feat(kv-router): add start positions to flat KV events (#8426)

Signed-off-by: PeaBrane <yanrpei@gmail.com>

lib/bench/kv_router/common/mod.rs

@@ -632,6 +632,7 @@ impl SequenceData {
                 event_id,
                 data: KvCacheEventData::Stored(KvCacheStoreData {
                     parent_hash: None,
+                    start_position: None,
                     blocks: self
                         .local_hashes
                         .iter()

lib/bindings/c/src/lib.rs

@@ -286,6 +286,7 @@ fn kv_event_create_stored_from_parts(
         data: KvCacheEventData::Stored(KvCacheStoreData {
             blocks,
             parent_hash: kv_params.parent_hash.map(ExternalSequenceBlockHash),
+            start_position: None,
         }),
         event_id: kv_params.event_id,
         dp_rank: 0,

lib/bindings/python/rust/llm/kv.rs

@@ -306,6 +306,7 @@ impl KvEventPublisher {
                 event_id,
                 data: KvCacheEventData::Stored(KvCacheStoreData {
                     parent_hash: parent_hash.map(ExternalSequenceBlockHash::from),
+                    start_position: None,
                     blocks: create_stored_blocks(
                         kv_block_size,
                         &token_ids,

lib/kv-router/src/indexer/README.md

@@ -255,7 +255,10 @@ worker_blocks (DashMap<Worker, RwLock<HashMap>>):
 ### How Operations Work
 
 **store_blocks(worker, parent_hash, blocks)**:
-1. Find starting position: `pos = worker_blocks[worker][parent_hash].position + 1`
+1. Find starting position:
+   - `start_position` if the batch provides one
+   - otherwise `worker_blocks[worker][parent_hash].position + 1`
+   - otherwise `0`
 2. For each block at position `i`:
    - Insert into `index[(pos+i, local_hash)]` → add worker to SeqEntry
    - Insert into `worker_blocks[worker][seq_hash] = (pos+i, local_hash)`
@@ -291,9 +294,9 @@ Query: [b0, b1, b2, ..., b63, b64, ..., b127, ...]
 
 **dump_events()**:
 1. Iterate `worker_blocks`, collecting all blocks per worker
-2. Sort each worker's blocks by position (parents before children)
-3. Emit one single-block `RouterEvent::Stored` per block, synthesizing
-   `parent_hash` from any seq_hash at the prior position
+2. Sort each worker's blocks by position
+3. Emit one single-block `RouterEvent::Stored` per block with
+   `start_position = Some(position)` and `parent_hash = None`
 4. Events can be replayed into a fresh `PositionalIndexer` to reconstruct
    the same index state
 

lib/kv-router/src/indexer/concurrent_radix_tree.rs

@@ -608,6 +608,7 @@ impl ConcurrentRadixTree {
                         event_id,
                         data: KvCacheEventData::Stored(KvCacheStoreData {
                             parent_hash,
+                            start_position: None,
                             blocks: vec![KvCacheStoredBlockData {
                                 block_hash,
                                 mm_extra_info: None,

lib/kv-router/src/indexer/concurrent_radix_tree_compressed.rs

@@ -1191,6 +1191,7 @@ impl ConcurrentRadixTreeCompressed {
                             event_id,
                             data: KvCacheEventData::Stored(KvCacheStoreData {
                                 parent_hash,
+                                start_position: None,
                                 blocks: full_blocks.clone(),
                             }),
                             dp_rank: worker.dp_rank,
@@ -1205,6 +1206,7 @@ impl ConcurrentRadixTreeCompressed {
                             event_id,
                             data: KvCacheEventData::Stored(KvCacheStoreData {
                                 parent_hash,
+                                start_position: None,
                                 blocks: full_blocks[..k].to_vec(),
                             }),
                             dp_rank: worker.dp_rank,

