fix

libs/checkpoint-sqlite/langgraph/checkpoint/sqlite/__init__.py

@@ -24,10 +24,12 @@ from langgraph.checkpoint.base import (
 from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer
 
 from langgraph.checkpoint.sqlite._delta import (
-    DELTA_STAGE1_SQL,
+    DELTA_WALK_SQL,
     build_delta_channels_writes_history,
-    build_delta_stage2_sql,
-    step_walk_with_row,
+    build_delta_writes_fetch_sql,
+    parse_supersteps_since_last_snapshot_by_channel,
+    resolve_delta_chains,
+    step_walk_supersteps,
 )
 from langgraph.checkpoint.sqlite.utils import search_where
 
@@ -505,68 +507,106 @@ class SqliteSaver(BaseCheckpointSaver[str]):
     ) -> Mapping[str, DeltaChannelHistory]:
         """Fast-path override of `BaseCheckpointSaver.get_delta_channel_history`.
 
-        Two-stage query:
+        Walks the parent chain `supersteps` hops (read from the target's
+        `counters_since_delta_snapshot` metadata) to each channel's seed
+        snapshot, then collects the writes between the seed and the target.
 
-        * Stage 1 (paged): newest-first slice of `checkpoints` returning
+        * WALK: stream a newest-first slice of `checkpoints` returning
           `(checkpoint_id, parent_checkpoint_id, type, checkpoint)` per
-          ancestor. Sqlite has no JSONB, so we ship the full serialized
-          checkpoint blob and inspect `channel_values` in Python. Pages
-          newest-first by `checkpoint_id` with a `< cursor` predicate;
-          page size is `DELTA_PAGE_SIZE`. Stops paging when every channel
-          has found its seed or the chain is exhausted.
+          ancestor. Sqlite has no JSONB, so we deserialize only the seed
+          checkpoints to read their inline `channel_values`. Stops once the
+          shared chain reaches the deepest requested `supersteps` or the
+          root is reached.
 
-        * Stage 2 (per-channel UNION ALL): one branch per channel reading
-          `writes` filtered to that channel's specific `chain_cids`. No
-          separate seed-blob fetch — sqlite stores `channel_values` inline
-          in the checkpoint blob, so seeds come back from stage 1.
+        * FETCH (per-channel UNION ALL): one branch per channel reading
+          `writes` filtered to that channel's `chain_cids`. No separate
+          seed-blob fetch — sqlite stores `channel_values` inline.
         """
         if not channels:
             return {}
         channels = list(channels)
         thread_id = str(config["configurable"]["thread_id"])
+        if not thread_id:
+            raise ValueError("empty thread ID")
         checkpoint_ns = config["configurable"].get("checkpoint_ns", "")
-        checkpoint_id = get_checkpoint_id(config)
-        if checkpoint_id is None:
-            target = self.get_tuple(config)
-            if target is None:
-                return {ch: {"writes": []} for ch in channels}
-            checkpoint_id = target.config["configurable"]["checkpoint_id"]
 
-        chain_by_ch: dict[str, list[str]] = {ch: [] for ch in channels}
-        seed_val_by_ch: dict[str, Any] = {}
+        # Resolve the target checkpoint id + its metadata (for supersteps).
+        checkpoint_id = get_checkpoint_id(config)
+        with self.cursor(transaction=False) as cur:
+            if checkpoint_id is None:
+                cur.execute(
+                    "SELECT checkpoint_id, metadata FROM checkpoints "
+                    "WHERE thread_id = ? AND checkpoint_ns = ? "
+                    "ORDER BY checkpoint_id DESC LIMIT 1",
+                    (thread_id, checkpoint_ns),
+                )
+            else:
+                cur.execute(
+                    "SELECT checkpoint_id, metadata FROM checkpoints "
+                    "WHERE thread_id = ? AND checkpoint_ns = ? "
+                    "AND checkpoint_id = ?",
+                    (thread_id, checkpoint_ns, checkpoint_id),
+                )
+            target_row = cur.fetchone()
+        if target_row is None:
+            return {ch: {"writes": []} for ch in channels}
+        target_id = str(target_row[0])
+        metadata = json.loads(target_row[1]) if target_row[1] is not None else {}
+        supersteps_by_ch = parse_supersteps_since_last_snapshot_by_channel(
+            metadata, channels
+        )
+        max_supersteps = max(supersteps_by_ch.values(), default=0)
+        needed_depths = set(supersteps_by_ch.values())
+
+        shared_cpid_chain: list[str] = []
         walk_state: dict[str, Any] = {}
-        seeded: set[str] = set()
+        seed_values_by_depth: dict[int, dict[str, Any]] = {}
 
