feat(distributed): add optional cfr flag for exact constant frame rate

Distributed-render output today uses -c:v copy through concat → mux →
faststart, which means PTS timestamps from each chunk pass through
unchanged. Container r_frame_rate is exact (#1040 + this PR's parent),
but stream-level avg_frame_rate stays PTS-derived and can land on
fractional rationals like 27648000/921677 over a 60s render. Same for
sub-ms duration drift.

This is the achievable bar within -c copy stream-copy concat. For most
consumers (browser playback, YouTube, etc.) the difference is invisible.
For downstream tools that strict-check avg_frame_rate or
ms-precision duration (broadcast workflows, frame-accurate compositors,
some third-party transcoders), it matters.

Adds an opt-in cfr config flag (default false). When true, the
assemble step's final pass re-encodes with -fps_mode cfr -r <fps>
instead of -c copy, producing exact CFR output. Trade-off: ~2-5x the
stitch time for a 60s 1080p clip; second-generation H.264 quality loss
is negligible at -crf 18 but is non-zero.

packages/aws-lambda/src/events.ts

@@ -81,6 +81,16 @@ export interface AssembleEvent {
   OutputS3Uri: string;
   /** Output container format; drives file vs frame-dir handling. */
   Format: DistributedFormat;
+  /**
+   * Optional exact-CFR re-encode at assemble time. When `true`, the final
+   * assembled video is re-encoded with `-fps_mode cfr -r <fps>` so the
+   * stream's `avg_frame_rate` matches the container's `r_frame_rate`
+   * exactly (and the file's duration is exact, not PTS-derived). Trade-off
+   * is ~2-5x the assemble wall-clock. mp4 only — webm / mov stream-copy
+   * paths already produce exact avg_frame_rate. Default `false` /
+   * unset preserves current `-c copy` behavior.
+   */
+  Cfr?: boolean;
 }
 
 /**

packages/aws-lambda/src/handler.ts

@@ -407,7 +407,9 @@ async function handleAssemble(
         ? join(work, "output-frames")
         : join(work, `output${formatExtension(event.Format)}`);
 
-    const result: AssembleResult = await primitive(planDir, chunkPaths, audioPath, finalOutput);
+    const result: AssembleResult = await primitive(planDir, chunkPaths, audioPath, finalOutput, {
+      cfr: event.Cfr === true,
+    });
 
     if (event.Format === "png-sequence") {
       const tarball = `${finalOutput}.tar.gz`;

packages/producer/src/services/distributed/assemble.test.ts

@@ -321,6 +321,147 @@ describe("assemble()", () => {
     TIMEOUT_MS,
   );
 
