diff --git a/Cargo.toml b/Cargo.toml
index 51269b9..41bcf8c 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -22,3 +22,4 @@ ratatui = "0.30"
 crossterm = "0.29"
 clap = { version = "4", features = ["derive"] }
 log-viewer-core = { path = "crates/core" }
+textwrap = "0.16"
diff --git a/crates/core/src/parser/json.rs b/crates/core/src/parser/json.rs
index 0e7227b..bd78578 100644
--- a/crates/core/src/parser/json.rs
+++ b/crates/core/src/parser/json.rs
@@ -26,7 +26,7 @@ pub fn parse_line(line: &str) -> Option<LogEntry> {
         .iter()
         .find_map(|key| fields.remove(*key))
         .and_then(|v| v.as_str().map(String::from))
-        .map(|s| s.parse::<LogLevel>().unwrap());
+        .map(|s| s.parse::<LogLevel>().unwrap_or_else(|e| match e {}));
 
     Some(LogEntry {
         line_number: 0,
diff --git a/crates/tui/Cargo.toml b/crates/tui/Cargo.toml
index dc46fb9..8ce5d1a 100644
--- a/crates/tui/Cargo.toml
+++ b/crates/tui/Cargo.toml
@@ -9,3 +9,4 @@ crossterm.workspace = true
 clap.workspace = true
 anyhow.workspace = true
 log-viewer-core.workspace = true
+textwrap.workspace = true
diff --git a/crates/tui/src/app.rs b/crates/tui/src/app.rs
index 1699767..74664c7 100644
--- a/crates/tui/src/app.rs
+++ b/crates/tui/src/app.rs
@@ -1,17 +1,546 @@
+use std::path::Path;
+use std::time::Instant;
+
+use log_viewer_core::io::file_reader::FileReader;
+
 pub struct App {
     pub should_quit: bool,
+
+    // File state
+    file_reader: Option<FileReader>,
+    pub(crate) file_path: Option<String>,
+
+    // Scroll state
+    pub(crate) cursor_line: usize,
+    pub(crate) v_offset: usize,
+
+    // Soft wrap cache
+    pub(crate) wrap_cache: Vec<Vec<String>>,
+    pub(crate) visual_heights: Vec<usize>,
+    pub(crate) total_visual_rows: usize,
+    pub(crate) cached_width: usize,
+
+    // Viewport
+    #[allow(dead_code)]
+    pub(crate) content_width: u16,
+    pub(crate) content_height: u16,
+
+    // gg state machine
+    pub(crate) last_g_press: Option<Instant>,
 }
 
 impl App {
     pub fn new() -> Self {
-        Self { should_quit: false }
-    }
-
-    pub fn handle_key(&mut self, key: crossterm::event::KeyEvent) {
-        match key.code {
-            crossterm::event::KeyCode::Char('q') => self.should_quit = true,
-            crossterm::event::KeyCode::Esc => self.should_quit = true,
-            _ => {}
+        Self {
+            should_quit: false,
+            file_reader: None,
+            file_path: None,
+            cursor_line: 0,
+            v_offset: 0,
+            wrap_cache: Vec::new(),
+            visual_heights: Vec::new(),
+            total_visual_rows: 0,
+            cached_width: 0,
+            content_width: 0,
+            content_height: 0,
+            last_g_press: None,
         }
     }
+
+    pub fn load_file(&mut self, path: &str) -> anyhow::Result<()> {
+        let reader = FileReader::open(Path::new(path)).map_err(|e| anyhow::anyhow!("{e}"))?;
+        self.file_reader = Some(reader);
+        self.file_path = Some(path.to_string());
+        self.cursor_line = 0;
+        self.v_offset = 0;
+        self.wrap_cache.clear();
+        self.visual_heights.clear();
+        self.total_visual_rows = 0;
+        self.cached_width = 0; // force wrap cache rebuild on next render
+        self.last_g_press = None; // reset gg state machine
+        Ok(())
+    }
+
+    #[allow(dead_code)]
+    pub fn recompute_wrap_cache(&mut self, width: usize) {
+        if !self.is_loaded() || width == 0 {
+            return;
+        }
+        if self.cached_width == width {
+            return;
+        }
+
+        let line_count = self.total_lines();
+        self.wrap_cache.clear();
+        self.visual_heights.clear();
+        self.visual_heights.reserve(line_count);
+
+        for i in 0..line_count {
+            let line = self.get_line(i).unwrap_or("");
+            let wrapped: Vec<std::borrow::Cow<'_, str>> = textwrap::wrap(line, width);
+            let wrapped: Vec<String> = wrapped.into_iter().map(|c| c.into_owned()).collect();
