,
+  longestMediaWindow,
   readStaticCompositionMeta,
+  resolveCompositionLength,
   resolveCompositionLengthSeconds,
   resolveContentDerivedDuration,
   resolveMediaWindowDurationSeconds,
@@ -20,6 +22,20 @@ describe("resolveCompositionLengthSeconds", () => {
     derived: () => 0,
   };
 
+  it("says which input set the length", () => {
+    expect(resolveCompositionLength({ ...base, declared: 9 }).from).toBe("declared");
+    expect(resolveCompositionLength({ ...base, timeline: () => 10, floors: () => [4] }).from).toBe(
+      "timeline",
+    );
+    expect(resolveCompositionLength({ ...base, floors: () => [null, 7] })).toEqual({
+      seconds: 7,
+      from: "floor",
+      floorIndex: 1,
+    });
+    expect(resolveCompositionLength({ ...base, fallback: 5 }).from).toBe("fallback");
+    expect(resolveCompositionLength({ ...base, derived: () => 2 }).from).toBe("derived");
+  });
+
   it("takes a declared length and reads nothing else", () => {
     const timeline = vi.fn(() => 10);
     const floors = vi.fn(() => [12]);
@@ -147,6 +163,34 @@ describe("rules moved unchanged from the runtime", () => {
     expect(readStaticCompositionMeta(parseHTMLContent(html))?.durationSeconds).toBe(3);
   });
 
+  it("names the clip missing its trim when its full media length sets the film's length", () => {
+    const doc = parseHTMLContent(
+      `<div data-composition-id="main">` +
+        `<video id="cut" data-start="0" data-duration="3" src="cut.mp4"></video>` +
+        `<video id="source" data-start="2" src="full-source.mp4"></video></div>`,
+    );
+    const natural: Record<string, number> = { cut: 3600, source: 5400 };
+    const media = {
+      mediaStart: (el: HTMLMediaElement) => Number(el.getAttribute("data-start")),
+      mediaDuration: (el: HTMLMediaElement) =>
+        el.hasAttribute("data-duration")
+          ? Number(el.getAttribute("data-duration"))
+          : natural[el.id]!,
+    };
+    const longest = longestMediaWindow(doc, media);
+    expect(longest?.element.id).toBe("source");
+    expect(longest?.seconds).toBe(5402);
+    expect(
+      resolveCompositionLength({
+        declared: null,
+        timeline: () => 3,
+        floors: () => [longest?.seconds ?? null, 3],
+        fallback: 0,
+        derived: () => 0,
+      }),
+    ).toEqual({ seconds: 5402, from: "floor", floorIndex: 0 });
+  });
+
   it("skips a media clip of one frame or less", () => {
     const doc = parseHTMLContent(
       `<div data-composition-id="main"><video data-start="5"></video></div>`,
diff --git a/packages/core/src/runtime/compositionLength.ts b/packages/core/src/runtime/compositionLength.ts
index cd8a14c1cc..975829c3d8 100644
--- a/packages/core/src/runtime/compositionLength.ts
+++ b/packages/core/src/runtime/compositionLength.ts
@@ -34,15 +34,17 @@ export function readCompositionSize(root: Element): {
   };
 }
 
-/** The latest end among the document's timed video and audio, at their absolute starts. */
-export function resolveMediaWindowDurationSeconds(
+type MediaTiming = {
+  mediaStart: (el: HTMLMediaElement) => number;
+  mediaDuration: (el: HTMLMediaElement) => number | null;
+};
+
+/** The timed `<video>`/`<audio>` whose window ends last, and where it ends. */
+export function longestMediaWindow(
   doc: ParentNode,
-  media: {
-    mediaStart: (el: HTMLMediaElement) => number;
-    mediaDuration: (el: HTMLMediaElement) => number | null;
-  },
-): number | null {
-  const clipEnds: number[] = [];
+  media: MediaTiming,
+): { seconds: number; element: HTMLMediaElement } | null {
+  let longest: { seconds: number; element: HTMLMediaElement } | null = null;
   for (const node of doc.querySelectorAll<HTMLMediaElement>(
     "video[data-start], audio[data-start]",
   )) {
@@ -50,12 +52,17 @@ export function resolveMediaWindowDurationSeconds(
     if (!Number.isFinite(start)) continue;
     const duration = media.mediaDuration(node);
     if (duration == null || duration <= MIN_VALID_TIMELINE_DURATION_SECONDS) continue;
-    clipEnds.push(Math.max(0, start) + duration);
+    const end = Math.max(0, start) + duration;
+    if (!longest || end > longest.seconds) longest = { seconds: end, element: node };
   }
-  return aboveOneFrame(
-    resolveCompositionDuration({ authoredDurationSeconds: null, clipEndsSeconds: clipEnds })
-      .seconds,
-  );
+  return longest && aboveOneFrame(longest.seconds) !== null ? longest : null;
+}
+
+export function resolveMediaWindowDurationSeconds(
+  doc: ParentNode,
+  media: MediaTiming,
+): number | null {
+  return longestMediaWindow(doc, media)?.seconds ?? null;
 }
 
