diff --git a/packages/producer/src/services/htmlCompiler.naturalDuration.test.ts b/packages/producer/src/services/htmlCompiler.naturalDuration.test.ts
index 9e7136083a..2467a1d272 100644
--- a/packages/producer/src/services/htmlCompiler.naturalDuration.test.ts
+++ b/packages/producer/src/services/htmlCompiler.naturalDuration.test.ts
@@ -79,6 +79,21 @@ describe("compileForRender natural media duration parity", () => {
     }
   });
 
+  it("marks only variable-bound media whose natural duration was inferred", async () => {
+    const { document } = await compile(`
+      <audio id="inferred" src="ten-seconds.wav" data-var-src="track"></audio>
+      <audio id="authored" src="ten-seconds.wav" data-var-src="track" data-duration="4"></audio>
+      <audio id="plain" src="ten-seconds.wav"></audio>`);
+
+    expect(document.getElementById("inferred")?.hasAttribute("data-hf-inferred-duration")).toBe(
+      true,
+    );
+    expect(document.getElementById("authored")?.hasAttribute("data-hf-inferred-duration")).toBe(
+      false,
+    );
+    expect(document.getElementById("plain")?.hasAttribute("data-hf-inferred-duration")).toBe(false);
+  });
+
   it("uses shared playback-start precedence and fallback semantics", async () => {
     const cases = [
       ["precedence", 'data-playback-start="2" data-media-start="7"', 8],
diff --git a/packages/producer/src/services/htmlCompiler.test.ts b/packages/producer/src/services/htmlCompiler.test.ts
index a13e65c56a..0981973132 100644
--- a/packages/producer/src/services/htmlCompiler.test.ts
+++ b/packages/producer/src/services/htmlCompiler.test.ts
@@ -34,12 +34,17 @@ describe("discoverMediaFromBrowser", () => {
     html: string,
     currentSrcById: Record<string, string>,
     serializeCallback = false,
+    intrinsicDurationById: Record<string, number> = {},
   ) {
     const { document } = parseHTML(html);
     for (const [id, currentSrc] of Object.entries(currentSrcById)) {
       const element = document.getElementById(id);
       if (element) Object.defineProperty(element, "currentSrc", { value: currentSrc });
     }
+    for (const [id, duration] of Object.entries(intrinsicDurationById)) {
+      const element = document.getElementById(id);
+      if (element) Object.defineProperty(element, "duration", { value: duration });
+    }
     const previousDocument = Reflect.get(globalThis, "document");
     Reflect.set(globalThis, "document", document);
     try {
@@ -82,6 +87,21 @@ describe("discoverMediaFromBrowser", () => {
     expect(media[0]).toMatchObject({ id: "hf-img-1", tagName: "image" });
   });
 
+  it("uses intrinsic duration only for variable media with an inferred duration", async () => {
+    const media = await discover(
+      `<audio id="inferred" src="fallback.wav" data-start="0" data-duration="3" data-end="3" data-var-src="track" data-hf-inferred-duration></audio>
+       <audio id="authored" src="fallback.wav" data-start="0" data-duration="3" data-end="3" data-var-src="track"></audio>`,
+      { inferred: "selected.wav", authored: "selected.wav" },
+      false,
+      { inferred: 6.530612, authored: 6.530612 },
+    );
+
+    expect(media.find((item) => item.id === "inferred")?.duration).toBe(6.530612);
+    expect(media.find((item) => item.id === "inferred")?.durationInferred).toBe(true);
+    expect(media.find((item) => item.id === "authored")?.duration).toBe(3);
+    expect(media.find((item) => item.id === "authored")?.durationInferred).toBe(false);
+  });
+
   it("discovers the owning image for a variable-bound picture source", async () => {
     const media = await discover(
       `<picture>
diff --git a/packages/producer/src/services/htmlCompiler.ts b/packages/producer/src/services/htmlCompiler.ts
index cf55a87a82..ce0f264ed0 100644
--- a/packages/producer/src/services/htmlCompiler.ts
+++ b/packages/producer/src/services/htmlCompiler.ts
@@ -104,6 +104,8 @@ export interface CompiledComposition {
   hasAncestorBackgroundImage: boolean;
 }
 
