diff --git a/.beans/claudbg-nq36--tui-app-state-model-and-screen-enum.md b/.beans/claudbg-nq36--tui-app-state-model-and-screen-enum.md
index 53259ac..f5bdb94 100644
--- a/.beans/claudbg-nq36--tui-app-state-model-and-screen-enum.md
+++ b/.beans/claudbg-nq36--tui-app-state-model-and-screen-enum.md
@@ -1,11 +1,11 @@
 ---
 # claudbg-nq36
 title: 'TUI: app state model and screen enum'
-status: todo
+status: in-progress
 type: task
 priority: normal
 created_at: 2026-03-30T04:45:19Z
-updated_at: 2026-03-30T04:49:03Z
+updated_at: 2026-03-30T16:37:25Z
 parent: claudbg-i6l2
 blocked_by:
     - claudbg-78xt
diff --git a/src/lib.rs b/src/lib.rs
index 3f44620..909ccbc 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -7,6 +7,7 @@ pub mod error;
 pub mod models;
 pub mod output;
 pub mod parser;
+pub mod tui;
 pub mod util;
 
 #[cfg(test)]
diff --git a/src/tui/mod.rs b/src/tui/mod.rs
new file mode 100644
index 0000000..ddf67f5
--- /dev/null
+++ b/src/tui/mod.rs
@@ -0,0 +1,6 @@
+//! TUI module — terminal user interface for claudbg.
+//!
+//! This module will grow to include rendering and event-handling logic.
+//! For now it exposes the application state model used by all TUI screens.
+
+pub mod state;
diff --git a/src/tui/state.rs b/src/tui/state.rs
new file mode 100644
index 0000000..2bba5e2
--- /dev/null
+++ b/src/tui/state.rs
@@ -0,0 +1,336 @@
+//! TUI application state model.
+//!
+//! Defines the central [`AppState`] struct and the [`Screen`] / [`Focus`] enums
+//! that drive the TUI navigation model.  This module holds pure data — no
+//! terminal I/O or rendering logic lives here.
+
+use crate::models::session::RawEntry;
+use crate::parser::discovery::AgentRef;
+
+// ---------------------------------------------------------------------------
+// Lightweight display types
+// ---------------------------------------------------------------------------
+
+/// Lightweight summary of a session, used in the session-list screen.
+///
+/// Carries only the fields needed to render the list rows; heavier data
+/// (transcript entries, sub-agent details) is loaded on demand when the user
+/// navigates into a session.
+#[derive(Debug, Clone)]
+pub struct SessionListItem {
+    /// 8-character prefix of the session UUID (display form).
+    pub short_id: String,
+    /// Full session UUID (used when resolving to load transcript entries).
+    pub full_id: String,
+    /// Human-readable date string (e.g. `"2025-03-30 14:22:01"`).
+    pub date: String,
+    /// Project path recovered from the `cwd` field of the first JSONL line.
+    /// Falls back to an empty string when unavailable.
+    pub project: String,
+    /// Model identifier from the first assistant message (e.g. `"claude-sonnet-4-6"`).
+    /// Empty string when no assistant message is present.
+    pub model: String,
+    /// Total number of user + assistant messages in the session.
+    pub msg_count: usize,
+    /// Number of sub-agent runs attached to this session.
+    pub agent_count: usize,
+}
+
+// ---------------------------------------------------------------------------
+// Screen enum
+// ---------------------------------------------------------------------------
+
+/// The active screen (navigation state) of the TUI.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Screen {
+    /// The session list overview — the TUI's home screen.
+    SessionList,
+    /// A session transcript view for the given session UUID.
+    Transcript {
+        /// Full UUID of the session being viewed.
+        session_id: String,
+    },
+    /// A sub-agent transcript view nested under a parent session.
+    SubagentTranscript {
+        /// Full UUID of the parent session.
+        parent_session_id: String,
+        /// The agent's UUID (as returned by [`AgentRef::agent_id`]).
+        agent_id: String,
+    },
+}
+
+// ---------------------------------------------------------------------------
+// Focus enum
+// ---------------------------------------------------------------------------
+
+/// Which panel currently holds keyboard focus on the transcript screen.
+///
+/// On the session-list screen `Focus` is not meaningful; it is tracked here
+/// so the value is preserved when the user navigates back and forth.
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum Focus {
+    /// The main chat-log pane (default).
+    ChatLog,
+    /// The sub-agents side panel (shown when sub-agents are present).