 /** The latest of the root's declared length and its direct sub-compositions' ends. */
@@ -118,14 +125,30 @@ export type CompositionLengthInputs = {
   derived: () => number;
 };
 
-/** A declared root length is the length; else the longest of timeline, floors and fallback (a
- *  loop-inflated timeline yields to a floor or fallback); else the length derived from the clips. */
+export type CompositionLengthFrom = "declared" | "timeline" | "floor" | "fallback" | "derived";
+
+export type CompositionLengthSource =
+  | { from: CompositionLengthFrom }
+  | { from: "media"; tag: string; src: string; trimmed: boolean };
+
 export function resolveCompositionLengthSeconds(input: CompositionLengthInputs): number {
+  return resolveCompositionLength(input).seconds;
+}
+
+/** A declared root length is the length; else the longest of timeline, floors and fallback (a
+ *  loop-inflated timeline yields to a floor or fallback); else the length derived from the clips.
+ *  `from` names the input that set it, and `floorIndex` which floor when a floor did. */
+export function resolveCompositionLength(input: CompositionLengthInputs): {
+  seconds: number;
+  from: CompositionLengthFrom;
+  floorIndex: number | null;
+} {
   if (input.declared !== null && Number.isFinite(input.declared) && input.declared > 0) {
-    return input.declared;
+    return { seconds: input.declared, from: "declared", floorIndex: null };
   }
   const rawTimeline = aboveOneFrame(input.timeline());
-  const floor = Math.max(0, ...input.floors().map((seconds) => seconds ?? 0));
+  const floors = input.floors().map((seconds) => seconds ?? 0);
+  const floor = Math.max(0, ...floors);
   const fallback =
     Number.isFinite(input.fallback) && input.fallback > MIN_VALID_TIMELINE_DURATION_SECONDS
       ? input.fallback
@@ -136,12 +159,21 @@ export function resolveCompositionLengthSeconds(input: CompositionLengthInputs):
     rawTimeline >= LOOP_INFLATED_TIMELINE_SECONDS &&
     aboveOneFrame(floorOrFallback) !== null;
   const timeline = loopInflated ? null : rawTimeline;
-  let seconds: number;
-  if (timeline !== null) seconds = Math.max(timeline, floorOrFallback);
-  else if (aboveOneFrame(floor) !== null) seconds = floorOrFallback;
-  else if (fallback > 0) seconds = fallback;
-  else seconds = input.derived();
-  return seconds > 0 ? seconds : 0;
+  const floorWins = floor >= fallback;
+  const fromFloorOrFallback = (seconds: number) => ({
+    seconds,
+    from: floorWins ? ("floor" as const) : ("fallback" as const),
+    floorIndex: floorWins ? floors.indexOf(floor) : null,
+  });
+  if (timeline !== null) {
+    return timeline >= floorOrFallback
+      ? { seconds: timeline, from: "timeline", floorIndex: null }
+      : fromFloorOrFallback(floorOrFallback);
+  }
+  if (aboveOneFrame(floor) !== null) return fromFloorOrFallback(floorOrFallback);
+  if (fallback > 0) return { seconds: fallback, from: "fallback", floorIndex: null };
+  const derived = input.derived();
+  return { seconds: derived > 0 ? derived : 0, from: "derived", floorIndex: null };
 }
 
