diff --git a/solitaire_core/src/game_state.rs b/solitaire_core/src/game_state.rs
new file mode 100644
index 0000000..857fe8c
--- /dev/null
+++ b/solitaire_core/src/game_state.rs
@@ -0,0 +1,541 @@
+use std::collections::HashMap;
+use serde::{Deserialize, Serialize};
+use crate::card::{Card, Suit};
+use crate::deck::{deal_klondike, Deck};
+use crate::error::MoveError;
+use crate::pile::{Pile, PileType};
+use crate::rules::{can_place_on_foundation, can_place_on_tableau};
+use crate::scoring::{compute_time_bonus as scoring_time_bonus, score_move, score_undo as scoring_undo};
+
+const MAX_UNDO_STACK: usize = 64;
+
+/// Whether cards are drawn one at a time or three at a time from the stock.
+#[derive(Debug, Clone, PartialEq, Eq, Serialize, Deserialize)]
+pub enum DrawMode {
+    DrawOne,
+    DrawThree,
+}
+
+/// Snapshot of game state used for undo.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct StateSnapshot {
+    piles: HashMap<PileType, Pile>,
+    score: i32,
+    move_count: u32,
+    stock_recycled: bool,
+}
+
+/// Full state of an in-progress Klondike Solitaire game.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct GameState {
+    pub piles: HashMap<PileType, Pile>,
+    pub draw_mode: DrawMode,
+    pub score: i32,
+    pub move_count: u32,
+    pub elapsed_seconds: u64,
+    pub seed: u64,
+    pub is_won: bool,
+    pub is_auto_completable: bool,
+    pub(crate) undo_stack: Vec<StateSnapshot>,
+    /// Whether the waste has already been recycled back to stock once.
+    /// A second recycle attempt returns `StockEmpty`.
+    stock_recycled: bool,
+}
+
+impl GameState {
+    /// Creates a new game dealt from the given seed and draw mode.
+    pub fn new(seed: u64, draw_mode: DrawMode) -> Self {
+        let mut deck = Deck::new();
+        deck.shuffle(seed);
+        let (tableau, stock) = deal_klondike(deck);
+
+        let mut piles: HashMap<PileType, Pile> = HashMap::new();
+        piles.insert(PileType::Stock, stock);
+        piles.insert(PileType::Waste, Pile::new(PileType::Waste));
+        for suit in [Suit::Clubs, Suit::Diamonds, Suit::Hearts, Suit::Spades] {
+            piles.insert(PileType::Foundation(suit), Pile::new(PileType::Foundation(suit)));
+        }
+        for (i, pile) in tableau.into_iter().enumerate() {
+            piles.insert(PileType::Tableau(i), pile);
+        }
+
+        Self {
+            piles,
+            draw_mode,
+            score: 0,
+            move_count: 0,
+            elapsed_seconds: 0,
+            seed,
+            is_won: false,
+            is_auto_completable: false,
+            undo_stack: Vec::new(),
+            stock_recycled: false,
+        }
+    }
+
+    /// Number of snapshots currently on the undo stack.
+    pub fn undo_stack_len(&self) -> usize {
+        self.undo_stack.len()
+    }
+
+    fn take_snapshot(&self) -> StateSnapshot {
+        StateSnapshot {
+            piles: self.piles.clone(),
+            score: self.score,
+            move_count: self.move_count,
+            stock_recycled: self.stock_recycled,
+        }
+    }
+
+    fn push_snapshot(&mut self) {
+        if self.undo_stack.len() >= MAX_UNDO_STACK {
+            self.undo_stack.remove(0);
+        }
+        self.undo_stack.push(self.take_snapshot());
+    }
+
+    /// Draw cards from stock to waste. When stock is empty, recycles waste back to stock.
