@@ -152,11 +152,48 @@ export function hlsArgs(o: HlsOptions): string[] {
   ];
 }
 
+/**
+ * RFC 6381 codec string for an H.264 stream: avc1.PPCCLL.
+ *
+ * PP is profile_idc, CC the constraint flags, LL the level, each as two hex
+ * digits. ffprobe reports the profile by name and the level as an integer, so
+ * the name is mapped back to its idc.
+ *
+ * This is derived from the encoded file rather than hardcoded because the
+ * declaration is load-bearing: a player reads CODECS to choose a decoder
+ * configuration before it fetches a segment, and a wrong level is a promise
+ * about the bitstream that the bitstream does not keep.
+ */
+const PROFILE_IDC: Record<string, number> = {
+  "Constrained Baseline": 66,
+  Baseline: 66,
+  Main: 77,
+  Extended: 88,
+  High: 100,
+  "High 10": 110,
+  "High 4:2:2": 122,
+  "High 4:4:4 Predictive": 244,
+};
+
+export function avcCodecString(profile: string | undefined, level: number | undefined): string {
+  const idc = PROFILE_IDC[profile ?? ""] ?? 100;
+  const lvl = typeof level === "number" && level > 0 ? level : 31;
+  const hex = (n: number) => n.toString(16).padStart(2, "0").toUpperCase();
+  // Constraint flags are 0 for the profiles we produce; libx264 sets none that
+  // belong in this field for High.
+  return `avc1.${hex(idc)}00${hex(lvl)}`;
+}
+
+/** AAC-LC, appended only when a rendition actually carries an audio track. */
+export const AAC_LC_CODEC = "mp4a.40.2";
+
 /**
  * The multivariant playlist.
  *
  * Written by hand rather than by ffmpeg's var_stream_map because the bandwidth
- * and codec declarations have to describe the renditions we actually produced,
+ * and codec declarations have to describe the renditions we actually produced
+ * — including not advertising an audio codec for a silent ad, which is a
+ * promise a player will wait on,
  * and because EXT-X-INDEPENDENT-SEGMENTS is deliberately absent: our segments
  * open on a keyframe but are not independently decodable in the sense that tag
  * asserts, and claiming it would be a lie a player acts on.
 
@@ -18,6 +18,8 @@ import {
 import { composeDocument, type ComposeAssets } from "./compose";
 import { validateSnapshot, type VideoDesignSnapshot } from "./snapshot";
 import {
+  AAC_LC_CODEC,
+  avcCodecString,
   encodeAudioCompanion,
   encodeMp4,
   extractPoster,
@@ -232,19 +234,30 @@ export async function renderPreroll(args: {
   for (const profile of ["mp4_720p", "mp4_480p"] as const) {
     const spec = videoProfile(profile);
     const name = `${spec.height}p`;
-    await packageHls({ inPath: path.join(outDir, `${profile}.mp4`), outDir: hlsDir, name });
+    const renditionPath = path.join(outDir, `${profile}.mp4`);
+    await packageHls({ inPath: renditionPath, outDir: hlsDir, name });
 
     const playlist = await readFile(path.join(hlsDir, `${name}.m3u8`), "utf8");
     problems.push(...validateMediaPlaylist(playlist));
 
-    const mp4Size = statSync(path.join(outDir, `${profile}.mp4`)).size;
+    // Declare what this rendition actually is. Both halves used to be
+    // hardcoded and both were wrong: the level said 4.0 for a stream encoded
+    // at 3.1, and every silent ad advertised an AAC track it did not have.
+    // A player reads CODECS before fetching a segment, so an audio codec named
+    // here is an audio track it will wait for.
+    const probe = await probeMedia(renditionPath);
+    const v = probe.streams.find((st) => st.codec_type === "video");
+    const hasAudio = probe.streams.some((st) => st.codec_type === "audio");
+    const codecs = [avcCodecString(v?.profile, v?.level), ...(hasAudio ? [AAC_LC_CODEC] : [])].join(",");
+
+    const mp4Size = statSync(renditionPath).size;
     renditions.push({
       name,
       width: spec.width!,
       height: spec.height!,
       // Peak bandwidth, derived from the rendition we actually produced.
       bandwidth: Math.round((mp4Size * 8) / 5),
-      codecs: "avc1.640028,mp4a.40.2",
+      codecs,
     });
   }
 