 /** Size, frame rate and length of a composition nothing is running (a server-parsed or detached
diff --git a/packages/core/src/runtime/init.ts b/packages/core/src/runtime/init.ts
index bfa60cac64..a93258938a 100644
--- a/packages/core/src/runtime/init.ts
+++ b/packages/core/src/runtime/init.ts
@@ -127,9 +127,10 @@ import {
   MIN_VALID_TIMELINE_DURATION_SECONDS,
   readCompositionSize,
   resolveAuthoredCompositionFloorSeconds,
-  resolveCompositionLengthSeconds,
+  longestMediaWindow as longestMediaWindowIn,
+  resolveCompositionLength,
+  type CompositionLengthFrom,
   resolveContentDerivedDuration as resolveContentDerivedDurationFor,
-  resolveMediaWindowDurationSeconds as resolveMediaWindowDurationSecondsIn,
 } from "./compositionLength";
 import { createRuntimeStartTimeResolver } from "./startResolver";
 import { createClipTree } from "./clipTree";
@@ -1170,7 +1171,7 @@ export function initSandboxRuntimeModular(): void {
   // `getInferredDurationSeconds()` (third-party runtimes). Foreign code inside
   // a cache scope can touch the DOM between two resolves. This loop is DOM
   // reads only, so nothing can change under it.
-  const resolveMediaWindowDurationSeconds = (): number | null => {
+  const longestMediaWindow = (): ReturnType<typeof longestMediaWindowIn> => {
     const mediaNodes = Array.from(
       document.querySelectorAll("video[data-start], audio[data-start]"),
     ) as HTMLMediaElement[];
@@ -1178,7 +1179,7 @@ export function initSandboxRuntimeModular(): void {
     // this every tick, and it should not pay for a resolver it never uses.
     if (mediaNodes.length === 0) return null;
     return withTimingResolver(() =>
-      resolveMediaWindowDurationSecondsIn(document, {
+      longestMediaWindowIn(document, {
         mediaStart: resolveAbsoluteMediaStartSeconds,
         mediaDuration: resolveMediaElementDurationSeconds,
       }),
@@ -1237,6 +1238,19 @@ export function initSandboxRuntimeModular(): void {
     return contentDerivedCache.result;
   };
 
+  /** Which input set the composition's length; a media clip is named by tag, src and whether it is trimmed. */
+  const publishLengthSource = (from: CompositionLengthFrom, media: HTMLMediaElement | null) => {
+    window.__hf = window.__hf || {};
+    window.__hf.lengthSource = media
+      ? {
+          from: "media",
+          tag: media.tagName.toLowerCase(),
+          src: media.getAttribute("src") ?? media.currentSrc ?? "",
+          trimmed: media.hasAttribute("data-duration") || media.hasAttribute("data-end"),
+        }
+      : { from };
+  };
+
   /** Carries how a length that no timeline supplied was found, for render telemetry. */
   const publishDerivedDuration = (result: ReturnType<typeof resolveCompositionDuration>) => {
     window.__hf = window.__hf || {};
@@ -1253,8 +1267,12 @@ export function initSandboxRuntimeModular(): void {
     );
   };
 
-  const resolveMediaDurationFloorSeconds = (): number | null => {
-    const mediaWindowDuration = resolveMediaWindowDurationSeconds();
+  const resolveMediaDurationFloor = (): { seconds: number; element: HTMLMediaElement } | null => {
+    const longest = longestMediaWindow();
+    return longest && resolveMediaDurationFloorSeconds(longest.seconds) !== null ? longest : null;
+  };
+
+  const resolveMediaDurationFloorSeconds = (mediaWindowDuration: number | null): number | null => {
     if (
       typeof mediaWindowDuration !== "number" ||
       !Number.isFinite(mediaWindowDuration) ||
@@ -1348,7 +1366,11 @@ export function initSandboxRuntimeModular(): void {
   //     signature compared on read
   // The bias is deliberate: a redundant derivation is a missed optimisation,
   // a missed one is a wrong duration.
-  type DurationFloors = { media: number | null; authoredComposition: number | null };
+  type DurationFloors = {
+    media: number | null;
+    mediaElement: HTMLMediaElement | null;
+    authoredComposition: number | null;
+  };
   const DURATION_FLOOR_INPUT_ATTRIBUTES = [
     "data-start",
     "data-duration",
@@ -1419,10 +1441,14 @@ export function initSandboxRuntimeModular(): void {
     wakeTransport();
   };
 