+    pub fn draw(&mut self) -> Result<(), MoveError> {
+        if self.is_won {
+            return Err(MoveError::GameAlreadyWon);
+        }
+
+        let stock_len = self.piles[&PileType::Stock].cards.len();
+
+        if stock_len == 0 {
+            let waste_len = self.piles[&PileType::Waste].cards.len();
+            if waste_len == 0 || self.stock_recycled {
+                return Err(MoveError::StockEmpty);
+            }
+            // Recycle: reverse waste back onto stock, face-down
+            let waste_cards: Vec<Card> = self.piles
+                .get_mut(&PileType::Waste)
+                .unwrap()
+                .cards
+                .drain(..)
+                .collect();
+            let stock = self.piles.get_mut(&PileType::Stock).unwrap();
+            for mut card in waste_cards.into_iter().rev() {
+                card.face_up = false;
+                stock.cards.push(card);
+            }
+            self.stock_recycled = true;
+            return Ok(());
+        }
+
+        self.push_snapshot();
+
+        let draw_count = match self.draw_mode {
+            DrawMode::DrawOne => 1,
+            DrawMode::DrawThree => 3,
+        };
+        let available = stock_len.min(draw_count);
+        let drain_start = stock_len - available;
+
+        let drawn: Vec<Card> = self.piles
+            .get_mut(&PileType::Stock)
+            .unwrap()
+            .cards
+            .drain(drain_start..)
+            .collect();
+
+        let waste = self.piles.get_mut(&PileType::Waste).unwrap();
+        for mut card in drawn {
+            card.face_up = true;
+            waste.cards.push(card);
+        }
+
+        self.move_count += 1;
+        Ok(())
+    }
+
+    /// Move `count` cards from pile `from` to pile `to`.
+    ///
+    /// Returns `Err(MoveError)` if the move is illegal. On success, updates score,
+    /// flips the newly exposed source card if needed, and checks win/auto-complete.
+    pub fn move_cards(&mut self, from: PileType, to: PileType, count: usize) -> Result<(), MoveError> {
+        if self.is_won {
+            return Err(MoveError::GameAlreadyWon);
+        }
+        if from == to {
+            return Err(MoveError::RuleViolation("source and destination must differ".into()));
+        }
+
+        // Validate via scoped immutable borrows
+        let move_start = {
+            let from_pile = self.piles.get(&from).ok_or(MoveError::InvalidSource)?;
+            if from_pile.cards.is_empty() {
+                return Err(MoveError::EmptySource);
+            }
+            if count == 0 || count > from_pile.cards.len() {
+                return Err(MoveError::RuleViolation("invalid card count".into()));
+            }
+            let start = from_pile.cards.len() - count;
+            for card in &from_pile.cards[start..] {
+                if !card.face_up {
+                    return Err(MoveError::RuleViolation("cannot move face-down card".into()));
+                }
+            }
+            let bottom_card = from_pile.cards[start].clone();
+
+            match &to {
+                PileType::Foundation(suit) => {
+                    if count != 1 {
+                        return Err(MoveError::RuleViolation(
+                            "only one card can move to foundation at a time".into(),
+                        ));
+                    }
+                    let dest = self.piles.get(&to).ok_or(MoveError::InvalidDestination)?;
+                    if !can_place_on_foundation(&bottom_card, dest, *suit) {
+                        return Err(MoveError::RuleViolation("invalid foundation placement".into()));
+                    }
+                }
+                PileType::Tableau(_) => {
+                    let dest = self.piles.get(&to).ok_or(MoveError::InvalidDestination)?;
+                    if !can_place_on_tableau(&bottom_card, dest) {
+                        return Err(MoveError::RuleViolation("invalid tableau placement".into()));
+                    }
+                }
+                _ => return Err(MoveError::InvalidDestination),
+            }
+            start
+        };
+
+        let score_delta = score_move(&from, &to);
+        self.push_snapshot();
+
+        // Execute move
+        let mut moved: Vec<Card> = self.piles
+            .get_mut(&from)
+            .unwrap()
+            .cards
+            .split_off(move_start);
+
+        // Flip the newly exposed top card of the source pile
+        if let Some(top) = self.piles.get_mut(&from).unwrap().cards.last_mut() {
+            if !top.face_up {
+                top.face_up = true;
+            }
+        }
+
+        self.piles.get_mut(&to).unwrap().cards.append(&mut moved);
+
+        self.score = (self.score + score_delta).max(0);
+        self.move_count += 1;
+
+        self.is_won = self.check_win();
+        if !self.is_won {
+            self.is_auto_completable = self.check_auto_complete();
+        }
+
+        Ok(())
+    }
+
+    /// Restore the most recent undo snapshot and apply the undo score penalty (-15).
