diff --git a/crates/zx_document_core/src/lib.rs b/crates/zx_document_core/src/lib.rs
index cdc78ed..60101ae 100644
--- a/crates/zx_document_core/src/lib.rs
+++ b/crates/zx_document_core/src/lib.rs
@@ -13,4 +13,5 @@ pub mod pdf;
 pub mod progress;
 pub mod reading_material;
 pub mod search;
+pub mod session_v2;
 pub mod text;
diff --git a/crates/zx_document_core/src/session_v2.rs b/crates/zx_document_core/src/session_v2.rs
new file mode 100644
index 0000000..676dbd4
--- /dev/null
+++ b/crates/zx_document_core/src/session_v2.rs
@@ -0,0 +1,254 @@
+use std::collections::HashMap;
+use std::sync::Mutex;
+
+use crate::progress::ReadingPosition;
+use crate::reading_material::ReadingMaterialRef;
+
+fn sessions() -> &'static Mutex<HashMap<String, ReadingSessionV2>> {
+    use std::sync::OnceLock;
+    static SESSIONS: OnceLock<Mutex<HashMap<String, ReadingSessionV2>>> = OnceLock::new();
+    SESSIONS.get_or_init(|| Mutex::new(HashMap::new()))
+}
+
+#[derive(Debug, Clone, PartialEq)]
+pub enum ReadingSessionStatus {
+    Active,
+    Paused,
+    Closed,
+}
+
+#[derive(Debug, Clone)]
+pub struct ReadingSessionV2 {
+    pub client_session_id: String,
+    pub material: ReadingMaterialRef,
+    pub started_at_ms: i64,
+    pub last_event_at_ms: i64,
+    pub next_sequence: u64,
+    pub total_active_seconds: u32,
+    pub last_position: Option<ReadingPosition>,
+    pub status: ReadingSessionStatus,
+}
+
+/// Start a new reading session. Returns the client_session_id as handle.
+pub fn start_reading_session_v2(
+    material: ReadingMaterialRef,
+    timestamp_ms: i64,
+) -> Result<String, SessionError> {
+    let session_id = uuid::Uuid::new_v4().to_string();
+
+    let session = ReadingSessionV2 {
+        client_session_id: session_id.clone(),
+        material,
+        started_at_ms: timestamp_ms,
+        last_event_at_ms: timestamp_ms,
+        next_sequence: 1,
+        total_active_seconds: 0,
+        last_position: None,
+        status: ReadingSessionStatus::Active,
+    };
+
+    match sessions().lock() {
+        Ok(mut map) => {
+            map.insert(session_id.clone(), session);
+            Ok(session_id)
+        }
+        Err(_) => Err(SessionError::LockPoisoned),
+    }
+}
+
+/// Pause the session. Stops active time tracking.
+pub fn pause_reading_session_v2(session_id: &str) -> Result<(), SessionError> {
+    with_session_mut(session_id, |s| {
+        if s.status == ReadingSessionStatus::Closed {
+            return Err(SessionError::AlreadyClosed);
+        }
+        s.status = ReadingSessionStatus::Paused;
+        Ok(())
+    })
+}
+
+/// Resume a paused session.
+pub fn resume_reading_session_v2(session_id: &str) -> Result<(), SessionError> {
+    with_session_mut(session_id, |s| {
+        if s.status == ReadingSessionStatus::Closed {
+            return Err(SessionError::AlreadyClosed);
+        }
+        s.status = ReadingSessionStatus::Active;
+        Ok(())
+    })
+}
+
+/// Close the session. No more events allowed.
+pub fn close_reading_session_v2(session_id: &str) -> Result<(), SessionError> {
+    with_session_mut(session_id, |s| {
+        if s.status == ReadingSessionStatus::Closed {
+            return Err(SessionError::AlreadyClosed);
+        }
+        s.status = ReadingSessionStatus::Closed;
+        Ok(())
+    })
+}
+
+/// Record an event on this session: bumps sequence and updates last_event_at_ms.
+/// Returns the sequence number that should be assigned to the event.
+pub fn record_session_event_v2(
+    session_id: &str,
+    timestamp_ms: i64,
+    active_seconds_delta: u32,
+    position: Option<ReadingPosition>,
+) -> Result<u64, SessionError> {
+    with_session_mut(session_id, |s| {
+        if s.status == ReadingSessionStatus::Closed {
+            return Err(SessionError::AlreadyClosed);
+        }
+        if s.status != ReadingSessionStatus::Active && active_seconds_delta > 0 {
+            return Err(SessionError::NotActive);
+        }
+        let seq = s.next_sequence;
+        s.next_sequence += 1;
+        s.last_event_at_ms = timestamp_ms;
+        if s.status == ReadingSessionStatus::Active {
+            s.total_active_seconds += active_seconds_delta;
+        }
+        if position.is_some() {
+            s.last_position = position;
+        }
+        Ok(seq)
+    })
+}
+
+/// Get a copy of the session info.
+pub fn get_session_v2(session_id: &str) -> Result<ReadingSessionV2, SessionError> {
+    match sessions().lock() {
+        Ok(map) => map.get(session_id).cloned().ok_or(SessionError::NotFound),
+        Err(_) => Err(SessionError::LockPoisoned),
+    }
+}
+
+/// Remove a closed session from memory.