         with self.cursor(transaction=False) as cur:
-            cur.execute(DELTA_STAGE1_SQL, (thread_id, checkpoint_ns, checkpoint_id))
-            for row in cur:
-                cid, parent_cid, type_tag, blob = row
-                if step_walk_with_row(
-                    cid=cid,
-                    parent_cid=parent_cid,
-                    type_tag=type_tag,
-                    blob=blob,
-                    target_id=checkpoint_id,
-                    serde=self.serde,
-                    chain_by_ch=chain_by_ch,
-                    seed_val_by_ch=seed_val_by_ch,
-                    walk_state=walk_state,
-                    seeded=seeded,
-                    channels=channels,
-                ):
-                    break
+            if max_supersteps > 0:
+                cur.execute(DELTA_WALK_SQL, (thread_id, checkpoint_ns, target_id))
+                for row in cur:
+                    cid, parent_cid, type_tag, blob = row
+                    if step_walk_supersteps(
+                        cid=cid,
+                        parent_cid=parent_cid,
+                        type_tag=type_tag,
+                        blob=blob,
+                        target_id=target_id,
+                        serde=self.serde,
+                        shared_cpid_chain=shared_cpid_chain,
+                        walk_state=walk_state,
+                        max_supersteps=max_supersteps,
+                        needed_depths=needed_depths,
+                        seed_values_by_depth=seed_values_by_depth,
+                        channels=channels,
+                    ):
+                        break
 
-            channels_with_chain = [ch for ch in channels if chain_by_ch[ch]]
-            stage2_sql = build_delta_stage2_sql(
-                chain_lens=[len(chain_by_ch[ch]) for ch in channels_with_chain],
+            (
+                chained_cpid_by_ch,
+                seed_cpid_by_ch,
+                seed_value_by_ch,
+            ) = resolve_delta_chains(
+                channels=channels,
+                supersteps_by_ch=supersteps_by_ch,
+                shared_cpid_chain=shared_cpid_chain,
+                seed_values_by_depth=seed_values_by_depth,
+                has_reached_root=bool(walk_state.get("reached_root")),
+                thread_id=thread_id,
             )
-            if stage2_sql:
-                stage2_params: list[Any] = []
+
+            channels_with_chain = [ch for ch in channels if chained_cpid_by_ch[ch]]
+            fetch_sql = build_delta_writes_fetch_sql(
+                chain_lens=[len(chained_cpid_by_ch[ch]) for ch in channels_with_chain],
+            )
+            if fetch_sql:
+                fetch_params: list[Any] = []
                 for ch in channels_with_chain:
-                    stage2_params.extend(
-                        [thread_id, checkpoint_ns, ch, *chain_by_ch[ch]]
+                    fetch_params.extend(
+                        [thread_id, checkpoint_ns, ch, *chained_cpid_by_ch[ch]]
                     )
-                cur.execute(stage2_sql, stage2_params)
+                cur.execute(fetch_sql, fetch_params)
                 stage2_rows = cast(
                     "list[tuple[str, str, str, int, str, bytes]]", cur.fetchall()
                 )
@@ -575,9 +615,9 @@ class SqliteSaver(BaseCheckpointSaver[str]):
 
         return build_delta_channels_writes_history(
             channels=channels,
-            chain_by_ch=chain_by_ch,
-            seed_val_by_ch=seed_val_by_ch,
-            seeded=seeded,
+            chained_cpid_by_ch=chained_cpid_by_ch,
+            seed_cpid_by_ch=seed_cpid_by_ch,
+            seed_value_by_ch=seed_value_by_ch,
             stage2_rows=stage2_rows,
             serde=self.serde,
         )

libs/checkpoint-sqlite/langgraph/checkpoint/sqlite/_delta.py

@@ -1,37 +1,50 @@
 """Shared helpers for `get_delta_channel_history` on sqlite savers.
 
