diff --git a/packages/engine/src/services/videoFrameExtractor.ts b/packages/engine/src/services/videoFrameExtractor.ts
index 989705f8d1..b0015b3ae3 100644
--- a/packages/engine/src/services/videoFrameExtractor.ts
+++ b/packages/engine/src/services/videoFrameExtractor.ts
@@ -14,6 +14,7 @@ import { createConcurrencyLimit } from "../utils/concurrencyLimit.js";
 import { parseHTML } from "linkedom";
 import { isPathInside } from "@hyperframes/parsers/asset-paths";
 import { resolveProjectRelativeSrc } from "@hyperframes/parsers/asset-resolution";
+import { alphaDecoderFor } from "@hyperframes/parsers/media-alpha";
 import {
   MEDIA_RENDER_ID_ATTR,
   isAudibleVideoElement,
@@ -1580,10 +1581,7 @@ export function codecMayHaveAlpha(codec: string | undefined): boolean {
 }
 
 export function decoderForCodec(codec: string | undefined): string {
-  const c = (codec ?? "").toLowerCase();
-  if (c === "vp9") return "libvpx-vp9";
-  if (c === "vp8") return "libvpx";
-  return c;
+  return alphaDecoderFor(codec) ?? (codec ?? "").toLowerCase();
 }
 
 export function resolveFrameFormat(
diff --git a/packages/engine/src/utils/ffprobe.test.ts b/packages/engine/src/utils/ffprobe.test.ts
index 969028ad2f..b1cdae9d6d 100644
--- a/packages/engine/src/utils/ffprobe.test.ts
+++ b/packages/engine/src/utils/ffprobe.test.ts
@@ -5,12 +5,7 @@ import { mkdtempSync, readFileSync, renameSync, rmSync, writeFileSync } from "fs
 import { tmpdir } from "os";
 import { basename, resolve } from "path";
 import { afterEach, describe, expect, it, vi } from "vitest";
-import {
-  extractMediaMetadata,
-  extractPngMetadataFromBuffer,
-  parseFrameRate,
-  pixelFormatHasAlpha,
-} from "./ffprobe.js";
+import { extractMediaMetadata, extractPngMetadataFromBuffer, parseFrameRate } from "./ffprobe.js";
 
 function crc32(buf: Buffer): number {
   let crc = 0xffffffff;
@@ -1737,27 +1732,6 @@ describe("crc32 works on every runtime the package declares", () => {
   });
 });
 
-describe("pix_fmt alpha detection", () => {
-  // The old pattern's (^|[^a-z]) anchor bound to `yuva` alone, and the list
-  // omitted formats real files actually use.
-  const ALPHA = [
-    "yuva420p",
-    "rgba",
-    "argb",
-    "bgra",
-    "abgr",
-    "gbrap",
-    "ya8",
-    "ya16le",
-    "ayuv64le",
-    "yuva444p12le",
-  ];
-  const OPAQUE = ["yuv420p", "rgb24", "gray", "gbrp", "nv12", "yuv444p10le", "bgr0", "rgb0"];
-
-  it.each(ALPHA)("detects alpha in %s", (fmt) => expect(pixelFormatHasAlpha(fmt)).toBe(true));
-  it.each(OPAQUE)("reports %s as opaque", (fmt) => expect(pixelFormatHasAlpha(fmt)).toBe(false));
-});
-
 describe("audio duration decode probe never fails the call and needs no profile allowlist", () => {
   afterEach(() => {
     vi.resetModules();
diff --git a/packages/engine/src/utils/ffprobe.ts b/packages/engine/src/utils/ffprobe.ts
index d45f64c15a..655aca9914 100644
--- a/packages/engine/src/utils/ffprobe.ts
+++ b/packages/engine/src/utils/ffprobe.ts
@@ -5,6 +5,7 @@ import { chunkCrc32 } from "./crc32.js";
 import { StringDecoder } from "node:string_decoder";
 import { basename } from "path";
 import { redactTelemetryString } from "@hyperframes/core";
+import { videoStreamHasAlpha } from "@hyperframes/parsers/media-alpha";
 import { urlInputArgs } from "./urlInput.js";
 import { FFPROBE_PATH_ENV, getFfprobeBinary } from "./ffmpegBinaries.js";
 import { ManagedChildProcess } from "./managedChildProcess.js";
@@ -604,23 +605,6 @@ export function extractPngMetadataFromBuffer(buf: Buffer): StillImageMetadata |
   return width > 0 && height > 0 ? { width, height, colorSpace: colorSpaceFromCicp } : null;
 }
 
