fix(windows): improve yt-dlp filename safety without changing defaults

src-tauri/src/commands/download.rs

@@ -1033,7 +1033,7 @@ pub async fn download_video(
         "--file-access-retries".to_string(),
         "2".to_string(),
     ];
-    add_safe_filename_args(&mut args);
+    add_safe_filename_args(&mut args, Some(&sanitized_path));
 
     if split_embedded_chapters {
         args.push("--split-chapters".to_string());

src-tauri/src/commands/metadata.rs

@@ -750,7 +750,7 @@ pub async fn fetch_metadata(
         "-o".to_string(),
         output_template.clone(),
     ];
-    add_safe_filename_args(&mut args);
+    add_safe_filename_args(&mut args, Some(&sanitized_path));
 
     // Description output template - yt-dlp adds .description automatically
     if write_description {

src-tauri/src/services/ytdlp.rs

@@ -928,15 +928,7 @@ pub fn build_cookie_args(
     args
 }
 
-/// Keep yt-dlp output filenames below common filesystem limits.
-///
-/// Some extractors, especially Facebook Reels, expose long captions as titles.
-/// macOS returns ENAMETOOLONG before yt-dlp can create the .part file unless
-/// yt-dlp trims the generated filename first.
-pub fn add_safe_filename_args(args: &mut Vec<String>) {
-    args.push("--trim-filenames".to_string());
-    args.push("180".to_string());
-}
+pub use crate::utils::add_safe_filename_args;
 
 fn parse_cookie_skip_rule(rule: &str) -> Option<(String, String)> {
     let trimmed = rule.trim();
@@ -1235,16 +1227,12 @@ mod tests {
     fn safe_filename_args_trim_long_titles_before_writing_files() {
         let mut args = vec!["--newline".to_string()];
 
-        add_safe_filename_args(&mut args);
+        add_safe_filename_args(&mut args, None);
 
-        assert_eq!(
-            args,
-            vec![
-                "--newline".to_string(),
-                "--trim-filenames".to_string(),
-                "180".to_string(),
-            ]
-        );
+        assert!(args.contains(&"--newline".to_string()));
+        assert!(args.contains(&"--trim-filenames".to_string()));
+        #[cfg(windows)]
+        assert!(args.contains(&"--windows-filenames".to_string()));
     }
 
     #[test]

src-tauri/src/utils/filename.rs

@@ -0,0 +1,99 @@
+//! yt-dlp filename safety helpers for long paths and Windows-invalid characters.
+
+const WINDOWS_MAX_PATH: usize = 260;
+const RESERVED_SUFFIX_BYTES: usize = 40;
+const MIN_TRIM_FILENAMES: u32 = 50;
+const MAX_TRIM_FILENAMES: u32 = 180;
+const DEFAULT_TRIM_FILENAMES: u32 = 180;
+
+/// Compute a safe `--trim-filenames` byte limit from the output directory length.
+///
+/// Reserves space for the directory prefix so the final path stays within common
+/// filesystem limits (especially `MAX_PATH` on Windows).
+pub fn calc_trim_filenames_bytes(output_path: &str) -> u32 {
+    let path_len = output_path.as_bytes().len();
+    let available = WINDOWS_MAX_PATH
+        .saturating_sub(path_len)
+        .saturating_sub(RESERVED_SUFFIX_BYTES);
+    (available as u32).clamp(MIN_TRIM_FILENAMES, MAX_TRIM_FILENAMES)
+}
+
+/// Append yt-dlp flags that keep generated filenames safe on the target platform.
+pub fn add_safe_filename_args(args: &mut Vec<String>, output_path: Option<&str>) {
+    #[cfg(windows)]
+    {
+        args.push("--windows-filenames".to_string());
+    }
+
+    let trim = output_path
+        .map(calc_trim_filenames_bytes)
+        .unwrap_or(DEFAULT_TRIM_FILENAMES);
+
+    args.push("--trim-filenames".to_string());
+    args.push(trim.to_string());
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn calc_trim_filenames_respects_long_output_paths() {
+        let long_path = "G:\\Youwee\\Downloads\\very-long-folder-name";
+        assert!(calc_trim_filenames_bytes(long_path) < DEFAULT_TRIM_FILENAMES);
+    }
+
+    #[test]
+    fn calc_trim_filenames_clamps_short_paths_to_default_cap() {
+        assert_eq!(calc_trim_filenames_bytes("/tmp"), DEFAULT_TRIM_FILENAMES);
+    }
+
+    #[test]
+    fn calc_trim_filenames_uses_byte_length_for_non_ascii_paths() {
+        let path = "G:\\下载\\Youwee\\very-long-non-ascii-output-directory-name";
+        assert!(path.as_bytes().len() > path.chars().count());
+
+        let expected = (WINDOWS_MAX_PATH
+            .saturating_sub(path.as_bytes().len())
+            .saturating_sub(RESERVED_SUFFIX_BYTES) as u32)
+            .clamp(MIN_TRIM_FILENAMES, MAX_TRIM_FILENAMES);
+        assert_eq!(calc_trim_filenames_bytes(path), expected);
+        assert!(expected < DEFAULT_TRIM_FILENAMES);
+    }
+
+    #[test]
+    fn add_safe_filename_args_uses_path_aware_trim() {
+        let mut args = Vec::new();
+        add_safe_filename_args(&mut args, Some("G:\\Youwee"));
+
+        assert!(args.contains(&"--trim-filenames".to_string()));
+        let trim_index = args.iter().position(|arg| arg == "--trim-filenames").unwrap();
+        let trim_value: u32 = args[trim_index + 1].parse().unwrap();
+        assert!(trim_value >= MIN_TRIM_FILENAMES);
+        assert!(trim_value <= MAX_TRIM_FILENAMES);
+    }
+
+    #[test]
+    fn add_safe_filename_args_falls_back_to_default_trim() {
+        let mut args = Vec::new();
+        add_safe_filename_args(&mut args, None);
+
+        assert_eq!(
+            args,
+            vec![
+                #[cfg(windows)]
+                "--windows-filenames".to_string(),
+                "--trim-filenames".to_string(),
+                DEFAULT_TRIM_FILENAMES.to_string(),
+            ]
+        );
+    }
+
+    #[cfg(windows)]
+    #[test]
+    fn add_safe_filename_args_enables_windows_filenames() {
+        let mut args = Vec::new();
+        add_safe_filename_args(&mut args, None);
+        assert!(args.contains(&"--windows-filenames".to_string()));
+    }
+}

src-tauri/src/utils/mod.rs

@@ -1,12 +1,14 @@
 mod command;
 mod extract;
+mod filename;
 mod format;
 mod path;
 mod progress;
 mod security;
 
 pub use command::*;
 pub use extract::*;
+pub use filename::*;
 pub use format::*;
 pub use path::*;
 pub use progress::*;