-Mirrors the two-stage shape of `BasePostgresSaver` (ancestor walk +
-per-channel UNION ALL writes fetch), but adapted for sqlite's
-constraints. The structural differences:
+Mirrors the supersteps-based two-pass shape of `BasePostgresSaver`
+(ancestor walk bounded by `counters_since_delta_snapshot` + per-channel
+UNION ALL writes fetch), adapted for sqlite's constraints:
 
 * No JSONB — to inspect `channel_values` for a checkpoint we must
-  deserialize the full blob. Stage 1 streams the cursor row-by-row and
-  deserializes only the rows the merged walk visits, freeing each blob
-  before advancing.
+  deserialize the full blob. The WALK streams the cursor row-by-row and
+  deserializes only the seed checkpoints (the ones at a channel's
+  `supersteps` depth), freeing each blob before advancing.
 * No separate blob table — `channel_values` lives inline in the
-  checkpoint, so seeds come back from stage 1 with no second fetch.
-* Single merged walk (not K independent walks): each visited cid is
-  deserialized exactly once, regardless of how many channels are still
-  seeking their seed.
+  checkpoint, so seeds come back from the WALK with no second fetch.
+* Single shared parent-chain walk: each requested channel slices the
+  same chain to its own `supersteps` depth.
 
-The streaming design keeps peak in-flight memory at roughly one
-deserialized checkpoint at a time, instead of holding the entire
-ancestor chain's worth of raw blobs as a `fetchall()`-materialized list.
+Walk depth is driven by the *supersteps since last snapshot* counter,
+not by scanning `channel_values` for the snapshot marker — the only
+reliable way to locate seeds that aren't a `_DeltaSnapshot` sentinel
+(e.g. legacy plain-value blobs from a thread migrated off a non-delta
+channel).
 """
 
 from __future__ import annotations
 
+import logging
 from collections.abc import Mapping, Sequence
 from typing import Any
 
-from langgraph.checkpoint.base import DeltaChannelHistory, PendingWrite
+from langgraph.checkpoint.base import (
+    DeltaChannelHistory,
+    PendingWrite,
+    _parse_supersteps_since_last_snapshot_by_channel,
+)
+from langgraph.checkpoint.serde.types import _DeltaSnapshot
 
-# Stage 1 streams ancestors of `target_cid` newest-first. The `<=`
+logger = logging.getLogger(__name__)
+
+# Re-exported under the package-local name used by the sqlite savers.
+parse_supersteps_since_last_snapshot_by_channel = (
+    _parse_supersteps_since_last_snapshot_by_channel
+)
+
+# The WALK streams ancestors of `target_id` newest-first. The `<=`
 # predicate keeps target itself in the stream so we can read its
-# `parent_checkpoint_id` from the first row without a separate lookup;
-# the caller skips target's own writes/seed (matches the
-# `BaseCheckpointSaver` contract).
-DELTA_STAGE1_SQL = (
+# `parent_checkpoint_id` from the first matching row without a separate
+# lookup; target's own writes/seed are not part of the contract.
+DELTA_WALK_SQL = (
     "SELECT checkpoint_id, parent_checkpoint_id, type, checkpoint "
     "FROM checkpoints "
     "WHERE thread_id = ? AND checkpoint_ns = ? AND checkpoint_id <= ? "
@@ -39,12 +52,12 @@ DELTA_STAGE1_SQL = (
 )
 
 
-def build_delta_stage2_sql(*, chain_lens: Sequence[int]) -> str:
-    """Stage-2 per-channel UNION ALL fetching writes from `writes`.
+def build_delta_writes_fetch_sql(*, chain_lens: Sequence[int]) -> str:
+    """Per-channel UNION ALL fetching writes from `writes`.
 
     One branch per channel with a non-empty chain. Each branch inlines its
     own `IN (?, ?, ...)` placeholder list because sqlite has no array-bind
-    equivalent of postgres's `= ANY(%s)`. Caller passes parameters in
+    equivalent of postgres's `= ANY(?)`. Caller passes parameters in
     matching order: `[thread_id, checkpoint_ns, channel, *chain_cids]` per
     branch.
 
@@ -65,7 +78,7 @@ def build_delta_stage2_sql(*, chain_lens: Sequence[int]) -> str:
     return " UNION ALL ".join(branches)
 
 
-def step_walk_with_row(
+def step_walk_supersteps(
     *,
     cid: str,
     parent_cid: str | None,
@@ -73,75 +86,142 @@ def step_walk_with_row(
     blob: bytes,
     target_id: str,
     serde: Any,
-    chain_by_ch: dict[str, list[str]],
-    seed_val_by_ch: dict[str, Any],
+    shared_cpid_chain: list[str],
     walk_state: dict[str, Any],
-    seeded: set[str],
+    max_supersteps: int,
+    needed_depths: set[int],
+    seed_values_by_depth: dict[int, dict[str, Any]],
     channels: Sequence[str],
 ) -> bool:
-    """Process one streamed stage-1 row in the merged ancestor walk.
+    """Process one streamed WALK row, extending the shared parent chain.
 
