diff --git a/packages/engine/src/index.ts b/packages/engine/src/index.ts
index d2dff7f6f..28e533c76 100644
--- a/packages/engine/src/index.ts
+++ b/packages/engine/src/index.ts
@@ -115,6 +115,7 @@ export {
   parseImageElements,
   extractVideoFramesRange,
   extractAllVideoFrames,
+  resolveProjectRelativeSrc,
   getFrameAtTime,
   createFrameLookupTable,
   FrameLookupTable,
diff --git a/packages/engine/src/services/audioMixer.ts b/packages/engine/src/services/audioMixer.ts
index a3ccb804c..b204a79f1 100644
--- a/packages/engine/src/services/audioMixer.ts
+++ b/packages/engine/src/services/audioMixer.ts
@@ -12,6 +12,7 @@ import { downloadToTemp, isHttpUrl } from "../utils/urlDownloader.js";
 import { DEFAULT_CONFIG, type EngineConfig } from "../config.js";
 import { runFfmpeg } from "../utils/runFfmpeg.js";
 import { unwrapTemplate } from "../utils/htmlTemplate.js";
+import { resolveProjectRelativeSrc } from "./videoFrameExtractor.js";
 import type { AudioElement, AudioTrack, MixResult } from "./audioMixer.types.js";
 
 export type { AudioElement, AudioTrack, MixResult } from "./audioMixer.types.js";
@@ -325,13 +326,10 @@ export async function processCompositionAudio(
       }
       try {
         let srcPath = element.src;
-        // Use isAbsolute() rather than startsWith("/"). On Windows, absolute paths
-        // like "C:\…" are not detected by the latter, so we'd re-join them under
-        // baseDir and produce duplicated, nonexistent paths.
         if (!isAbsolute(srcPath) && !isHttpUrl(srcPath)) {
-          const fromCompiled = compiledDir ? join(compiledDir, srcPath) : null;
-          srcPath =
-            fromCompiled && existsSync(fromCompiled) ? fromCompiled : join(baseDir, srcPath);
+          // Same browser-vs-filesystem path semantics as videos — see
+          // resolveProjectRelativeSrc in videoFrameExtractor for the full why.
+          srcPath = resolveProjectRelativeSrc(element.src, baseDir, compiledDir);
         }
 
         if (isHttpUrl(srcPath)) {
diff --git a/packages/engine/src/services/videoFrameExtractor.test.ts b/packages/engine/src/services/videoFrameExtractor.test.ts
index f03c63fdf..84635f12b 100644
--- a/packages/engine/src/services/videoFrameExtractor.test.ts
+++ b/packages/engine/src/services/videoFrameExtractor.test.ts
@@ -9,6 +9,7 @@ import {
   parseImageElements,
   extractAllVideoFrames,
   createFrameLookupTable,
+  resolveProjectRelativeSrc,
   type VideoElement,
   type ExtractedFrames,
 } from "./videoFrameExtractor.js";
@@ -23,6 +24,67 @@ import { runFfmpeg } from "../utils/runFfmpeg.js";
 // synthesized VFR fixture.
 const HAS_FFMPEG = spawnSync("ffmpeg", ["-version"]).status === 0;
 
+// Regression: a long-standing footgun where `<video src="../assets/foo">`
+// inside a sub-composition silently dropped the video from extraction. The
+// browser's URL resolver clamps `..` at the served origin's root (so the
+// page renders fine in the studio), but `path.join(projectDir, "../assets/foo")`
+// normalizes to <parentOfProjectDir>/assets/foo, which doesn't exist. Result:
+// no extracted frames, no per-frame injection, the rendered output shows the
+// <video>'s first decoded frame for the whole clip duration. The resolver
+// now mirrors browser semantics by stripping leading `..` segments as a
+// fallback when the literal join doesn't exist.
+describe("resolveProjectRelativeSrc — sub-composition path clamping", () => {
+  let tmp: string;
+
+  beforeAll(() => {
+    tmp = mkdtempSync(join(tmpdir(), "hf-resolver-"));
+    mkdirSync(join(tmp, "project", "assets"), { recursive: true });
+    // Empty file is enough for existsSync — this test is about path resolution.
+    require("node:fs").writeFileSync(join(tmp, "project", "assets", "foo.mp4"), "");
+  });
+  afterAll(() => {
+    rmSync(tmp, { recursive: true, force: true });
+  });
+
+  it("returns the literal join when the file exists at projectDir/src", () => {
+    const projectDir = join(tmp, "project");
+    expect(resolveProjectRelativeSrc("assets/foo.mp4", projectDir)).toBe(
+      join(projectDir, "assets/foo.mp4"),
+    );
+  });
+
+  it("clamps a leading `../` (sub-comp authoring) so `../assets/foo.mp4` resolves to assets/foo.mp4", () => {
+    const projectDir = join(tmp, "project");
+    expect(resolveProjectRelativeSrc("../assets/foo.mp4", projectDir)).toBe(
+      join(projectDir, "assets/foo.mp4"),
+    );
+  });
+
+  it("clamps multiple leading `../../../` segments", () => {
+    const projectDir = join(tmp, "project");
+    expect(resolveProjectRelativeSrc("../../../assets/foo.mp4", projectDir)).toBe(
+      join(projectDir, "assets/foo.mp4"),
+    );
+  });
+
+  it("returns the (non-existent) base-dir path on miss so callers get a stable error message", () => {
+    const projectDir = join(tmp, "project");
+    expect(resolveProjectRelativeSrc("../assets/missing.mp4", projectDir)).toBe(
+      join(projectDir, "../assets/missing.mp4"),
+    );
+  });
+
+  it("prefers compiled-dir over base-dir when the file exists in both", () => {
+    const projectDir = join(tmp, "project");
+    const compiledDir = join(tmp, "compiled");
+    mkdirSync(join(compiledDir, "assets"), { recursive: true });
+    require("node:fs").writeFileSync(join(compiledDir, "assets", "foo.mp4"), "");
+    expect(resolveProjectRelativeSrc("assets/foo.mp4", projectDir, compiledDir)).toBe(
+      join(compiledDir, "assets/foo.mp4"),
+    );
+  });
+});
+
 describe("parseVideoElements", () => {
   it("parses videos without an id or data-start attribute", () => {
     const videos = parseVideoElements('<video src="clip.mp4"></video>');
diff --git a/packages/engine/src/services/videoFrameExtractor.ts b/packages/engine/src/services/videoFrameExtractor.ts
index 4dfb775b3..9992d9966 100644
--- a/packages/engine/src/services/videoFrameExtractor.ts
+++ b/packages/engine/src/services/videoFrameExtractor.ts
@@ -459,6 +459,37 @@ async function convertVfrToCfr(
   }
 }
 
