diff --git a/src/lib/exportGraph.ts b/src/lib/exportGraph.ts
new file mode 100644
index 000000000..aad68743f
--- /dev/null
+++ b/src/lib/exportGraph.ts
@@ -0,0 +1,289 @@
+import { EditRecipe, BackgroundMusicOptions, ImageOverlayOptions } from "./types";
+import { buildTextFilter } from "./text-overlay";
+
+/**
+ * The FFmpeg filter graph and argv for one export.
+ *
+ * This lives outside `ffmpeg.worker.ts` so it is a single implementation that
+ * both the worker and the unit tests execute — the export arguments used to be
+ * duplicated (one copy here, one copy in `ffmpeg.ts` that nothing called), and
+ * the copy that actually ran silently drifted behind it. Keep every export
+ * graph decision in this file.
+ *
+ * Everything here is pure: no worker globals, no DOM, no FFmpeg instance.
+ */
+
+export function buildVideoFilter(recipe: EditRecipe, targetW: number, targetH: number): string {
+  const filters: string[] = [];
+
+  if (recipe.trimStart > 0 || recipe.trimEnd !== null) {
+    // Only use trim filter with precise bounds to avoid scanning entire file.
+    // If trimEnd is null, let FFmpeg infer the end instead of a large placeholder.
+    if (recipe.trimEnd !== null) {
+      filters.push(`trim=start=${recipe.trimStart}:end=${recipe.trimEnd}`);
+    } else if (recipe.trimStart > 0) {
+      filters.push(`trim=start=${recipe.trimStart}`);
+    }
+  }
+
+  if (recipe.stabilization) {
+    filters.push("deshake");
+  }
+
+  if (recipe.rotate === 90) {
+    filters.push("transpose=1");
+  } else if (recipe.rotate === 180) {
+    filters.push("transpose=1,transpose=1");
+  } else if (recipe.rotate === 270) {
+    filters.push("transpose=2");
+  }
+
+  if (recipe.framing === "fit") {
+    filters.push(
+      `scale=${targetW}:${targetH}:force_original_aspect_ratio=decrease`,
+      `pad=${targetW}:${targetH}:(ow-iw)/2:(oh-ih)/2:color=black`
+    );
+  } else {
+    filters.push(
+      `scale=${targetW}:${targetH}:force_original_aspect_ratio=increase`,
+      `crop=${targetW}:${targetH}`
+    );
+  }
+
+  if (recipe.sharpness !== 0) {
+    filters.push(`unsharp=5:5:${recipe.sharpness}:5:5:0.0`);
+  }
+
+  // Normalize timestamps only when needed — trim or speed change both
+  // require a clean 0-based timeline to produce correct output duration.
+  if (recipe.trimStart > 0 || recipe.trimEnd !== null || recipe.speed !== 1) {
+    filters.push("setpts=PTS-STARTPTS");
+  }
+
+  if (recipe.speed !== 1) {
+    const pts = (1 / recipe.speed).toFixed(4);
+    filters.push(`setpts=${pts}*PTS`);
+  }
+
+  if (recipe.denoise) {
+    filters.push("hqdn3d=1.5:1.5:6:6");
+  }
+
+  const needsEq =
+    recipe.brightness !== 0 ||
+    recipe.contrast !== 1 ||
+    recipe.saturation !== 1;
+
+  if (needsEq) {
+    filters.push(
+      `eq=brightness=${recipe.brightness}:contrast=${recipe.contrast}:saturation=${recipe.saturation}`
+    );
+  }
+
+  const textOverlays = recipe.textOverlays || [];
+  textOverlays.forEach((overlay) => {
+    filters.push(buildTextFilter(overlay, targetW, targetH));
+  });
+
+  return filters.join(",");
+}
+
+export function buildAudioFilter(recipe: EditRecipe): string {
+  if (recipe.speed <= 0) return "";
+  const filters: string[] = [];
+
+  let remaining = recipe.speed;
+  while (remaining < 0.5) {
+    filters.push("atempo=0.5");
+    remaining /= 0.5;
+  }
+
+  while (remaining > 2.0) {
+    filters.push("atempo=2.0");
+    remaining /= 2.0;
+  }
+
+  if (Math.abs(remaining - 1.0) > 0.001) {
+    filters.push(`atempo=${Number(remaining.toFixed(4))}`);
+  }
+
+  if (recipe.volume !== undefined && recipe.volume !== 100) {
+    filters.push(`volume=${(recipe.volume / 100).toFixed(2)}`);
+  }
+
+  if (recipe.normalizeAudio) filters.push("loudnorm=I=-14:TP=-1.5:LRA=11");
+
+  return filters.join(",");
+}
+
+export function buildAudioTrimFilter(recipe: EditRecipe): string {
+  if (recipe.trimStart === 0 && recipe.trimEnd === null) return "";
+
+  // Avoid scanning entire audio with large placeholder values;
+  // use precise trim bounds when available.
+  let trimFilter = `atrim=start=${recipe.trimStart}`;
+  if (recipe.trimEnd !== null) {
+    trimFilter += `:end=${recipe.trimEnd}`;
+  }
+  // asetpts normalizes timestamps after trim for correct stream positioning
+  return `${trimFilter},asetpts=PTS-STARTPTS`;
+}
+
+/**
+ * Output duration cap, as FFmpeg arguments.
+ *
+ * Without `-t`, an audio input that outlasts the video decides when encoding
+ * stops: a music track longer than the video extends the output, and looped
+ * music (`-stream_loop -1`) on a video with no audio track never terminates.