-    The cursor returns (cid, parent_cid, type, blob) rows in
-    `checkpoint_id` DESC order starting at target. The first row is
-    target itself; we read its parent_cid to seed the walk and otherwise
-    skip it (target's own writes/seed are not part of the contract).
+    The cursor returns `(cid, parent_cid, type, checkpoint)` rows in
+    `checkpoint_id` DESC order starting at target. The first row is target
+    itself; we read its `parent_cid` to seed the walk and skip it (target's
+    own writes/seed are not part of the contract). Off-path rows (a sibling
+    branch on the same thread) advance the cursor without doing work.
 
-    For each subsequent row, if `cid` matches the walk's current
-    position, we deserialize the blob, append the cid to every
-    not-yet-seeded channel's chain, and check `channel_values` for
-    seeds. The deserialized checkpoint is dropped before advancing — no
-    cross-row cache, so peak in-flight is one deserialized checkpoint.
+    For each on-path ancestor we append its cid to `shared_cpid_chain`
+    (newest first). When the ancestor sits at a depth some channel needs as
+    its seed (`len(chain)` ∈ `needed_depths`), we deserialize it once and
+    record its `channel_values` for the requested channels. The
+    deserialized checkpoint is dropped immediately — peak in-flight is one
+    deserialized checkpoint.
 
-    Off-path rows (different branch on the same thread) advance the
-    cursor without doing any work.
-
-    Returns True when every requested channel is seeded — the caller
-    can stop iterating and close the cursor.
+    Sets `walk_state["reached_root"]` when an ancestor has no parent.
+    Returns True when the walk can stop: chain reached `max_supersteps`, or
+    the root was reached.
     """
     if "started" not in walk_state:
         if cid == target_id:
             walk_state["started"] = True
             walk_state["cur_cid"] = parent_cid
-            walk_state["active"] = {ch for ch in channels if ch not in seeded}
+            if parent_cid is None:
+                walk_state["reached_root"] = True
+                return True
         # Not target yet (or target not present): keep streaming.
         return False
-    active: set[str] = walk_state["active"]
-    if not active:
+    if len(shared_cpid_chain) >= max_supersteps:
         return True
     if cid != walk_state["cur_cid"]:
         # Off-path row from a sibling branch — skip without deserializing.
         return False
-    for ch in active:
-        chain_by_ch[ch].append(cid)
-    ckpt = serde.loads_typed((type_tag, blob))
-    channel_values: Mapping[str, Any] = ckpt.get("channel_values") or {}
-    for ch in [ch for ch in active if ch in channel_values]:
-        seed_val_by_ch[ch] = channel_values[ch]
-        seeded.add(ch)
-        active.discard(ch)
-    del ckpt, channel_values
+    shared_cpid_chain.append(cid)
+    depth = len(shared_cpid_chain)  # 1-indexed position along the chain
+    # Capture channel_values at any depth a channel may use as its seed: the
+    # exact `supersteps` depths, plus the root-most checkpoint (the seed
+    # candidate when the chain is shorter than `supersteps`).
+    if depth in needed_depths or parent_cid is None:
+        ckpt = serde.loads_typed((type_tag, blob))
+        channel_values: Mapping[str, Any] = ckpt.get("channel_values") or {}
+        seed_values_by_depth[depth] = {
+            ch: channel_values[ch] for ch in channels if ch in channel_values
+        }
+        del ckpt, channel_values
+    if parent_cid is None:
+        walk_state["reached_root"] = True
+        return True
     walk_state["cur_cid"] = parent_cid
-    return not active
+    return len(shared_cpid_chain) >= max_supersteps
+
+
+def resolve_delta_chains(
+    *,
+    channels: Sequence[str],
+    supersteps_by_ch: Mapping[str, int],
+    shared_cpid_chain: Sequence[str],
+    seed_values_by_depth: Mapping[int, Mapping[str, Any]],
+    has_reached_root: bool,
+    thread_id: str,
+) -> tuple[
+    dict[str, list[str]],
+    dict[str, str | None],
+    dict[str, Any],
+]:
+    """Slice the shared parent chain into per-channel chain/seed mappings.
+
+    For each channel the seed snapshot sits `supersteps` hops back: the seed
+    checkpoint is `shared_cpid_chain[supersteps - 1]` and the chain is
+    `shared_cpid_chain[:supersteps]` (newest first), with the seed's inline
+    `channel_values[ch]` captured during the walk.
+
+    When the chain is shorter than `supersteps` but the walk reached the
+    root, the persisted chain is "compressed" relative to the logical
+    superstep count (`durability="exit"`, or a thread that never
+    snapshotted). The seed candidate is then the oldest persisted checkpoint
+    (`shared_cpid_chain[-1]`); if its `channel_values[ch]` is absent the
+    channel has no seed and replays the full chain on an empty baseline.
+    """
+    chained_cpid_by_ch: dict[str, list[str]] = {ch: [] for ch in channels}
+    seed_cpid_by_ch: dict[str, str | None] = {ch: None for ch in channels}
+    seed_value_by_ch: dict[str, Any] = {}
+    for ch in channels:
+        bound = supersteps_by_ch.get(ch, 0)
+        if bound <= 0:
+            continue
+        if len(shared_cpid_chain) >= bound:
+            seed_depth = bound
+        elif has_reached_root:
+            seed_depth = len(shared_cpid_chain)
+        else:
+            logger.warning(
+                "cannot find seed snapshot for delta channel "
+                "(thread_id=%s, channel=%s)",
+                thread_id,
+                ch,
+            )
+            continue
+        if seed_depth <= 0:
+            continue
+        chained_cpid_by_ch[ch] = list(shared_cpid_chain[:seed_depth])
+        seed_cpid_by_ch[ch] = shared_cpid_chain[seed_depth - 1]
+        seed_vals = seed_values_by_depth.get(seed_depth, {})
+        if ch in seed_vals:
+            seed_value_by_ch[ch] = seed_vals[ch]
+    return chained_cpid_by_ch, seed_cpid_by_ch, seed_value_by_ch
 
