diff --git a/packages/engine/src/services/audioMixer.ts b/packages/engine/src/services/audioMixer.ts
index f920e709d0..8fb5404dc9 100644
--- a/packages/engine/src/services/audioMixer.ts
+++ b/packages/engine/src/services/audioMixer.ts
@@ -5,7 +5,12 @@ import { renameWithRetry } from "@hyperframes/core/rename-retry";
 import { removeScratch } from "@hyperframes/core/remove-scratch";
 import { join, dirname, isAbsolute, relative } from "path";
 import { parseHTML } from "linkedom";
-import { extractAudioMetadata, ffmpegSeekSeconds, type AudioMetadata } from "../utils/ffprobe.js";
+import {
+  extractAudioMetadata,
+  ffmpegSeekSeconds,
+  formatFfmpegSeconds,
+  type AudioMetadata,
+} from "../utils/ffprobe.js";
 import { isNotMediaPayload } from "../utils/notMediaPayload.js";
 import { urlInputArgs } from "../utils/urlInput.js";
 import { clampAudioGain, clipGain } from "@hyperframes/core/audio-gain";
@@ -739,7 +744,7 @@ async function extractAudioSegment(
           "-stream_loop",
           "-1",
           "-t",
-          String(sourceDuration),
+          formatFfmpegSeconds(sourceDuration),
           "-i",
           cyclePath,
           "-vn",
@@ -749,7 +754,7 @@ async function extractAudioSegment(
           "48000",
           ...outputArgs,
           "-t",
-          String(duration),
+          formatFfmpegSeconds(duration),
           "-y",
           outputPath,
         ],
@@ -795,13 +800,19 @@ async function extractAudioSegment(
     ...(readsUrl ? FAIL_ON_READ_ERROR_INSTEAD_OF_ENDING_THERE : []),
     ...urlInputArgs(readFrom),
     ...(inputSeek
-      ? ["-ss", String(ffmpegStart - warmUp), "-t", String(sourceDuration + warmUp)]
+      ? [
+          "-ss",
+          formatFfmpegSeconds(ffmpegStart - warmUp),
+          "-t",
+          formatFfmpegSeconds(sourceDuration + warmUp),
+        ]
       : []),
     "-i",
     readFrom,
   ];
   args.push("-vn", "-acodec", "pcm_s16le", "-ar", "48000", ...outputArgs);
-  if (normalizedPlaybackRate !== 1 || options?.padToDuration) args.push("-t", String(duration));
+  if (normalizedPlaybackRate !== 1 || options?.padToDuration)
+    args.push("-t", formatFfmpegSeconds(duration));
   args.push("-y", outputPath);
 
   const result = await runFfmpeg(args, { signal, timeout: ffmpegProcessTimeout });
@@ -870,7 +881,7 @@ async function generateSilence(
     "-i",
     "anullsrc=r=48000:cl=stereo",
     "-t",
-    String(duration),
+    formatFfmpegSeconds(duration),
     "-acodec",
     "pcm_s16le",
     "-y",
@@ -983,7 +994,7 @@ async function mixAudioTracksFlat(
       "-ar",
       "48000",
       "-t",
-      String(totalDuration),
+      formatFfmpegSeconds(totalDuration),
       "-y",
       outputPath,
     ];
@@ -1168,7 +1179,7 @@ async function mixGroupMembersFlat(
         "-ar",
         "48000",
         "-t",
-        String(totalDuration),
+        formatFfmpegSeconds(totalDuration),
         "-y",
         outputPath,
       ];
diff --git a/packages/engine/src/services/videoFrameExtractor.test.ts b/packages/engine/src/services/videoFrameExtractor.test.ts
index c70421e05b..dc23e387b6 100644
--- a/packages/engine/src/services/videoFrameExtractor.test.ts
+++ b/packages/engine/src/services/videoFrameExtractor.test.ts
@@ -24,6 +24,7 @@ import {
   createFrameLookupTable,
   FrameLookupTable,
   rebaseVideoToWindow,
+  alignClipStartToFirstOutputFrame,
   resolveFrameFormat,
   codecMayHaveAlpha,
   decoderForCodec,
@@ -193,6 +194,14 @@ describe("resolveVideoExtractionDuration", () => {
     expect(seconds).toBeCloseTo(4.6586, 3);
   });
 