+const INFERRED_MEDIA_DURATION_ATTR = "data-hf-inferred-duration";
+
 /** Adapts linkedom's `parseHTML` to the `checkSubCompositionUsability` contract. */
 function parseSubCompHtmlForValidity(html: string): ParsableDocumentLike {
   return parseHTML(html).document as unknown as ParsableDocumentLike;
@@ -534,6 +536,23 @@ async function resolveMediaDuration(
  * Compile a single HTML file: static pass + ffprobe for unresolved media.
  * Returns compiled HTML and any unresolved composition elements that need browser resolution.
  */
+function markInferredVariableMediaDurations(
+  html: string,
+  unresolvedMedia: readonly UnresolvedElement[],
+): string {
+  const unresolvedIds = new Set(unresolvedMedia.map((element) => element.id));
+  if (unresolvedIds.size === 0) return html;
+
+  const { document } = parseHTML(html);
+  let changed = false;
+  for (const element of document.querySelectorAll("video[data-var-src], audio[data-var-src]")) {
+    if (!unresolvedIds.has(element.id)) continue;
+    element.setAttribute(INFERRED_MEDIA_DURATION_ATTR, "");
+    changed = true;
+  }
+  return changed ? document.toString() : html;
+}
+
 async function compileHtmlFile(
   html: string,
   baseDir: string,
@@ -567,8 +586,9 @@ async function compileHtmlFile(
     (r): r is ResolvedDuration => r.duration != null && Number.isFinite(r.duration),
   );
 
+  const markedStaticHtml = markInferredVariableMediaDurations(staticCompiled, mediaUnresolved);
   let compiledHtml =
-    resolutions.length > 0 ? injectDurations(staticCompiled, resolutions) : staticCompiled;
+    resolutions.length > 0 ? injectDurations(markedStaticHtml, resolutions) : markedStaticHtml;
 
   // Phase 2: Bound authored audio to playable source (parallel ffprobe).
   // Explicit video slots may outlive their source and hold the final frame.
@@ -2101,6 +2121,8 @@ export interface BrowserMediaElement {
   start: number;
   end: number;
   duration: number;
+  /** True when compilation inferred duration from the fallback source. */
+  durationInferred: boolean;
   mediaStart: number;
   loop: boolean;
   hasAudio: boolean;
@@ -2123,6 +2145,8 @@ export async function discoverMediaFromBrowser(page: Page): Promise<BrowserMedia
       start: number;
       endRaw: string | null;
       durationRaw: string | null;
+      intrinsicDuration: number;
+      durationInferred: boolean;
       playbackStartRaw: string | null;
       mediaStartRaw: string | null;
       loop: boolean;
@@ -2169,6 +2193,10 @@ export async function discoverMediaFromBrowser(page: Page): Promise<BrowserMedia
       const start = parseFloat(htmlEl.getAttribute("data-start") || "0");
       const endRaw = htmlEl.getAttribute("data-end");
       const durationRaw = htmlEl.getAttribute("data-duration");
+      const durationInferred = htmlEl.hasAttribute("data-hf-inferred-duration");
+      const intrinsicDuration = isImage
+        ? 0
+        : (htmlEl as HTMLVideoElement | HTMLAudioElement).duration;
       const playbackStartRaw = htmlEl.getAttribute("data-playback-start");
       const mediaStartRaw = htmlEl.getAttribute("data-media-start");
       const loop = htmlEl.hasAttribute("loop");
@@ -2185,6 +2213,8 @@ export async function discoverMediaFromBrowser(page: Page): Promise<BrowserMedia
         start,
         endRaw,
         durationRaw,
+        intrinsicDuration,
+        durationInferred,
         playbackStartRaw,
         mediaStartRaw,
         loop,
@@ -2197,18 +2227,23 @@ export async function discoverMediaFromBrowser(page: Page): Promise<BrowserMedia
     return results;
   });
 
