Fix FCPXML import failing DTD validation in Final Cut Pro

lib/serializeTimeline.ts

@@ -175,6 +175,23 @@ export function timelineExtension(format: TimelineExportFormat): string {
   return TIMELINE_FORMATS.find((f) => f.value === format)?.ext ?? format;
 }
 
+/**
+ * Drop `modDate` from the exported project.
+ *
+ * Final Cut Pro rejects the entire document with "DTD validation failed" when
+ * it cannot parse this attribute. The upstream writer stamps an IANA zone name
+ * (`2026-08-29 12:37:45 America/Los_Angeles`) where FCP wants a numeric UTC
+ * offset (`-0700`), and FCP 10.6.x additionally only accepts the clock format
+ * matching the user's system 12/24-hour setting. modDate is optional and we
+ * have nothing meaningful to put in it, so the safe answer is to omit it.
+ *
+ * Attribute values are XML-escaped by the writer, so `"` and `>` cannot appear
+ * inside one — matching up to the tag's `>` is safe even for odd file names.
+ */
+export function stripFcpxmlModDate(xml: string): string {
+  return xml.replace(/(<project\b[^>]*?)\s+modDate="[^"]*"/g, "$1");
+}
+
 export function serializeTimelineXml(
   options: TimelineExportOptions,
   format: Exclude<TimelineExportFormat, "aaf">
@@ -193,7 +210,15 @@ export function serializeTimelineXml(
     return writeXMEML(timeline);
   }
   if (format === "premiere") return writeXMEML(timeline);
-  return writeFCPXML(timeline);
+
+  // A single FCP asset-clip carries both the video and the audio of its asset.
+  // Our parallel V1/A1 tracks are the same media over the same ranges, so
+  // keeping both makes the writer attach A1 as connected clips in lane 1 —
+  // FCP imports that as a duplicate video overlay with doubled audio.
+  if (timeline.tracks.length > 1) {
+    timeline.tracks = timeline.tracks.filter((t) => t.kind === "video");
+  }
+  return stripFcpxmlModDate(writeFCPXML(timeline));
 }
 
 export async function serializeTimelineAaf(

tests/serialize-timeline-test.ts

@@ -8,6 +8,7 @@ import {
   buildNleTimeline,
   mediaFileUrl,
   serializeTimelineXml,
+  stripFcpxmlModDate,
 } from "../lib/serializeTimeline";
 import {
   AAF_MAX_CLIPS,
@@ -134,9 +135,74 @@ async function main() {
   assert(fcpx.includes("<fcpxml"), "fcpxml root");
   assert(fcpx.includes("<spine>") || fcpx.includes("<spine "), "fcpxml spine");
   assert(fcpx.includes("asset-clip") || fcpx.includes("asset"), "fcpxml assets");
+  // FCP fails DTD validation on an unparseable modDate, so we omit it.
+  assert(!fcpx.includes("modDate"), "fcpxml has no modDate");
+  // V1/A1 are the same media over the same ranges; a single asset-clip per
+  // range carries both, so no duplicate connected clips in lane 1.
+  assert(!fcpx.includes("lane="), "fcpxml has no connected-clip lanes");
+  assert(
+    fcpx.match(/<asset-clip/g)?.length === keeps.length,
+    "one fcpxml asset-clip per keep range"
+  );
   console.log("fcpxml: ok");
 }
 
+{
+  // Audio-only projects have a single track and must still land on the spine.
+  const fcpx = serializeTimelineXml(
+    {
+      keepRanges: keeps,
+      duration: 5,
+      mediaFileName: "podcast.m4a",
+      frameRate: "30",
+      withVideo: false,
+      withAudio: true,
+    },
+    "fcpx"
+  );
+  assert(fcpx.includes('hasAudio="1"'), "fcpxml audio-only asset");
+  assert(!fcpx.includes('hasVideo="1"'), "fcpxml audio-only has no video");
+  assert(
+    fcpx.match(/<asset-clip/g)?.length === keeps.length,
+    "audio-only fcpxml asset-clips"
+  );
+  console.log("fcpxml audio-only: ok");
+}
+
+{
+  assert(
+    stripFcpxmlModDate('<project name="a" modDate="2026-01-01 00:00:00 UTC">') ===
+      '<project name="a">',
+    "strips modDate"
+  );
+  assert(
+    stripFcpxmlModDate('<project modDate="x" uid="u"/>') === '<project uid="u"/>',
+    "strips modDate mid-tag"
+  );
+  // Only the project tag; a clip named "modDate=..." must survive untouched.
+  const kept = '<asset-clip name="my modDate=&quot;x&quot; take.mp4"/>';
+  assert(stripFcpxmlModDate(kept) === kept, "leaves non-project tags alone");
+  console.log("strip modDate: ok");
+}
+
+{
+  // Premiere/Resolve still need the parallel audio track.
+  const premiere = serializeTimelineXml(
+    {
+      keepRanges: keeps,
+      duration: 5,
+      mediaFileName: "interview.mp4",
+      frameRate: "24",
+      withVideo: true,
+      withAudio: true,
+    },
+    "premiere"
+  );
+  assert(premiere.includes("<video>"), "premiere keeps video track");
+  assert(premiere.includes("<audio>"), "premiere keeps audio track");
+  console.log("xmeml tracks intact: ok");
+}
+
 {
   // Audio-only timeline
   const xml = serializeTimelineXml(