+/**
+ * Resolve a relative `<video src>` to a filesystem path the way the browser
+ * resolves it as a URL. Browsers clamp `..` segments at the served origin's
+ * root; `path.join(projectDir, "../assets/foo")` does not. So a sub-comp
+ * `<video src="../assets/foo">` loads in the page (browser clamps to
+ * `<projectDir>/assets/foo`) but the filesystem-side resolver lands at
+ * `<parentOfProjectDir>/assets/foo` — file missing, extraction skipped,
+ * the rendered output shows the video's first frame for the whole clip.
+ * Mirror the clamp here.
+ *
+ * Returns the first existing candidate, or the base-dir join on miss so
+ * the caller's existsSync check produces a stable error path.
+ */
+export function resolveProjectRelativeSrc(
+  src: string,
+  baseDir: string,
+  compiledDir?: string,
+): string {
+  const candidates = [
+    compiledDir && join(compiledDir, src),
+    join(baseDir, src),
+    ...(src.startsWith("..")
+      ? (() => {
+          const clamped = src.replace(/^(\.\.[\\/])+/, "");
+          return [compiledDir && join(compiledDir, clamped), join(baseDir, clamped)];
+        })()
+      : []),
+  ].filter(Boolean) as string[];
+  return candidates.find(existsSync) ?? join(baseDir, src);
+}
+
 export async function extractAllVideoFrames(
   videos: VideoElement[],
   baseDir: string,
@@ -496,9 +527,7 @@ export async function extractAllVideoFrames(
       // baseDir and produce duplicated, nonexistent paths
       // (e.g. C:\tmp\hf-vfr-test-X\C:\tmp\hf-vfr-test-X\vfr_screen.mp4).
       if (!isAbsolute(videoPath) && !isHttpUrl(videoPath)) {
-        const fromCompiled = compiledDir ? join(compiledDir, videoPath) : null;
-        videoPath =
-          fromCompiled && existsSync(fromCompiled) ? fromCompiled : join(baseDir, videoPath);
+        videoPath = resolveProjectRelativeSrc(video.src, baseDir, compiledDir);
       }
 
       if (isHttpUrl(videoPath)) {
@@ -508,6 +537,15 @@ export async function extractAllVideoFrames(
       }
 
       if (!existsSync(videoPath)) {
+        // Loud: silent miss leaves the rendered video frozen at frame 0 with
+        // no error in stdout — extremely confusing for authors.
+        process.stderr.write(
+          `[hyperframes:render] WARNING: video <${video.id}> src="${video.src}" ` +
+            `could not be resolved on disk (looked for ${videoPath}). ` +
+            `The rendered output will show this video's first frame for the entire clip duration. ` +
+            `If your <video> lives inside a sub-composition, prefer project-root-relative paths ` +
+            `(e.g. src="assets/foo.mp4") over "../assets/foo.mp4".\n`,
+        );
         errors.push({ videoId: video.id, error: `Video file not found: ${videoPath}` });
         continue;
       }
diff --git a/packages/producer/src/services/renderOrchestrator.ts b/packages/producer/src/services/renderOrchestrator.ts
index b3a60eeee..16a73f782 100644
--- a/packages/producer/src/services/renderOrchestrator.ts
+++ b/packages/producer/src/services/renderOrchestrator.ts
@@ -33,6 +33,7 @@ import {
   type EngineConfig,
   resolveConfig,
   extractAllVideoFrames,
+  resolveProjectRelativeSrc,
   type ExtractedFrames,
   type ExtractionPhaseBreakdown,
   createFrameLookupTable,
@@ -2312,13 +2313,15 @@ export async function executeRenderJob(
     if (job.config.hdrMode !== "force-sdr" && composition.videos.length > 0) {
       await Promise.all(
         composition.videos.map(async (v) => {
-          let videoPath = v.src;
-          if (!videoPath.startsWith("/")) {
-            const fromCompiled = existsSync(join(compiledDir, videoPath))
-              ? join(compiledDir, videoPath)
-              : join(projectDir, videoPath);
-            videoPath = fromCompiled;
-          }
+          // Use the shared resolver so a `<video src="../assets/foo">` in a
+          // sub-composition resolves the same way the browser would (see
+          // resolveProjectRelativeSrc in videoFrameExtractor for the full
+          // explanation). Without this, the HDR probe silently no-ops on
+          // those videos and the downstream extraction bug shows up
+          // amplified — videos register but never extract.
+          const videoPath = v.src.startsWith("/")
+            ? v.src
+            : resolveProjectRelativeSrc(v.src, projectDir, compiledDir);
           if (!existsSync(videoPath)) return;
           const meta = await extractMediaMetadata(videoPath);
           if (isHdrColorSpace(meta.colorSpace)) {