lib/kv-router/src/indexer/kv_indexer.rs

@@ -255,6 +255,7 @@ impl KvIndexer {
                             let hashes = routing_req.local_hashes.iter().zip(routing_req.sequence_hashes.iter());
                             let stored_event = KvCacheEventData::Stored(KvCacheStoreData {
                                 parent_hash: None,
+                                start_position: None,
                                 blocks: hashes.map(|(local_hash, sequence_hash)| KvCacheStoredBlockData {
                                     tokens_hash: *local_hash,
                                     block_hash: ExternalSequenceBlockHash(*sequence_hash),

lib/kv-router/src/indexer/positional.rs

@@ -237,30 +237,37 @@ impl PositionalIndexer {
         store_data: KvCacheStoreData,
         event_id: u64,
     ) -> Result<(), KvCacheEventError> {
+        let KvCacheStoreData {
+            parent_hash,
+            start_position,
+            blocks,
+        } = store_data;
         let worker_map = worker_blocks.entry(worker).or_default();
-        // Determine starting position based on parent_hash
-        let start_pos = match store_data.parent_hash {
-            Some(parent_hash) => {
-                let Some(entry) = worker_map.get(&parent_hash) else {
-                    tracing::warn!(
-                        worker_id = worker.worker_id.to_string(),
-                        dp_rank = worker.dp_rank,
-                        event_id,
-                        parent_hash = ?parent_hash,
-                    );
-                    return Err(KvCacheEventError::ParentBlockNotFound);
-                };
-
-                entry.0 + 1 // parent position + 1
-            }
-            None => 0, // Start from position 0
+        let start_pos = match start_position {
+            Some(start_position) => start_position as usize,
+            None => match parent_hash {
+                Some(parent_hash) => {
+                    let Some(entry) = worker_map.get(&parent_hash) else {
+                        tracing::warn!(
+                            worker_id = worker.worker_id.to_string(),
+                            dp_rank = worker.dp_rank,
+                            event_id,
+                            parent_hash = ?parent_hash,
+                        );
+                        return Err(KvCacheEventError::ParentBlockNotFound);
+                    };
+
+                    entry.0 + 1 // parent position + 1
+                }
+                None => 0, // Start from position 0
+            },
         };
 
         let worker_blocks_entry = worker_blocks.entry(worker).or_default();
 
-        let num_stored_blocks = store_data.blocks.len();
+        let num_stored_blocks = blocks.len();
 
-        for (i, block_data) in store_data.blocks.into_iter().enumerate() {
+        for (i, block_data) in blocks.into_iter().enumerate() {
             let position = start_pos + i;
             let local_hash = block_data.tokens_hash;
             let seq_hash = block_data.block_hash;
@@ -411,47 +418,22 @@ impl PositionalIndexer {
         let mut event_id = 0u64;
 
         for (worker, worker_map) in worker_blocks.iter() {
-            // Collect (position, local_hash, seq_hash) and sort by position
-            // so parents are emitted before children during replay.
+            // Collect (position, local_hash, seq_hash) and sort by position.
             let mut blocks: Vec<_> = worker_map
                 .iter()
                 .map(|(seq_hash, (pos, local_hash))| (*pos, *local_hash, *seq_hash))
                 .collect();
             blocks.sort_unstable_by_key(|(pos, _, _)| *pos);
 
