fix: explain age-verification quality limit for YouTube downloads

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - **Linux launcher category** - Add `AudioVideo`, `Video`, and `Network` categories to the Linux desktop entry so Youwee no longer appears under "Other" in application menus
+- **Age-restricted YouTube quality notice** - Log a clear explanation when YouTube requires account age-verification for an age-restricted video, which limits yt-dlp to low-quality formats (usually 360p) instead of silently downloading them
 
 ## [0.20.4] - 2026-09-20
 

docs/CHANGELOG.vi.md

@@ -12,6 +12,7 @@ và dự án tuân theo [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 
 ### Sửa lỗi
 - **Danh mục launcher Linux** - Thêm các danh mục `AudioVideo`, `Video` và `Network` vào desktop entry trên Linux để Youwee không còn nằm trong mục "Other" của menu ứng dụng
+- **Thông báo chất lượng video YouTube giới hạn độ tuổi** - Ghi log giải thích rõ ràng khi YouTube yêu cầu xác minh tuổi tài khoản cho video giới hạn độ tuổi, khiến yt-dlp chỉ lấy được định dạng chất lượng thấp (thường là 360p) thay vì âm thầm tải bản đó
 
 ## [0.20.4] - 2026-09-20
 

docs/CHANGELOG.zh-CN.md

@@ -12,6 +12,7 @@ Youwee 的所有重要更改都将记录在此文件中。
 
 ### 修复
 - **Linux 启动器分类** - 为 Linux 桌面入口添加 `AudioVideo`、`Video` 和 `Network` 分类，使 Youwee 不再显示在应用菜单的“其他”分类中
+- **YouTube 年龄限制视频画质提示** - 当 YouTube 要求账号进行年龄验证时记录清晰的日志说明；此时 yt-dlp 只能获取低画质格式（通常为 360p），不再静默下载而不提示
 
 ## [0.20.4] - 2026-09-20
 

src-tauri/src/commands/download.rs

@@ -564,6 +564,16 @@ mod playlist_chapter_tests {
         );
     }
 
+    #[test]
+    fn detects_youtube_age_verification_notice() {
+        assert!(is_youtube_age_verification_notice(
+            "[youtube] YCuT6oXJX7A: This video is age-restricted and YouTube is requiring account age-verification; some formats may be missing"
+        ));
+        assert!(!is_youtube_age_verification_notice(
+            "[youtube] YCuT6oXJX7A: This video is age-restricted; some formats may be missing without authentication."
+        ));
+    }
+
     #[test]
     fn output_path_arg_uses_ytdlp_home_path_slot() {
         assert_eq!(
@@ -1108,6 +1118,26 @@ fn is_aria2_not_found_line(line: &str) -> bool {
             || lower.contains("is not recognized"))
 }
 
+/// yt-dlp prints this when a logged-in account is not age-verified for an
+/// age-restricted YouTube video. It then falls back to clients that need a
+/// GVS PO Token, so without one only format 18 (360p) remains.
+fn is_youtube_age_verification_notice(line: &str) -> bool {
+    line.contains("age-restricted") && line.contains("requiring account age-verification")
+}
+
+fn log_youtube_age_verification_notice(line: &str, logged: &AtomicBool, url: &str) {
+    if !is_youtube_age_verification_notice(line) || logged.swap(true, Ordering::SeqCst) {
+        return;
+    }
+    add_log_internal(
+        "info",
+        "YouTube requires account age-verification for this age-restricted video; only low-quality formats (usually 360p) may be available. Verify your age on YouTube with the account used for cookies, then refresh cookies.",
+        None,
+        Some(url),
+    )
+    .ok();
+}
+
 fn download_cancelled_error() -> BackendError {
     BackendError::new(crate::types::code::DOWNLOAD_CANCELLED, "Download cancelled")
         .with_retryable(false)
@@ -1784,6 +1814,7 @@ pub async fn download_video(
             let mut printed_filepaths: Vec<String> = Vec::new();
             let mut observed_filepaths: Vec<String> = Vec::new();
             let mut recent_output: VecDeque<String> = VecDeque::new();
+            let age_verification_logged = AtomicBool::new(false);
 
             let quality_display = match quality.as_str() {
                 "8k" => Some("8K".to_string()),
@@ -1812,6 +1843,7 @@ pub async fn download_video(
                         if let Some(filepath) = parse_ytdlp_output_filepath(&line) {
                             push_unique_filepath(&mut observed_filepaths, &filepath);
                         }
+                        log_youtube_age_verification_notice(&line, &age_verification_logged, &url);
 
                         // Parse playlist item info
                         if line.contains("Downloading item") {
@@ -1935,6 +1967,11 @@ pub async fn download_video(
                         if let Some(filepath) = parse_ytdlp_output_filepath(&stderr_line) {
                             push_unique_filepath(&mut observed_filepaths, &filepath);
                         }
+                        log_youtube_age_verification_notice(
+                            &stderr_line,
+                            &age_verification_logged,
+                            &url,
+                        );
 
                         if let Some((percent, speed, eta, pi, pc, downloaded_size, elapsed_time)) =
                             parse_progress(&stderr_line)
@@ -2424,6 +2461,10 @@ async fn handle_tokio_download(
     let stderr_url = url.clone();
     let stderr_recent_output = recent_output.clone();
     let stderr_observed_filepaths = observed_filepaths.clone();
+    // yt-dlp prints the age-verification notice to stdout, but check stderr too
+    // in case output is redirected (e.g. quiet/logtostderr); log it only once.
+    let age_verification_logged = Arc::new(AtomicBool::new(false));
+    let stderr_age_verification_logged = age_verification_logged.clone();
     let stderr_task = if let Some(stderr_handle) = stderr {
         Some(tokio::spawn(async move {
             let mut stderr_reader = BufReader::new(stderr_handle);
@@ -2449,6 +2490,11 @@ async fn handle_tokio_download(
                         push_unique_filepath(&mut paths, &filepath);
                     }
                 }
+                log_youtube_age_verification_notice(
+                    &line,
+                    &stderr_age_verification_logged,
+                    &stderr_url,
+                );
 
                 // Parse progress from stderr (live streams output here)
                 if let Some((percent, speed, eta, pi, pc, downloaded_size, elapsed_time)) =
@@ -2516,6 +2562,7 @@ async fn handle_tokio_download(
                 push_unique_filepath(&mut paths, &filepath);
             }
         }
+        log_youtube_age_verification_notice(&line, &age_verification_logged, &url);
 
         // Parse progress and emit events
         if let Some((percent, speed, eta, pi, pc, downloaded_size, elapsed_time)) =