+ *
+ * Returns no arguments when the source duration cannot be trusted —
+ * `videoDuration` falls back to 0 when metadata fails to load, and containers
+ * such as MediaRecorder WebM report no duration at all (`Infinity`). Either
+ * value would otherwise reach FFmpeg as `-t 0` (a zero-length export) or
+ * `-t Infinity` ("Invalid duration specification for t: Infinity").
+ */
+export function buildDurationArgs(recipe: EditRecipe, videoDuration: number): string[] {
+  const sourceDuration = (recipe.trimEnd ?? videoDuration) - recipe.trimStart;
+  if (!Number.isFinite(sourceDuration) || sourceDuration <= 0) return [];
+  return ["-t", (sourceDuration / recipe.speed).toFixed(6)];
+}
+
+export function buildArguments(
+  recipe: EditRecipe,
+  format: "mp4" | "webm" | "mkv" | "gif",
+  outputName: string,
+  inputName: string,
+  targetW: number,
+  targetH: number,
+  hasMusicTrack: boolean,
+  musicInputName: string,
+  musicOptions: BackgroundMusicOptions | undefined,
+  hasOverlay: boolean,
+  overlayInputName: string,
+  overlayOptions: ImageOverlayOptions | undefined,
+  hasOriginalAudio: boolean,
+  videoDuration: number
+): string[] {
+  const vf = buildVideoFilter(recipe, targetW, targetH);
+  const audioTrim = hasOriginalAudio ? buildAudioTrimFilter(recipe) : "";
+  const audioSpeed = hasOriginalAudio ? buildAudioFilter(recipe) : "";
+  const afParts = [audioTrim, audioSpeed].filter(Boolean);
+  const af = afParts.join(",");
+
+  const musicIdx = 1;
+  const overlayIdx = hasMusicTrack ? 2 : 1;
+
+  const args: string[] = [];
+  args.push("-i", inputName);
+  if (hasMusicTrack) {
+    if (musicOptions!.loopMusic) args.push("-stream_loop", "-1");
+    args.push("-i", musicInputName);
+  }
+  if (hasOverlay) {
+    args.push("-i", overlayInputName);
+  }
+
+  const needsFilterComplex = hasOverlay || hasMusicTrack;
+  const shouldKeepAudio = recipe.keepAudio && (hasOriginalAudio || hasMusicTrack);
+
+  if (needsFilterComplex) {
+    const filterParts: string[] = [];
+    let videoOut = "[0:v]";
+
+    if (vf) {
+      filterParts.push(`[0:v]${vf}[vbase]`);
+      videoOut = "[vbase]";
+    }
+
+    if (hasOverlay) {
+      const scaledW = overlayOptions!.size;
+      const alpha = (overlayOptions!.opacity / 100).toFixed(2);
+      const posMap: Record<string, string> = {
+        "top-left": "20:20",
+        "top-right": "W-w-20:20",
+        "bottom-left": "20:H-h-20",
+        "bottom-right": "W-w-20:H-h-20",
+      };
+
+      interface PositionCoords {
+        x: number;
+        y: number;
+      }
+
+      const pos = typeof overlayOptions?.position === "string"
+        ? (posMap[overlayOptions.position] ?? "W-w-20:H-h-20")
+        : overlayOptions?.position
+        ? `(W-w)*${(overlayOptions.position as PositionCoords).x}/100:(H-h)*${(overlayOptions.position as PositionCoords).y}/100`
+        : "W-w-20:H-h-20";
+
+      filterParts.push(`[${overlayIdx}:v]scale=${scaledW}:-2,format=rgba,colorchannelmixer=aa=${alpha}[logo]`);
+      filterParts.push(`${videoOut}[logo]overlay=${pos}[vout]`);
+      videoOut = "[vout]";
+    }
+
+    let audioOut = "";
+    if (shouldKeepAudio) {
+      if (hasMusicTrack) {
+        const musicVol = (musicOptions!.musicVolume / 100).toFixed(2);
+        if (hasOriginalAudio) {
+          const origVol = (musicOptions!.originalAudioVolume / 100).toFixed(2);
+          const origChain = afParts.length > 0
+            ? `[0:a]${afParts.join(",")},volume=${origVol}[orig]`
+            : `[0:a]volume=${origVol}[orig]`;
+          filterParts.push(origChain);
+          filterParts.push(`[${musicIdx}:a]volume=${musicVol}[music]`);
+          filterParts.push(`[orig][music]amix=inputs=2:duration=first:dropout_transition=0[aout]`);
+          audioOut = "[aout]";
+        } else {
+          filterParts.push(`[${musicIdx}:a]volume=${musicVol}[aout]`);
+          audioOut = "[aout]";
+        }
+      } else if (hasOriginalAudio && af) {
+        filterParts.push(`[0:a]${af}[aout]`);
+        audioOut = "[aout]";
+      }
+    }
+
+    if (filterParts.length > 0) {
+      args.push("-filter_complex", filterParts.join(";"));
+    }
+    args.push("-map", videoOut === "[0:v]" ? "0:v" : videoOut);
+
+    if (!shouldKeepAudio) {
+      args.push("-an");
+    } else if (audioOut) {
+      args.push("-map", audioOut);
+    } else if (hasOriginalAudio) {
+      args.push("-map", "0:a");
+    }
+  } else {
+    if (vf) args.push("-vf", vf);
+    if (!shouldKeepAudio) {
+      args.push("-an");
+    } else if (af && hasOriginalAudio) {
+      args.push("-af", af);
+    }
+  }
+
+  if (format === "webm") {
+    args.push(
+      "-c:v", "libvpx-vp9",
+      "-b:v", "0",
+      "-crf", String(recipe.quality),
+      "-cpu-used", "4",
+      "-deadline", "realtime"
