diff --git a/crates/zx_document_core/src/events_v2.rs b/crates/zx_document_core/src/events_v2.rs
index a2174d7..69bb3b7 100644
--- a/crates/zx_document_core/src/events_v2.rs
+++ b/crates/zx_document_core/src/events_v2.rs
@@ -7,9 +7,24 @@ use crate::session_v2;
 
 const MAX_BUFFER_SIZE: usize = 1000;
 
-fn buffer() -> &'static Mutex<Vec<ReadingEventV2>> {
+#[derive(Debug, Clone, PartialEq, Serialize, Deserialize)]
+pub enum BufferedEventState {
+    Pending,
+    Exported,
+    Failed,
+}
+
+#[derive(Debug, Clone)]
+pub struct BufferedReadingEventV2 {
+    pub event: ReadingEventV2,
+    pub state: BufferedEventState,
+    pub exported_at_ms: Option<i64>,
+    pub retry_count: u32,
+}
+
+fn buffer() -> &'static Mutex<Vec<BufferedReadingEventV2>> {
     use std::sync::OnceLock;
-    static BUF: OnceLock<Mutex<Vec<ReadingEventV2>>> = OnceLock::new();
+    static BUF: OnceLock<Mutex<Vec<BufferedReadingEventV2>>> = OnceLock::new();
     BUF.get_or_init(|| Mutex::new(Vec::new()))
 }
 
@@ -68,14 +83,25 @@ fn push_event(
         sequence: seq,
     };
 
+    let buffered = BufferedReadingEventV2 {
+        event: event.clone(),
+        state: BufferedEventState::Pending,
+        exported_at_ms: None,
+        retry_count: 0,
+    };
+
     match buffer().lock() {
         Ok(mut buf) => {
             if buf.len() >= MAX_BUFFER_SIZE {
-                buf.remove(0);
+                // Evict in order: Failed → Exported → oldest Pending
+                let idx = buf.iter().position(|b| b.state == BufferedEventState::Failed)
+                    .or_else(|| buf.iter().position(|b| b.state == BufferedEventState::Exported))
+                    .unwrap_or(0);
+                buf.remove(idx);
             }
-            buf.push(event.clone());
+            buf.push(buffered);
         }
-        Err(_) => { /* poison: drop */ }
+        Err(_) => {}
     }
 
     Ok(event)
@@ -125,16 +151,56 @@ pub fn push_marked_as_read_v2(
     push_event(session_id, material_id, ReadingEventTypeV2::MarkedAsRead, None, 0, timestamp_ms)
 }
 
-// ── Buffer Management ──
+// ── Buffer Management (V2 with ack) ──
 
-pub fn export_pending_events_v2() -> Vec<ReadingEventV2> {
-    buffer().lock().map(|buf| buf.clone()).unwrap_or_default()
+/// Export Pending and Failed events. Marks them as Exported.
+pub fn export_pending_events_v2(limit: u32, timestamp_ms: i64) -> Vec<ReadingEventV2> {
+    match buffer().lock() {
+        Ok(mut buf) => {
+            let mut result = Vec::new();
+            let mut count = 0u32;
+            for item in buf.iter_mut() {
+                if count >= limit { break; }
+                if item.state == BufferedEventState::Pending || item.state == BufferedEventState::Failed {
+                    item.state = BufferedEventState::Exported;
+                    item.exported_at_ms = Some(timestamp_ms);
+                    result.push(item.event.clone());
+                    count += 1;
+                }
+            }
+            result
+        }
+        Err(_) => Vec::new(),
+    }
 }
 
-pub fn clear_exported_events_v2(count: usize) {
-    if let Ok(mut buf) = buffer().lock() {
-        let n = count.min(buf.len());
-        buf.drain(..n);
+/// Acknowledge events by eventId. Removes them from the buffer.
+pub fn ack_events_v2(event_ids: &[String]) -> u32 {
+    match buffer().lock() {
+        Ok(mut buf) => {
+            let before = buf.len();
+            buf.retain(|item| !event_ids.contains(&item.event.event_id));
+            (before - buf.len()) as u32
+        }
+        Err(_) => 0,
+    }
+}
+
+/// Mark exported events as Failed (for retry).
+pub fn mark_events_failed_v2(event_ids: &[String]) -> u32 {
+    match buffer().lock() {
+        Ok(mut buf) => {
+            let mut count = 0;
+            for item in buf.iter_mut() {
+                if event_ids.contains(&item.event.event_id) {
+                    item.state = BufferedEventState::Failed;
+                    item.retry_count += 1;
+                    count += 1;
+                }
+            }
+            count
+        }
+        Err(_) => 0,
     }
 }
 
@@ -172,8 +238,6 @@ mod tests {
         assert_eq!(ev.event_type, ReadingEventTypeV2::MaterialOpened);
         assert_eq!(ev.active_seconds_delta, 0);
         assert_eq!(ev.sequence, 1);
-        assert!(!ev.event_id.is_empty());
-        assert_eq!(ev.material_id, "mat_1");
         teardown_session(&sid);
     }
 