-/**
- * Does this pix_fmt carry an alpha channel?
- *
- * Exported so the test asserts the shipped predicate rather than a copy of
- * the pattern. Anchored at the start, matching studio-server's
- * mediaMetadata.ts: the previous inline pattern bound its `(^|[^a-z])` anchor
- * to the first alternative only — `|` is looser than concatenation — so the
- * guard was decorative for every other name. It also missed abgr, ya8/ya16
- * and ayuv64, and its `gray[a-z0-9]*a` branch matched only gray8a/gray16a,
- * names FFmpeg renamed to ya8/ya16 in 2013. A `ya8` grayscale-plus-alpha PNG
- * reported hasAlpha:false, resolveFrameFormat picked jpg, and the overlay
- * flattened to an opaque rectangle.
- */
-export function pixelFormatHasAlpha(pixelFormat: string): boolean {
-  return /^(?:yuva|rgba|argb|bgra|abgr|gbrap|ya|ayuv)/i.test(pixelFormat);
-}
-
 function extractStillImageMetadata(filePath: string): StillImageMetadata | null {
   try {
     return extractPngMetadataFromBuffer(readFileSync(filePath));
@@ -791,16 +775,7 @@ export async function extractMediaMetadata(filePath: string): Promise<VideoMetad
     const colorSpace =
       merged.colorTransfer || merged.colorPrimaries || merged.colorSpace ? merged : null;
     const pixelFormat = videoStream.pix_fmt || "";
-    const alphaMode = readTagCI(videoStream.tags, "alpha_mode");
-    const hasAlpha = pixelFormatHasAlpha(pixelFormat) || alphaMode === "1";
-    // Anchored at the start, matching studio-server's mediaMetadata.ts. The
-    // previous pattern bound its `(^|[^a-z])` anchor to the FIRST alternative
-    // only — `|` is looser than concatenation — so the guard was decorative
-    // for every other name. It also missed abgr, ya8/ya16 and ayuv64, and its
-    // `gray[a-z0-9]*a` branch matched only gray8a/gray16a, names FFmpeg
-    // renamed to ya8/ya16 in 2013. A `ya8` grayscale-plus-alpha PNG reported
-    // hasAlpha:false, resolveFrameFormat picked jpg, and the overlay
-    // flattened to an opaque rectangle.
+    const hasAlpha = videoStreamHasAlpha(videoStream);
 