+    pub fn undo(&mut self) -> Result<(), MoveError> {
+        if self.is_won {
+            return Err(MoveError::GameAlreadyWon);
+        }
+        let snapshot = self.undo_stack.pop().ok_or(MoveError::UndoStackEmpty)?;
+        self.piles = snapshot.piles;
+        self.score = (snapshot.score + scoring_undo()).max(0);
+        self.move_count = snapshot.move_count;
+        self.stock_recycled = snapshot.stock_recycled;
+        self.is_won = false;
+        self.is_auto_completable = false;
+        Ok(())
+    }
+
+    /// Returns `true` when all four foundations each contain 13 cards.
+    pub fn check_win(&self) -> bool {
+        [Suit::Clubs, Suit::Diamonds, Suit::Hearts, Suit::Spades]
+            .iter()
+            .all(|&suit| {
+                self.piles
+                    .get(&PileType::Foundation(suit))
+                    .is_some_and(|p| p.cards.len() == 13)
+            })
+    }
+
+    /// Returns `true` when stock and waste are empty and all tableau cards are face-up.
+    /// At that point the game can be completed without further player input.
+    pub fn check_auto_complete(&self) -> bool {
+        if !self.piles[&PileType::Stock].cards.is_empty() {
+            return false;
+        }
+        if !self.piles[&PileType::Waste].cards.is_empty() {
+            return false;
+        }
+        (0..7).all(|i| {
+            self.piles[&PileType::Tableau(i)]
+                .cards
+                .iter()
+                .all(|c| c.face_up)
+        })
+    }
+
+    /// Time bonus added to score on win: `700_000 / elapsed_seconds` (0 if elapsed is 0).
+    pub fn compute_time_bonus(&self) -> i32 {
+        scoring_time_bonus(self.elapsed_seconds)
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::card::{Card, Rank};
+
+    fn new_game() -> GameState {
+        GameState::new(42, DrawMode::DrawOne)
+    }
+
+    // --- Initial state ---
+
+    #[test]
+    fn new_game_has_correct_tableau_sizes() {
+        let g = new_game();
+        let total: usize = (0..7).map(|i| g.piles[&PileType::Tableau(i)].cards.len()).sum();
+        assert_eq!(total, 28);
+        for i in 0..7 {
+            assert_eq!(g.piles[&PileType::Tableau(i)].cards.len(), i + 1);
+        }
+    }
+
+    #[test]
+    fn new_game_stock_has_24_cards() {
+        assert_eq!(new_game().piles[&PileType::Stock].cards.len(), 24);
+    }
+
+    #[test]
+    fn new_game_waste_is_empty() {
+        assert!(new_game().piles[&PileType::Waste].cards.is_empty());
+    }
+
+    #[test]
+    fn new_game_foundations_are_empty() {
+        let g = new_game();
+        for suit in [Suit::Clubs, Suit::Diamonds, Suit::Hearts, Suit::Spades] {
+            assert!(g.piles[&PileType::Foundation(suit)].cards.is_empty());
+        }
+    }
+
+    #[test]
+    fn new_game_is_not_won() {
+        assert!(!new_game().is_won);
+    }
+
+    // --- Seeded reproducibility ---
+
+    #[test]
+    fn same_seed_produces_identical_layout() {
+        let g1 = GameState::new(12345, DrawMode::DrawOne);
+        let g2 = GameState::new(12345, DrawMode::DrawOne);
+        for i in 0..7 {
+            assert_eq!(
+                g1.piles[&PileType::Tableau(i)].cards,
+                g2.piles[&PileType::Tableau(i)].cards
+            );
+        }
+        assert_eq!(
+            g1.piles[&PileType::Stock].cards,