+            let height = wrapped.len().max(1);
+            self.wrap_cache.push(wrapped);
+            self.visual_heights.push(height);
+        }
+
+        self.total_visual_rows = self.visual_heights.iter().sum();
+        self.cached_width = width;
+
+        // Clamp v_offset
+        let max_offset = self
+            .total_visual_rows
+            .saturating_sub(self.content_height as usize);
+        self.v_offset = self.v_offset.min(max_offset);
+    }
+
+    // ── Scroll methods ──────────────────────────────────────────────
+
+    pub fn scroll_down_line(&mut self) {
+        if !self.is_loaded() || self.total_lines() == 0 {
+            return;
+        }
+        let last = self.total_lines() - 1;
+        if self.cursor_line < last {
+            self.cursor_line += 1;
+        }
+        self.ensure_cursor_visible();
+    }
+
+    pub fn scroll_up_line(&mut self) {
+        if !self.is_loaded() || self.total_lines() == 0 {
+            return;
+        }
+        self.cursor_line = self.cursor_line.saturating_sub(1);
+        self.ensure_cursor_visible();
+    }
+
+    pub fn scroll_down_half_page(&mut self) {
+        if !self.is_loaded() || self.total_lines() == 0 {
+            return;
+        }
+        let half = self.content_height as usize / 2;
+        self.v_offset = self.v_offset.saturating_add(half);
+        let center_visual = self
+            .v_offset
+            .saturating_add(self.content_height as usize / 2);
+        self.cursor_line = self.visual_row_to_logical_row(center_visual);
+        self.clamp_v_offset();
+    }
+
+    pub fn scroll_up_half_page(&mut self) {
+        if !self.is_loaded() || self.total_lines() == 0 {
+            return;
+        }
+        let half = self.content_height as usize / 2;
+        self.v_offset = self.v_offset.saturating_sub(half);
+        let center_visual = self
+            .v_offset
+            .saturating_add(self.content_height as usize / 2);
+        self.cursor_line = self.visual_row_to_logical_row(center_visual);
+        self.clamp_v_offset();
+    }
+
+    pub fn scroll_down_page(&mut self) {
+        if !self.is_loaded() || self.total_lines() == 0 {
+            return;
+        }
+        let page = self.content_height as usize;
+        self.v_offset = self.v_offset.saturating_add(page);
+        let center_visual = self
+            .v_offset
+            .saturating_add(self.content_height as usize / 2);
+        self.cursor_line = self.visual_row_to_logical_row(center_visual);
+        self.clamp_v_offset();
+    }
+
+    pub fn scroll_up_page(&mut self) {
+        if !self.is_loaded() || self.total_lines() == 0 {
+            return;
+        }
+        let page = self.content_height as usize;
+        self.v_offset = self.v_offset.saturating_sub(page);
+        let center_visual = self
+            .v_offset
+            .saturating_add(self.content_height as usize / 2);
+        self.cursor_line = self.visual_row_to_logical_row(center_visual);
+        self.clamp_v_offset();
+    }
+
+    pub fn scroll_to_top(&mut self) {
+        if !self.is_loaded() || self.total_lines() == 0 {
+            return;
+        }
+        self.cursor_line = 0;
+        self.v_offset = 0;
+    }
+
+    pub fn scroll_to_bottom(&mut self) {
+        if !self.is_loaded() || self.total_lines() == 0 {
+            return;
+        }
+        self.cursor_line = self.total_lines().saturating_sub(1);
+        self.ensure_cursor_visible();
+        self.clamp_v_offset();
+    }
+
+    // ── Internal helpers ────────────────────────────────────────────
+
+    fn ensure_cursor_visible(&mut self) {
+        if !self.is_loaded() || self.total_lines() == 0 {
+            return;
+        }
+        let cursor_first = self.cursor_to_first_visual_row(self.cursor_line);
+        let height = self
+            .visual_heights
+            .get(self.cursor_line)
+            .copied()
+            .unwrap_or(1);
+        let cursor_last = cursor_first + height.saturating_sub(1);
+
+        let content_h = self.content_height as usize;
+