     const containerDuration = output?.format.duration ? parseFloat(output.format.duration) : 0;
     const streamDuration = videoStream.duration ? parseFloat(videoStream.duration) : 0;
diff --git a/packages/parsers/package-subpaths.json b/packages/parsers/package-subpaths.json
index 879bb703aa..1472a2fee2 100644
--- a/packages/parsers/package-subpaths.json
+++ b/packages/parsers/package-subpaths.json
@@ -116,6 +116,12 @@
       "types": "./dist/ffBinaries.d.ts",
       "environments": ["bun", "node"]
     },
+    "./media-alpha": {
+      "source": "./src/mediaAlpha.ts",
+      "runtime": "./dist/mediaAlpha.js",
+      "types": "./dist/mediaAlpha.d.ts",
+      "environments": ["browser", "bun", "node"]
+    },
     "./asset-resolution": {
       "source": "./src/assetResolution.ts",
       "runtime": "./dist/assetResolution.js",
diff --git a/packages/parsers/package.json b/packages/parsers/package.json
index 7470d11288..78ffa7b6b3 100644
--- a/packages/parsers/package.json
+++ b/packages/parsers/package.json
@@ -124,6 +124,12 @@
       "import": "./src/ffBinaries.ts",
       "types": "./src/ffBinaries.ts"
     },
+    "./media-alpha": {
+      "bun": "./src/mediaAlpha.ts",
+      "node": "./dist/mediaAlpha.js",
+      "import": "./src/mediaAlpha.ts",
+      "types": "./src/mediaAlpha.ts"
+    },
     "./asset-resolution": {
       "bun": "./src/assetResolution.ts",
       "node": "./dist/assetResolution.js",
@@ -237,6 +243,10 @@
         "import": "./dist/ffBinaries.js",
         "types": "./dist/ffBinaries.d.ts"
       },
+      "./media-alpha": {
+        "import": "./dist/mediaAlpha.js",
+        "types": "./dist/mediaAlpha.d.ts"
+      },
       "./asset-resolution": {
         "import": "./dist/assetResolution.js",
         "types": "./dist/assetResolution.d.ts"
diff --git a/packages/parsers/src/mediaAlpha.test.ts b/packages/parsers/src/mediaAlpha.test.ts
new file mode 100644
index 0000000000..7124355e48
--- /dev/null
+++ b/packages/parsers/src/mediaAlpha.test.ts
@@ -0,0 +1,70 @@
+import { describe, expect, it } from "vitest";
+import { alphaDecoderFor, pixelFormatHasAlpha, videoStreamHasAlpha } from "./mediaAlpha.js";
+
+describe("pixelFormatHasAlpha", () => {
+  const ALPHA = [
+    "yuva420p",
+    "yuva444p10le",
+    "yuva444p12le",
+    "rgba",
+    "argb",
+    "bgra",
+    "abgr",
+    "gbrap",
+    "gbrap12le",
+    "ya8",
+    "ya16le",
+    "ayuv64le",
+    "YUVA420P",
+  ];
+  const OPAQUE = [
+    "yuv420p",
+    "yuv422p10le",
+    "yuv444p10le",
+    "rgb24",
+    "gray",
+    "gbrp",
+    "nv12",
+    "bgr0",
+    "rgb0",
+  ];
+
+  it.each(ALPHA)("detects alpha in %s", (fmt) => expect(pixelFormatHasAlpha(fmt)).toBe(true));
+  it.each(OPAQUE)("reports %s as opaque", (fmt) => expect(pixelFormatHasAlpha(fmt)).toBe(false));
+  it("reports a missing pix_fmt as opaque", () =>
+    expect(pixelFormatHasAlpha(undefined)).toBe(false));
+});
+
+describe("videoStreamHasAlpha", () => {
+  // ffprobe 8.1 on a libvpx-vp9 yuva420p encode: the alpha lives only in the tag.
+  const vp9Alpha = { codec_name: "vp9", pix_fmt: "yuv420p", tags: { ALPHA_MODE: "1" } };
+
+  it("reads the VP8/VP9 alpha_mode tag in any casing", () => {
+    expect(videoStreamHasAlpha(vp9Alpha)).toBe(true);
+    expect(videoStreamHasAlpha({ pix_fmt: "yuv420p", tags: { alpha_mode: "1" } })).toBe(true);