+            g2.piles[&PileType::Stock].cards
+        );
+    }
+
+    #[test]
+    fn different_seeds_produce_different_layouts() {
+        let g1 = GameState::new(1, DrawMode::DrawOne);
+        let g2 = GameState::new(2, DrawMode::DrawOne);
+        let t1: Vec<u32> = g1.piles[&PileType::Tableau(0)].cards.iter().map(|c| c.id).collect();
+        let t2: Vec<u32> = g2.piles[&PileType::Tableau(0)].cards.iter().map(|c| c.id).collect();
+        assert_ne!(t1, t2);
+    }
+
+    // --- Draw ---
+
+    #[test]
+    fn draw_one_moves_one_card_to_waste() {
+        let mut g = new_game();
+        let stock_before = g.piles[&PileType::Stock].cards.len();
+        g.draw().unwrap();
+        assert_eq!(g.piles[&PileType::Stock].cards.len(), stock_before - 1);
+        assert_eq!(g.piles[&PileType::Waste].cards.len(), 1);
+    }
+
+    #[test]
+    fn drawn_card_is_face_up() {
+        let mut g = new_game();
+        g.draw().unwrap();
+        assert!(g.piles[&PileType::Waste].cards.last().unwrap().face_up);
+    }
+
+    #[test]
+    fn draw_three_moves_up_to_three_cards() {
+        let mut g = GameState::new(42, DrawMode::DrawThree);
+        g.draw().unwrap();
+        assert_eq!(g.piles[&PileType::Waste].cards.len(), 3);
+        assert_eq!(g.piles[&PileType::Stock].cards.len(), 21);
+    }
+
+    #[test]
+    fn draw_from_empty_stock_recycles_waste() {
+        let mut g = new_game();
+        while !g.piles[&PileType::Stock].cards.is_empty() {
+            g.draw().unwrap();
+        }
+        let waste_count = g.piles[&PileType::Waste].cards.len();
+        assert!(waste_count > 0);
+        g.draw().unwrap(); // recycle
+        assert_eq!(g.piles[&PileType::Stock].cards.len(), waste_count);
+        assert!(g.piles[&PileType::Waste].cards.is_empty());
+    }
+
+    #[test]
+    fn draw_from_empty_stock_and_waste_returns_error() {
+        let mut g = new_game();
+        while !g.piles[&PileType::Stock].cards.is_empty() {
+            g.draw().unwrap();
+        }
+        g.draw().unwrap(); // recycle
+        while !g.piles[&PileType::Stock].cards.is_empty() {
+            g.draw().unwrap();
+        }
+        assert_eq!(g.draw(), Err(MoveError::StockEmpty));
+    }
+
+    // --- Move validation ---
+
+    #[test]
+    fn move_zero_cards_returns_rule_violation() {
+        let mut g = new_game();
+        let result = g.move_cards(PileType::Tableau(0), PileType::Tableau(1), 0);
+        assert!(matches!(result, Err(MoveError::RuleViolation(_))));
+    }
+
+    #[test]
+    fn move_to_stock_returns_invalid_destination() {
+        let mut g = new_game();
+        let result = g.move_cards(PileType::Tableau(0), PileType::Stock, 1);
+        assert_eq!(result, Err(MoveError::InvalidDestination));
+    }
+
+    #[test]
+    fn move_to_waste_returns_invalid_destination() {
+        let mut g = new_game();
+        let result = g.move_cards(PileType::Tableau(0), PileType::Waste, 1);
+        assert_eq!(result, Err(MoveError::InvalidDestination));
+    }
+
+    #[test]
+    fn move_same_source_and_dest_returns_rule_violation() {
+        let mut g = new_game();
+        let result = g.move_cards(PileType::Tableau(0), PileType::Tableau(0), 1);
+        assert!(matches!(result, Err(MoveError::RuleViolation(_))));
+    }
+
+    // --- Win detection ---
+
+    #[test]