+    SubagentsPanel,
+}
+
+impl Default for Focus {
+    fn default() -> Self {
+        Self::ChatLog
+    }
+}
+
+// ---------------------------------------------------------------------------
+// AppState
+// ---------------------------------------------------------------------------
+
+/// Central application state for the TUI.
+///
+/// All mutable state that drives rendering and event handling lives here.
+/// Rendering code reads this struct; event handlers mutate it.
+#[derive(Debug)]
+pub struct AppState {
+    // ── Navigation ──────────────────────────────────────────────────────────
+    /// Currently displayed screen.
+    pub screen: Screen,
+
+    // ── Session list ────────────────────────────────────────────────────────
+    /// All sessions, sorted most-recent-first.  Loaded once at startup.
+    pub sessions: Vec<SessionListItem>,
+    /// Index of the highlighted row in the session list.
+    pub list_selected: usize,
+
+    // ── Transcript ──────────────────────────────────────────────────────────
+    /// Parsed JSONL entries for the currently viewed session or sub-agent.
+    pub transcript_entries: Vec<RawEntry>,
+    /// Vertical scroll offset (lines from the top of the chat log).
+    pub transcript_scroll: usize,
+    /// Horizontal scroll offset (columns from the left edge of the chat log).
+    pub transcript_h_scroll: usize,
+
+    // ── Sub-agents panel ────────────────────────────────────────────────────
+    /// Sub-agent references for the currently viewed session.
+    pub subagents: Vec<AgentRef>,
+    /// Index of the highlighted row in the sub-agents panel.
+    pub subagent_selected: usize,
+
+    // ── Focus ───────────────────────────────────────────────────────────────
+    /// Which panel currently holds keyboard focus.
+    pub focus: Focus,
+
+    // ── Modals ──────────────────────────────────────────────────────────────
+    /// Whether the "quit?" confirmation dialog is visible.
+    pub show_quit_dialog: bool,
+    /// Whether the keyboard-shortcut help overlay is visible.
+    pub show_help: bool,
+}
+
+impl AppState {
+    /// Create a fresh [`AppState`] ready for the session-list screen.
+    ///
+    /// All data fields are initialised to empty / zero so the TUI can render
+    /// immediately; the caller is responsible for populating [`AppState::sessions`]
+    /// before the first frame is drawn.
+    pub fn new() -> Self {
+        Self {
+            screen: Screen::SessionList,
+            sessions: Vec::new(),
+            list_selected: 0,
+            transcript_entries: Vec::new(),
+            transcript_scroll: 0,
+            transcript_h_scroll: 0,
+            subagents: Vec::new(),
+            subagent_selected: 0,
+            focus: Focus::default(),
+            show_quit_dialog: false,
+            show_help: false,
+        }
+    }
+
+    /// Transition to the transcript screen for a session.
+    ///
+    /// Resets all transcript / sub-agent scroll and selection state so that
+    /// the new view starts from the top.  The caller is responsible for
+    /// loading [`AppState::transcript_entries`] and [`AppState::subagents`]
+    /// after calling this method.
+    pub fn enter_transcript(&mut self, session_id: impl Into<String>) {
+        self.screen = Screen::Transcript {
+            session_id: session_id.into(),
+        };
+        self.transcript_entries.clear();
+        self.transcript_scroll = 0;
+        self.transcript_h_scroll = 0;
+        self.subagents.clear();
+        self.subagent_selected = 0;
+        self.focus = Focus::ChatLog;
+    }
+
+    /// Transition to the sub-agent transcript screen.
+    ///
+    /// Resets transcript scroll state.  The caller is responsible for loading
+    /// [`AppState::transcript_entries`] for the agent after calling this method.
+    pub fn enter_subagent_transcript(
+        &mut self,
+        parent_session_id: impl Into<String>,
+        agent_id: impl Into<String>,
+    ) {
+        self.screen = Screen::SubagentTranscript {
+            parent_session_id: parent_session_id.into(),
+            agent_id: agent_id.into(),
+        };
+        self.transcript_entries.clear();
+        self.transcript_scroll = 0;
+        self.transcript_h_scroll = 0;
+        self.focus = Focus::ChatLog;
+    }
+
+    /// Return to the session-list screen.
+    ///
+    /// Clears transcript and sub-agent data.  The session list and the
+    /// previously selected row are preserved so the cursor position is
+    /// restored after navigating back.