-            // Track one valid seq_hash per position for parent_hash synthesis.
-            // Note: The synthesized parent_hash doesn't need to be the true logical
-            // parent — during replay it's only used to derive `start_pos = parent.position + 1`,
-            // so any seq_hash at the previous position is sufficient. The PositionalIndexer
-            // is position-based, not tree-topology-based.
-            let mut last_at_position: FxHashMap<usize, ExternalSequenceBlockHash> =
-                FxHashMap::default();
-
             for (pos, local_hash, seq_hash) in blocks {
-                let parent_hash = if pos == 0 {
-                    None
-                } else {
-                    match last_at_position.get(&(pos - 1)) {
-                        Some(&parent) => Some(parent),
-                        None => {
-                            tracing::warn!(
-                                worker_id = worker.worker_id.to_string(),
-                                dp_rank = worker.dp_rank,
-                                position = pos,
-                                "Orphaned block at position with no parent; skipping in dump"
-                            );
-                            continue;
-                        }
-                    }
-                };
-
                 events.push(RouterEvent {
                     worker_id: worker.worker_id,
                     storage_tier: crate::protocols::StorageTier::Device,
                     event: KvCacheEvent {
                         event_id,
                         data: KvCacheEventData::Stored(KvCacheStoreData {
-                            parent_hash,
+                            parent_hash: None,
+                            start_position: Some(pos as u32),
                             blocks: vec![KvCacheStoredBlockData {
                                 block_hash: seq_hash,
                                 tokens_hash: local_hash,
@@ -462,7 +444,6 @@ impl PositionalIndexer {
                     },
                 });
                 event_id += 1;
-                last_at_position.insert(pos, seq_hash);
             }
         }
 

lib/kv-router/src/indexer/radix_tree.rs

@@ -542,6 +542,7 @@ impl RadixTree {
                         event_id,
                         data: KvCacheEventData::Stored(KvCacheStoreData {
                             parent_hash,
+                            start_position: None,
                             blocks: vec![KvCacheStoredBlockData {
                                 block_hash,
                                 mm_extra_info: None,

lib/kv-router/src/indexer/tests.rs

@@ -32,7 +32,7 @@ fn make_store_event_with_dp_rank(
     local_hashes: &[u64],
     dp_rank: u32,
 ) -> RouterEvent {
-    make_store_event_full(worker_id, local_hashes, dp_rank, None)
+    make_store_event_full(worker_id, local_hashes, dp_rank, None, None)
 }
 
 /// Create a store event with parent hash for continuation sequences.
@@ -72,17 +72,27 @@ fn make_store_event_with_parent(
         0,
         KvCacheEventData::Stored(KvCacheStoreData {
             parent_hash,
+            start_position: None,
             blocks: stored_blocks_with_sequence_hashes(&new_block_hashes, new_seq_hashes),
         }),
     )
 }
 
+fn make_store_event_with_start_position(
+    worker_id: u64,
+    local_hashes: &[u64],
+    start_position: u32,
+) -> RouterEvent {
+    make_store_event_full(worker_id, local_hashes, 0, None, Some(start_position))
+}
+
 /// Create a store event with all options.
 fn make_store_event_full(
     worker_id: u64,
     local_hashes: &[u64],
     dp_rank: u32,
     parent_hash: Option<ExternalSequenceBlockHash>,
+    start_position: Option<u32>,
 ) -> RouterEvent {
     let local_block_hashes: Vec<LocalBlockHash> =
         local_hashes.iter().map(|&h| LocalBlockHash(h)).collect();
@@ -94,6 +104,7 @@ fn make_store_event_full(
         dp_rank,
         KvCacheEventData::Stored(KvCacheStoreData {
             parent_hash,
+            start_position,
             blocks: stored_blocks_with_sequence_hashes(&local_block_hashes, &seq_hashes),
         }),
     )
@@ -781,6 +792,46 @@ mod interface_tests {
         assert_score(index.as_ref(), &[1, 2, 3], WorkerWithDpRank::new(0, 0), 3).await;
     }
 
