diff --git a/catalog/src/manifest.rs b/catalog/src/manifest.rs
index b3ddf92..aec0083 100644
--- a/catalog/src/manifest.rs
+++ b/catalog/src/manifest.rs
@@ -281,6 +281,38 @@ impl Manifest {
         .unwrap();
     }
 
+    /// Correct a segment's recorded size, but only if the row still exists and
+    /// still records `observed_size`. Returns true when the update was applied.
+    ///
+    /// Unlike [`update`](Self::update), this never re-creates a row: if retention
+    /// has removed the segment since it was observed, the conditional `WHERE`
+    /// matches nothing and we leave the manifest untouched. This makes it safe to
+    /// apply size fixes from a reconciliation snapshot that may have gone stale
+    /// while writes and retention ran concurrently.
+    pub(crate) async fn update_size_if_unchanged(
+        &self,
+        id: &PartitionId,
+        index: SegmentIndex,
+        observed_size: usize,
+        corrected_size: usize,
+    ) -> bool {
+        let result = sqlx::query(
+            "
+            UPDATE segments SET size = ?4
+            WHERE topic = ?1 AND partition = ?2 AND segment_index = ?3 AND size = ?5
+        ",
+        )
+        .bind(id.topic())
+        .bind(id.partition())
+        .bind(index.to_row())
+        .bind(i64::try_from(corrected_size).unwrap())
+        .bind(i64::try_from(observed_size).unwrap())
+        .execute(&self.pool)
+        .await
+        .unwrap();
+        result.rows_affected() > 0
+    }
+
     /// Inverse of upsert, gets any previously stored data for a segment.
     pub async fn get_segment_data(&self, id: SegmentId<&PartitionId>) -> Option<SegmentData> {
         sqlx::query(
@@ -849,6 +881,58 @@ mod test {
         assert_eq!(state.get_partition_range(&id).await, None);
     }
 
+    #[tokio::test]
+    async fn test_update_size_if_unchanged() {
+        let id = PartitionId::new("topic", "cas");
+        let root = tempdir().unwrap();
+        let path = root.path().join("testing.sqlite");
+        let state = Manifest::attach(path).await;
+
+        let time = Utc.timestamp_opt(0, 0).unwrap()..=Utc.timestamp_opt(0, 0).unwrap();
+        state
+            .update(
+                &id,
+                &SegmentData {
+                    index: SegmentIndex(0),
+                    time,
+                    records: RecordIndex(0)..RecordIndex(10),
+                    size: 12,
+                    version: SEGMENT_FORMAT_VERSION,
+                },
+            )
+            .await;
+        assert_eq!(state.get_size(Scope::Partition(&id)).await, 12);
+
+        // Stale observation: the row no longer records the size we think it does,
+        // so the conditional update must not apply.
+        assert!(
+            !state
+                .update_size_if_unchanged(&id, SegmentIndex(0), 999, 34)
+                .await
+        );
+        assert_eq!(state.get_size(Scope::Partition(&id)).await, 12);
+
+        // Matching observation: the fix applies.
+        assert!(
+            state
+                .update_size_if_unchanged(&id, SegmentIndex(0), 12, 34)
+                .await
+        );
+        assert_eq!(state.get_size(Scope::Partition(&id)).await, 34);
+
+        // Simulate retention removing the segment, then a stale fix racing in. The
+        // update must not resurrect the deleted row.
+        state.remove_segment(SegmentIndex(0).to_id(&id)).await;
+        assert_eq!(state.get_min_segment(&id).await, None);
+        assert!(
+            !state
+                .update_size_if_unchanged(&id, SegmentIndex(0), 34, 56)
+                .await
+        );
+        assert_eq!(state.get_min_segment(&id).await, None);
+        assert_eq!(state.get_size(Scope::Partition(&id)).await, 0);
+    }
+
     #[tokio::test]
     async fn test_time_query() {
         let topic = "topic";
diff --git a/catalog/src/reconcile.rs b/catalog/src/reconcile.rs
index e17a05a..4ec629d 100644
--- a/catalog/src/reconcile.rs
+++ b/catalog/src/reconcile.rs
@@ -512,15 +512,29 @@ impl ReconcileJob {
                         .fixes
                         .contains(&ReconcileFix::UpdateManifestSizes)
                     {
-                        info!("Fixing size mismatch for segment {}", segment_file_name);
-                        let mut corrected_segment = segment.clone();
-                        corrected_segment.size = total_actual_size;
-
-                        // Update the manifest with the correct size
-                        self.catalog
+                        // Apply the fix conditionally on the size we observed. If
+                        // retention removed this segment (or another writer changed
+                        // it) since we snapshotted the manifest, the update is a
+                        // no-op rather than re-creating a stale row. This keeps the
+                        // fix safe to run while writes and retention are in flight.
+                        let applied = self
+                            .catalog
                             .manifest()
-                            .update(&partition_id, &corrected_segment)
+                            .update_size_if_unchanged(
+                                &partition_id,
+                                segment.index,
+                                segment.size,
+                                total_actual_size,
+                            )
                             .await;
+                        if applied {
+                            info!("Fixed size mismatch for segment {}", segment_file_name);
+                        } else {
+                            info!(
+                                "Skipped stale size fix for segment {} (concurrently removed or changed)",
+                                segment_file_name
+                            );
+                        }
                     }
                 } else {
                     debug!(