+    );
+    if (shouldKeepAudio) args.push("-c:a", "libopus");
+  } else if (format === "mkv") {
+    args.push("-c:v", "libx264", "-crf", String(recipe.quality), "-preset", "ultrafast");
+    if (shouldKeepAudio) args.push("-c:a", "aac", "-b:a", "128k");
+  } else {
+    args.push("-c:v", "libx264", "-crf", String(recipe.quality), "-preset", "ultrafast", "-movflags", "+faststart");
+    if (shouldKeepAudio) args.push("-c:a", "aac", "-b:a", "128k");
+  }
+
+  args.push(...buildDurationArgs(recipe, videoDuration));
+
+  args.push(outputName);
+  return args;
+}
diff --git a/src/lib/ffmpeg.ts b/src/lib/ffmpeg.ts
index 1251ac076..9d8540a5e 100644
--- a/src/lib/ffmpeg.ts
+++ b/src/lib/ffmpeg.ts
@@ -1,7 +1,7 @@
 import { EditRecipe, ExportResult, BackgroundMusicOptions, ImageOverlayOptions, MAX_FILE_SIZE } from "./types";
 import { getPresetById } from "./presets";
-import { buildTextFilter } from "./text-overlay";
 import { trackAction, trackTiming } from "./telemetry";
+export { buildVideoFilter, buildAudioFilter } from "./exportGraph";
 
 export class FFmpegLoadError extends Error {}
 
@@ -376,268 +376,6 @@ function buildSessionId(): string {
   return `${timestamp}-${randomPart}${counterPart}`;
 }
 
-export function buildVideoFilter(recipe: EditRecipe, targetW: number, targetH: number): string {
-  const filters: string[] = [];
-
-  if (recipe.trimStart > 0 || recipe.trimEnd !== null) {
-    // Only use trim filter with precise bounds to avoid scanning entire file
-    // If trimEnd is null, use duration parameter instead of large placeholder value
-    if (recipe.trimEnd !== null) {
-      filters.push(`trim=start=${recipe.trimStart}:end=${recipe.trimEnd}`);
-    } else if (recipe.trimStart > 0) {
-      // When only trimStart is set, let FFmpeg infer end (don't use 999999 placeholder)
-      filters.push(`trim=start=${recipe.trimStart}`);
-    }
-  }
-
-  if (recipe.stabilization) {
-    filters.push("deshake");
-  }
-
-  if (recipe.rotate === 90) {
-    filters.push("transpose=1");
-  } else if (recipe.rotate === 180) {
-    filters.push("transpose=1,transpose=1");
-  } else if (recipe.rotate === 270) {
-    filters.push("transpose=2");
-  }
-
-  if (recipe.framing === "fit") {
-    filters.push(
-      `scale=${targetW}:${targetH}:force_original_aspect_ratio=decrease`,
-      `pad=${targetW}:${targetH}:(ow-iw)/2:(oh-ih)/2:color=black`
-    );
-  } else {
-    filters.push(
-      `scale=${targetW}:${targetH}:force_original_aspect_ratio=increase`,
-      `crop=${targetW}:${targetH}`
-    );
-  }
-  if (recipe.sharpness !== 0) {
-    filters.push(`unsharp=5:5:${recipe.sharpness}:5:5:0.0`);
-  }
-
-  // Normalize timestamps only when needed — trim or speed change both
-  // require a clean 0-based timeline to produce correct output duration.
-  if (recipe.trimStart > 0 || recipe.trimEnd !== null || recipe.speed !== 1) {
-    filters.push("setpts=PTS-STARTPTS");
-  }
-
-  if (recipe.speed !== 1) {
-    const pts = (1 / recipe.speed).toFixed(4);
-    filters.push(`setpts=${pts}*PTS`);
-  }
-
-  if (recipe.denoise) {
-    filters.push("hqdn3d=1.5:1.5:6:6");
-  }
-
-  const needsEq =
-    recipe.brightness !== 0 ||
-    recipe.contrast !== 1 ||
-    recipe.saturation !== 1;
-
-  if (needsEq) {
-    filters.push(
-      `eq=brightness=${recipe.brightness}:contrast=${recipe.contrast}:saturation=${recipe.saturation}`
-    );
-  }
-
-  // Add text overlays
-  const textOverlays = recipe.textOverlays || [];
-  textOverlays.forEach((overlay) => {
-    filters.push(buildTextFilter(overlay, targetW, targetH));
-  });
-
-  return filters.join(",");
-}
-
-export function buildAudioFilter(recipe: EditRecipe): string {
-  if (recipe.speed <= 0) return "";
-  const filters: string[] = [];
-
-  let remaining = recipe.speed;
-  while (remaining < 0.5) {
-    filters.push("atempo=0.5");
-    remaining /= 0.5;
-  }
-
-  while (remaining > 2.0) {
-    filters.push("atempo=2.0");
-    remaining /= 2.0;
-  }
-
-  if (Math.abs(remaining - 1.0) > 0.001) {
-    filters.push(`atempo=${Number(remaining.toFixed(4))}`);
-  }
-
-  if (recipe.volume !== undefined && recipe.volume !== 100) {
-    filters.push(`volume=${(recipe.volume / 100).toFixed(2)}`);
-  }
-
-  if (recipe.normalizeAudio) filters.push("loudnorm=I=-14:TP=-1.5:LRA=11");
-
-  return filters.join(",");
-}
-
-function buildAudioTrimFilter(recipe: EditRecipe): string {
-  if (recipe.trimStart === 0 && recipe.trimEnd === null) return "";
-
-  // Avoid scanning entire audio with large placeholder values
-  // Use precise trim bounds when available
-  let trimFilter = `atrim=start=${recipe.trimStart}`;
-  if (recipe.trimEnd !== null) {
-    trimFilter += `:end=${recipe.trimEnd}`;
-  }
-  // asetpts normalizes timestamps after trim for correct stream positioning