+    fn win_detection_all_foundations_complete() {
+        let mut g = new_game();
+        for suit in [Suit::Clubs, Suit::Diamonds, Suit::Hearts, Suit::Spades] {
+            let f = g.piles.get_mut(&PileType::Foundation(suit)).unwrap();
+            f.cards.clear();
+            for rank in [
+                Rank::Ace, Rank::Two, Rank::Three, Rank::Four, Rank::Five,
+                Rank::Six, Rank::Seven, Rank::Eight, Rank::Nine, Rank::Ten,
+                Rank::Jack, Rank::Queen, Rank::King,
+            ] {
+                f.cards.push(Card { id: 0, suit, rank, face_up: true });
+            }
+        }
+        assert!(g.check_win());
+    }
+
+    #[test]
+    fn win_detection_incomplete_is_false() {
+        assert!(!new_game().check_win());
+    }
+
+    // --- Undo ---
+
+    #[test]
+    fn undo_empty_stack_returns_error() {
+        let mut g = new_game();
+        assert_eq!(g.undo(), Err(MoveError::UndoStackEmpty));
+    }
+
+    #[test]
+    fn undo_after_draw_restores_pile_sizes() {
+        let mut g = new_game();
+        let stock_before = g.piles[&PileType::Stock].cards.len();
+        let waste_before = g.piles[&PileType::Waste].cards.len();
+        g.draw().unwrap();
+        g.undo().unwrap();
+        assert_eq!(g.piles[&PileType::Stock].cards.len(), stock_before);
+        assert_eq!(g.piles[&PileType::Waste].cards.len(), waste_before);
+    }
+
+    #[test]
+    fn undo_applies_score_penalty() {
+        let mut g = new_game();
+        let score_before = g.score;
+        g.draw().unwrap();
+        g.undo().unwrap();
+        let expected = (score_before + scoring_undo()).max(0);
+        assert_eq!(g.score, expected);
+    }
+
+    #[test]
+    fn undo_stack_capped_at_64() {
+        let mut g = new_game();
+        for _ in 0..70 {
+            let _ = g.draw();
+        }
+        assert!(g.undo_stack_len() <= 64);
+    }
+
+    // --- Scoring ---
+
+    #[test]
+    fn score_never_goes_below_zero() {
+        let mut g = new_game();
+        for _ in 0..5 {
+            g.draw().unwrap();
+            g.undo().unwrap();
+        }
+        assert!(g.score >= 0);
+    }
+
+    // --- Auto-complete ---
+
+    #[test]
+    fn auto_complete_false_when_stock_not_empty() {
+        assert!(!new_game().check_auto_complete());
+    }
+
+    #[test]
+    fn auto_complete_false_when_face_down_cards_remain() {
+        let mut g = new_game();
+        g.piles.get_mut(&PileType::Stock).unwrap().cards.clear();
+        g.piles.get_mut(&PileType::Waste).unwrap().cards.clear();
+        // Tableau(1) has a face-down card at index 0
+        assert!(!g.check_auto_complete());
+    }
+
+    // --- Time bonus ---
+
+    #[test]
+    fn time_bonus_zero_when_elapsed_is_zero() {
+        let mut g = new_game();
+        g.elapsed_seconds = 0;
+        assert_eq!(g.compute_time_bonus(), 0);
+    }
+
+    #[test]
+    fn time_bonus_at_100_seconds() {
+        let mut g = new_game();
+        g.elapsed_seconds = 100;
+        assert_eq!(g.compute_time_bonus(), 7000);
+    }
+}
diff --git a/solitaire_core/src/lib.rs b/solitaire_core/src/lib.rs
index 79d71b0..2c600b3 100644
--- a/solitaire_core/src/lib.rs
+++ b/solitaire_core/src/lib.rs
@@ -1,6 +1,7 @@
 pub mod card;
 pub mod deck;
 pub mod error;
+pub mod game_state;
 pub mod pile;
 pub mod rules;
 pub mod scoring;