+  });
+
+  it("reads an alpha pix_fmt with no tag", () => {
+    expect(videoStreamHasAlpha({ pix_fmt: "yuva444p10le" })).toBe(true);
+  });
+
+  it("reports opaque streams", () => {
+    expect(videoStreamHasAlpha({ pix_fmt: "yuv420p" })).toBe(false);
+    expect(videoStreamHasAlpha({ pix_fmt: "yuv420p", tags: { ALPHA_MODE: "0" } })).toBe(false);
+    expect(videoStreamHasAlpha({ pix_fmt: "yuv420p", tags: { ENCODER: "Lavc" } })).toBe(false);
+  });
+});
+
+describe("alphaDecoderFor", () => {
+  it("picks the libvpx decoders that read the alpha side channel", () => {
+    expect(alphaDecoderFor("vp9")).toBe("libvpx-vp9");
+    expect(alphaDecoderFor("VP9")).toBe("libvpx-vp9");
+    expect(alphaDecoderFor("vp8")).toBe("libvpx");
+  });
+
+  it("leaves other codecs on ffmpeg's default decoder", () => {
+    expect(alphaDecoderFor("prores")).toBeNull();
+    expect(alphaDecoderFor("h264")).toBeNull();
+    expect(alphaDecoderFor(undefined)).toBeNull();
+  });
+});
diff --git a/packages/parsers/src/mediaAlpha.ts b/packages/parsers/src/mediaAlpha.ts
new file mode 100644
index 0000000000..4f208fc2c8
--- /dev/null
+++ b/packages/parsers/src/mediaAlpha.ts
@@ -0,0 +1,27 @@
+const ALPHA_PIX_FMT_RE = /^(?:yuva|rgba|argb|bgra|abgr|gbrap|ya|ayuv)/i;
+
+export function pixelFormatHasAlpha(pixFmt: string | undefined): boolean {
+  return pixFmt !== undefined && ALPHA_PIX_FMT_RE.test(pixFmt);
+}
+
+export interface FfprobeAlphaFields {
+  pix_fmt?: string;
+  tags?: Record<string, string | undefined>;
+}
+
+/** True when an ffprobe video stream carries alpha. VP8/VP9 alpha rides in a side channel:
+ * ffprobe reports `yuv420p` plus an `alpha_mode=1` tag, whose casing differs across builds. */
+export function videoStreamHasAlpha(stream: FfprobeAlphaFields): boolean {
+  if (pixelFormatHasAlpha(stream.pix_fmt)) return true;
+  return Object.entries(stream.tags ?? {}).some(
+    ([key, value]) => key.toLowerCase() === "alpha_mode" && value === "1",
+  );
+}
+
+/** The decoder that reads a VP8/VP9 alpha side channel; ffmpeg's native ones drop it. */
+export function alphaDecoderFor(codecName: string | undefined): "libvpx-vp9" | "libvpx" | null {
+  const codec = codecName?.toLowerCase();
+  if (codec === "vp9") return "libvpx-vp9";
+  if (codec === "vp8") return "libvpx";
+  return null;
+}
diff --git a/packages/parsers/tsup.config.ts b/packages/parsers/tsup.config.ts
index 75bd2cf17f..c07f14c8f9 100644
--- a/packages/parsers/tsup.config.ts
+++ b/packages/parsers/tsup.config.ts
@@ -21,6 +21,7 @@ export default defineConfig({
     colorGradingContract: "src/colorGradingContract.ts",
     subCompositionValidity: "src/subCompositionValidity.ts",
     ffBinaries: "src/ffBinaries.ts",
+    mediaAlpha: "src/mediaAlpha.ts",
     assetResolution: "src/assetResolution.ts",
     htmlAttributeSpans: "src/htmlAttributeSpans.ts",
     timeSpans: "src/timeSpans.ts",
diff --git a/packages/studio-server/src/helpers/mediaCodecMap.test.ts b/packages/studio-server/src/helpers/mediaCodecMap.test.ts
index cad1265f3b..3ab8ab549e 100644
--- a/packages/studio-server/src/helpers/mediaCodecMap.test.ts
+++ b/packages/studio-server/src/helpers/mediaCodecMap.test.ts
@@ -22,7 +22,10 @@ import {
 const FAKE_FFPROBE_PATH = process.execPath;
 