 
 def build_delta_channels_writes_history(
     *,
     channels: Sequence[str],
-    chain_by_ch: Mapping[str, list[str]],
-    seed_val_by_ch: Mapping[str, Any],
-    seeded: set[str],
+    chained_cpid_by_ch: Mapping[str, Sequence[str]],
+    seed_cpid_by_ch: Mapping[str, str | None],
+    seed_value_by_ch: Mapping[str, Any],
     stage2_rows: Sequence[tuple[str, str, str, int, str, bytes]],
     serde: Any,
 ) -> dict[str, DeltaChannelHistory]:
-    """Demux stage-2 rows per channel; produce per-channel histories.
+    """Demux writes rows per channel; produce per-channel histories.
 
-    Stage-2 rows are `(checkpoint_id, channel, task_id, idx, type, value)`.
+    `stage2_rows` are `(checkpoint_id, channel, task_id, idx, type, value)`.
     Final write order is oldest→newest globally and `(task_id, idx)` within
     a checkpoint, matching the contract on `DeltaChannelHistory.writes`.
 
-    `seed` is omitted when the walk reached a true root with no snapshot
-    found (channel never entered `seeded`); consumers treat absence as
-    "start empty".
+    The seed checkpoint's own writes are replayed on top of a
+    `_DeltaSnapshot` seed (the snapshot is the value *prior* to its own
+    writes), but skipped for a migrated plain-value seed (a legacy
+    non-delta blob already incorporates those writes). `seed` is omitted
+    when no seed was located (implicit empty baseline); consumers treat
+    absence as "start empty".
     """
     writes_by_ch_by_cid: dict[str, dict[str, list[tuple[str, bytes, str, int]]]] = {
         ch: {} for ch in channels
@@ -156,17 +236,26 @@ def build_delta_channels_writes_history(
 