+        if cursor_first < self.v_offset {
+            self.v_offset = cursor_first;
+        } else if cursor_last >= self.v_offset.saturating_add(content_h) {
+            self.v_offset = cursor_last.saturating_sub(content_h).saturating_add(1);
+        }
+        self.clamp_v_offset();
+    }
+
+    fn clamp_v_offset(&mut self) {
+        let max_offset = self
+            .total_visual_rows
+            .saturating_sub(self.content_height as usize);
+        self.v_offset = self.v_offset.min(max_offset);
+    }
+
+    pub(crate) fn cursor_to_first_visual_row(&self, line: usize) -> usize {
+        self.visual_heights.iter().take(line).sum()
+    }
+
+    pub(crate) fn visual_row_to_logical_row(&self, visual_row: usize) -> usize {
+        let mut acc: usize = 0;
+        for (i, &h) in self.visual_heights.iter().enumerate() {
+            if acc.saturating_add(h) > visual_row {
+                return i;
+            }
+            acc += h;
+        }
+        // Boundary: visual_row >= total_visual_rows → return last line
+        self.visual_heights.len().saturating_sub(1)
+    }
+
+    // ── Key handling ────────────────────────────────────────────────
+
+    pub fn handle_key(&mut self, key: crossterm::event::KeyEvent) {
+        use crossterm::event::{KeyCode, KeyModifiers};
+
+        match key.code {
+            KeyCode::Char('q') | KeyCode::Esc => {
+                self.should_quit = true;
+                self.last_g_press = None;
+            }
+            KeyCode::Char('j') | KeyCode::Down => {
+                self.scroll_down_line();
+                self.last_g_press = None;
+            }
+            KeyCode::Char('k') | KeyCode::Up => {
+                self.scroll_up_line();
+                self.last_g_press = None;
+            }
+            KeyCode::Char('d') if key.modifiers.contains(KeyModifiers::CONTROL) => {
+                self.scroll_down_half_page();
+                self.last_g_press = None;
+            }
+            KeyCode::Char('u') if key.modifiers.contains(KeyModifiers::CONTROL) => {
+                self.scroll_up_half_page();
+                self.last_g_press = None;
+            }
+            KeyCode::Char('f') if key.modifiers.contains(KeyModifiers::CONTROL) => {
+                self.scroll_down_page();
+                self.last_g_press = None;
+            }
+            KeyCode::PageDown => {
+                self.scroll_down_page();
+                self.last_g_press = None;
+            }
+            KeyCode::Char('b') if key.modifiers.contains(KeyModifiers::CONTROL) => {
+                self.scroll_up_page();
+                self.last_g_press = None;
+            }
+            KeyCode::PageUp => {
+                self.scroll_up_page();
+                self.last_g_press = None;
+            }
+            KeyCode::Char('G') | KeyCode::End => {
+                self.scroll_to_bottom();
+                self.last_g_press = None;
+            }
+            KeyCode::Char('g') => {
+                if let Some(instant) = self.last_g_press
+                    && instant.elapsed().as_millis() < 500
+                {
+                    self.scroll_to_top();
+                    self.last_g_press = None;
+                    return;
+                }
+                self.last_g_press = Some(Instant::now());
+            }
+            KeyCode::Home => {
+                self.scroll_to_top();
+                self.last_g_press = None;
+            }
+            _ => {
+                self.last_g_press = None;
+            }
+        }
+    }
+
+    // ── Utility methods ─────────────────────────────────────────────
+
+    #[allow(dead_code)]
+    pub fn get_line(&self, idx: usize) -> Option<&str> {
+        self.file_reader.as_ref().and_then(|r| r.get_line(idx))
+    }
+
+    #[allow(dead_code)]
+    pub fn file_name(&self) -> Option<&str> {
+        self.file_path
+            .as_ref()
+            .and_then(|p| std::path::Path::new(p).file_name().and_then(|n| n.to_str()))
+    }
+
+    pub fn total_lines(&self) -> usize {