 function makeRunner(
-  codecByPath: Record<string, string | { codecName: string; pixFmt?: string }>,
+  codecByPath: Record<
+    string,
+    string | { codecName: string; pixFmt?: string; tags?: Record<string, string> }
+  >,
 ): FfprobeRunner {
   return (_command, args) => {
     const filePath = args[args.length - 1] ?? "";
@@ -34,6 +37,7 @@ function makeRunner(
             codec_type: "video",
             codec_name: normalized.codecName,
             pix_fmt: normalized.pixFmt,
+            tags: normalized.tags,
           },
         ]
       : [];
@@ -164,6 +168,23 @@ describe("probeAssetCodec", () => {
     });
   });
 
+  it("flags a VP9 whose alpha is only in the ALPHA_MODE tag, so it gets the VP8 proxy", async () => {
+    const project = tmpProject();
+    const videoPath = join(project, "cutout.webm");
+    writeFileSync(videoPath, "fake video bytes");
+
+    // What ffprobe prints for a libvpx-vp9 yuva420p encode.
+    const facts = await probeAssetCodec(
+      videoPath,
+      makeRunner({
+        [videoPath]: { codecName: "vp9", pixFmt: "yuv420p", tags: { ALPHA_MODE: "1" } },
+      }),
+    );
+
+    expect(facts?.hasAlpha).toBe(true);
+    expect(proxyVariantFor(facts!)).toBe("vp8");
+  });
+
   it("returns null (never throws) when ffprobe is unresolvable", async () => {
     const project = tmpProject();
     const videoPath = join(project, "clip.mp4");
diff --git a/packages/studio-server/src/helpers/mediaCodecMap.ts b/packages/studio-server/src/helpers/mediaCodecMap.ts
index 7650661abe..8966e22422 100644
--- a/packages/studio-server/src/helpers/mediaCodecMap.ts
+++ b/packages/studio-server/src/helpers/mediaCodecMap.ts
@@ -9,7 +9,7 @@ import {
   isUnresolvedAssetPlaceholder,
   resolveExistingLocalAsset as resolveAsset,
 } from "@hyperframes/parsers/asset-resolution";
-import { pixelFormatHasAlpha, probeMediaMetadata, type FfprobeRunner } from "./mediaMetadata.js";
+import { probeMediaMetadata, type FfprobeRunner } from "./mediaMetadata.js";
 
 /**
  * One reusable answer to "what codec is this asset, and is it browser-hostile?",
@@ -24,7 +24,7 @@ export interface AssetCodecFacts {
    * when no representative mime exists (ProRes: browsers never decode it, so
    * the runtime always proxies rather than probing `canPlayType`). */
   representativeMime: string | null;
-  /** Source carries an alpha channel (ffprobe pix_fmt). Alpha sources use a
+  /** Source carries an alpha channel (`MediaMetadata.hasAlpha`). Alpha sources use a
    * VP8/WebM proxy so their transparency is preserved across Chromium builds. */
   hasAlpha: boolean;
 }
@@ -205,7 +205,7 @@ async function probeCodecFacts(
   if (metadata.kind !== "video") return null;
   const codecName = metadata.color.codecName;
   if (!codecName) return null;
-  return codecFactsFor(codecName, pixelFormatHasAlpha(metadata.color.pixelFormat));
+  return codecFactsFor(codecName, metadata.hasAlpha === true);
 }
 
 interface CachedAssetProbe {
diff --git a/packages/studio-server/src/helpers/mediaMetadata.test.ts b/packages/studio-server/src/helpers/mediaMetadata.test.ts
index 414368c595..73006c0e4c 100644
--- a/packages/studio-server/src/helpers/mediaMetadata.test.ts
+++ b/packages/studio-server/src/helpers/mediaMetadata.test.ts
@@ -1,5 +1,5 @@
 import { afterEach, describe, expect, it, vi } from "vitest";
-import { classifyMediaColor, pixelFormatHasAlpha, probeMediaMetadata } from "./mediaMetadata.js";
+import { classifyMediaColor, probeMediaMetadata } from "./mediaMetadata.js";
 
 afterEach(() => vi.unstubAllEnvs());
 