+  it(
+    "cfr:true re-encodes for exact avg_frame_rate matching r_frame_rate",
+    async () => {
+      if (!hasFfmpeg) {
+        console.warn("[assemble.test] skipping cfr test — ffmpeg not available on this host");
+        return;
+      }
+
+      // Opt-in CFR: the re-encode pass with `-fps_mode cfr -r <fps>` must
+      // land the stream's `avg_frame_rate` on the requested rational
+      // exactly, not a PTS-derived fraction. Default `cfr=false` path is
+      // covered by the existing concat-copy tests above.
+      const chunks: ChunkSliceJson[] = [
+        { index: 0, startFrame: 0, endFrame: 5 },
+        { index: 1, startFrame: 5, endFrame: 10 },
+      ];
+      const planDir = buildPlanDir("mp4", chunks, 10, false);
+
+      const chunkAPath = join(planDir, "chunk-0.mp4");
+      const chunkBPath = join(planDir, "chunk-1.mp4");
+      makeMp4Chunk(chunkAPath, 5);
+      makeMp4Chunk(chunkBPath, 5);
+
+      const outputPath = join(planDir, "output-cfr.mp4");
+      const result = await assemble(planDir, [chunkAPath, chunkBPath], null, outputPath, {
+        cfr: true,
+      });
+
+      expect(result.outputPath).toBe(outputPath);
+      expect(existsSync(outputPath)).toBe(true);
+      expect(result.framesEncoded).toBe(10);
+
+      // ffprobe both r_frame_rate AND avg_frame_rate — the CFR re-encode's
+      // contract is that they're equal and both exactly match the
+      // requested rate.
+      const probe = spawnSync(
+        "ffprobe",
+        [
+          "-v",
+          "error",
+          "-select_streams",
+          "v:0",
+          "-show_entries",
+          "stream=r_frame_rate,avg_frame_rate,duration",
+          "-of",
+          "json",
+          outputPath,
+        ],
+        { stdio: "pipe" },
+      );
+      expect(probe.status).toBe(0);
+      const parsed = JSON.parse(probe.stdout.toString()) as {
+        streams?: Array<{ r_frame_rate?: string; avg_frame_rate?: string; duration?: string }>;
+      };
+      const stream = parsed.streams?.[0];
+      expect(stream).toBeDefined();
+      expect(stream?.r_frame_rate).toBe("30/1");
+      expect(stream?.avg_frame_rate).toBe("30/1");
+      const expectedDuration = 10 / 30;
+      const probedDuration = Number(stream?.duration ?? 0);
+      expect(Math.abs(probedDuration - expectedDuration)).toBeLessThan(0.001);
+    },
+    TIMEOUT_MS,
+  );
+
+  it(
+    "cfr:true rejects non-mp4 formats with a clear error",
+    async () => {
+      const chunks: ChunkSliceJson[] = [{ index: 0, startFrame: 0, endFrame: 5 }];
+      // png-sequence path short-circuits before the cfr check; webm/mov
+      // would hit the runtime guard. We rebuild plan.json with a non-mp4
+      // format manually so this test runs without a webm encoder.
+      const planDir = mkdtempSync(join(runRoot, "plan-webm-cfr-"));
+      mkdirSync(join(planDir, "meta"), { recursive: true });
+      writeFileSync(
+        join(planDir, "plan.json"),
+        JSON.stringify({
+          planHash: "fake",
+          totalFrames: 5,
+          hasAudio: false,
+          dimensions: { fpsNum: 30, fpsDen: 1, width: 160, height: 120, format: "webm" },
+        }),
+        "utf-8",
+      );
+      writeFileSync(join(planDir, "meta", "chunks.json"), JSON.stringify(chunks), "utf-8");
+      // Fabricate a placeholder file so the existence check passes — the
+      // cfr-guard error fires before we actually run the concat invocation
+      // in the multi-chunk branch; the single-chunk remux path runs first
+      // here, then we hit the cfr guard. Since the remux is real, only
+      // run this test when ffmpeg is present.
+      if (!hasFfmpeg) {
+        console.warn("[assemble.test] skipping cfr-non-mp4 test — ffmpeg not available");
+        return;
+      }
+      const chunkPath = join(planDir, "chunk-0.webm");
+      // Build a real 5-frame webm chunk so the concat step succeeds and
+      // the cfr guard is what actually trips.
+      const buildResult = spawnSync("ffmpeg", [
+        "-v",
+        "error",
+        "-f",
+        "lavfi",
+        "-i",
+        "testsrc=size=160x120:rate=30:duration=0.166666",
+        "-c:v",
+        "libvpx-vp9",
+        "-row-mt",
+        "1",
+        "-deadline",
+        "realtime",
+        "-cpu-used",
+        "8",
+        "-g",
+        "5",
+        "-keyint_min",
+        "5",
+        "-pix_fmt",
+        "yuv420p",
+        "-vframes",
+        "5",
+        "-y",
+        chunkPath,
+      ]);
+      if (buildResult.status !== 0) {
+        console.warn(
+          "[assemble.test] skipping cfr-non-mp4 test — libvpx-vp9 not available on this host",
+        );
+        return;
+      }
+      let caught: unknown;
+      try {
+        await assemble(planDir, [chunkPath], null, join(planDir, "out.webm"), { cfr: true });
+      } catch (err) {
+        caught = err;
+      }
+      expect(caught).toBeDefined();
+      expect((caught as Error).message).toContain("cfr=true is only supported");
+    },
+    TIMEOUT_MS,
+  );
+
   it(
     "merges png-sequence chunk directories with continuous global numbering",
     () => {

packages/producer/src/services/distributed/assemble.ts

@@ -87,11 +87,29 @@ export async function assemble(
   chunkPaths: readonly string[],
   audioPath: string | null,
   outputPath: string,
-  options?: { logger?: ProducerLogger; abortSignal?: AbortSignal },
+  options?: {
+    logger?: ProducerLogger;
+    abortSignal?: AbortSignal;
+    /**
+     * Opt-in exact-CFR re-encode. When `true`, the assembled video is
+     * re-encoded once at the end of the concat/single-chunk step with
+     * `-fps_mode cfr -r <fps>` so the stream-level `avg_frame_rate`
+     * matches the container's `r_frame_rate` exactly (and the file's
+     * duration lands on the requested `frameCount / fps` to ms
+     * precision, with no PTS-derived drift). Trade-off: ~2-5x the
+     * stitch time for a 60s 1080p clip plus second-generation H.264
+     * quality loss (negligible at `-crf 18` but non-zero). Default
+     * `false` preserves the existing `-c copy` behavior. mp4 only
+     * (libx264); webm / mov pass through unchanged because their
+     * stream-copy paths don't exhibit the same avg-frame-rate drift.
+     */
+    cfr?: boolean;
+  },
 ): Promise<AssembleResult> {
   const start = Date.now();
   const log = options?.logger ?? defaultLogger;
   const abortSignal = options?.abortSignal;
+  const cfr = options?.cfr === true;
 