+  it("leaves media start untouched for a nested clip whose start sums to a frame boundary with float dust", () => {
+    // 3.3 + 1.3666666666666667 = 4.666666666666666, frame 140 at 30 fps
+    const clip = video({ start: 3.3 + 1.3666666666666667, end: 8, mediaStart: 1.35 });
+    alignClipStartToFirstOutputFrame(clip, 30);
+    expect(clip.mediaStart).toBe(1.35);
+    expect(clip.start).toBe(140 / 30);
+  });
+
   it("snaps a float-noise composition start to the timeline origin so the first frame is kept", () => {
     const clip = video({ start: -1, end: 8, playbackRate: 0.7 });
     rebaseVideoToWindow(clip, { compositionStart: 2.2e-16, mediaStart: 0.7, durationSeconds: 5 });
diff --git a/packages/engine/src/services/videoFrameExtractor.ts b/packages/engine/src/services/videoFrameExtractor.ts
index 606f6bf833..1ebe2c60ff 100644
--- a/packages/engine/src/services/videoFrameExtractor.ts
+++ b/packages/engine/src/services/videoFrameExtractor.ts
@@ -46,6 +46,7 @@ import {
   extractFinalVideoFrameTimestamp,
   extractMediaMetadata,
   ffmpegSeekSeconds,
+  formatFfmpegSeconds,
   keyframeGapSeconds,
   mediaFileIdentity,
   readMetadataCache,
@@ -1059,12 +1060,12 @@ async function runSegmentedExtraction(job: SegmentedExtraction): Promise<RunFfmp
               ...decodeArgs,
               "-noaccurate_seek",
               "-ss",
-              String(job.ffmpegSeek(seek)),
+              formatFfmpegSeconds(job.ffmpegSeek(seek)),
               "-i",
               videoPath,
               // Read one frame past the segment and keep exactly its own frames.
               "-t",
-              String((frames + 1) / fps),
+              formatFfmpegSeconds((frames + 1) / fps),
               "-frames:v",
               String(frames),
               "-start_number",
@@ -1207,11 +1208,25 @@ async function extractVideoFramesRangeUnredacted(
     // sample. This is intentionally reserved for the one-frame path: input
     // seeking is faster, but valid unindexed transports (notably MPEG-TS with
     // a negative timestamp base) can seek to EOF and emit zero frames.
-    args.push("-i", videoPath, "-ss", String(Math.max(0, ffmpegSeek(startTime))), "-frames:v", "1");
+    args.push(
+      "-i",
+      videoPath,
+      "-ss",
+      formatFfmpegSeconds(Math.max(0, ffmpegSeek(startTime))),
+      "-frames:v",
+      "1",
+    );
   } else {
     // An accurate seek drops the frame on screen at startTime; the fps sampling below needs it.
     if (sampleAtOutputFps) args.push("-noaccurate_seek");
-    args.push("-ss", String(ffmpegSeek(startTime)), "-i", videoPath, "-t", String(duration));
+    args.push(
+      "-ss",
+      formatFfmpegSeconds(ffmpegSeek(startTime)),
+      "-i",
+      videoPath,
+      "-t",
+      formatFfmpegSeconds(duration),
+    );
   }
 
   const vfFilters: string[] = [];
@@ -1510,13 +1525,17 @@ export function isVideoPastTimelineEnd(video: Pick<VideoElement, "start">, timel
   return video.start >= timelineEnd;
 }
 