-  const deriveDurationFloors = (): DurationFloors => ({
-    media: resolveMediaDurationFloorSeconds(),
-    authoredComposition: resolveAuthoredCompositionDurationFloorSeconds(),
-  });
+  const deriveDurationFloors = (): DurationFloors => {
+    const media = resolveMediaDurationFloor();
+    return {
+      media: media?.seconds ?? null,
+      mediaElement: media?.element ?? null,
+      authoredComposition: resolveAuthoredCompositionDurationFloorSeconds(),
+    };
+  };
 
   const readTimelineRegistrySignature = (): string => {
     const timelines = (window.__timelines ?? {}) as Record<string, RuntimeTimelineLike | undefined>;
@@ -1545,13 +1571,16 @@ export function initSandboxRuntimeModular(): void {
     timingRevision?: number,
   ): number => {
     let derivedDuration: ReturnType<typeof resolveContentDerivedDuration> | null = null;
-    const seconds = resolveCompositionLengthSeconds({
+    let mediaElement: HTMLMediaElement | null = null;
+    const length = resolveCompositionLength({
       declared: parseStrictFiniteTimingNumber(
         resolveRootCompositionElement()?.getAttribute("data-duration"),
       ),
       timeline: () => getTimelineDurationSeconds(timeline),
       floors: () => {
-        const { media, authoredComposition } = resolveDurationFloors(timingRevision);
+        const floors = resolveDurationFloors(timingRevision);
+        const { media, authoredComposition } = floors;
+        mediaElement = floors.mediaElement;
         // Deliberately NOT cached: adapters infer their duration from live animation objects
         // (CSS/WAAPI/Lottie), which can change without any DOM mutation to observe.
         return [media, authoredComposition, resolveAdapterDurationFloorSeconds()];
@@ -1565,7 +1594,8 @@ export function initSandboxRuntimeModular(): void {
     // The published source describes only a length that was derived; any other source clears it.
     if (derivedDuration) publishDerivedDuration(derivedDuration);
     else if (window.__hf?.durationSource) delete window.__hf.durationSource;
-    return seconds;
+    publishLengthSource(length.from, length.floorIndex === 0 ? mediaElement : null);
+    return length.seconds;
   };
 
   // Sub-composition timelines the runtime nested into the root: the host composition id, and where.
diff --git a/packages/core/src/runtime/window.d.ts b/packages/core/src/runtime/window.d.ts
index 277672652f..3b94f4b8e2 100644
--- a/packages/core/src/runtime/window.d.ts
+++ b/packages/core/src/runtime/window.d.ts
@@ -9,6 +9,7 @@ import type { RuntimeColorGradingApi } from "./colorGrading";
 import type { HyperframePickerApi } from "../inline-scripts/pickerApi";
 import type { PlayerAPI } from "../core.types";
 import type { ClipTree } from "./clipTree";
+import type { CompositionLengthSource } from "./compositionLength";
 import type { SvgSelectorAliases } from "../compiler/svgSelectorAliases";
 
 type ThreeClockLike = {
@@ -59,6 +60,8 @@ declare global {
       onSwallowed?: (label: string, err: unknown) => void;
       seek?: (timeSeconds: number, options?: RuntimeSeekOptions) => void;
       duration?: number;
+      /** Which input set the length; the producer reports it with the render. */
+      lengthSource?: CompositionLengthSource;
       /** How a length that no timeline supplied was found: the render telemetry reads this. */
       durationSource?: {
         source: "authored" | "derived" | "unresolved";
diff --git a/packages/engine/src/index.ts b/packages/engine/src/index.ts
index 697b226d3f..6949db251a 100644
--- a/packages/engine/src/index.ts
+++ b/packages/engine/src/index.ts
@@ -133,6 +133,8 @@ export {
   writeCapturedFrame,
   discardWarmupCapture,
   getCompositionDuration,
+  getCompositionLengthSource,
+  type CompositionLengthSource,
   getCapturePerfSummary,
   percentileOf,
   prepareCaptureSessionForReuse,
diff --git a/packages/engine/src/services/frameCapture.ts b/packages/engine/src/services/frameCapture.ts
index ab4c1efcc2..693565d056 100644
--- a/packages/engine/src/services/frameCapture.ts
+++ b/packages/engine/src/services/frameCapture.ts
@@ -122,6 +122,7 @@ import {
   type ChromePids,
 } from "./chromeMemorySampler.js";
 import { sampleProcessRss } from "../utils/processRss.js";
+import type { CompositionLengthSource } from "@hyperframes/core/runtime/composition-length";
 export { isMemoryExhaustionError, isTransientBrowserError } from "./captureFailure.js";
 
 export type { CaptureOptions, CaptureResult, CaptureBufferResult, CapturePerfSummary };
@@ -5345,6 +5346,16 @@ export async function getCompositionDuration(session: CaptureSession): Promise<n
   });
 }
 