+    #[tokio::test]
+    async fn test_flat_dump_replay_preserves_start_positions() {
+        let index = make_indexer("flat");
+        index
+            .apply_event(make_store_event_with_start_position(0, &[11, 12], 10))
+            .await;
+
+        flush_and_settle(index.as_ref()).await;
+
+        let dumped = index.dump_events().await.unwrap();
+        let stored = dumped
+            .iter()
+            .filter_map(|event| match &event.event.data {
+                KvCacheEventData::Stored(data) => Some(data),
+                _ => None,
+            })
+            .collect::<Vec<_>>();
+        assert_eq!(stored.len(), 2);
+        assert_eq!(
+            stored
+                .iter()
+                .map(|data| data.start_position)
+                .collect::<Vec<_>>(),
+            vec![Some(10), Some(11)]
+        );
+        assert!(stored.iter().all(|data| data.parent_hash.is_none()));
+
+        let replay = make_indexer("flat");
+        for event in dumped {
+            replay.apply_event(event).await;
+        }
+
+        flush_and_settle(replay.as_ref()).await;
+
+        assert_eq!(
+            snapshot_tree(index.as_ref()).await,
+            snapshot_tree(replay.as_ref()).await
+        );
+    }
+
     #[tokio::test]
     #[apply(indexer_template)]
     async fn test_multiple_dp_ranks(variant: &str) {
@@ -1081,6 +1132,7 @@ mod lora_tests {
             0,
             KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: None,
+                start_position: None,
                 blocks: stored_blocks_with_sequence_hashes(&base_hashes, &base_seq),
             }),
         );
@@ -1093,6 +1145,7 @@ mod lora_tests {
             0,
             KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: None,
+                start_position: None,
                 blocks: stored_blocks_with_sequence_hashes(&lora_hashes, &lora_seq),
             }),
         );
@@ -1177,6 +1230,7 @@ mod lora_tests {
                 0,
                 KvCacheEventData::Stored(KvCacheStoreData {
                     parent_hash: None,
+                    start_position: None,
                     blocks: stored_blocks_with_sequence_hashes(&base_local, &base_seq),
                 }),
             ))
@@ -1191,6 +1245,7 @@ mod lora_tests {
                 0,
                 KvCacheEventData::Stored(KvCacheStoreData {
                     parent_hash: None,
+                    start_position: None,
                     blocks: stored_blocks_with_sequence_hashes(&lora_local, &lora_seq),
                 }),
             ))
@@ -1262,6 +1317,7 @@ mod lora_tests {
                 0,
                 KvCacheEventData::Stored(KvCacheStoreData {
                     parent_hash: None,
+                    start_position: None,
                     blocks: stored_blocks_with_sequence_hashes(&hashes_a, &seq_a),
                 }),
             ))
@@ -1275,6 +1331,7 @@ mod lora_tests {
                 0,
                 KvCacheEventData::Stored(KvCacheStoreData {
                     parent_hash: None,
+                    start_position: None,
                     blocks: stored_blocks_with_sequence_hashes(&hashes_b, &seq_b),
                 }),
             ))
@@ -2126,6 +2183,7 @@ mod local_indexer_tests {
                 event_id,
                 data: KvCacheEventData::Stored(KvCacheStoreData {
                     parent_hash: None,
+                    start_position: None,
                     blocks: vec![KvCacheStoredBlockData {
                         block_hash: ExternalSequenceBlockHash(block_hash),
                         tokens_hash: LocalBlockHash(block_hash),
@@ -2234,6 +2292,7 @@ mod local_indexer_tests {
                     event_id: id,
                     data: KvCacheEventData::Stored(KvCacheStoreData {
                         parent_hash: None,
+                        start_position: None,
                         blocks: vec![KvCacheStoredBlockData {
                             block_hash: ExternalSequenceBlockHash(id * 100),
                             tokens_hash: LocalBlockHash(id * 200),
@@ -2335,6 +2394,7 @@ mod local_indexer_tests {
                     event_id: id,
                     data: KvCacheEventData::Stored(KvCacheStoreData {
                         parent_hash: None,
+                        start_position: None,
                         blocks: vec![KvCacheStoredBlockData {
                             block_hash: ExternalSequenceBlockHash(id * 100),
                             tokens_hash: LocalBlockHash(id * 200),
@@ -2423,6 +2483,7 @@ mod local_indexer_tests {
                 event_id: 1,
                 data: KvCacheEventData::Stored(KvCacheStoreData {
                     parent_hash: None,
+                    start_position: None,
                     blocks: vec![KvCacheStoredBlockData {
                         block_hash: ExternalSequenceBlockHash(100),
                         tokens_hash: LocalBlockHash(200),
@@ -2479,6 +2540,7 @@ mod local_indexer_tests {
                 event_id: 1,
                 data: KvCacheEventData::Stored(KvCacheStoreData {
                     parent_hash: None,
+                    start_position: None,
                     blocks: vec![KvCacheStoredBlockData {
                         block_hash: ExternalSequenceBlockHash(100),
                         tokens_hash: LocalBlockHash(200),