@@ -175,23 +175,3 @@ describe("probeMediaMetadata", () => {
     expect(metadata).toMatchObject({ kind: "video", color: { codecName: "h264" } });
   });
 });
-
-describe("pixelFormatHasAlpha", () => {
-  it("detects alpha-bearing pixel formats", () => {
-    for (const pixFmt of [
-      "yuva420p",
-      "yuva444p10le",
-      "rgba",
-      "argb",
-      "bgra",
-      "abgr",
-      "gbrap12le",
-      "ya8",
-    ]) {
-      expect(pixelFormatHasAlpha(pixFmt)).toBe(true);
-    }
-    for (const pixFmt of ["yuv420p", "yuv422p10le", "rgb24", "gbrp", "gray", undefined]) {
-      expect(pixelFormatHasAlpha(pixFmt)).toBe(false);
-    }
-  });
-});
diff --git a/packages/studio-server/src/helpers/mediaMetadata.ts b/packages/studio-server/src/helpers/mediaMetadata.ts
index d99b0f53aa..ccfb415fa0 100644
--- a/packages/studio-server/src/helpers/mediaMetadata.ts
+++ b/packages/studio-server/src/helpers/mediaMetadata.ts
@@ -6,6 +6,7 @@ import {
   type ToneMapSourceColour,
 } from "@hyperframes/core";
 import { findFfBinary } from "@hyperframes/parsers/ff-binaries";
+import { videoStreamHasAlpha } from "@hyperframes/parsers/media-alpha";
 
 export interface FfprobeRunResult {
   status: number | null;
@@ -73,6 +74,8 @@ export interface MediaMetadata {
   color: MediaColorMetadata;
   /** Video audio stream, when probed. Absent means the drop path stays muted. */
   hasAudio?: boolean;
+  /** Video stream carries alpha (pix_fmt or the VP8/VP9 alpha_mode tag). */
+  hasAlpha?: boolean;
   probeError?: string;
 }
 
@@ -86,6 +89,7 @@ interface FfprobeStream {
   color_primaries?: string;
   bits_per_raw_sample?: string;
   disposition?: { attached_pic?: number };
+  tags?: Record<string, string | undefined>;
 }
 
 const VIDEO_EXT = new Set([
@@ -137,17 +141,6 @@ function colorLabel(input: {
   return "SDR/unknown";
 }
 
-// Conservative alpha-bearing pix_fmt list: yuva* (yuva420p, yuva444p10le...),
-// rgba/argb/bgra/abgr (packed RGB+alpha), gbrap* (planar GBR+alpha, ProRes
-// 4444 decodes to these), ya* (gray+alpha). Prefix match keeps bit-depth /
-// endianness suffixes covered.
-const ALPHA_PIX_FMT_RE = /^(?:yuva|rgba|argb|bgra|abgr|gbrap|ya)/;
-
-/** True when an ffprobe `pix_fmt` carries an alpha component. */
-export function pixelFormatHasAlpha(pixFmt: string | undefined): boolean {
-  return pixFmt !== undefined && ALPHA_PIX_FMT_RE.test(pixFmt.toLowerCase());
-}
-
 export function classifyMediaColor(stream: FfprobeStream | null | undefined): MediaColorMetadata {
   const colorPrimaries = lower(stream?.color_primaries);
   const colorSpace = lower(stream?.color_space);
@@ -222,7 +215,7 @@ export async function probeMediaMetadata(
       "-v",
       "error",
       "-show_entries",
-      "stream=codec_type,codec_name,profile,pix_fmt,color_space,color_transfer,color_primaries,bits_per_raw_sample:stream_disposition=attached_pic",
+      "stream=codec_type,codec_name,profile,pix_fmt,color_space,color_transfer,color_primaries,bits_per_raw_sample:stream_disposition=attached_pic:stream_tags=alpha_mode",
       "-of",
       "json",
       "--",
@@ -247,6 +240,7 @@ export async function probeMediaMetadata(
     const metadata: MediaMetadata = { kind, color: classifyMediaColor(stream) };
     if (kind === "video") {
       metadata.hasAudio = (parsed.streams ?? []).some((item) => item.codec_type === "audio");
+      metadata.hasAlpha = stream !== undefined && videoStreamHasAlpha(stream);
     }
     return metadata;
   } catch {
diff --git a/packages/studio-server/src/helpers/proxyTranscoder.test.ts b/packages/studio-server/src/helpers/proxyTranscoder.test.ts
index 74e7785354..f4ba57d4af 100644
--- a/packages/studio-server/src/helpers/proxyTranscoder.test.ts
+++ b/packages/studio-server/src/helpers/proxyTranscoder.test.ts
@@ -23,13 +23,21 @@ const realFfmpegFilters = realFfmpeg
   : "";
 const canToneMapForReal =
   Boolean(findFfBinary("ffprobe")) && /\szscale\s/.test(realFfmpegFilters ?? "");
+const realFfmpegEncoders = realFfmpeg
+  ? spawnSync(realFfmpeg, ["-hide_banner", "-encoders"], { encoding: "utf8" }).stdout
+  : "";
+const canEncodeVpxAlphaForReal =
+  Boolean(findFfBinary("ffprobe")) &&
+  /\slibvpx\s/.test(realFfmpegEncoders ?? "") &&
+  /\slibvpx-vp9\s/.test(realFfmpegEncoders ?? "");
 