-  return `${trimFilter},asetpts=PTS-STARTPTS`;
-}
-
-function buildArguments(
-  recipe: EditRecipe,
-  format: "mp4" | "webm" | "mkv" | "gif",
-  outputName: string,
-  inputName: string,
-  targetW: number,
-  targetH: number,
-  hasMusicTrack: boolean,
-  musicInputName: string,
-  musicOptions: BackgroundMusicOptions | undefined,
-  hasOverlay: boolean,
-  overlayInputName: string,
-  overlayOptions: ImageOverlayOptions | undefined,
-  hasOriginalAudio: boolean,
-  videoDuration: number
-): string[] {
-  const vf = buildVideoFilter(recipe, targetW, targetH);
-  const audioTrim = hasOriginalAudio ? buildAudioTrimFilter(recipe) : "";
-  const audioSpeed = hasOriginalAudio ? buildAudioFilter(recipe) : "";
-  const afParts = [audioTrim, audioSpeed].filter(Boolean);
-  const af = afParts.join(",");
-
-  const musicIdx = 1;
-  const overlayIdx = hasMusicTrack ? 2 : 1;
-
-  const args: string[] = [];
-  args.push("-i", inputName);
-  if (hasMusicTrack) {
-    if (musicOptions!.loopMusic) args.push("-stream_loop", "-1");
-    args.push("-i", musicInputName);
-  }
-  if (hasOverlay) {
-    args.push("-i", overlayInputName);
-  }
-
-  const needsFilterComplex = hasOverlay || hasMusicTrack;
-  const shouldKeepAudio = recipe.keepAudio && (hasOriginalAudio || hasMusicTrack);
-
-  if (needsFilterComplex) {
-    const filterParts: string[] = [];
-    let videoOut = "[0:v]";
-
-    if (vf) {
-      filterParts.push(`[0:v]${vf}[vbase]`);
-      videoOut = "[vbase]";
-    }
-
-if (hasOverlay) {
-  const scaledW = overlayOptions!.size;
-  const alpha = (overlayOptions!.opacity / 100).toFixed(2);
-  const posMap: Record<string, string> = {
-    "top-left":     "20:20",
-    "top-right":    "main_w-w-20:20",
-    "bottom-left":  "20:main_h-h-20",
-    "bottom-right": "main_w-w-20:main_h-h-20",
-  };
-
-interface PositionCoords {
-    x: number;
-    y: number;
-  }
-
-  const pos = typeof overlayOptions?.position === "string"
-    ? (posMap[overlayOptions.position] ?? "main_w-w-20:main_h-h-20")
-    : overlayOptions?.position
-    ? `(main_w)*${(overlayOptions.position as PositionCoords).x}/100:(main_h)*${(overlayOptions.position as PositionCoords).y}/100`
-    : "main_w-w-20:main_h-h-20";
-
-  filterParts.push(`[${overlayIdx}:v]scale=${scaledW}:-2,format=rgba,colorchannelmixer=aa=${alpha}[logo]`);
-  filterParts.push(`${videoOut}[logo]overlay=${pos}[vout]`);
-  videoOut = "[vout]";
-}
-
-    let audioOut = "";
-    if (shouldKeepAudio) {
-      if (hasMusicTrack) {
-        const musicVol = (musicOptions!.musicVolume / 100).toFixed(2);
-        if (hasOriginalAudio) {
-          const origVol  = (musicOptions!.originalAudioVolume / 100).toFixed(2);
-          const origChain = afParts.length > 0
-            ? `[0:a]${afParts.join(",")},volume=${origVol}[orig]`
-            : `[0:a]volume=${origVol}[orig]`;
-          filterParts.push(origChain);
-          filterParts.push(`[${musicIdx}:a]volume=${musicVol}[music]`);
-          filterParts.push(`[orig][music]amix=inputs=2:duration=first:dropout_transition=0[aout]`);
-          audioOut = "[aout]";
-        } else {
-          filterParts.push(`[${musicIdx}:a]volume=${musicVol}[aout]`);
-          audioOut = "[aout]";
-        }
-      } else if (hasOriginalAudio && af) {
-        filterParts.push(`[0:a]${af}[aout]`);
-        audioOut = "[aout]";
-      }
-    }
-
-    if (filterParts.length > 0) {
-      args.push("-filter_complex", filterParts.join(";"));
-    }
-    args.push("-map", videoOut === "[0:v]" ? "0:v" : videoOut);
-
-    if (!shouldKeepAudio) {
-      args.push("-an");
-    } else if (audioOut) {
-      args.push("-map", audioOut);
-    } else if (hasOriginalAudio) {
-      args.push("-map", "0:a");
-    }
-  } else {
-    if (vf) args.push("-vf", vf);
-    if (!shouldKeepAudio) {
-      args.push("-an");
-    } else if (af && hasOriginalAudio) {
-      args.push("-af", af);
-    }
-  }
-
-  if (format === "webm") {
-    args.push(
-      "-c:v", "libvpx-vp9",
-      "-b:v", "0",
-      "-crf", String(recipe.quality),
-      "-cpu-used", "4",
-      "-deadline", "realtime"
-    );
-    if (shouldKeepAudio) args.push("-c:a", "libopus");
-  } else if (format === "mkv") {
-    args.push("-c:v", "libx264", "-crf", String(recipe.quality), "-preset", "ultrafast");
-    if (shouldKeepAudio) args.push("-c:a", "aac", "-b:a", "128k");
-  } else {
-    args.push("-c:v", "libx264", "-crf", String(recipe.quality), "-preset", "ultrafast", "-movflags", "+faststart");
-    if (shouldKeepAudio) args.push("-c:a", "aac", "-b:a", "128k");
-  }
-
-  // Add explicit output duration to prevent slight duration overshoot
-  // caused by encoder/filter pipeline frame flush at stream end,
-  // and to prevent infinite loops when adding looped audio to silent videos.