lib/kv-router/src/protocols.rs

@@ -505,6 +505,9 @@ pub enum KvCacheEventData {
 pub struct KvCacheStoreData {
     /// The optional hash of the parent block.
     pub parent_hash: Option<ExternalSequenceBlockHash>,
+    /// Absolute position of the first block in this batch for positional replay.
+    #[serde(default)]
+    pub start_position: Option<u32>,
     /// A list of stored blocked data.
     pub blocks: Vec<KvCacheStoredBlockData>,
 }
@@ -932,6 +935,7 @@ mod tests {
             event_id: 1,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: None,
+                start_position: None,
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(0),
                     mm_extra_info: None,
@@ -1135,6 +1139,7 @@ mod tests {
     fn test_kv_cache_events_serialization() {
         let event_data = KvCacheEventData::Stored(KvCacheStoreData {
             parent_hash: Some(ExternalSequenceBlockHash(1)),
+            start_position: None,
             blocks: vec![KvCacheStoredBlockData {
                 block_hash: ExternalSequenceBlockHash(2),
                 tokens_hash: LocalBlockHash(3),

lib/kv-router/src/test_utils.rs

@@ -72,9 +72,18 @@ pub fn make_blocks(hashes: Vec<u64>) -> Vec<KvCacheStoredBlockData> {
 pub fn add_blocks(
     hashes: Vec<u64>,
     parent_hash: Option<ExternalSequenceBlockHash>,
+) -> KvCacheEventData {
+    add_blocks_with_start_position(hashes, parent_hash, None)
+}
+
+pub fn add_blocks_with_start_position(
+    hashes: Vec<u64>,
+    parent_hash: Option<ExternalSequenceBlockHash>,
+    start_position: Option<u32>,
 ) -> KvCacheEventData {
     KvCacheEventData::Stored(KvCacheStoreData {
         parent_hash,
+        start_position,
         blocks: make_blocks(hashes),
     })
 }
@@ -88,6 +97,21 @@ pub fn create_store_event(
     router_event(worker_id, event_id, 0, add_blocks(hashes, parent))
 }
 
+pub fn create_store_event_with_start_position(
+    worker_id: WorkerId,
+    event_id: u64,
+    hashes: Vec<u64>,
+    parent: Option<ExternalSequenceBlockHash>,
+    start_position: Option<u32>,
+) -> RouterEvent {
+    router_event(
+        worker_id,
+        event_id,
+        0,
+        add_blocks_with_start_position(hashes, parent, start_position),
+    )
+}
+
 pub fn create_remove_event(worker_id: WorkerId, event_id: u64, hashes: Vec<u64>) -> RouterEvent {
     remove_event(
         worker_id,

lib/kv-router/src/zmq_wire.rs

@@ -464,6 +464,7 @@ pub fn convert_event(
                     parent_hash: parent_block_hash
                         .map(BlockHashValue::into_u64)
                         .map(ExternalSequenceBlockHash::from),
+                    start_position: None,
                     blocks: create_stored_blocks(
                         kv_block_size,
                         &token_ids,

lib/llm/benches/kv_router_bench.rs

@@ -557,6 +557,7 @@ fn sequence_to_router_event(sequence: &SequenceData, event_id: u64) -> RouterEve
         event_id,
         data: KvCacheEventData::Stored(KvCacheStoreData {
             parent_hash: None,
+            start_position: None,
             blocks: sequence
                 .local_hashes
                 .iter()

lib/llm/src/kv_router/indexer/worker_query.rs

@@ -947,6 +947,7 @@ mod tests {
                 event_id,
                 data: KvCacheEventData::Stored(KvCacheStoreData {
                     parent_hash: None,
+                    start_position: None,
                     blocks: vec![KvCacheStoredBlockData {
                         block_hash: ExternalSequenceBlockHash(event_id),
                         tokens_hash: LocalBlockHash(event_id),

