diff --git a/catalog/src/partition.rs b/catalog/src/partition.rs
index 0b87d2b..4a6c56a 100644
--- a/catalog/src/partition.rs
+++ b/catalog/src/partition.rs
@@ -56,6 +56,13 @@ pub struct Partition {
     state: RwLock<State>,
     config: Config,
     manifest: Manifest,
+    /// High-water mark for the highest [SegmentIndex] whose manifest update has
+    /// been confirmed durable for a *sealed* (no longer active) segment, or
+    /// `None` until the first segment is sealed durably.
+    ///
+    /// Monotonically non-decreasing: retention removing segments never moves it
+    /// backward, and the currently active (writeable) segment is never included.
+    sealed_ix: watch::Receiver<Option<SegmentIndex>>,
 }
 
 impl std::fmt::Debug for Partition {
@@ -104,10 +111,23 @@ impl Partition {
         let (commit_writer, commits) = watch::channel(record);
         let commit_manifest = manifest.clone();
         let commit_id = id.clone();
+        let (sealed_tx, sealed_ix) = watch::channel(None);
         tokio::spawn(async move {
             while let Some(r) = writes.recv().await {
                 trace!("{} checkpoint: {:?}", commit_id, &r);
                 commit_manifest.update(&commit_id, &r.data).await;
+                // the manifest update above is now durable. if it sealed the
+                // segment, advance the watermark, holding it monotonic against
+                // any out-of-order update.
+                if r.sealed {
+                    sealed_tx.send_if_modified(|current| match current {
+                        Some(ix) if *ix >= r.data.index => false,
+                        _ => {
+                            *current = Some(r.data.index);
+                            true
+                        }
+                    });
+                }
                 // ok if no receivers, that means nothing is awaiting a commit
                 commit_writer.send(r.data.records.end).ok();
             }
@@ -127,6 +147,7 @@ impl Partition {
             state: RwLock::new(state),
             id,
             config,
+            sealed_ix,
         }
     }
 
@@ -170,6 +191,18 @@ impl Partition {
         &self.id
     }
 
+    /// The highest [SegmentIndex] whose manifest update has been confirmed
+    /// durable for a sealed (no longer active) segment, or `None` if no segment
+    /// has been sealed durably yet.
+    ///
+    /// This is a high-water mark: it never moves backward, even after retention
+    /// removes the segment it points at. The currently active (writeable)
+    /// segment is never included. Cheap to call; intended to be read once per
+    /// reconcile pass per partition.
+    pub fn sealed_ix(&self) -> Option<SegmentIndex> {
+        *self.sealed_ix.borrow()
+    }
+
     pub(crate) fn slog_name(id: &PartitionId) -> String {
         format!("{}-{}", id.topic(), id.partition())
     }
@@ -1713,4 +1746,66 @@ pub mod test {
 
         Ok(())
     }
+
+    #[tokio::test]
+    async fn test_sealed_ix_fresh_is_none() -> Result<()> {
+        let (_dir, part) = partition(segment_3s()).await?;
+        assert_eq!(part.sealed_ix(), None);
+        part.close().await;
+        Ok(())
+    }
+
+    #[tokio::test]
+    async fn test_sealed_ix_active_not_sealed() -> Result<()> {
+        // segment_3s rolls at >2 rows; two writes stay in the active segment.
+        // checkpoint makes that segment's data durable without sealing it, so
+        // the watermark stays None.
+        let (_dir, part) = partition(segment_3s()).await?;
+        init_records(&part, 2).await?;
+        part.checkpoint().await;
+        part.ensure_index(RecordIndex(2)).await?;
+        assert_eq!(part.sealed_ix(), None);
+        part.close().await;
+        Ok(())
+    }
+
+    #[tokio::test]
+    async fn test_sealed_ix_advances_after_roll() -> Result<()> {
+        // the fourth write rolls segment 0 (records 0..3); once that seal lands
+        // durably, sealed_ix points at segment 0 (never the active segment 1).
+        let (_dir, part) = partition(segment_3s()).await?;
+        init_records(&part, 4).await?;
+        part.ensure_index(RecordIndex(3)).await?;
+        assert_eq!(part.sealed_ix(), Some(SegmentIndex(0)));
+        part.close().await;
+        Ok(())
+    }
+
+    #[tokio::test]
+    async fn test_sealed_ix_tracks_most_recent_sealed() -> Result<()> {
+        // ten writes seal segments 0 (0..3), 1 (3..6) and 2 (6..9), leaving an
+        // active segment 3. sealed_ix tracks the most recent *sealed* segment.
+        let (_dir, part) = partition(segment_3s()).await?;
+        init_records(&part, 10).await?;
+        part.ensure_index(RecordIndex(9)).await?;
+        assert_eq!(part.sealed_ix(), Some(SegmentIndex(2)));
+        part.close().await;
+        Ok(())
+    }
+
+    #[tokio::test]
+    async fn test_sealed_ix_monotonic_under_retention() -> Result<()> {
+        let (_dir, part) = partition(segment_3s()).await?;
+        init_records(&part, 10).await?;
+        part.ensure_index(RecordIndex(9)).await?;
+        assert_eq!(part.sealed_ix(), Some(SegmentIndex(2)));
+
+        // retention removing the oldest segment is an observation that must not
+        // advance the watermark, and crucially must not move it backward.
+        part.remove_oldest().await;
+        assert_eq!(part.sealed_ix(), Some(SegmentIndex(2)));
+
+        part.close().await;
+        Ok(())
+    }
 }
diff --git a/catalog/src/slog.rs b/catalog/src/slog.rs
index f11741d..2268e90 100644
--- a/catalog/src/slog.rs
+++ b/catalog/src/slog.rs
@@ -300,6 +300,9 @@ struct AppendRequest {
 #[derive(Debug)]
 pub(crate) struct WriteResult {
     pub(crate) data: SegmentData,
+    /// True when this write sealed (finalized) the segment, i.e. the segment is
+    /// now immutable and no longer the active, writeable one.
+    pub(crate) sealed: bool,
 }
 
 impl Slog {
@@ -732,6 +735,7 @@ fn spawn_slog_thread(
                             size,
                             version: crate::manifest::SEGMENT_FORMAT_VERSION,
                         },
+                        sealed: seal,
                     };
                     trace!("{}: commit {:?}/{:?} send", name, segment, records.end);
                     tx_commits.blocking_send(response).expect("channel closed");