-  const sourceDuration = (recipe.trimEnd ?? videoDuration) - recipe.trimStart;
-  const outputDuration = sourceDuration / recipe.speed;
-  args.push("-t", outputDuration.toFixed(6));
-
-  args.push(outputName);
-  return args;
-}
-
 export function formatBytes(bytes: number): string {
   if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`;
   return `${(bytes / (1024 * 1024)).toFixed(1)} MB`;
diff --git a/src/lib/ffmpeg.worker.ts b/src/lib/ffmpeg.worker.ts
index b0e33a683..4c17a2c44 100644
--- a/src/lib/ffmpeg.worker.ts
+++ b/src/lib/ffmpeg.worker.ts
@@ -2,7 +2,7 @@ import { FFmpeg } from "@ffmpeg/ffmpeg";
 import { toBlobURL } from "@ffmpeg/util";
 import { EditRecipe, BackgroundMusicOptions, ImageOverlayOptions } from "./types";
 import { getPresetById } from "./presets";
-import { buildTextFilter } from "./text-overlay";
+import { buildArguments, buildDurationArgs, buildVideoFilter } from "./exportGraph";
 
 const CORE_BASE_URL = "https://cdn.jsdelivr.net/npm/@ffmpeg/core@0.12.10/dist/umd";
 // MT core aligned to the same version and distribution format as CORE_BASE_URL.
@@ -92,249 +92,6 @@ async function fetchWithIntegrity(url: string, mimeType: string): Promise<string
   return URL.createObjectURL(blob);
 }
 
-function buildVideoFilter(recipe: EditRecipe, targetW: number, targetH: number): string {
-  const filters: string[] = [];
-
-  if (recipe.trimStart > 0 || recipe.trimEnd !== null) {
-    const end = recipe.trimEnd !== null ? recipe.trimEnd : 999999;
-    filters.push(`trim=start=${recipe.trimStart}:end=${end}`);
-  }
-
-  if (recipe.stabilization) {
-    filters.push("deshake");
-  }
-
-  if (recipe.rotate === 90) {
-    filters.push("transpose=1");
-  } else if (recipe.rotate === 180) {
-    filters.push("transpose=1,transpose=1");
-  } else if (recipe.rotate === 270) {
-    filters.push("transpose=2");
-  }
-
-  if (recipe.framing === "fit") {
-    filters.push(
-      `scale=${targetW}:${targetH}:force_original_aspect_ratio=decrease`,
-      `pad=${targetW}:${targetH}:(ow-iw)/2:(oh-ih)/2:color=black`
-    );
-  } else {
-    filters.push(
-      `scale=${targetW}:${targetH}:force_original_aspect_ratio=increase`,
-      `crop=${targetW}:${targetH}`
-    );
-  }
-
-  if (recipe.trimStart > 0 || recipe.trimEnd !== null || recipe.speed !== 1) {
-    filters.push("setpts=PTS-STARTPTS");
-  }
-
-  if (recipe.speed !== 1) {
-    const pts = (1 / recipe.speed).toFixed(4);
-    filters.push(`setpts=${pts}*PTS`);
-  }
-
-  if (recipe.denoise) {
-    filters.push("hqdn3d=1.5:1.5:6:6");
-  }
-
-  const needsEq =
-    recipe.brightness !== 0 ||
-    recipe.contrast !== 1 ||
-    recipe.saturation !== 1;
-
-  if (needsEq) {
-    filters.push(
-      `eq=brightness=${recipe.brightness}:contrast=${recipe.contrast}:saturation=${recipe.saturation}`
-    );
-  }
-
-  const textOverlays = recipe.textOverlays || [];
-  textOverlays.forEach((overlay) => {
-    filters.push(buildTextFilter(overlay, targetW, targetH));
-  });
-
-  return filters.join(",");
-}
-
-function buildAudioFilter(speed: number, normalizeAudio: boolean): string {
-  if (speed <= 0) return "";
-  const filters: string[] = [];
-
-  let remaining = speed;
-  while (remaining < 0.5) {
-    filters.push("atempo=0.5");
-    remaining /= 0.5;
-  }
-
-  while (remaining > 2.0) {
-    filters.push("atempo=2.0");
-    remaining /= 2.0;
-  }
-
-  if (Math.abs(remaining - 1.0) > 0.001) {
-    filters.push(`atempo=${Number(remaining.toFixed(4))}`);
-  }
-
-  if (normalizeAudio) filters.push("loudnorm=I=-14:TP=-1.5:LRA=11");
-
-  return filters.join(",");
-}
-
-function buildAudioTrimFilter(recipe: EditRecipe): string {
-  if (recipe.trimStart === 0 && recipe.trimEnd === null) return "";
-  const end = recipe.trimEnd !== null ? recipe.trimEnd : 999999;
-  return `atrim=start=${recipe.trimStart}:end=${end},asetpts=PTS-STARTPTS`;
-}
-
-function buildArguments(
-  recipe: EditRecipe,
-  format: "mp4" | "webm" | "mkv" | "gif",
-  outputName: string,
-  inputName: string,
-  targetW: number,
-  targetH: number,
-  hasMusicTrack: boolean,
-  musicInputName: string,
-  musicOptions: BackgroundMusicOptions | undefined,
-  hasOverlay: boolean,
-  overlayInputName: string,
-  overlayOptions: ImageOverlayOptions | undefined,
-  hasOriginalAudio: boolean,
-  videoDuration: number
-): string[] {
-  const vf = buildVideoFilter(recipe, targetW, targetH);