lib/llm/src/kv_router/publisher/tests.rs

@@ -606,6 +606,7 @@ mod tests_startup_helpers {
             event_id: 1,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: None,
+                start_position: None,
                 blocks: vec![
                     KvCacheStoredBlockData {
                         block_hash: ExternalSequenceBlockHash(100),
@@ -698,6 +699,7 @@ mod tests_startup_helpers {
             event_id: 1,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: None,
+                start_position: None,
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(100),
                     tokens_hash: LocalBlockHash(200),
@@ -780,6 +782,7 @@ mod tests_startup_helpers {
             event_id: 1,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: None,
+                start_position: None,
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(100),
                     tokens_hash: LocalBlockHash(200),
@@ -968,6 +971,7 @@ mod tests_startup_helpers {
 
         let KvCacheEventData::Stored(KvCacheStoreData {
             parent_hash,
+            start_position,
             blocks,
         }) = event.data
         else {
@@ -975,6 +979,7 @@ mod tests_startup_helpers {
         };
 
         assert!(parent_hash.is_none());
+        assert!(start_position.is_none());
         assert_eq!(blocks.len(), 1);
         assert_eq!(blocks[0].block_hash.0, 42);
 
@@ -1094,6 +1099,7 @@ mod tests_startup_helpers {
             event_id: 1,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: None,
+                start_position: None,
                 blocks: vec![
                     KvCacheStoredBlockData {
                         block_hash: ExternalSequenceBlockHash(100),
@@ -1166,6 +1172,7 @@ mod tests_startup_helpers {
             event_id: 2,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: None,
+                start_position: None,
                 blocks: vec![
                     KvCacheStoredBlockData {
                         block_hash: ExternalSequenceBlockHash(100), // Shared prefix
@@ -1280,6 +1287,7 @@ mod test_event_dedup_filter {
     fn store_data(hashes: &[u64]) -> KvCacheStoreData {
         KvCacheStoreData {
             parent_hash: None,
+            start_position: None,
             blocks: hashes
                 .iter()
                 .map(|&h| KvCacheStoredBlockData {
@@ -1556,6 +1564,7 @@ mod batching_state_tests {
 
         state.pending_stored = Some(KvCacheStoreData {
             parent_hash: None,
+            start_position: None,
             blocks: vec![],
         });
         assert!(
@@ -1659,6 +1668,7 @@ mod batching_state_tests {
         };
         let first = KvCacheStoreData {
             parent_hash: Some(ExternalSequenceBlockHash(0)),
+            start_position: None,
             blocks: vec![block1],
         };
 