+        self.file_reader.as_ref().map_or(0, |r| r.line_count())
+    }
+
+    pub fn is_loaded(&self) -> bool {
+        self.file_reader.is_some()
+    }
+
+    #[allow(dead_code)]
+    pub fn file_size(&self) -> u64 {
+        self.file_reader.as_ref().map_or(0, |r| r.file_size())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    use std::sync::atomic::{AtomicU64, Ordering};
+    static COUNTER: AtomicU64 = AtomicU64::new(0);
+
+    fn make_temp_file(content: &str) -> std::path::PathBuf {
+        let id = COUNTER.fetch_add(1, Ordering::Relaxed);
+        let dir = std::env::temp_dir();
+        let name = format!("log_viewer_test_{}_{}", std::process::id(), id);
+        let path = dir.join(name);
+        std::fs::write(&path, content).unwrap();
+        path
+    }
+
+    fn cleanup(path: &std::path::Path) {
+        let _ = std::fs::remove_file(path);
+    }
+
+    #[test]
+    fn test_app_new_defaults() {
+        let app = App::new();
+        assert!(!app.should_quit);
+        assert!(app.file_reader.is_none());
+        assert!(app.file_path.is_none());
+        assert_eq!(app.cursor_line, 0);
+        assert_eq!(app.v_offset, 0);
+        assert!(app.wrap_cache.is_empty());
+        assert!(app.visual_heights.is_empty());
+        assert_eq!(app.total_visual_rows, 0);
+        assert_eq!(app.cached_width, 0);
+        assert_eq!(app.content_width, 0);
+        assert_eq!(app.content_height, 0);
+        assert!(app.last_g_press.is_none());
+    }
+
+    #[test]
+    fn test_load_file_success() {
+        let path = make_temp_file("line1\nline2\nline3\n");
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            assert!(app.load_file(path.to_str().unwrap()).is_ok());
+            assert!(app.is_loaded());
+            assert_eq!(app.total_lines(), 3);
+            assert_eq!(app.cursor_line, 0);
+            assert_eq!(app.v_offset, 0);
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_load_file_resets_state() {
+        let path = make_temp_file("a\nb\nc\nd\n");
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            app.load_file(path.to_str().unwrap()).unwrap();
+            app.cursor_line = 3;
+            app.v_offset = 2;
+
+            let path2 = make_temp_file("x\ny\n");
+            app.load_file(path2.to_str().unwrap()).unwrap();
+            assert_eq!(app.cursor_line, 0);
+            assert_eq!(app.v_offset, 0);
+            assert_eq!(app.total_lines(), 2);
+            cleanup(&path2);
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_load_file_nonexistent() {
+        let mut app = App::new();
+        let result = app.load_file("/tmp/no_such_file_log_viewer_test_xyz");
+        assert!(result.is_err());
+        assert!(!app.is_loaded());
+    }
+
+    #[test]
+    fn test_unloaded_scroll_safety() {
+        let mut app = App::new();
+        // None of these should panic
+        app.scroll_down_line();
+        app.scroll_up_line();
+        app.scroll_down_half_page();
+        app.scroll_up_half_page();
+        app.scroll_down_page();
+        app.scroll_up_page();
+        app.scroll_to_top();
+        app.scroll_to_bottom();
+        assert_eq!(app.cursor_line, 0);
+        assert_eq!(app.v_offset, 0);
+    }
+
+    #[test]
+    fn test_empty_file_scroll_safety() {
+        let path = make_temp_file("");
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            // FileReader on empty file should have 0 lines (or 1 empty line depending on impl)
+            app.load_file(path.to_str().unwrap()).unwrap();
+
+            app.scroll_down_line();
+            app.scroll_up_line();
+            app.scroll_down_half_page();
+            app.scroll_up_half_page();
+            app.scroll_down_page();
+            app.scroll_up_page();
+            app.scroll_to_top();
+            app.scroll_to_bottom();
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_scroll_down_at_bottom() {
+        let path = make_temp_file("a\nb\nc\n");
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            app.load_file(path.to_str().unwrap()).unwrap();