+export type { CompositionLengthSource };
+
+/** Which input set the composition's length, as the runtime published it; null when it did not. */
+export async function getCompositionLengthSource(
+  session: CaptureSession,
+): Promise<CompositionLengthSource | null> {
+  if (!session.isInitialized) throw new Error("[FrameCapture] Session not initialized");
+  return session.page.evaluate(() => window.__hf?.lengthSource ?? null);
+}
+
 /**
  * Ungated-release safety net, part 1: capture K screenshot ground-truth frames
  * BEFORE the drawElement canvas is injected (the only window where a page
diff --git a/packages/engine/src/types.ts b/packages/engine/src/types.ts
index 64d1ba3a6e..606f2bbec9 100644
--- a/packages/engine/src/types.ts
+++ b/packages/engine/src/types.ts
@@ -6,6 +6,7 @@
  */
 import type { AudioFailureDiagnostic } from "./services/audioMixer.types.js";
 import type { Fps } from "@hyperframes/core";
+import type { CompositionLengthSource } from "@hyperframes/core/runtime/composition-length";
 import type { ChromeMemoryStats } from "./services/chromeMemorySampler.js";
 import type { MotionBlurOptions } from "./services/motionBlur.js";
 
@@ -130,6 +131,8 @@ export interface HfSeekOptions {
 export interface HfProtocol {
   /** Total duration of the composition in seconds */
   duration: number;
+  /** Which input set `duration`. */
+  lengthSource?: CompositionLengthSource;
   /** Seek to a specific time. Must produce deterministic visual output. */
   seek(time: number, options?: HfSeekOptions): void;
   /** Optional: media elements the engine should handle */
diff --git a/packages/producer/src/services/render/stages/probeStage.test.ts b/packages/producer/src/services/render/stages/probeStage.test.ts
index c84e606e2e..fe927ed223 100644
--- a/packages/producer/src/services/render/stages/probeStage.test.ts
+++ b/packages/producer/src/services/render/stages/probeStage.test.ts
@@ -18,6 +18,7 @@ import {
   hasAutoStartVideos,
   hasScriptedAudioVolumeAutomation,
   hasVariableBoundMedia,
+  reportedLengthSource,
 } from "./probeStage.js";
 
 // ── Mocks for runProbeStage tests ────────────────────────────────────────────
@@ -160,6 +161,12 @@ mock.module("@hyperframes/engine", () => ({
     durationProbeSessions.push(session);
     return 5;
   },
+  getCompositionLengthSource: async () => ({
+    from: "media",
+    tag: "video",
+    src: "/private/customer/Full Source.mp4?token=x",
+    trimmed: false,
+  }),
   closeCaptureSession: async (session: MockSession) => {
     closeCaptureSessionCallCount++;
     closedSessions.push(session);
@@ -1457,3 +1464,23 @@ describe("runProbeStage — transient browser error retry (#1687)", () => {
     expect(result.probeSession).not.toBeNull();
   });
 });
+
+describe("reportedLengthSource", () => {
+  it("names a media clip by tag, trim and a hash of its file name, never its path", () => {
+    const reported = reportedLengthSource({
+      from: "media",
+      tag: "video",
+      src: "https://cdn.example/private/customer/Full Source.mp4?token=secret#t=2",
+      trimmed: false,
+    });
+    expect(reported).toEqual({
+      from: "media",
+      tag: "video",
+      trimmed: false,
+      srcHash: createHash("sha256").update("Full Source.mp4").digest("hex").slice(0, 12),
+    });
+    expect(JSON.stringify(reported)).not.toMatch(/customer|secret|Full Source/);
+    expect(reportedLengthSource({ from: "timeline" })).toEqual({ from: "timeline" });
+    expect(reportedLengthSource(null)).toBeNull();
+  });
+});
diff --git a/packages/producer/src/services/render/stages/probeStage.ts b/packages/producer/src/services/render/stages/probeStage.ts
index 4a1dae3a77..eb50fecf69 100644
--- a/packages/producer/src/services/render/stages/probeStage.ts
+++ b/packages/producer/src/services/render/stages/probeStage.ts
@@ -28,11 +28,13 @@
  *     prefix.
  */
 
