diff --git a/packages/core/src/bridge/element-builder.ts b/packages/core/src/bridge/element-builder.ts
index 8158a319..b07f7980 100644
--- a/packages/core/src/bridge/element-builder.ts
+++ b/packages/core/src/bridge/element-builder.ts
@@ -208,11 +208,14 @@ async function makePttElem(ctx: SendContext, element: MessageElement): Promise<P
 
   const msgInfo = await uploadPttMsgInfo(ctx.bridge, isGroup, targetIdOrUid, element);
 
+  // commonElem.businessType is the QQ NT scene tag the receive-side
+  // decoder pairs with: 12=c2c, 22=group. Sending the group tag on a
+  // c2c message bounces with PbSendMsg result=79.
   return {
     commonElem: {
       serviceType: 48,
       pbElem: msgInfo,
-      businessType: 22,
+      businessType: isGroup ? 22 : 12,
     } as any,
   };
 }
@@ -226,11 +229,14 @@ async function makeVideoElem(ctx: SendContext, element: MessageElement): Promise
 
   const msgInfo = await uploadVideoMsgInfo(ctx.bridge, isGroup, targetIdOrUid, element);
 
+  // commonElem.businessType is the QQ NT scene tag the receive-side
+  // decoder pairs with: 11=c2c, 21=group. Sending the group tag on a
+  // c2c message bounces with PbSendMsg result=79.
   return {
     commonElem: {
       serviceType: 48,
       pbElem: msgInfo,
-      businessType: 21,
+      businessType: isGroup ? 21 : 11,
     } as any,
   };
 }
diff --git a/packages/core/src/bridge/highway/video-upload.ts b/packages/core/src/bridge/highway/video-upload.ts
index 94c8e743..b8990ce3 100644
--- a/packages/core/src/bridge/highway/video-upload.ts
+++ b/packages/core/src/bridge/highway/video-upload.ts
@@ -487,7 +487,11 @@ export async function uploadVideoMsgInfo(
           subFileType: 100,
         },
       ],
-      compatQmsgSceneType: 2,
+      // Scene tag the NTV2 upload registers under. image-upload and
+      // ptt-upload both differentiate (group=2, c2c=1); video was
+      // hardcoded to the group value, which contributed to the c2c
+      // send-back rejection (PbSendMsg result=79).
+      compatQmsgSceneType: isGroup ? 2 : 1,
       extBizInfo: {
         pic: { bizType: 0, textSummary: 'Nya~' },
         video: { bytesPbReserve: new Uint8Array([0x80, 0x01, 0x00]) },
diff --git a/packages/core/tests/element-builder.test.ts b/packages/core/tests/element-builder.test.ts
new file mode 100644
index 00000000..ebe4e639
--- /dev/null
+++ b/packages/core/tests/element-builder.test.ts
@@ -0,0 +1,94 @@
+// Regression test for the c2c-vs-group businessType asymmetry that made
+// private-chat video / record sends bounce with `send private message
+// rejected: result=79`. The receive-side decoder
+// (msg-push/rich-body-decoder.ts) explicitly treats the businessType
+// pairs as:
+//
+//   image  : c2c=10, group=20
+//   video  : c2c=11, group=21
+//   record : c2c=12, group=22
+//
+// `makeImageElem` always honoured this split (`isGroup ? 20 : 10`).
+// `makeVideoElem` and `makePttElem` used to hardcode the group value
+// for both scenes, so any c2c video / private voice send arrived at the
+// QQ NT server with a businessType the c2c routing path did not
+// recognise and got rejected with result=79.
+
+import { describe, expect, it, vi } from 'vitest';
+
+vi.mock('../src/bridge/highway/image-upload', () => ({
+  uploadImageMsgInfo: vi.fn(async () => new Uint8Array([7, 8, 9])),
+}));
+vi.mock('../src/bridge/highway/ptt-upload', () => ({
+  uploadPttMsgInfo: vi.fn(async () => new Uint8Array([4, 5, 6])),
+}));
+vi.mock('../src/bridge/highway/video-upload', () => ({
+  uploadVideoMsgInfo: vi.fn(async () => new Uint8Array([1, 2, 3])),
+}));
+
+import { buildSendElems } from '../src/bridge/element-builder';
+
+const fakeBridge = {} as any;
+
+function commonElem(elem: any): { serviceType: number; businessType: number; pbElem: Uint8Array } {
+  return elem.commonElem;
+}
+
+describe('element-builder / commonElem.businessType per scene', () => {
+  describe('image', () => {
+    it('c2c → businessType 10', async () => {
+      const [elem] = await buildSendElems(
+        [{ type: 'image', url: 'file:///tmp/a.png' } as any],
+        { bridge: fakeBridge, userUid: 'u_peer' },
+      );
+      expect(commonElem(elem).serviceType).toBe(48);
+      expect(commonElem(elem).businessType).toBe(10);
+    });
+
+    it('group → businessType 20', async () => {
+      const [elem] = await buildSendElems(
+        [{ type: 'image', url: 'file:///tmp/a.png' } as any],
+        { bridge: fakeBridge, groupId: 12345 },
+      );
+      expect(commonElem(elem).businessType).toBe(20);
+    });
+  });
+
+  describe('video', () => {
+    it('c2c → businessType 11 (regression: was 21, server returned result=79)', async () => {
+      const [elem] = await buildSendElems(
+        [{ type: 'video', url: 'file:///tmp/clip.mp4' } as any],
+        { bridge: fakeBridge, userUid: 'u_peer' },
+      );
+      expect(commonElem(elem).serviceType).toBe(48);
+      expect(commonElem(elem).businessType).toBe(11);
+    });
+
+    it('group → businessType 21', async () => {
+      const [elem] = await buildSendElems(
+        [{ type: 'video', url: 'file:///tmp/clip.mp4' } as any],
+        { bridge: fakeBridge, groupId: 12345 },
+      );
+      expect(commonElem(elem).businessType).toBe(21);
+    });
+  });
+
+  describe('record', () => {
+    it('c2c → businessType 12 (regression: was 22)', async () => {
+      const [elem] = await buildSendElems(
+        [{ type: 'record', url: 'file:///tmp/voice.amr' } as any],
+        { bridge: fakeBridge, userUid: 'u_peer' },
+      );
+      expect(commonElem(elem).serviceType).toBe(48);
+      expect(commonElem(elem).businessType).toBe(12);
+    });
+
+    it('group → businessType 22', async () => {
+      const [elem] = await buildSendElems(
+        [{ type: 'record', url: 'file:///tmp/voice.amr' } as any],
+        { bridge: fakeBridge, groupId: 12345 },
+      );
+      expect(commonElem(elem).businessType).toBe(22);
+    });
+  });
+});