diff --git a/src/components/KGOnePanel.tsx b/src/components/KGOnePanel.tsx
index 59d5de6..db30a23 100644
--- a/src/components/KGOnePanel.tsx
+++ b/src/components/KGOnePanel.tsx
@@ -10,6 +10,7 @@ import { KGAudioFileStorage } from '../core/io/KGAudioFileStorage';
 import { ConfigManager } from '../core/config/ConfigManager';
 import { DEBUG_MODE } from '../constants/uiConstants';
 import { fetchWithRetry } from '../util/retryUtil';
+import { sliceAudioToWav } from '../util/audioUtil';
 import type { KeySignature } from '../core/KGProject';
 import { ImportStemsCommand } from '../core/commands';
 import type { StemImportEntry } from '../core/commands';
@@ -915,18 +916,40 @@ const SeparatorTab: React.FC = () => {
 
       const audioFileId = selectedAudioRegion.region.getAudioFileId();
       const audioFileName = selectedAudioRegion.region.getAudioFileName();
-      const arrayBuffer = await KGAudioFileStorage.loadAudioFile(projectName, audioFileId);
-      const audioFile = new File([arrayBuffer], audioFileName, { type: 'audio/mpeg' });
+      const clipStart = selectedAudioRegion.region.getClipStartOffsetSeconds();
+      const fullDuration = selectedAudioRegion.region.getAudioDurationSeconds();
+      const regionLengthSec = selectedAudioRegion.region.getLength() * (60 / bpm);
+      const effectiveDuration = Math.min(regionLengthSec, fullDuration - clipStart);
+
+      const rawBuffer = await KGAudioFileStorage.loadAudioFile(projectName, audioFileId);
+      const needsSlice = clipStart > 0.01 || effectiveDuration < fullDuration - 0.01;
+
+      let uploadBuffer: ArrayBuffer;
+      let uploadFileName: string;
+      let uploadMimeType: string;
+
+      if (needsSlice) {
+        setGenHint('Trimming audio to region range...');
+        uploadBuffer = await sliceAudioToWav(rawBuffer, clipStart, effectiveDuration);
+        uploadFileName = audioFileName.replace(/\.[^.]+$/, '.wav');
+        uploadMimeType = 'audio/wav';
+      } else {
+        uploadBuffer = rawBuffer;
+        uploadFileName = audioFileName;
+        uploadMimeType = 'audio/mpeg';
+      }
+
+      const audioFile = new File([uploadBuffer], uploadFileName, { type: uploadMimeType });
 
       const formData = new FormData();
-      formData.append('file', audioFile, audioFileName);
+      formData.append('file', audioFile, uploadFileName);
       formData.append('model_filename', model);
 
       if (signal.aborted) return;
 
       setGenHint('Submitting separation request...');
 
-      kgoneLog('REQ', 'POST /v1/separator/separate', { file: audioFileName, model_filename: model });
+      kgoneLog('REQ', 'POST /v1/separator/separate', { file: uploadFileName, model_filename: model });
       const sepResp = await fetch(`${baseUrl}/v1/separator/separate`, {
         method: 'POST',
         body: formData,
diff --git a/src/util/audioUtil.ts b/src/util/audioUtil.ts
new file mode 100644
index 0000000..e3ec6ca
--- /dev/null
+++ b/src/util/audioUtil.ts
@@ -0,0 +1,52 @@
+function writeString(view: DataView, offset: number, str: string) {
+  for (let i = 0; i < str.length; i++)
+    view.setUint8(offset + i, str.charCodeAt(i));
+}
+
+export async function sliceAudioToWav(
+  arrayBuffer: ArrayBuffer,
+  startSeconds: number,
+  durationSeconds: number
+): Promise<ArrayBuffer> {
+  const audioCtx = new AudioContext();
+  const decoded = await audioCtx.decodeAudioData(arrayBuffer);
+  audioCtx.close();
+
+  const { sampleRate, numberOfChannels: numCh } = decoded;
+  const startSample = Math.floor(startSeconds * sampleRate);
+  const numSamples = Math.min(
+    Math.floor(durationSeconds * sampleRate),
+    decoded.length - startSample
+  );
+
+  const bytesPerSample = 2; // 16-bit PCM
+  const dataSize = numSamples * numCh * bytesPerSample;
+  const buf = new ArrayBuffer(44 + dataSize);
+  const view = new DataView(buf);
+
+  writeString(view, 0, 'RIFF');
+  view.setUint32(4, 36 + dataSize, true);
+  writeString(view, 8, 'WAVE');
+  writeString(view, 12, 'fmt ');
+  view.setUint32(16, 16, true);
+  view.setUint16(20, 1, true); // PCM
+  view.setUint16(22, numCh, true);
+  view.setUint32(24, sampleRate, true);
+  view.setUint32(28, sampleRate * numCh * bytesPerSample, true);
+  view.setUint16(32, numCh * bytesPerSample, true);
+  view.setUint16(34, 16, true);
+  writeString(view, 36, 'data');
+  view.setUint32(40, dataSize, true);
+
+  let offset = 44;
+  for (let i = 0; i < numSamples; i++) {
+    for (let ch = 0; ch < numCh; ch++) {
+      const s = decoded.getChannelData(ch)[startSample + i];
+      const c = Math.max(-1, Math.min(1, s));
+      view.setInt16(offset, c < 0 ? c * 32768 : c * 32767, true);
+      offset += 2;
+    }
+  }
+
+  return buf;
+}