diff --git a/src/main.rs b/src/main.rs
index 65a1041..177daf6 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -224,7 +224,8 @@ fn format_duration(dur: Duration) -> String {
 mod tests {
     use clap::Parser;
 
-    use super::{Args, split_csv};
+    use super::{Args, calc_savings, parse_compression, split_csv};
+    use crate::precompress::Algorithm;
 
     #[test]
     fn args_respect_ignore_by_default() {
@@ -244,4 +245,73 @@ mod tests {
         let values = split_csv(vec![String::from("a,b"), String::from("c")]).collect::<Vec<_>>();
         assert_eq!(values, vec!["a", "b", "c"]);
     }
+
+    #[test]
+    fn args_accept_repeated_and_comma_separated_filters() {
+        let args = Args::parse_from([
+            "precompress",
+            "--extensions",
+            "js,css",
+            "--extensions",
+            "html",
+            "--exclude",
+            "dist/**,build/**",
+            "--exclude",
+            "*.map",
+            ".",
+        ]);
+
+        assert_eq!(
+            split_csv(args.extensions.expect("extensions should be parsed")).collect::<Vec<_>>(),
+            vec!["js", "css", "html"]
+        );
+        assert_eq!(
+            split_csv(args.exclude.expect("exclude should be parsed")).collect::<Vec<_>>(),
+            vec!["dist/**", "build/**", "*.map"]
+        );
+    }
+
+    #[test]
+    fn parse_compression_supports_aliases_and_quality_overrides() {
+        let (algorithms, quality) = parse_compression(Some(vec![
+            String::from("br:11,gzip:5"),
+            String::from("zst:-3"),
+        ]));
+
+        assert!(algorithms.brotli);
+        assert!(!algorithms.deflate);
+        assert!(algorithms.gzip);
+        assert!(algorithms.zstd);
+        assert_eq!(quality.brotli, 11);
+        assert_eq!(quality.gzip, 5);
+        assert_eq!(quality.zstd, -3);
+    }
+
+    #[test]
+    fn parse_compression_defaults_match_enabled_algorithms() {
+        let (algorithms, quality) = parse_compression(None);
+        let enabled = algorithms
+            .iter()
+            .map(|alg| alg.to_string())
+            .collect::<Vec<_>>();
+
+        assert_eq!(
+            enabled,
+            vec![
+                Algorithm::Brotli.to_string(),
+                Algorithm::Gzip.to_string(),
+                Algorithm::Zstd.to_string(),
+            ]
+        );
+        assert_eq!(quality.brotli, 10);
+        assert_eq!(quality.gzip, 7);
+        assert_eq!(quality.zstd, 19);
+    }
+
+    #[test]
+    fn calc_savings_handles_zero_positive_and_negative_values() {
+        assert_eq!(calc_savings(0, 0), 0);
+        assert_eq!(calc_savings(50, 50), 50);
+        assert_eq!(calc_savings(-50, 100), 0);
+    }
 }
diff --git a/src/precompress.rs b/src/precompress.rs
index 4dcab96..8daa1ca 100644
--- a/src/precompress.rs
+++ b/src/precompress.rs
@@ -440,6 +440,7 @@ static EXTENSIONS: phf::Set<&'static str> = phf::phf_set! {
 #[cfg(test)]
 mod tests {
     use std::{
+        collections::HashSet,
         fs,
         io::{self, Write},
         path::{Path, PathBuf},
@@ -555,6 +556,123 @@ mod tests {
         Ok(())
     }
 
+    #[test]
+    fn compressor_uses_default_and_custom_extension_filters() {
+        let default = Compressor::new(1, 1, Quality::default(), Algorithms::default(), None, false);
+        let custom = Compressor::new(
+            1,
+            1,
+            Quality::default(),
+            Algorithms::default(),
+            Some(HashSet::from([String::from("bin")])),
+            false,
+        );
+
+        assert!(default.should_compress(Path::new("asset.js")));
+        assert!(!default.should_compress(Path::new("asset.bin")));
+        assert!(!default.should_compress(Path::new("LICENSE")));
+        assert!(custom.should_compress(Path::new("asset.bin")));
+        assert!(!custom.should_compress(Path::new("asset.js")));
+    }
+
+    #[test]
+    fn compressor_skips_small_and_filtered_out_files() -> Result<()> {
+        let root = test_dir("skip-files");
+        fs::write(root.join("small.js"), "tiny")?;
+        fs::write(root.join("note.txt"), "ignored extension")?;
+
+        let compressor = Compressor::new(
+            1,
+            32,
+            Quality::default(),
+            Algorithms {
+                brotli: false,
+                deflate: false,
+                gzip: true,
+                zstd: false,
+            },
+            Some(HashSet::from([String::from("js")])),
+            false,
+        );
+        compressor.precompress(&root, &WalkOptions::default())?;
+        let stats = compressor.finish();
+
+        assert_eq!(stats.num_source_files, 0);
+        assert_eq!(stats.num_errors, 0);
+        assert!(!root.join("small.js.gz").exists());
+        assert!(!root.join("note.txt.gz").exists());
+
+        fs::remove_dir_all(root)?;
+        Ok(())
+    }
+
+    #[test]
+    fn compressor_overwrites_existing_outputs() -> Result<()> {
+        let root = test_dir("overwrite-output");
+        let src_path = root.join("asset.js");
+        fs::write(&src_path, "const payload = 'hello world';\n".repeat(256))?;
+        let dst_path = root.join("asset.js.gz");
+        fs::write(&dst_path, b"stale artifact")?;
+
+        let original = fs::read(&dst_path)?;
+        let compressor = Compressor::new(
+            1,
+            1,
+            Quality::default(),
+            Algorithms {
+                brotli: false,
+                deflate: false,
+                gzip: true,
+                zstd: false,
+            },
+            None,
+            false,
+        );
+        compressor.precompress(&root, &WalkOptions::default())?;
+        let stats = compressor.finish();
+
+        assert_eq!(stats.num_source_files, 1);
+        assert_eq!(stats.num_errors, 0);
+        assert_ne!(fs::read(&dst_path)?, original);
+
+        fs::remove_dir_all(root)?;
+        Ok(())
+    }
+
+    #[test]
+    fn compressor_cleans_up_temp_output_after_failed_replace() -> Result<()> {
+        let root = test_dir("cleanup-failed-replace");
+        let src_path = root.join("asset.js");
+        fs::write(&src_path, "const payload = 'hello world';\n".repeat(256))?;
+        let dst_path = root.join("asset.js.gz");
+        let tmp_path = tmp_output_path(&dst_path);
+        fs::create_dir(&dst_path)?;
+
+        let compressor = Compressor::new(
+            1,
+            1,
+            Quality::default(),
+            Algorithms {
+                brotli: false,
+                deflate: false,
+                gzip: true,
+                zstd: false,
+            },
+            None,
+            false,
+        );
+        compressor.precompress(&root, &WalkOptions::default())?;
+        let stats = compressor.finish();
+
+        assert_eq!(stats.num_source_files, 0);
+        assert_eq!(stats.num_errors, 1);
+        assert!(dst_path.is_dir());
+        assert!(!tmp_path.exists());
+
+        fs::remove_dir_all(root)?;
+        Ok(())
+    }
+
     fn walk_paths(root: &Path, options: &WalkOptions) -> Result<Vec<String>> {
         let mut paths = build_walk(root, options)?
             .filter_map(|entry| entry.ok())