@@ -1865,6 +1875,7 @@ mod event_processor_tests {
                 event_id: i as u64,
                 data: KvCacheEventData::Stored(KvCacheStoreData {
                     parent_hash,
+                    start_position: None,
                     blocks: vec![KvCacheStoredBlockData {
                         block_hash: ExternalSequenceBlockHash(i as u64),
                         tokens_hash: LocalBlockHash(i as u64 * 100),
@@ -1957,6 +1968,7 @@ mod event_processor_tests {
                 event_id: i as u64,
                 data: KvCacheEventData::Stored(KvCacheStoreData {
                     parent_hash: Some(ExternalSequenceBlockHash((i + 1) as u64 * 100)),
+                    start_position: None,
                     blocks: vec![KvCacheStoredBlockData {
                         block_hash: ExternalSequenceBlockHash(i as u64),
                         tokens_hash: LocalBlockHash(i as u64 * 100),
@@ -2023,6 +2035,7 @@ mod event_processor_tests {
             event_id: 0,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: None,
+                start_position: Some(10),
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(1),
                     tokens_hash: LocalBlockHash(100),
@@ -2038,6 +2051,7 @@ mod event_processor_tests {
             event_id: 1,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: Some(ExternalSequenceBlockHash(1)),
+                start_position: Some(11_111),
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(2),
                     tokens_hash: LocalBlockHash(200),
@@ -2053,6 +2067,7 @@ mod event_processor_tests {
             event_id: 2,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: Some(ExternalSequenceBlockHash(1)),
+                start_position: Some(20),
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(3),
                     tokens_hash: LocalBlockHash(300),
@@ -2086,6 +2101,11 @@ mod event_processor_tests {
                 data.parent_hash, None,
                 "First batch should preserve the original root parent"
             );
+            assert_eq!(
+                data.start_position,
+                Some(10),
+                "First batch should preserve the original start position"
+            );
         } else {
             panic!("Expected first event to be Stored");
         }
@@ -2101,6 +2121,11 @@ mod event_processor_tests {
                 Some(ExternalSequenceBlockHash(1)),
                 "Second batch should preserve the reused parent hash"
             );
+            assert_eq!(
+                data.start_position,
+                Some(20),
+                "Second batch should keep the new root's start position"
+            );
         } else {
             panic!("Expected second event to be Stored");
         }
@@ -2235,6 +2260,7 @@ mod event_processor_tests {
             event_id: 1,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: Some(ExternalSequenceBlockHash(0)),
+                start_position: None,
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(1),
                     tokens_hash: LocalBlockHash(100),
@@ -2537,6 +2563,7 @@ mod event_processor_tests {
             event_id: 100,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: Some(ExternalSequenceBlockHash(0)),
+                start_position: None,
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(1),
                     tokens_hash: LocalBlockHash(100),
@@ -2552,6 +2579,7 @@ mod event_processor_tests {
             event_id: 101,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: Some(ExternalSequenceBlockHash(1)),
+                start_position: None,
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(2),
                     tokens_hash: LocalBlockHash(200),
@@ -2569,6 +2597,7 @@ mod event_processor_tests {
             event_id: 200,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: Some(ExternalSequenceBlockHash(0)),
+                start_position: None,
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(100),
                     tokens_hash: LocalBlockHash(1000),
@@ -2654,6 +2683,7 @@ mod event_processor_tests {
             event_id: 0,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: None, // Root block with no parent
+                start_position: Some(10),
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(1),
                     tokens_hash: LocalBlockHash(100),
@@ -2670,6 +2700,7 @@ mod event_processor_tests {
             event_id: 1,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: Some(ExternalSequenceBlockHash(1)), // Points to previous block
+                start_position: Some(999),
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(2),
                     tokens_hash: LocalBlockHash(200),
@@ -2686,6 +2717,7 @@ mod event_processor_tests {
             event_id: 2,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: Some(ExternalSequenceBlockHash(2)),
+                start_position: Some(1_234),
                 blocks: vec![KvCacheStoredBlockData {
                     block_hash: ExternalSequenceBlockHash(3),
                     tokens_hash: LocalBlockHash(300),
@@ -2716,6 +2748,11 @@ mod event_processor_tests {
                 data.parent_hash, None,
                 "Batch parent_hash should remain None (from first event), NOT overwritten by subsequent events"
             );
+            assert_eq!(
+                data.start_position,
+                Some(10),
+                "Batch start_position should remain anchored to the first event"
+            );
         } else {
             panic!("Expected Stored event");
         }

lib/mocker/src/kv_manager/sglang_backend.rs

@@ -310,6 +310,7 @@ impl SglangKvManager {
             event_id: self.next_event_id,
             data: KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash,
+                start_position: None,
                 blocks,
             }),
             dp_rank: self.dp_rank,

lib/mocker/src/kv_manager/vllm_backend.rs

@@ -116,6 +116,7 @@ impl KvManager {
 
             KvCacheEventData::Stored(KvCacheStoreData {
                 parent_hash: parent_hash.map(ExternalSequenceBlockHash),
+                start_position: None,
                 blocks: full_blocks
                     .into_iter()
                     .zip(local_hashes_slice.iter())