diff --git a/rust/agents/validator/src/submit.rs b/rust/agents/validator/src/submit.rs
index c80d590ef8..3de0a798af 100644
--- a/rust/agents/validator/src/submit.rs
+++ b/rust/agents/validator/src/submit.rs
@@ -167,7 +167,7 @@ impl ValidatorSubmitter {
         //
         // tree.index() will panic if the tree is empty, so we use tree.count() instead
         // and convert the correctness_checkpoint.index to a count by adding 1.
-        while correctness_checkpoint.index + 1 > tree.count() as u32 {
+        while tree.count() as u32 <= correctness_checkpoint.index {
             if let Some(insertion) = self
                 .message_db
                 .retrieve_merkle_tree_insertion_by_leaf_index(&(tree.count() as u32))?
@@ -239,6 +239,8 @@ impl ValidatorSubmitter {
         &self,
         checkpoints: Vec<CheckpointWithMessageId>,
     ) -> Result<()> {
+        let last_checkpoint = checkpoints.as_slice()[checkpoints.len() - 1];
+
         for queued_checkpoint in checkpoints {
             let existing = self
                 .checkpoint_syncer
@@ -260,9 +262,15 @@ impl ValidatorSubmitter {
                 "Signed and submitted checkpoint"
             );
 
+            // TODO: move these into S3 implementations
             // small sleep before signing next checkpoint to avoid rate limiting
             sleep(Duration::from_millis(100)).await;
         }
+
+        self.checkpoint_syncer
+            .update_latest_index(last_checkpoint.index)
+            .await?;
+
         Ok(())
     }
 }
diff --git a/rust/hyperlane-base/src/traits/checkpoint_syncer.rs b/rust/hyperlane-base/src/traits/checkpoint_syncer.rs
index e017b3f5aa..abec982c7d 100644
--- a/rust/hyperlane-base/src/traits/checkpoint_syncer.rs
+++ b/rust/hyperlane-base/src/traits/checkpoint_syncer.rs
@@ -10,6 +10,16 @@ use hyperlane_core::{SignedAnnouncement, SignedCheckpointWithMessageId};
 pub trait CheckpointSyncer: Debug + Send + Sync {
     /// Read the highest index of this Syncer
     async fn latest_index(&self) -> Result<Option<u32>>;
+    /// Writes the highest index of this Syncer
+    async fn write_latest_index(&self, index: u32) -> Result<()>;
+    /// Update the latest index of this syncer if necessary
+    async fn update_latest_index(&self, index: u32) -> Result<()> {
+        let curr = self.latest_index().await?.unwrap_or(0);
+        if index > curr {
+            self.write_latest_index(index).await?;
+        }
+        Ok(())
+    }
     /// Attempt to fetch the signed (checkpoint, messageId) tuple at this index
     async fn fetch_checkpoint(&self, index: u32) -> Result<Option<SignedCheckpointWithMessageId>>;
     /// Write the signed (checkpoint, messageId) tuple to this syncer
diff --git a/rust/hyperlane-base/src/types/local_storage.rs b/rust/hyperlane-base/src/types/local_storage.rs
index 76c02a38cd..38397c1bf6 100644
--- a/rust/hyperlane-base/src/types/local_storage.rs
+++ b/rust/hyperlane-base/src/types/local_storage.rs
@@ -62,6 +62,14 @@ impl CheckpointSyncer for LocalStorage {
         }
     }
 
+    async fn write_latest_index(&self, index: u32) -> Result<()> {
+        let path = self.latest_index_file_path();
+        tokio::fs::write(&path, index.to_string())
+            .await
+            .with_context(|| format!("Writing index to {path:?}"))?;
+        Ok(())
+    }
+
     async fn fetch_checkpoint(&self, index: u32) -> Result<Option<SignedCheckpointWithMessageId>> {
         let Ok(data) = tokio::fs::read(self.checkpoint_file_path(index)).await else {
             return Ok(None);
diff --git a/rust/hyperlane-base/src/types/s3_storage.rs b/rust/hyperlane-base/src/types/s3_storage.rs
index cfce7ff3e5..2d09a18a86 100644
--- a/rust/hyperlane-base/src/types/s3_storage.rs
+++ b/rust/hyperlane-base/src/types/s3_storage.rs
@@ -131,7 +131,7 @@ impl S3Storage {
         format!("checkpoint_{index}_with_id.json")
     }
 
-    fn index_key() -> String {
+    fn latest_index_key() -> String {
         "checkpoint_latest_index.json".to_owned()
     }
 
@@ -144,7 +144,7 @@ impl S3Storage {
 impl CheckpointSyncer for S3Storage {
     async fn latest_index(&self) -> Result<Option<u32>> {
         let ret = self
-            .anonymously_read_from_bucket(S3Storage::index_key())
+            .anonymously_read_from_bucket(S3Storage::latest_index_key())
             .await?
             .map(|data| serde_json::from_slice(&data))
             .transpose()
@@ -159,6 +159,13 @@ impl CheckpointSyncer for S3Storage {
         ret
     }
 
+    async fn write_latest_index(&self, index: u32) -> Result<()> {
+        let serialized_index = serde_json::to_string(&index)?;
+        self.write_to_bucket(S3Storage::latest_index_key(), &serialized_index)
+            .await?;
+        Ok(())
+    }
+
     async fn fetch_checkpoint(&self, index: u32) -> Result<Option<SignedCheckpointWithMessageId>> {
         self.anonymously_read_from_bucket(S3Storage::checkpoint_key(index))
             .await?