-  const audioTrim = hasOriginalAudio ? buildAudioTrimFilter(recipe) : "";
-  const audioSpeed = hasOriginalAudio ? buildAudioFilter(recipe.speed, recipe.normalizeAudio ?? false) : "";
-  const afParts = [audioTrim, audioSpeed].filter(Boolean);
-  const af = afParts.join(",");
-
-  const musicIdx = 1;
-  const overlayIdx = hasMusicTrack ? 2 : 1;
-
-  const args: string[] = [];
-  args.push("-i", inputName);
-  if (hasMusicTrack) {
-    if (musicOptions!.loopMusic) args.push("-stream_loop", "-1");
-    args.push("-i", musicInputName);
-  }
-  if (hasOverlay) {
-    args.push("-i", overlayInputName);
-  }
-
-  const needsFilterComplex = hasOverlay || hasMusicTrack;
-  const shouldKeepAudio = recipe.keepAudio && (hasOriginalAudio || hasMusicTrack);
-
-  if (needsFilterComplex) {
-    const filterParts: string[] = [];
-    let videoOut = "[0:v]";
-
-    if (vf) {
-      filterParts.push(`[0:v]${vf}[vbase]`);
-      videoOut = "[vbase]";
-    }
-
-if (hasOverlay) {
-  const scaledW = overlayOptions!.size;
-  const alpha = (overlayOptions!.opacity / 100).toFixed(2);
-  const posMap: Record<string, string> = {
-    "top-left":     "20:20",
-    "top-right":    "W-w-20:20",
-    "bottom-left":  "20:H-h-20",
-    "bottom-right": "W-w-20:H-h-20",
-  };
-
-interface PositionCoords {
-    x: number;
-    y: number;
-  }
-
-  const pos = typeof overlayOptions?.position === "string"
-    ? (posMap[overlayOptions.position] ?? "W-w-20:H-h-20")
-    : overlayOptions?.position
-    ? `(W-w)*${(overlayOptions.position as PositionCoords).x}/100:(H-h)*${(overlayOptions.position as PositionCoords).y}/100`
-    : "W-w-20:H-h-20";
-
-  filterParts.push(`[${overlayIdx}:v]scale=${scaledW}:-2,format=rgba,colorchannelmixer=aa=${alpha}[logo]`);
-  filterParts.push(`${videoOut}[logo]overlay=${pos}[vout]`);
-  videoOut = "[vout]";
-}
-
-    let audioOut = "";
-    if (shouldKeepAudio) {
-      if (hasMusicTrack) {
-        const musicVol = (musicOptions!.musicVolume / 100).toFixed(2);
-        if (hasOriginalAudio) {
-          const origVol = (musicOptions!.originalAudioVolume / 100).toFixed(2);
-          const origChain = afParts.length > 0
-            ? `[0:a]${afParts.join(",")},volume=${origVol}[orig]`
-            : `[0:a]volume=${origVol}[orig]`;
-          filterParts.push(origChain);
-          filterParts.push(`[${musicIdx}:a]volume=${musicVol}[music]`);
-          filterParts.push(`[orig][music]amix=inputs=2:duration=first:dropout_transition=0[aout]`);
-          audioOut = "[aout]";
-        } else {
-          filterParts.push(`[${musicIdx}:a]volume=${musicVol}[aout]`);
-          audioOut = "[aout]";
-        }
-      } else if (hasOriginalAudio && af) {
-        filterParts.push(`[0:a]${af}[aout]`);
-        audioOut = "[aout]";
-      }
-    }
-
-    if (filterParts.length > 0) {
-      args.push("-filter_complex", filterParts.join(";"));
-    }
-    args.push("-map", videoOut === "[0:v]" ? "0:v" : videoOut);
-
-    if (!shouldKeepAudio) {
-      args.push("-an");
-    } else if (audioOut) {
-      args.push("-map", audioOut);
-    } else if (hasOriginalAudio) {
-      args.push("-map", "0:a");
-    }
-  } else {
-    if (vf) args.push("-vf", vf);
-    if (!shouldKeepAudio) {
-      args.push("-an");
-    } else if (af && hasOriginalAudio) {
-      args.push("-af", af);
-    }
-  }
-
-  if (format === "webm") {
-    args.push(
-      "-c:v", "libvpx-vp9",
-      "-b:v", "0",
-      "-crf", String(recipe.quality),
-      "-cpu-used", "4",
-      "-deadline", "realtime"
-    );
-    if (shouldKeepAudio) args.push("-c:a", "libopus");
-  } else if (format === "mkv") {
-    args.push("-c:v", "libx264", "-crf", String(recipe.quality), "-preset", "ultrafast");
-    if (shouldKeepAudio) args.push("-c:a", "aac", "-b:a", "128k");
-  } else {
-    args.push("-c:v", "libx264", "-crf", String(recipe.quality), "-preset", "ultrafast", "-movflags", "+faststart");
-    if (shouldKeepAudio) args.push("-c:a", "aac", "-b:a", "128k");
-  }
-
-  if (recipe.speed !== 1) {
-    const sourceDuration = (recipe.trimEnd ?? videoDuration) - recipe.trimStart;
-    // Skip -t when sourceDuration <= 0 (happens when video metadata fails to
-    // load and videoDuration falls back to 0). Passing -t 0 to FFmpeg produces
-    // a zero-length output file; omitting it lets FFmpeg encode the full stream.