@@ -263,7 +276,7 @@ export async function renderPreroll(args: {
     width: null,
     height: null,
     durationMs: null,
-    codecs: "avc1.640028,mp4a.40.2",
+    codecs: renditions.map((r) => r.codecs).join(" "),
     validation: { ok: true, problems: [] },
     extraFiles: hlsFiles.filter((f) => f !== masterPath),
   });
 
@@ -30,6 +30,8 @@ export type ProbeStream = {
   avg_frame_rate?: string;
   /** Present only with -count_frames; it is a decode, not a header read. */
   nb_read_frames?: string;
+  /** h264 level as an integer, e.g. 31 for level 3.1. */
+  level?: number;
   duration?: string;
   sample_rate?: string;
   channels?: number;
 
@@ -7,7 +7,7 @@ import {
   timeline,
 } from "@/lib/ads/video/compose";
 import type { VideoDesignSnapshot } from "@/lib/ads/video/snapshot";
-import { GOP_FRAMES, hlsArgs, mp4Args, multivariantPlaylist, posterArgs } from "@/lib/ads/video/encode";
+import { AAC_LC_CODEC, avcCodecString, GOP_FRAMES, hlsArgs, mp4Args, multivariantPlaylist, posterArgs } from "@/lib/ads/video/encode";
 import {
   evaluateProbe,
   parseRational,
@@ -195,6 +195,32 @@ describe("encoder arguments carry the media contract", () => {
     expect(poster[poster.indexOf("-ss") + 1]).toBe("2");
   });
 
+it("derives the avc1 codec string from what was actually encoded", () => {
+    // Shipped renditions probe as High @ 3.1. The hardcoded string said level
+    // 4.0 (0x28), which is a promise about the bitstream it does not keep.
+    expect(avcCodecString("High", 31)).toBe("avc1.64001F");
+    expect(avcCodecString("High", 40)).toBe("avc1.640028");
+    expect(avcCodecString("Main", 31)).toBe("avc1.4D001F");
+    expect(avcCodecString("Constrained Baseline", 30)).toBe("avc1.42001E");
+    // Unknown input falls back to High @ 3.1 rather than emitting nonsense.
+    expect(avcCodecString(undefined, undefined)).toBe("avc1.64001F");
+  });
+
+  it("never advertises audio for a silent rendition", () => {
+    // A player reads CODECS before fetching a segment, so an audio codec named
+    // here is an audio track it will wait for. Every ad is silent today.
+    const silent = multivariantPlaylist([
+      { name: "720p", width: 1280, height: 720, bandwidth: 700000, codecs: avcCodecString("High", 31) },
+    ]);
+    expect(silent).toContain('CODECS="avc1.64001F"');
+    expect(silent).not.toContain(AAC_LC_CODEC);
+
+    const narrated = multivariantPlaylist([
+      { name: "720p", width: 1280, height: 720, bandwidth: 700000, codecs: `${avcCodecString("High", 31)},${AAC_LC_CODEC}` },
+    ]);
+    expect(narrated).toContain(AAC_LC_CODEC);
+  });
+
   it("does not claim independent segments in the multivariant playlist", () => {
     const m = multivariantPlaylist([
       { name: "720p", width: 1280, height: 720, bandwidth: 2_400_000, codecs: "avc1.640028" },
 
@@ -148,6 +148,17 @@ describe("a snapshot renders to validated media", () => {
       // Never claimed, because we do not guarantee it.
       expect(master).not.toContain("EXT-X-INDEPENDENT-SEGMENTS");
 
+      // The CODECS declaration must describe what was actually encoded. These
+      // renditions are silent, so naming an audio codec would be an audio
+      // track a player waits for and never receives; and the level has to be
+      // the one in the bitstream, not a guess.
+      expect(master).not.toContain("mp4a");
+      const declared = [...master.matchAll(/CODECS="([^"]+)"/g)].map((m) => m[1]);
+      expect(declared.length).toBe(2);
+      for (const codec of declared) {
+        expect(codec, `codecs=${codec}`).toMatch(/^avc1\.[0-9A-F]{6}$/);
+      }
+
       expect(byProfile.poster.byteSize).toBeGreaterThan(0);
       expect(byProfile.poster.contentType).toBe("image/webp");
     },