-  return elements.map(({ endRaw, durationRaw, playbackStartRaw, mediaStartRaw, ...element }) => ({
-    ...element,
-    end: parseStrictFiniteTimingNumber(endRaw) ?? 0,
-    duration: parseStrictFiniteTimingNumber(durationRaw) ?? 0,
-    mediaStart: readMediaStart({
-      getAttribute(name: string) {
-        if (name === "data-playback-start") return playbackStartRaw;
-        if (name === "data-media-start") return mediaStartRaw;
-        return null;
-      },
+  return elements.map(
+    ({ endRaw, durationRaw, intrinsicDuration, playbackStartRaw, mediaStartRaw, ...element }) => ({
+      ...element,
+      end: parseStrictFiniteTimingNumber(endRaw) ?? 0,
+      duration:
+        element.durationInferred && Number.isFinite(intrinsicDuration) && intrinsicDuration > 0
+          ? intrinsicDuration
+          : (parseStrictFiniteTimingNumber(durationRaw) ?? 0),
+      mediaStart: readMediaStart({
+        getAttribute(name: string) {
+          if (name === "data-playback-start") return playbackStartRaw;
+          if (name === "data-media-start") return mediaStartRaw;
+          return null;
+        },
+      }),
     }),
-  }));
+  );
 }
 
 export async function discoverAudioVolumeAutomationFromTimeline(
diff --git a/packages/producer/src/services/render/stages/probeStage.test.ts b/packages/producer/src/services/render/stages/probeStage.test.ts
index 82945f2241..dd0052388d 100644
--- a/packages/producer/src/services/render/stages/probeStage.test.ts
+++ b/packages/producer/src/services/render/stages/probeStage.test.ts
@@ -444,6 +444,79 @@ describe("runProbeStage — forceScreenshot threading", () => {
     expect(mediaPreflightComposition).toBe(input.composition);
   });
 
+  it("uses selected intrinsic duration only when the variable-bound duration was inferred", async () => {
+    resetRetryMocks();
+    const media = (
+      id: string,
+      duration: number,
+      durationInferred: boolean,
+    ): Record<string, unknown> => ({
+      id,
+      tagName: "audio",
+      src: `${id}-selected.wav`,
+      start: 0,
+      end: duration,
+      duration,
+      durationInferred,
+      mediaStart: 0,
+      loop: false,
+      hasAudio: true,
+      volume: 1,
+      muted: false,
+    });
+    browserMediaResults = [
+      media("longer-inferred", 6.530612, true),
+      media("longer-authored", 6.530612, false),
+      media("shorter-inferred", 3.836939, true),
+    ];
+    const { runProbeStage } = await import("./probeStage.js");
+    const input = makeProbeInput({});
+    input.composition.duration = 7;
+    input.composition.audios.push(
+      {
+        id: "longer-inferred",
+        src: "short.wav",
+        start: 0,
+        end: 3.836939,
+        mediaStart: 0,
+        layer: 0,
+        volume: 1,
+        type: "audio",
+      },
+      {
+        id: "longer-authored",
+        src: "short.wav",
+        start: 0,
+        end: 3.836939,
+        mediaStart: 0,
+        layer: 0,
+        volume: 1,
+        type: "audio",
+      },
+      {
+        id: "shorter-inferred",
+        src: "long.wav",
+        start: 0,
+        end: 6.530612,
+        mediaStart: 0,
+        layer: 0,
+        volume: 1,
+        type: "audio",
+      },
+    );
+    input.compiled.html = `
+      <audio id="longer-inferred" src="short.wav" data-var-src="a" data-hf-inferred-duration></audio>
+      <audio id="longer-authored" src="short.wav" data-var-src="b" data-duration="3.836939"></audio>
+      <audio id="shorter-inferred" src="long.wav" data-var-src="c" data-hf-inferred-duration></audio>`;
+    input.job.config.variables = { a: "a.wav", b: "b.wav", c: "c.wav" };
+
+    await runProbeStage(input);
+
+    expect(input.composition.audios.map((audio) => audio.end)).toEqual([
+      6.530612, 3.836939, 3.836939,
+    ]);
+  });
+
   it("passes cancellation through and closes probe-owned resources when preflight rejects", async () => {
     resetRetryMocks();
     mediaPreflightError = new Error("ASSET_MEDIA_TYPE_MISMATCH");
diff --git a/packages/producer/src/services/render/stages/probeStage.ts b/packages/producer/src/services/render/stages/probeStage.ts
index 7b4de32b27..3e234f24a2 100644
--- a/packages/producer/src/services/render/stages/probeStage.ts
+++ b/packages/producer/src/services/render/stages/probeStage.ts
@@ -176,6 +176,17 @@ export function hasVariableBoundMedia(
   });
 }
 