-    if (sourceDuration > 0) {
-      const outputDuration = sourceDuration / recipe.speed;
-      args.push("-t", outputDuration.toFixed(6));
-    }
-  }
-
-  args.push(outputName);
-  return args;
-}
-
 async function loadCore(onProgress?: (percent: number) => void): Promise<void> {
   if (ffmpegLoaded) {
     onProgress?.(100);
@@ -465,16 +222,10 @@ async function runExport(request: ExportRequest): Promise<ResultPayload> {
         ? `[0:v]${vf}[x];[x][1:v]paletteuse`
         : "[0:v][1:v]paletteuse";
 
-      const gifDurationArgs = recipe.speed !== 1
-        ? (() => {
-            const sourceDuration = (recipe.trimEnd ?? videoDuration) - recipe.trimStart;
-            // Skip -t when sourceDuration <= 0 (metadata load failure fallback)
-            // to avoid producing a zero-length GIF.
-            if (sourceDuration <= 0) return [];
-            const outputDuration = sourceDuration / recipe.speed;
-            return ["-t", outputDuration.toFixed(6)];
-          })()
-        : [];
+      // GIF only needs -t for speed changes; the shared helper also rejects the
+      // unknown-duration cases (0 from failed metadata, Infinity from containers
+      // that report no duration) that would otherwise abort the export.
+      const gifDurationArgs = recipe.speed !== 1 ? buildDurationArgs(recipe, videoDuration) : [];
 
       const pass1Code = await ffmpeg.exec(
         ["-i", inputName, "-vf", vfWithPalette, ...gifDurationArgs, "-y", paletteName],
diff --git a/src/lib/tests/exportGraph.test.ts b/src/lib/tests/exportGraph.test.ts
new file mode 100644
index 000000000..223585368
--- /dev/null
+++ b/src/lib/tests/exportGraph.test.ts
@@ -0,0 +1,209 @@
+import { describe, it, expect } from "vitest";
+import {
+  buildArguments,
+  buildAudioFilter,
+  buildAudioTrimFilter,
+  buildDurationArgs,
+  buildVideoFilter,
+} from "../exportGraph";
+import { DEFAULT_RECIPE } from "../constants";
+import { BackgroundMusicOptions, EditRecipe, ImageOverlayOptions } from "../types";
+
+const MUSIC: BackgroundMusicOptions = {
+  file: null,
+  musicVolume: 70,
+  originalAudioVolume: 100,
+  loopMusic: false,
+};
+
+const OVERLAY: ImageOverlayOptions = {
+  file: null,
+  position: "bottom-right",
+  size: 200,
+  opacity: 100,
+};
+
+interface ArgsOverrides {
+  recipe?: Partial<EditRecipe>;
+  format?: "mp4" | "webm" | "mkv" | "gif";
+  hasMusicTrack?: boolean;
+  musicOptions?: BackgroundMusicOptions;
+  hasOverlay?: boolean;
+  overlayOptions?: ImageOverlayOptions;
+  hasOriginalAudio?: boolean;
+  videoDuration?: number;
+}
+
+/** The worker's call shape: one recipe, every branch flag overridable. */
+function build(overrides: ArgsOverrides = {}): string[] {
+  const recipe = { ...DEFAULT_RECIPE, ...(overrides.recipe ?? {}) } as EditRecipe;
+  return buildArguments(
+    recipe,
+    overrides.format ?? "mp4",
+    "output.mp4",
+    "input.mp4",
+    1080,
+    1920,
+    overrides.hasMusicTrack ?? false,
+    "music_input.mp3",
+    overrides.musicOptions,
+    overrides.hasOverlay ?? false,
+    "overlay.png",
+    overrides.overlayOptions,
+    overrides.hasOriginalAudio ?? true,
+    overrides.videoDuration ?? 10
+  );
+}
+
+function valueAfter(args: string[], flag: string): string | null {
+  const index = args.indexOf(flag);
+  return index === -1 ? null : args[index + 1] ?? null;
+}
+
+function filterComplex(args: string[]): string {
+  return valueAfter(args, "-filter_complex") ?? "";
+}
+
+const recipe = (patch: Partial<EditRecipe>) => ({ ...DEFAULT_RECIPE, ...patch } as EditRecipe);
+
+describe("export graph: video filters", () => {
+  it("applies the Sharpness setting with unsharp", () => {
+    expect(buildVideoFilter(recipe({ sharpness: 2 }), 1080, 1920)).toContain("unsharp=5:5:2:5:5:0.0");
+  });
+
+  it("omits unsharp when sharpness is at its default", () => {
+    expect(buildVideoFilter(recipe({ sharpness: 0 }), 1080, 1920)).not.toContain("unsharp");
+  });
+
+  it("trims to the end of the stream without a placeholder bound", () => {
+    const filter = buildVideoFilter(recipe({ trimStart: 5 }), 1080, 1920);
+    expect(filter).toContain("trim=start=5");
+    expect(filter).not.toContain("999999");
+  });
+});
+
+describe("export graph: audio filters", () => {
+  it("applies the Volume setting to the original audio", () => {
+    expect(buildAudioFilter(recipe({ volume: 50 }))).toBe("volume=0.50");
+    expect(buildAudioFilter(recipe({ volume: 200 }))).toBe("volume=2.00");
+    expect(buildAudioFilter(recipe({ volume: 0 }))).toBe("volume=0.00");
+  });
+
+  it("leaves the chain untouched at the default volume", () => {
+    expect(buildAudioFilter(recipe({ volume: 100 }))).toBe("");
+  });
+
+  it("combines speed and volume", () => {