   // ── 1. Validate planDir manifest matches chunkPaths shape ──────────────
   const planJsonPath = join(planDir, "plan.json");
@@ -193,12 +211,66 @@ export async function assemble(
       }
     }
 
+    // ── 2c. Optional exact-CFR re-encode ──────────────────────────────────
+    // The concat / single-chunk step produces a stream-copy intermediate
+    // whose container `r_frame_rate` is exact but whose stream-level
+    // `avg_frame_rate` stays PTS-derived (concat-copy carries each chunk's
+    // original PTS unmodified). For consumers that strict-check
+    // `avg_frame_rate` or ms-precision duration (broadcast workflows,
+    // frame-accurate compositors, some third-party transcoders), an
+    // opt-in re-encode with `-fps_mode cfr -r <fps>` lands the stream's
+    // avg-frame-rate on the requested rational exactly. Restricted to
+    // mp4 / libx264 — webm and mov go through their own stream-copy
+    // paths that don't exhibit the same avg-frame-rate drift.
+    let postConcatPath = concatOutputPath;
+    if (cfr) {
+      if (plan.dimensions.format !== "mp4") {
+        throw new Error(
+          `[assemble] cfr=true is only supported for format="mp4" (got ` +
+            `"${plan.dimensions.format}"). Stream-copy paths for webm and mov ` +
+            `already produce exact avg_frame_rate; cfr re-encode is not needed.`,
+        );
+      }
+      const cfrOutputPath = join(workDir, `cfr.${plan.dimensions.format}`);
+      const cfrArgs = [
+        "-i",
+        concatOutputPath,
+        "-c:v",
+        "libx264",
+        "-preset",
+        "medium",
+        "-crf",
+        "18",
+        "-pix_fmt",
+        "yuv420p",
+        "-fps_mode",
+        "cfr",
+        "-r",
+        fpsArg,
+        "-y",
+        cfrOutputPath,
+      ];
+      const cfrResult = await runFfmpeg(cfrArgs, { signal: abortSignal });
+      if (!cfrResult.success) {
+        throw new Error(
+          `[assemble] ffmpeg cfr re-encode failed (exit ${cfrResult.exitCode}): ` +
+            `${cfrResult.stderr.slice(-400)}`,
+        );
+      }
+      postConcatPath = cfrOutputPath;
+      log.info("[assemble] cfr re-encode applied", {
+        format: plan.dimensions.format,
+        fpsNum: plan.dimensions.fpsNum,
+        fpsDen: plan.dimensions.fpsDen,
+      });
+    }
+
     // ── 3. Audio: pad-or-trim then mux ────────────────────────────────────
     let audioForMux: string | null = null;
     if (audioPath !== null && existsSync(audioPath)) {
       const paddedAudioPath = join(workDir, "audio-padded.aac");
       const padTrimResult = await padOrTrimAudioToVideoFrameCount({
-        videoPath: concatOutputPath,
+        videoPath: postConcatPath,
         audioPath,
         outputPath: paddedAudioPath,
       });
@@ -218,10 +290,10 @@ export async function assemble(
     // because it operates on a `RenderJob` and emits `updateJobStatus`
     // payloads — the distributed activity has no job to thread through.
     const muxOutputPath =
-      audioForMux !== null ? join(workDir, `mux.${plan.dimensions.format}`) : concatOutputPath;
+      audioForMux !== null ? join(workDir, `mux.${plan.dimensions.format}`) : postConcatPath;
     if (audioForMux !== null) {
       const muxResult = await muxVideoWithAudio(
-        concatOutputPath,
+        postConcatPath,
         audioForMux,
         muxOutputPath,
         abortSignal,

packages/producer/src/services/distributed/plan.ts

@@ -146,6 +146,20 @@ export interface DistributedRenderConfig {
   /** HDR is not supported in distributed mode; `force-hdr` trips a `FormatNotSupportedInDistributedError`. Defaults to `force-sdr`. */
   hdrMode?: "auto" | "force-sdr";
 
+  /**
+   * Opt-in exact-CFR re-encode at the assemble stage. When `true`, the
+   * stitched output is re-encoded once with `-fps_mode cfr -r <fps>` so
+   * the stream-level `avg_frame_rate` matches the container's
+   * `r_frame_rate` exactly (and the file duration is exact, not
+   * PTS-derived). Useful for downstream consumers that strict-check
+   * `avg_frame_rate` or ms-precision duration. Default `false` retains
+   * the existing `-c copy` stitch path, which is faster and lossless.
+   * mp4 only — webm / mov stream-copy paths already produce exact
+   * avg_frame_rate. Consumed by `assemble`; does not affect `planHash`
+   * (chunks render identically; only the final stitch step differs).
+   */
+  cfr?: boolean;
+
   logger?: ProducerLogger;
   /** Optional engine config override (env vars are not read when provided). */
   producerConfig?: EngineConfig;