From a70eab3a9cdbdeb1a10324795a9a405733b59769 Mon Sep 17 00:00:00 2001
From: Claude <noreply@anthropic.com>
Date: Sun, 7 Jun 2026 23:02:18 +0000
Subject: [PATCH] Harden YouTube download fallback to pick the largest landed
 file
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

When yt-dlp's prepared filename doesn't exist (post-processing changed the
extension), download_audio fell back to the first entry from iterdir(), whose
order is arbitrary. yt-dlp can leave sidecars (thumbnail, .info.json, a
leftover .part fragment) in the destination directory, so the fallback could
return a non-audio file that then fails downstream with a confusing decode
error. Pick the largest file instead — the decoded audio track dwarfs any
metadata sidecar.

https://claude.ai/code/session_01QVfdQDhreXWkEHQoArh71E
---
 aai_cli/youtube.py    |  5 ++++-
 tests/test_youtube.py | 28 ++++++++++++++++++++++++++++
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/aai_cli/youtube.py b/aai_cli/youtube.py
index 51ba20a1..86af557a 100644
--- a/aai_cli/youtube.py
+++ b/aai_cli/youtube.py
@@ -57,6 +57,9 @@ def download_audio(url: str, dest_dir: Path) -> Path:
 
     if not path.is_file():
         # Post-processing can change the extension; fall back to whatever landed.
+        # yt-dlp may also drop sidecars (thumbnail, .info.json, a leftover .part)
+        # in dest_dir, and iterdir() order is arbitrary, so pick the largest file:
+        # the decoded audio track dwarfs any metadata sidecar.
         files = [p for p in dest_dir.iterdir() if p.is_file()]
         if not files:
             raise CLIError(
@@ -64,5 +67,5 @@ def download_audio(url: str, dest_dir: Path) -> Path:
                 error_type="youtube_error",
                 exit_code=1,
             )
-        path = files[0]
+        path = max(files, key=lambda p: p.stat().st_size)
     return path
diff --git a/tests/test_youtube.py b/tests/test_youtube.py
index 3353a534..f9625a9d 100644
--- a/tests/test_youtube.py
+++ b/tests/test_youtube.py
@@ -80,6 +80,34 @@ def prepare_filename(self, info):
     assert youtube.download_audio("https://youtu.be/x", tmp_path) == landed
 
 
+def test_download_audio_falls_back_to_largest_file(tmp_path, monkeypatch):
+    # yt-dlp can leave sidecars (thumbnail, .info.json) next to the audio track;
+    # the fallback must pick the audio (largest), not an arbitrary iterdir() entry.
+    audio = tmp_path / "actual.webm"
+    thumb = tmp_path / "actual.webp"
+
+    class FakeYDL:
+        def __init__(self, opts):
+            pass
+
+        def __enter__(self):
+            return self
+
+        def __exit__(self, *exc):
+            return False
+
+        def extract_info(self, url, download):
+            thumb.write_bytes(b"\x00" * 16)  # small sidecar
+            audio.write_bytes(b"\x00" * 4096)  # the real, much larger track
+            return {"id": "x"}
+
+        def prepare_filename(self, info):
+            return str(tmp_path / "guessed.m4a")  # wrong extension; file doesn't exist
+
+    _fake_ytdlp(monkeypatch, FakeYDL)
+    assert youtube.download_audio("https://youtu.be/x", tmp_path) == audio
+
+
 def test_download_audio_no_file_produced_raises(tmp_path, monkeypatch):
     # prepare_filename points at a missing file and nothing landed in dest_dir.
     class FakeYDL: