diff --git a/TODO.md b/TODO.md
index 201c46d..f8e653d 100644
--- a/TODO.md
+++ b/TODO.md
@@ -1,7 +1,7 @@
 - [x] update the project to no more use features
 - [x] update last_model to be only used during one run
 - [x] rename project to "PolyScribe"
-- add tests
+- [x] add tests
 - update local models using hashes (--update-models)
 - create folder models/ if not present -> use /usr/share/polyscribe/models/ for release version, use ./models/ for development version
 - create missing folders for output files
diff --git a/src/main.rs b/src/main.rs
index f0a3caa..916c11d 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -512,3 +512,77 @@ fn main() -> Result<()> {
     Ok(())
 }
 
+
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use std::path::Path;
+
+    #[test]
+    fn test_format_srt_time_basic_and_rounding() {
+        assert_eq!(format_srt_time(0.0), "00:00:00,000");
+        assert_eq!(format_srt_time(1.0), "00:00:01,000");
+        assert_eq!(format_srt_time(61.0), "00:01:01,000");
+        assert_eq!(format_srt_time(3661.789), "01:01:01,789");
+        // rounding
+        assert_eq!(format_srt_time(0.0014), "00:00:00,001");
+        assert_eq!(format_srt_time(0.0015), "00:00:00,002");
+    }
+
+    #[test]
+    fn test_render_srt_with_and_without_speaker() {
+        let items = vec![
+            OutputEntry { id: 0, speaker: "Alice".to_string(), start: 0.0, end: 1.0, text: "Hello".to_string() },
+            OutputEntry { id: 1, speaker: String::new(), start: 1.0, end: 2.0, text: "World".to_string() },
+        ];
+        let srt = render_srt(&items);
+        let expected = "1\n00:00:00,000 --> 00:00:01,000\nAlice: Hello\n\n2\n00:00:01,000 --> 00:00:02,000\nWorld\n\n";
+        assert_eq!(srt, expected);
+    }
+
+    #[test]
+    fn test_sanitize_speaker_name() {
+        assert_eq!(sanitize_speaker_name("123-bob"), "bob");
+        assert_eq!(sanitize_speaker_name("00123-alice"), "alice");
+        assert_eq!(sanitize_speaker_name("abc-bob"), "abc-bob");
+        assert_eq!(sanitize_speaker_name("123"), "123");
+        assert_eq!(sanitize_speaker_name("-bob"), "-bob");
+        assert_eq!(sanitize_speaker_name("123-"), "");
+    }
+
+    #[test]
+    fn test_is_json_file_and_is_audio_file() {
+        assert!(is_json_file(Path::new("foo.json")));
+        assert!(is_json_file(Path::new("foo.JSON")));
+        assert!(!is_json_file(Path::new("foo.txt")));
+        assert!(!is_json_file(Path::new("foo")));
+
+        assert!(is_audio_file(Path::new("a.mp3")));
+        assert!(is_audio_file(Path::new("b.WAV")));
+        assert!(is_audio_file(Path::new("c.m4a")));
+        assert!(!is_audio_file(Path::new("d.txt")));
+    }
+
+    #[test]
+    fn test_normalize_lang_code() {
+        assert_eq!(normalize_lang_code("en"), Some("en".to_string()));
+        assert_eq!(normalize_lang_code("German"), Some("de".to_string()));
+        assert_eq!(normalize_lang_code("en_US.UTF-8"), Some("en".to_string()));
+        assert_eq!(normalize_lang_code("AUTO"), None);
+        assert_eq!(normalize_lang_code("  \t  "), None);
+        assert_eq!(normalize_lang_code("zh"), Some("zh".to_string()));
+    }
+
+    #[test]
+    fn test_date_prefix_format_shape() {
+        let d = date_prefix();
+        assert_eq!(d.len(), 10);
+        let bytes = d.as_bytes();
+        assert!(bytes[0].is_ascii_digit() && bytes[1].is_ascii_digit() && bytes[2].is_ascii_digit() && bytes[3].is_ascii_digit());
+        assert_eq!(bytes[4], b'-');
+        assert!(bytes[5].is_ascii_digit() && bytes[6].is_ascii_digit());
+        assert_eq!(bytes[7], b'-');
+        assert!(bytes[8].is_ascii_digit() && bytes[9].is_ascii_digit());
+    }
+}
diff --git a/tests/integration_cli.rs b/tests/integration_cli.rs
new file mode 100644
index 0000000..7f247a5
--- /dev/null
+++ b/tests/integration_cli.rs
@@ -0,0 +1,111 @@
+use std::fs;
+use std::io::Read;
+use std::path::{Path, PathBuf};
+use std::process::Command;
+