+            app.cursor_line = 2; // last line
+
+            app.scroll_down_line();
+            assert_eq!(app.cursor_line, 2); // stays at last
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_scroll_up_at_top() {
+        let path = make_temp_file("a\nb\nc\n");
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            app.load_file(path.to_str().unwrap()).unwrap();
+            assert_eq!(app.cursor_line, 0);
+
+            app.scroll_up_line();
+            assert_eq!(app.cursor_line, 0); // stays at 0
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_wrap_cache_correctness() {
+        let long_line = "a".repeat(200);
+        let path = make_temp_file(&format!("{long_line}\nshort\n"));
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            app.load_file(path.to_str().unwrap()).unwrap();
+            app.content_height = 50; // enough viewport
+
+            app.recompute_wrap_cache(20);
+            // Long line should wrap into multiple visual rows
+            assert!(
+                app.visual_heights[0] > 1,
+                "expected wrapping but got height {}",
+                app.visual_heights[0]
+            );
+            // Short line is 1 row
+            assert_eq!(app.visual_heights[1], 1);
+            assert!(app.total_visual_rows > 2);
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_recompute_wrap_cache_not_loaded() {
+        let mut app = App::new();
+        app.recompute_wrap_cache(80);
+        // Should not panic and cache should stay empty
+        assert!(app.wrap_cache.is_empty());
+    }
+
+    #[test]
+    fn test_recompute_wrap_cache_zero_width() {
+        let path = make_temp_file("hello\n");
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            app.load_file(path.to_str().unwrap()).unwrap();
+            app.recompute_wrap_cache(0);
+            // Should not panic, cache should stay empty (width 0 is guarded)
+            assert!(app.wrap_cache.is_empty());
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_load_file_resets_cached_width() {
+        let path = make_temp_file("hello\n");
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            app.cached_width = 80;
+            app.load_file(path.to_str().unwrap()).unwrap();
+            assert_eq!(app.cached_width, 0);
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
+
+    #[test]
+    fn test_load_file_resets_gg_state() {
+        let path = make_temp_file("hello\n");
+        let result = std::panic::catch_unwind(|| {
+            let mut app = App::new();
+            app.last_g_press = Some(Instant::now());
+            app.load_file(path.to_str().unwrap()).unwrap();
+            assert!(app.last_g_press.is_none());
+        });
+        cleanup(&path);
+        assert!(result.is_ok());
+    }
 }
diff --git a/crates/tui/src/main.rs b/crates/tui/src/main.rs
index a021895..67c09c4 100644
--- a/crates/tui/src/main.rs
+++ b/crates/tui/src/main.rs
@@ -11,7 +11,7 @@ struct Cli {
 }
 
 fn main() -> anyhow::Result<()> {
-    let _cli = Cli::parse();
+    let cli = Cli::parse();
 
     crossterm::terminal::enable_raw_mode()?;
     let mut stdout = std::io::stdout();
@@ -21,12 +21,21 @@ fn main() -> anyhow::Result<()> {
 
     let mut app = app::App::new();
 
+    if let Some(file) = cli.files.first()
+        && let Err(e) = app.load_file(file)
+    {
+        eprintln!("Error loading file: {e}");
+        std::process::exit(1);
+    }
+
     while !app.should_quit {
         terminal.draw(|frame| ui::render(frame, &app))?;
-        if crossterm::event::poll(std::time::Duration::from_millis(100))?
-            && let crossterm::event::Event::Key(key) = crossterm::event::read()?
-        {
-            app.handle_key(key);
+        if crossterm::event::poll(std::time::Duration::from_millis(100))? {
+            match crossterm::event::read()? {
+                crossterm::event::Event::Key(key) => app.handle_key(key),
+                crossterm::event::Event::Resize(_w, _h) => {}
+                _ => {}
+            }
         }
     }