diff --git a/apps/notes/feishu/server.py b/apps/notes/feishu/server.py
index 5aaef8d..d35f112 100644
--- a/apps/notes/feishu/server.py
+++ b/apps/notes/feishu/server.py
@@ -6,6 +6,7 @@
 import json
 import logging
 import os
+import re
 import shutil
 import subprocess
 import tempfile
@@ -17,6 +18,33 @@ import requests
 from fastapi import FastAPI, HTTPException
 from pydantic import BaseModel
 
+
+def probe_duration(src: Path) -> float:
+    """browser-recorded webm/m4a 经常没在 metadata 里写 duration（录到一半结束没法 finalize）。
+    先 try ffprobe format.duration，N/A 时 fallback 让 ffmpeg null-muxer 解码一遍统计。
+    """
+    try:
+        out = subprocess.check_output(
+            ['ffprobe', '-v', 'quiet', '-show_entries', 'format=duration',
+             '-of', 'csv=p=0', str(src)],
+            timeout=60,
+        ).decode().strip()
+        if out and out != 'N/A':
+            return float(out)
+    except (subprocess.CalledProcessError, ValueError, subprocess.TimeoutExpired):
+        pass
+    log.info("ffprobe format.duration=N/A, decoding to count time")
+    proc = subprocess.run(
+        ['ffmpeg', '-i', str(src), '-f', 'null', '-'],
+        stderr=subprocess.PIPE, stdout=subprocess.DEVNULL,
+        timeout=900,
+    )
+    matches = re.findall(rb'time=(\d+):(\d+):(\d+(?:\.\d+)?)', proc.stderr)
+    if not matches:
+        raise HTTPException(500, f'cannot determine duration; ffmpeg stderr tail: {proc.stderr[-300:].decode("utf-8","replace")}')
+    h, m, s = matches[-1]
+    return int(h) * 3600 + int(m) * 60 + float(s)
+
 logging.basicConfig(level=logging.INFO,
                     format='%(asctime)s %(levelname)s %(name)s: %(message)s')
 log = logging.getLogger('feishu')
@@ -49,16 +77,8 @@ def transcribe(req: TranscribeReq):
     tmp = Path(tempfile.gettempdir()) / f'transcribe-{uuid.uuid4().hex}'
     tmp.mkdir(parents=True)
     try:
-        # 1) 用 ffprobe 拿总时长
-        out = subprocess.check_output(
-            ['ffprobe', '-v', 'quiet', '-show_entries', 'format=duration',
-             '-of', 'csv=p=0', str(src)],
-            timeout=60,
-        )
-        try:
-            duration = float(out.decode().strip())
-        except ValueError:
-            raise HTTPException(500, f'ffprobe duration parse: {out!r}')
+        # 1) 拿总时长（ffprobe N/A 时回退 null-muxer 解码）
+        duration = probe_duration(src)
         log.info("duration=%.1fs", duration)
 
         # 2) 切 chunk_seconds 段，stride = chunk_seconds - overlap_seconds