-function alignClipStartToFirstOutputFrame(video: VideoElement, fps: number): void {
+export function alignClipStartToFirstOutputFrame(video: VideoElement, fps: number): void {
   if (video.loop || !Number.isFinite(video.start) || video.start < 0 || !(fps > 0)) return;
   const position = exportClipWindow(video.start, Infinity, fps).start * fps;
   const firstFrame =
     Math.abs(position - Math.round(position)) < 1e-6 ? Math.round(position) : Math.ceil(position);
   const shift = firstFrame / fps - video.start;
-  if (shift === 0) return;
+  // Float dust (a sum landing a hair off the grid) snaps the start; it never reaches mediaStart.
+  if (Math.abs(shift) < 1e-9) {
+    video.start = firstFrame / fps;
+    return;
+  }
   const playbackRate = normalizeRateSpec(video.playbackRate);
   video.mediaStart += sourceTimeAt(playbackRate, shift);
   video.playbackRate = shiftRateLane(playbackRate, shift);
diff --git a/packages/engine/src/utils/ffprobe.test.ts b/packages/engine/src/utils/ffprobe.test.ts
index f87bd70e2f..708b1221d8 100644
--- a/packages/engine/src/utils/ffprobe.test.ts
+++ b/packages/engine/src/utils/ffprobe.test.ts
@@ -5,7 +5,12 @@ import { mkdtempSync, readFileSync, renameSync, rmSync, writeFileSync } from "fs
 import { tmpdir } from "os";
 import { basename, join, resolve } from "path";
 import { afterAll, afterEach, beforeAll, describe, expect, it, vi } from "vitest";
-import { extractMediaMetadata, extractPngMetadataFromBuffer, parseFrameRate } from "./ffprobe.js";
+import {
+  extractMediaMetadata,
+  extractPngMetadataFromBuffer,
+  formatFfmpegSeconds,
+  parseFrameRate,
+} from "./ffprobe.js";
 
 function crc32(buf: Buffer): number {
   let crc = 0xffffffff;
@@ -2235,3 +2240,17 @@ describe.skipIf(!HAS_FFPROBE)("VP8/VP9 decoded alpha plane probe on real files",
     expect("alphaPlaneDecoded" in other).toBe(false);
   });
 });
+
+describe("formatFfmpegSeconds", () => {
+  it("never writes an exponent, which ffmpeg and ffprobe reject", () => {
+    expect(formatFfmpegSeconds(8.881784197001252e-16)).toBe("0");
+    expect(formatFfmpegSeconds(-8.881784197001252e-16)).toBe("0");
+    expect(formatFfmpegSeconds(1.5e-7)).toBe("0");
+  });
+
+  it("keeps ordinary times exactly as String() writes them", () => {
+    expect(formatFfmpegSeconds(1.35)).toBe("1.35");
+    expect(formatFfmpegSeconds(4.666666666666667)).toBe("4.666666666666667");
+    expect(formatFfmpegSeconds(0)).toBe("0");
+  });
+});
diff --git a/packages/engine/src/utils/ffprobe.ts b/packages/engine/src/utils/ffprobe.ts
index 0df8d35785..c55eeb6812 100644
--- a/packages/engine/src/utils/ffprobe.ts
+++ b/packages/engine/src/utils/ffprobe.ts
@@ -522,6 +522,11 @@ export function videoStartOnMediaTimeline(
   return (metadata.videoStreamLeadSeconds ?? 0) + (metadata.mediaTimeOffsetSeconds ?? 0);
 }
 
+export function formatFfmpegSeconds(seconds: number): string {
+  const text = String(seconds);
+  return text.includes("e") ? String(+seconds.toFixed(6)) : text;
+}
+
 export function ffmpegSeekSeconds(
   metadata: Pick<VideoMetadata, "mediaTimeOffsetSeconds">,
   mediaTime: number,
@@ -1104,7 +1109,7 @@ async function extractFinalPacketDuration(
       "-select_streams",
       "v:0",
       "-read_intervals",
-      `${intervalStart}%${streamEnd}`,
+      `${formatFfmpegSeconds(intervalStart)}%${formatFfmpegSeconds(streamEnd)}`,
       "-show_entries",
       "packet=pts_time,duration_time",
       "-of",
@@ -1200,7 +1205,8 @@ export async function extractFinalVideoFrameTimestamp(
       return parseFinalTimestamp(stdout);
     };
     const timestamp =
-      (await probe(`${intervalStart}%${streamEnd}`)) ?? (await probe(/* full scan */));
+      (await probe(`${formatFfmpegSeconds(intervalStart)}%${formatFfmpegSeconds(streamEnd)}`)) ??
+      (await probe(/* full scan */));
     if (timestamp === undefined) {
       throw new Error("[FFmpeg] ffprobe found no decodable final video frame");
     }
@@ -1442,7 +1448,7 @@ export async function keyframeGapSeconds(
     filePath,
     [
       ...["-select_streams", "v:0", "-show_entries", "packet=pts_time", "-of", "csv=p=0"],
-      ...["-read_intervals", `${seekSeconds}%+#1`],
+      ...["-read_intervals", `${formatFfmpegSeconds(seekSeconds)}%+#1`],
     ],
     signal,
   );