+    expect(buildAudioFilter(recipe({ speed: 2, volume: 200 }))).toBe("atempo=2,volume=2.00");
+  });
+
+  it("keeps normalization after the manual volume", () => {
+    expect(buildAudioFilter(recipe({ volume: 50, normalizeAudio: true }))).toBe(
+      "volume=0.50,loudnorm=I=-14:TP=-1.5:LRA=11"
+    );
+  });
+
+  it("trims audio without a placeholder bound", () => {
+    expect(buildAudioTrimFilter(recipe({}))).toBe("");
+    expect(buildAudioTrimFilter(recipe({ trimStart: 5 }))).toBe("atrim=start=5,asetpts=PTS-STARTPTS");
+    expect(buildAudioTrimFilter(recipe({ trimStart: 2, trimEnd: 8 }))).toBe(
+      "atrim=start=2:end=8,asetpts=PTS-STARTPTS"
+    );
+  });
+});
+
+describe("export graph: duration arguments", () => {
+  it("rejects durations that cannot be trusted", () => {
+    expect(buildDurationArgs(recipe({}), 0)).toEqual([]);
+    expect(buildDurationArgs(recipe({}), Infinity)).toEqual([]);
+    expect(buildDurationArgs(recipe({}), NaN)).toEqual([]);
+  });
+
+  it("divides the trimmed range by the playback speed", () => {
+    expect(buildDurationArgs(recipe({}), 12)).toEqual(["-t", "12.000000"]);
+    expect(buildDurationArgs(recipe({ trimStart: 2, trimEnd: 8, speed: 2 }), 30)).toEqual(["-t", "3.000000"]);
+  });
+});
+
+describe("export graph: output argv", () => {
+  it("passes the volume setting through to the encoded audio", () => {
+    expect(valueAfter(build({ recipe: { volume: 50 } }), "-af")).toBe("volume=0.50");
+  });
+
+  it("adds no audio filter at the default volume", () => {
+    expect(build()).not.toContain("-af");
+  });
+
+  it("caps the output at the video duration", () => {
+    expect(valueAfter(build({ videoDuration: 10 }), "-t")).toBe("10.000000");
+  });
+
+  it("caps trimmed and sped-up output at the expected duration", () => {
+    const args = build({ recipe: { trimStart: 2, trimEnd: 8, speed: 2 }, videoDuration: 30 });
+    expect(valueAfter(args, "-t")).toBe("3.000000");
+    expect(valueAfter(args, "-vf")).toContain("trim=start=2:end=8");
+    expect(args[args.length - 1]).toBe("output.mp4");
+  });
+
+  it("omits -t when the duration cannot be trusted", () => {
+    // 0 = metadata failed to load, Infinity = container reports no duration
+    // (MediaRecorder WebM). `-t Infinity` aborts the export outright.
+    expect(build({ videoDuration: 0 })).not.toContain("-t");
+    expect(build({ videoDuration: Infinity })).not.toContain("-t");
+    expect(build({ videoDuration: NaN })).not.toContain("-t");
+  });
+
+  it("caps the duration from an explicit trim even when the total is unknown", () => {
+    const args = build({ recipe: { trimStart: 1, trimEnd: 6 }, videoDuration: Infinity });
+    expect(valueAfter(args, "-t")).toBe("5.000000");
+  });
+
+  it("caps a looped music export so it cannot outrun the video (#1493)", () => {
+    const args = build({
+      hasMusicTrack: true,
+      musicOptions: { ...MUSIC, loopMusic: true },
+      hasOriginalAudio: false,
+      videoDuration: 10,
+    });
+    expect(valueAfter(args, "-stream_loop")).toBe("-1");
+    expect(filterComplex(args)).toContain("[1:a]volume=0.70[aout]");
+    expect(valueAfter(args, "-t")).toBe("10.000000");
+  });
+
+  it("mixes original audio and music at their configured volumes", () => {
+    const args = build({
+      hasMusicTrack: true,
+      musicOptions: { ...MUSIC, originalAudioVolume: 40 },
+    });
+    const graph = filterComplex(args);
+    expect(graph).toContain("[0:a]volume=0.40[orig]");
+    expect(graph).toContain("[1:a]volume=0.70[music]");
+    expect(graph).toContain("amix=inputs=2:duration=first:dropout_transition=0[aout]");
+    expect(valueAfter(args, "-map")).toBe("[vbase]");
+  });
+
+  it("drops audio entirely when the recipe is muted", () => {
+    const args = build({ recipe: { keepAudio: false } });
+    expect(args).toContain("-an");
+    expect(args).not.toContain("-af");
+    expect(args).not.toContain("-c:a");
+  });
+
+  it("drops audio when the source is silent and no music is attached", () => {
+    const args = build({ hasOriginalAudio: false });
+    expect(args).toContain("-an");
+    expect(args).not.toContain("-af");
+  });
+
+  it("keeps valid overlay positioning variables", () => {
+    const graph = filterComplex(build({ hasOverlay: true, overlayOptions: OVERLAY }));
+    expect(graph).toContain("overlay=W-w-20:H-h-20");
+    expect(graph).not.toContain("main_w");
+  });
+
+  it("uses the audio codec that matches the container", () => {
+    expect(valueAfter(build({ format: "webm" }), "-c:a")).toBe("libopus");
+    expect(valueAfter(build({ format: "mkv" }), "-c:a")).toBe("aac");
+    expect(valueAfter(build({ format: "mp4" }), "-c:a")).toBe("aac");
+  });
+});