+    pub fn go_back(&mut self) {
+        self.screen = Screen::SessionList;
+        self.transcript_entries.clear();
+        self.transcript_scroll = 0;
+        self.transcript_h_scroll = 0;
+        self.subagents.clear();
+        self.subagent_selected = 0;
+        self.focus = Focus::ChatLog;
+    }
+}
+
+impl Default for AppState {
+    fn default() -> Self {
+        Self::new()
+    }
+}
+
+// ---------------------------------------------------------------------------
+// Tests
+// ---------------------------------------------------------------------------
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// `AppState::new()` starts on `Screen::SessionList`.
+    #[test]
+    fn new_starts_on_session_list() {
+        let state = AppState::new();
+        assert_eq!(state.screen, Screen::SessionList);
+    }
+
+    /// `AppState::new()` starts with empty sessions and zero selection.
+    #[test]
+    fn new_starts_with_empty_sessions() {
+        let state = AppState::new();
+        assert!(state.sessions.is_empty());
+        assert_eq!(state.list_selected, 0);
+    }
+
+    /// `AppState::new()` starts with modals closed.
+    #[test]
+    fn new_starts_with_modals_closed() {
+        let state = AppState::new();
+        assert!(!state.show_quit_dialog);
+        assert!(!state.show_help);
+    }
+
+    /// `enter_transcript` switches to `Screen::Transcript` and resets scroll.
+    #[test]
+    fn enter_transcript_switches_screen() {
+        let mut state = AppState::new();
+        state.transcript_scroll = 42;
+        state.transcript_h_scroll = 7;
+        state.enter_transcript("abc12345-0000-0000-0000-000000000000");
+        assert_eq!(
+            state.screen,
+            Screen::Transcript {
+                session_id: "abc12345-0000-0000-0000-000000000000".to_string(),
+            }
+        );
+        assert_eq!(state.transcript_scroll, 0);
+        assert_eq!(state.transcript_h_scroll, 0);
+    }
+
+    /// `enter_transcript` clears previous transcript entries.
+    #[test]
+    fn enter_transcript_clears_entries() {
+        let mut state = AppState::new();
+        // Insert a dummy entry via direct field access (simplest approach).
+        state.transcript_entries.push(RawEntry {
+            entry_type: Some("user".to_string()),
+            session_id: None,
+            parent_session_id: None,
+            message: None,
+            system_message: None,
+            cwd: None,
+            timestamp: None,
+            duration_ms: None,
+            extra: Default::default(),
+        });
+        state.enter_transcript("new-session");
+        assert!(state.transcript_entries.is_empty());
+    }
+
+    /// `enter_subagent_transcript` sets `Screen::SubagentTranscript`.
+    #[test]
+    fn enter_subagent_transcript_sets_screen() {
+        let mut state = AppState::new();
+        state.enter_subagent_transcript("parent-session", "agent-001");
+        assert_eq!(
+            state.screen,
+            Screen::SubagentTranscript {
+                parent_session_id: "parent-session".to_string(),
+                agent_id: "agent-001".to_string(),
+            }
+        );
+    }
+
+    /// `go_back` returns to `Screen::SessionList` and clears transcript state.
+    #[test]
+    fn go_back_returns_to_session_list() {
+        let mut state = AppState::new();
+        state.enter_transcript("some-session");
+        state.transcript_scroll = 10;
+        state.go_back();
+        assert_eq!(state.screen, Screen::SessionList);
+        assert_eq!(state.transcript_scroll, 0);
+        assert!(state.transcript_entries.is_empty());
+    }
+
+    /// `go_back` preserves `list_selected`.
+    #[test]
+    fn go_back_preserves_list_selected() {
+        let mut state = AppState::new();
+        state.list_selected = 5;
+        state.enter_transcript("some-session");
+        state.go_back();
+        assert_eq!(state.list_selected, 5);
+    }
+
+    /// `Focus::default()` is `ChatLog`.
+    #[test]
+    fn focus_default_is_chat_log() {
+        assert_eq!(Focus::default(), Focus::ChatLog);
+    }
+
+    /// `SessionListItem` implements `Debug` and `Clone`.
+    #[test]
+    fn session_list_item_debug_clone() {
+        let item = SessionListItem {
+            short_id: "abc12345".to_string(),
+            full_id: "abc12345-0000-0000-0000-000000000000".to_string(),
+            date: "2025-03-30 14:00:00".to_string(),
+            project: "/home/user/project".to_string(),
+            model: "claude-sonnet-4-6".to_string(),
+            msg_count: 10,
+            agent_count: 2,
+        };
+        let cloned = item.clone();
+        assert_eq!(cloned.short_id, item.short_id);
+        let _ = format!("{item:?}");
+    }
+}