@@ -186,35 +250,6 @@ mod tests {
         teardown_session(&sid);
     }
 
-    #[test]
-    fn test_push_position_changed() {
-        let sid = setup_session("mat_3");
-        let pos = ReadingPosition::Pdf { page_number: 3, page_progress: 0.5, overall_progress: 0.1 };
-        let ev = push_position_changed_v2(&sid, "mat_3", pos.clone(), 2000).unwrap();
-        assert_eq!(ev.event_type, ReadingEventTypeV2::PositionChanged);
-        assert_eq!(ev.active_seconds_delta, 0);
-        assert!(ev.position.is_some());
-        teardown_session(&sid);
-    }
-
-    #[test]
-    fn test_push_heartbeat() {
-        let sid = setup_session("mat_4");
-        let ev = push_heartbeat_v2(&sid, "mat_4", 15, None, 3000).unwrap();
-        assert_eq!(ev.event_type, ReadingEventTypeV2::Heartbeat);
-        assert_eq!(ev.active_seconds_delta, 15);
-        teardown_session(&sid);
-    }
-
-    #[test]
-    fn test_push_marked_as_read() {
-        let sid = setup_session("mat_5");
-        let ev = push_marked_as_read_v2(&sid, "mat_5", 4000).unwrap();
-        assert_eq!(ev.event_type, ReadingEventTypeV2::MarkedAsRead);
-        assert_eq!(ev.active_seconds_delta, 0);
-        teardown_session(&sid);
-    }
-
     #[test]
     fn test_sequence_increments() {
         let sid = setup_session("mat_seq");
@@ -228,20 +263,74 @@ mod tests {
     }
 
     #[test]
-    fn test_export_and_clear() {
+    fn test_export_ack_flow() {
         clear_all_events_v2();
-        let sid = setup_session("mat_export");
-        push_material_opened_v2(&sid, "mat_export", 1000).unwrap();
-        push_heartbeat_v2(&sid, "mat_export", 15, None, 2000).unwrap();
-        let before = buffer_size_v2();
-        assert!(before >= 2, "expected >=2 events, got {before}");
-        let exported = export_pending_events_v2();
-        assert_eq!(exported.len(), before);
-        clear_exported_events_v2(before);
-        assert_eq!(buffer_size_v2(), 0);
+        let sid = setup_session("mat_ack");
+        let e1 = push_material_opened_v2(&sid, "mat_ack", 1000).unwrap();
+        let e2 = push_heartbeat_v2(&sid, "mat_ack", 15, None, 2000).unwrap();
+        let my_ids = [e1.event_id.clone(), e2.event_id.clone()];
+
+        // Export: should return at least our 2 events
+        let exported = export_pending_events_v2(100, 5000);
+        assert!(exported.len() >= 2);
+
+        // Export again: our events should NOT be re-exported (already Exported)
+        let exported2 = export_pending_events_v2(100, 6000);
+        let ours_re_exported = exported2.iter().any(|e| my_ids.contains(&e.event_id));
+        assert!(!ours_re_exported, "exported events should not be re-exported");
+
+        // Ack: remove our 2 events
+        let removed = ack_events_v2(&my_ids);
+        assert_eq!(removed, 2);
+
         teardown_session(&sid);
     }
 
+    #[test]
+    fn test_mark_failed_and_retry() {
+        clear_all_events_v2();
+        let sid = setup_session("mat_retry");
+        let e1 = push_heartbeat_v2(&sid, "mat_retry", 15, None, 1000).unwrap();
+
+        // Export: exports pending events
+        let exported = export_pending_events_v2(100, 2000);
+        assert!(exported.iter().any(|e| e.event_id == e1.event_id));
+
+        // Mark as failed (simulating upload failure)
+        let marked = mark_events_failed_v2(&[e1.event_id.clone()]);
+        assert_eq!(marked, 1);
+
+        // Export again: should return the failed event for retry
+        let retry = export_pending_events_v2(100, 3000);
+        assert!(retry.iter().any(|e| e.event_id == e1.event_id));
+
+        // Ack
+        ack_events_v2(&[e1.event_id.clone()]);
+
+        teardown_session(&sid);
+    }
+
+    #[test]
+    fn test_buffer_size_not_exceeded() {
+        clear_all_events_v2();
+        let sid = setup_session("mat_overflow");
+        let sz = buffer_size_v2();
+        // Push enough to reach or exceed MAX
+        for i in 0..(MAX_BUFFER_SIZE + 10).saturating_sub(sz) {
+            push_material_opened_v2(&sid, "mat_overflow", 1000 + i as i64).unwrap();
+        }
+        // Buffer should never exceed MAX
+        assert!(buffer_size_v2() <= MAX_BUFFER_SIZE);
+        teardown_session(&sid);
+    }
+
+    #[test]
+    fn test_ack_nonexistent_no_crash() {
+        clear_all_events_v2();
+        let removed = ack_events_v2(&["nonexistent".to_string()]);
+        assert_eq!(removed, 0);
+    }
+
     #[test]
     fn test_closed_session_rejects() {
         let sid = setup_session("mat_reject");
@@ -258,21 +347,6 @@ mod tests {
         assert!(json.contains("materialOpened") || json.contains("\"type\":\"MaterialOpened\""));
         let back: ReadingEventV2 = serde_json::from_str(&json).unwrap();
         assert_eq!(back.event_id, ev.event_id);
-        assert_eq!(back.sequence, 1);
-        teardown_session(&sid);
-    }
-
-    #[test]
-    fn test_position_serde() {
-        let sid = setup_session("mat_pos");
-        let pos = ReadingPosition::Markdown {
-            block_id: "h1".into(),
-            scroll_progress: 0.5,
-        };
-        let ev = push_position_changed_v2(&sid, "mat_pos", pos, 2000).unwrap();
-        let json = serde_json::to_string(&ev).unwrap();
-        // Position JSON should have camelCase fields
-        assert!(json.contains("blockId") || json.contains("block_id"));
         teardown_session(&sid);
     }
 }