+use chrono::Local;
+use serde::Deserialize;
+
+#[derive(Deserialize)]
+struct OutputEntry {
+    id: u64,
+    speaker: String,
+    start: f64,
+    end: f64,
+    text: String,
+}
+
+#[derive(Deserialize)]
+struct OutputRoot {
+    items: Vec<OutputEntry>,
+}
+
+struct TestDir(PathBuf);
+impl TestDir {
+    fn new() -> Self {
+        let mut p = std::env::temp_dir();
+        let ts = Local::now().format("%Y%m%d%H%M%S%3f");
+        let pid = std::process::id();
+        p.push(format!("polyscribe_test_{}_{}", pid, ts));
+        fs::create_dir_all(&p).expect("Failed to create temp dir");
+        TestDir(p)
+    }
+    fn path(&self) -> &Path { &self.0 }
+}
+impl Drop for TestDir {
+    fn drop(&mut self) {
+        let _ = fs::remove_dir_all(&self.0);
+    }
+}
+
+fn manifest_path(relative: &str) -> PathBuf {
+    let mut p = PathBuf::from(env!("CARGO_MANIFEST_DIR"));
+    p.push(relative);
+    p
+}
+
+#[test]
+fn cli_merges_json_inputs_and_writes_outputs_to_temp_dir() {
+    let exe = env!("CARGO_BIN_EXE_polyscribe");
+    let tmp = TestDir::new();
+    let base = tmp.path().join("out");
+
+    let input1 = manifest_path("input/1-s0wlz.json");
+    let input2 = manifest_path("input/2-vikingowl.json");
+
+    // Run the CLI to write outputs into temp directory
+    let status = Command::new(exe)
+        .arg(input1.as_os_str())
+        .arg(input2.as_os_str())
+        .arg("-o")
+        .arg(base.as_os_str())
+        .status()
+        .expect("failed to spawn polyscribe");
+    assert!(status.success(), "CLI did not exit successfully");
+
+    // Expect files with today's date prefix
+    let date = Local::now().format("%Y-%m-%d").to_string();
+    let stem = format!("{}_{}", date, "out");
+    let json_path = tmp.path().join(format!("{}.json", stem));
+    let toml_path = tmp.path().join(format!("{}.toml", stem));
+    let srt_path = tmp.path().join(format!("{}.srt", stem));
+
+    assert!(json_path.is_file(), "missing JSON output: {}", json_path.display());
+    assert!(toml_path.is_file(), "missing TOML output: {}", toml_path.display());
+    assert!(srt_path.is_file(), "missing SRT output: {}", srt_path.display());
+
+    // Parse JSON and perform sanity checks
+    let mut json_str = String::new();
+    fs::File::open(&json_path).unwrap().read_to_string(&mut json_str).unwrap();
+    let parsed: OutputRoot = serde_json::from_str(&json_str).expect("invalid JSON in output");
+    assert!(!parsed.items.is_empty(), "no items in JSON output");
+    // Speakers should include sanitized stems from inputs
+    let speakers: std::collections::HashSet<_> = parsed.items.iter().map(|e| e.speaker.as_str()).collect();
+    assert!(speakers.contains("s0wlz"), "expected speaker s0wlz");
+    assert!(speakers.contains("vikingowl"), "expected speaker vikingowl");
+
+    // Check SRT has expected basic structure and speaker label present at least once
+    let mut srt = String::new();
+    fs::File::open(&srt_path).unwrap().read_to_string(&mut srt).unwrap();
+    assert!(srt.starts_with("1\n"), "SRT should start with index 1");
+    assert!(srt.contains("s0wlz:") || srt.contains("vikingowl:"), "SRT should contain at least one speaker label");
+}
+
+#[test]
+fn cli_prints_json_to_stdout_when_no_output_path() {
+    let exe = env!("CARGO_BIN_EXE_polyscribe");
+    let input1 = manifest_path("input/1-s0wlz.json");
+    let input2 = manifest_path("input/2-vikingowl.json");
+
+    let output = Command::new(exe)
+        .arg(input1.as_os_str())
+        .arg(input2.as_os_str())
+        .output()
+        .expect("failed to spawn polyscribe");
+    assert!(output.status.success(), "CLI failed");
+
+    let stdout = String::from_utf8(output.stdout).expect("stdout not UTF-8");
+    assert!(stdout.contains("\"items\""), "stdout should contain items JSON array");
+    // Ensure no files were created in repo output/ by default in this mode
+    // (Program writes to stdout only when -o omitted.)
+}