-function meanLuma(path: string, filters = ""): number {
+function meanLuma(path: string, filters = "", inputArgs: string[] = []): number {
   const stats = execFileSync(
     realFfmpeg!,
     [
       "-v",
       "error",
+      ...inputArgs,
       "-i",
       path,
       "-frames:v",
@@ -134,6 +142,7 @@ async function loadModule(
   ffmpegPath: string | undefined,
   isHdr = false,
   hdrTransfer: string | null = isHdr ? "pq" : null,
+  codecName?: string,
 ): Promise<typeof import("./proxyTranscoder.js")> {
   vi.resetModules();
   vi.doMock("node:child_process", () => {
@@ -146,7 +155,7 @@ async function loadModule(
   vi.doMock("./mediaMetadata.js", () => ({
     probeMediaMetadata: async () => ({
       kind: "video",
-      color: { isHdr, hdrTransfer },
+      color: { isHdr, hdrTransfer, codecName },
     }),
     probeFirstFrameColour: async () => ({}),
   }));
@@ -267,6 +276,57 @@ describe("resolveProxy", () => {
     await expect(resultPromise).resolves.toBe(vp8Path);
   });
 
+  it("decodes a VP9 alpha source with libvpx-vp9 so the VP8 proxy keeps its alpha", async () => {
+    const { spawn, calls } = createSpawnSpy();
+    const { resolveProxy } = await loadModule(spawn, FFMPEG_PATH, false, null, "vp9");
+    const projectDir = tmpProject();
+    const sourcePath = join(projectDir, "cutout.webm");
+    writeFileSync(sourcePath, "source-bytes");
+
+    const vp8 = resolveProxy(projectDir, sourcePath, "vp8");
+    await flush();
+    const vp8Args = calls[0]!.args;
+    expect(vp8Args.slice(vp8Args.indexOf("-i") - 2, vp8Args.indexOf("-i"))).toEqual([
+      "-c:v",
+      "libvpx-vp9",
+    ]);
+    succeed(calls[0]!, "fake-vp8-bytes");
+    await vp8;
+
+    const h264 = resolveProxy(projectDir, sourcePath, "h264");
+    await flush();
+    expect(calls[1]!.args.slice(0, calls[1]!.args.indexOf("-i"))).not.toContain("-c:v");
+    succeed(calls[1]!);
+    await h264;
+  });
+
+  it.skipIf(!canEncodeVpxAlphaForReal)(
+    "keeps a real VP9 alpha webm transparent through probe, variant and transcode",
+    async () => {
+      vi.resetModules();
+      vi.doUnmock("./mediaMetadata.js");
+      const { resolveProxy } = await import("./proxyTranscoder.js");
+      const { probeAssetCodec, proxyVariantFor } = await import("./mediaCodecMap.js");
+      const projectDir = tmpProject();
+      const sourcePath = join(projectDir, "cutout.webm");
+      // Left half fully transparent, right half opaque: mean alpha 127.5.
+      execFileSync(realFfmpeg!, [
+        ...["-v", "error", "-f", "lavfi", "-i"],
+        "testsrc2=s=64x64:d=0.2,format=rgba,geq=r='r(X,Y)':g='g(X,Y)':b='b(X,Y)':a='if(lt(X,32),0,255)'",
+        ...["-c:v", "libvpx-vp9", "-pix_fmt", "yuva420p", "-auto-alt-ref", "0"],