+function reconcileBrowserMediaEnd(
+  existingEnd: number,
+  projectedEnd: number,
+  sourceChanged: boolean,
+  durationInferred: boolean,
+): number {
+  if (projectedEnd <= 0) return existingEnd;
+  if (sourceChanged && durationInferred) return projectedEnd;
+  return existingEnd <= 0 ? projectedEnd : Math.min(existingEnd, projectedEnd);
+}
+
 /**
  * Runtime-created media does not exist when the static compiler scans the HTML.
  * Launch a browser probe so discoverMediaFromBrowser can reconcile it before
@@ -500,7 +511,8 @@ export async function runProbeStage(input: ProbeStageInput): Promise<ProbeStageR
             // Reconcile to browser/runtime media metadata (runtime src can differ from static HTML).
             const existing = composition.videos.find((v) => v.id === el.id);
             if (existing) {
-              if (existing.src !== src) {
+              const sourceChanged = existing.src !== src;
+              if (sourceChanged) {
                 existing.src = src;
               }
               const projectedEnd = projectBrowserEndToCompositionTimeline(
@@ -508,10 +520,12 @@ export async function runProbeStage(input: ProbeStageInput): Promise<ProbeStageR
                 el.start,
                 resolveBrowserMediaEnd(el.start, el.end, el.duration),
               );
-              if (projectedEnd > 0) {
-                existing.end =
-                  existing.end <= 0 ? projectedEnd : Math.min(existing.end, projectedEnd);
-              }
+              existing.end = reconcileBrowserMediaEnd(
+                existing.end,
+                projectedEnd,
+                sourceChanged,
+                el.durationInferred,
+              );
               if (
                 el.mediaStart > 0 &&
                 (existing.mediaStart <= 0 ||
@@ -544,7 +558,8 @@ export async function runProbeStage(input: ProbeStageInput): Promise<ProbeStageR
           if (existingAudioIds.has(el.id)) {
             const existing = composition.audios.find((a) => a.id === el.id);
             if (existing) {
-              if (existing.src !== src) {
+              const sourceChanged = existing.src !== src;
+              if (sourceChanged) {
                 existing.src = src;
               }
               const projectedEnd = projectBrowserEndToCompositionTimeline(
@@ -552,10 +567,12 @@ export async function runProbeStage(input: ProbeStageInput): Promise<ProbeStageR
                 el.start,
                 resolveBrowserMediaEnd(el.start, el.end, el.duration),
               );
-              if (projectedEnd > 0) {
-                existing.end =
-                  existing.end <= 0 ? projectedEnd : Math.min(existing.end, projectedEnd);
-              }
+              existing.end = reconcileBrowserMediaEnd(
+                existing.end,
+                projectedEnd,
+                sourceChanged,
+                el.durationInferred,
+              );
               if (
                 el.mediaStart > 0 &&
                 (existing.mediaStart <= 0 ||