+pub fn remove_session_v2(session_id: &str) -> Result<(), SessionError> {
+    match sessions().lock() {
+        Ok(mut map) => {
+            map.remove(session_id).ok_or(SessionError::NotFound)?;
+            Ok(())
+        }
+        Err(_) => Err(SessionError::LockPoisoned),
+    }
+}
+
+// ── helpers ──
+
+fn with_session_mut<F, T>(session_id: &str, f: F) -> Result<T, SessionError>
+where
+    F: FnOnce(&mut ReadingSessionV2) -> Result<T, SessionError>,
+{
+    match sessions().lock() {
+        Ok(mut map) => {
+            let session = map.get_mut(session_id).ok_or(SessionError::NotFound)?;
+            f(session)
+        }
+        Err(_) => Err(SessionError::LockPoisoned),
+    }
+}
+
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum SessionError {
+    NotFound,
+    AlreadyClosed,
+    NotActive,
+    LockPoisoned,
+}
+
+impl std::fmt::Display for SessionError {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        match self {
+            Self::NotFound => write!(f, "Session not found"),
+            Self::AlreadyClosed => write!(f, "Session already closed"),
+            Self::NotActive => write!(f, "Session is not active (paused or closed)"),
+            Self::LockPoisoned => write!(f, "Session lock poisoned"),
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn test_material() -> ReadingMaterialRef {
+        ReadingMaterialRef::new("test_mat_001")
+    }
+
+    #[test]
+    fn test_start_session() {
+        let id = start_reading_session_v2(test_material(), 1000).unwrap();
+        assert!(!id.is_empty());
+        let s = get_session_v2(&id).unwrap();
+        assert_eq!(s.material.material_id, "test_mat_001");
+        assert_eq!(s.status, ReadingSessionStatus::Active);
+        assert_eq!(s.next_sequence, 1);
+        remove_session_v2(&id).unwrap();
+    }
+
+    #[test]
+    fn test_pause_resume_close() {
+        let id = start_reading_session_v2(test_material(), 1000).unwrap();
+        pause_reading_session_v2(&id).unwrap();
+        assert_eq!(get_session_v2(&id).unwrap().status, ReadingSessionStatus::Paused);
+        resume_reading_session_v2(&id).unwrap();
+        assert_eq!(get_session_v2(&id).unwrap().status, ReadingSessionStatus::Active);
+        close_reading_session_v2(&id).unwrap();
+        assert_eq!(get_session_v2(&id).unwrap().status, ReadingSessionStatus::Closed);
+        remove_session_v2(&id).unwrap();
+    }
+
+    #[test]
+    fn test_sequence_increments() {
+        let id = start_reading_session_v2(test_material(), 1000).unwrap();
+        let s1 = record_session_event_v2(&id, 2000, 15, None).unwrap();
+        let s2 = record_session_event_v2(&id, 3000, 15, None).unwrap();
+        assert_eq!(s1, 1);
+        assert_eq!(s2, 2);
+        assert_eq!(get_session_v2(&id).unwrap().total_active_seconds, 30);
+        remove_session_v2(&id).unwrap();
+    }
+
+    #[test]
+    fn test_closed_rejects_events() {
+        let id = start_reading_session_v2(test_material(), 1000).unwrap();
+        close_reading_session_v2(&id).unwrap();
+        assert!(record_session_event_v2(&id, 2000, 10, None).is_err());
+        assert!(close_reading_session_v2(&id).is_err());
+        remove_session_v2(&id).unwrap();
+    }
+
+    #[test]
+    fn test_paused_rejects_active_seconds() {
+        let id = start_reading_session_v2(test_material(), 1000).unwrap();
+        pause_reading_session_v2(&id).unwrap();
+        // PositionChanged (delta=0) should be allowed when paused
+        let seq = record_session_event_v2(&id, 2000, 0, None).unwrap();
+        assert_eq!(seq, 1);
+        // Heartbeat (delta>0) should be rejected when paused
+        assert!(record_session_event_v2(&id, 3000, 15, None).is_err());
+        remove_session_v2(&id).unwrap();
+    }
+
+    #[test]
+    fn test_not_found() {
+        assert!(get_session_v2("nonexistent").is_err());
+        assert!(close_reading_session_v2("nonexistent").is_err());
+    }
+
+    #[test]
+    fn test_two_sessions_independent() {
+        let a = start_reading_session_v2(test_material(), 1000).unwrap();
+        let b = start_reading_session_v2(ReadingMaterialRef::new("mat_b"), 2000).unwrap();
+        assert_ne!(a, b);
+        record_session_event_v2(&a, 3000, 10, None).unwrap();
+        record_session_event_v2(&b, 4000, 20, None).unwrap();
+        assert_eq!(get_session_v2(&a).unwrap().total_active_seconds, 10);
+        assert_eq!(get_session_v2(&b).unwrap().total_active_seconds, 20);
+        remove_session_v2(&a).unwrap();
+        remove_session_v2(&b).unwrap();
+    }
+}