+        ...["-metadata:s:v:0", "alpha_mode=1", sourcePath],
+      ]);
+
+      const facts = await probeAssetCodec(sourcePath);
+      expect(facts).toMatchObject({ codecName: "vp9", hasAlpha: true });
+      const proxyPath = await resolveProxy(projectDir, sourcePath, proxyVariantFor(facts!));
+      expect(proxyPath).toMatch(/\.webm$/);
+      const meanAlpha = meanLuma(proxyPath, "alphaextract,", ["-c:v", "libvpx"]);
+      expect(Math.abs(meanAlpha - 127.5)).toBeLessThan(8);
+    },
+    60_000,
+  );
+
   it("returns without spawning on a cache hit", async () => {
     const { spawn, calls } = createSpawnSpy();
     const { resolveProxy, getProxyCachePath } = await loadModule(spawn, FFMPEG_PATH);
diff --git a/packages/studio-server/src/helpers/proxyTranscoder.ts b/packages/studio-server/src/helpers/proxyTranscoder.ts
index 79c2a94235..70855fd1fe 100644
--- a/packages/studio-server/src/helpers/proxyTranscoder.ts
+++ b/packages/studio-server/src/helpers/proxyTranscoder.ts
@@ -8,6 +8,7 @@ import {
   type PreviewProxyBox,
 } from "@hyperframes/core";
 import { findFfBinary } from "@hyperframes/parsers/ff-binaries";
+import { alphaDecoderFor } from "@hyperframes/parsers/media-alpha";
 import { probeFirstFrameColour, probeMediaMetadata } from "./mediaMetadata.js";
 import { cleanupProxyCache } from "./proxyCache.js";
 import { PROXY_VARIANT_CONFIG, type ProxyVariant } from "./mediaCodecMap.js";
@@ -444,7 +445,9 @@ async function runFfmpeg(
     : [evenScale, `format=${pixelFormat}`].join(",");
 
   return new Promise((resolvePromise, reject) => {
-    const commonArgs = ["-y", "-i", sourcePath, "-vf", videoFilter];
+    const alphaDecoder = keepsAlpha ? alphaDecoderFor(metadata.color.codecName) : null;
+    const decoderArgs = alphaDecoder ? ["-c:v", alphaDecoder] : [];
+    const commonArgs = ["-y", ...decoderArgs, "-i", sourcePath, "-vf", videoFilter];
     const h264Args = [
       "-c:v",
       "libx264",
diff --git a/scripts/package-size-budgets.json b/scripts/package-size-budgets.json
index 38f896dcc5..97b4031c72 100644
--- a/scripts/package-size-budgets.json
+++ b/scripts/package-size-budgets.json
@@ -6,7 +6,7 @@
   "@hyperframes/gcp-cloud-run": { "packed": 49412, "unpacked": 192672 },
   "@hyperframes/inspector": { "packed": 808905, "unpacked": 3645432 },
   "@hyperframes/lint": { "packed": 168745, "unpacked": 654607 },
-  "@hyperframes/parsers": { "packed": 190591, "unpacked": 831243 },
+  "@hyperframes/parsers": { "packed": 191000, "unpacked": 831243 },
   "@hyperframes/player": { "packed": 157145, "unpacked": 541601 },
   "@hyperframes/producer": { "packed": 768210, "unpacked": 2807938 },
   "@hyperframes/sdk": { "packed": 90662, "unpacked": 325069 },