-import { join } from "node:path";
+import { createHash } from "node:crypto";
+import { basename, join } from "node:path";
 import { parseHTML } from "linkedom";
 import {
   type CaptureOptions,
   type CaptureSession,
+  type CompositionLengthSource,
   type EngineConfig,
   type SubTimelineWaitMemo,
   closeCaptureSession,
@@ -40,6 +42,7 @@ import {
   createCaptureSession,
   deriveBeginFrameProbeTimeTicks,
   getCompositionDuration,
+  getCompositionLengthSource,
   initializeSession,
   isTransientBrowserError,
   probeBeginFrameLiveness,
@@ -129,6 +132,8 @@ export interface ProbeStageResult {
   lastBrowserConsole: string[];
   /** Composition duration (post-probe). Guaranteed > 0 — the stage throws on <= 0. */
   duration: number;
+  /** Which input set `duration`; null when the runtime did not say. */
+  lengthSource: ReportedLengthSource | null;
   totalFrames: number;
   /** Wall-clock ms for the entire probe phase (near-zero when `needsBrowser` was false). */
   browserProbeMs: number;
@@ -334,6 +339,21 @@ export function probeRequiresBrowser(args: {
   );
 }
 
+export type ReportedLengthSource =
+  | { from: Exclude<CompositionLengthSource["from"], "media"> }
+  | { from: "media"; tag: string; trimmed: boolean; srcHash: string };
+
+/** The length source with a media clip's src reduced to a hash of its file name, so no path leaves the render. */
+export function reportedLengthSource(
+  source: CompositionLengthSource | null,
+): ReportedLengthSource | null {
+  if (!source) return null;
+  if (source.from !== "media") return { from: source.from };
+  const name = basename(source.src.split(/[?#]/, 1)[0] ?? "");
+  const srcHash = createHash("sha256").update(name).digest("hex").slice(0, 12);
+  return { from: "media", tag: source.tag, trimmed: source.trimmed, srcHash };
+}
+
 export async function runProbeStage(input: ProbeStageInput): Promise<ProbeStageResult> {
   const {
     projectDir,
@@ -358,6 +378,8 @@ export async function runProbeStage(input: ProbeStageInput): Promise<ProbeStageR
 
   let fileServer: FileServerHandle | null = null;
   let probeSession: CaptureSession | null = null;
+  let lengthSource: ReportedLengthSource | null =
+    input.composition.duration > 0 ? { from: "declared" } : null;
   let beginFrameStalled = false;
   let lastBrowserConsole: string[] = [];
 
@@ -552,10 +574,12 @@ export async function runProbeStage(input: ProbeStageInput): Promise<ProbeStageR
       if (composition.duration <= 0) {
         log.info("Discovering composition duration...");
         const discoveredDuration = await getCompositionDuration(session);
+        lengthSource = reportedLengthSource(await getCompositionLengthSource(session));
         assertNotAborted();
         log.info("Probed composition duration from browser", {
           discoveredDuration,
           staticDuration: compiled.staticDuration,
+          lengthSource,
         });
         composition.duration = discoveredDuration;
       } else {
@@ -892,6 +916,7 @@ export async function runProbeStage(input: ProbeStageInput): Promise<ProbeStageR
     probeSession,
     lastBrowserConsole,
     duration,
+    lengthSource,
     totalFrames,
     browserProbeMs,
     beginFrameStalled,
diff --git a/packages/producer/src/services/renderOrchestrator.ts b/packages/producer/src/services/renderOrchestrator.ts
index 78a5618c9f..9d4abc0ba1 100644
--- a/packages/producer/src/services/renderOrchestrator.ts
+++ b/packages/producer/src/services/renderOrchestrator.ts
@@ -3429,6 +3429,14 @@ async function executeRenderPipeline(input: {
     }
     observability.checkpoint("browser_probe", "duration resolved", {
       durationSeconds: probeResult.duration,
+      lengthFrom: probeResult.lengthSource?.from ?? null,
+      ...(probeResult.lengthSource?.from === "media"
+        ? {
+            lengthTag: probeResult.lengthSource.tag,
+            lengthTrimmed: probeResult.lengthSource.trimmed,
+            lengthSrcHash: probeResult.lengthSource.srcHash,
+          }
+        : {}),
       totalFrames,
       compositionHash,
       beginFrameStalled: probeResult.beginFrameStalled,