     result: dict[str, DeltaChannelHistory] = {}
     for ch in channels:
-        chain_cids = chain_by_ch.get(ch, [])
+        entry: DeltaChannelHistory = {"writes": []}
+
+        skip_seed_checkpoint_writes = False
+        if ch in seed_value_by_ch:
+            seed_value = seed_value_by_ch[ch]
+            entry["seed"] = seed_value
+            skip_seed_checkpoint_writes = not isinstance(seed_value, _DeltaSnapshot)
+
         cid_writes = writes_by_ch_by_cid.get(ch, {})
-        collected: list[PendingWrite] = []
-        # Chain is newest-first; iterate oldest-first for the public order.
-        for cid in reversed(chain_cids):
-            for type_tag, value_blob, task_id, _idx in cid_writes.get(cid, []):
-                collected.append(
-                    (task_id, ch, serde.loads_typed((type_tag, value_blob)))
-                )
-        entry: DeltaChannelHistory = {"writes": collected}
-        if ch in seeded:
-            entry["seed"] = seed_val_by_ch[ch]
+        if cid_writes:
+            collected: list[PendingWrite] = []
+            seed_cpid = seed_cpid_by_ch.get(ch)
+            # Chain is newest→oldest; replay oldest→newest.
+            for cid in reversed(list(chained_cpid_by_ch.get(ch, []))):
+                if skip_seed_checkpoint_writes and cid == seed_cpid:
+                    continue
+                for type_tag, value_blob, task_id, _idx in cid_writes.get(cid, []):
+                    collected.append(
+                        (task_id, ch, serde.loads_typed((type_tag, value_blob)))
+                    )
+            entry["writes"] = collected
         result[ch] = entry
     return result

libs/checkpoint-sqlite/langgraph/checkpoint/sqlite/aio.py

@@ -25,10 +25,12 @@ from langgraph.checkpoint.base import (
 from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer
 
 from langgraph.checkpoint.sqlite._delta import (
-    DELTA_STAGE1_SQL,
+    DELTA_WALK_SQL,
     build_delta_channels_writes_history,
-    build_delta_stage2_sql,
-    step_walk_with_row,
+    build_delta_writes_fetch_sql,
+    parse_supersteps_since_last_snapshot_by_channel,
+    resolve_delta_chains,
+    step_walk_supersteps,
 )
 from langgraph.checkpoint.sqlite.utils import search_where
 
@@ -625,61 +627,98 @@ class AsyncSqliteSaver(BaseCheckpointSaver[str]):
         """Fast-path override of `BaseCheckpointSaver.aget_delta_channel_history`.
 
         See `SqliteSaver.get_delta_channel_history` for design notes; this
-        is the async equivalent using `aiosqlite` cursors. Stage 1 pages
-        the parent chain newest-first and Python-deserializes each
-        checkpoint blob to find per-channel snapshots; stage 2 fetches
-        only the relevant writes via per-channel UNION ALL.
+        is the async equivalent using `aiosqlite` cursors. The WALK streams
+        the parent chain newest-first, bounded by the target's
+        `counters_since_delta_snapshot` supersteps, and deserializes only
+        seed checkpoints; FETCH pulls the relevant writes via per-channel
+        UNION ALL.
         """
         if not channels:
             return {}
         channels = list(channels)
         await self.setup()
         thread_id = str(config["configurable"]["thread_id"])
+        if not thread_id:
+            raise ValueError("empty thread ID")
         checkpoint_ns = config["configurable"].get("checkpoint_ns", "")
-        checkpoint_id = get_checkpoint_id(config)
-        if checkpoint_id is None:
-            target = await self.aget_tuple(config)
-            if target is None:
-                return {ch: {"writes": []} for ch in channels}
-            checkpoint_id = target.config["configurable"]["checkpoint_id"]
 
-        chain_by_ch: dict[str, list[str]] = {ch: [] for ch in channels}
-        seed_val_by_ch: dict[str, Any] = {}
+        # Resolve the target checkpoint id + its metadata (for supersteps).
+        checkpoint_id = get_checkpoint_id(config)
+        async with self.lock, self.conn.cursor() as cur:
+            if checkpoint_id is None:
+                await cur.execute(
+                    "SELECT checkpoint_id, metadata FROM checkpoints "
+                    "WHERE thread_id = ? AND checkpoint_ns = ? "
+                    "ORDER BY checkpoint_id DESC LIMIT 1",
+                    (thread_id, checkpoint_ns),
+                )
+            else:
+                await cur.execute(
+                    "SELECT checkpoint_id, metadata FROM checkpoints "
+                    "WHERE thread_id = ? AND checkpoint_ns = ? "
+                    "AND checkpoint_id = ?",
+                    (thread_id, checkpoint_ns, checkpoint_id),
+                )
+            target_row = await cur.fetchone()
+        if target_row is None:
+            return {ch: {"writes": []} for ch in channels}
+        target_id = str(target_row[0])
+        metadata = json.loads(target_row[1]) if target_row[1] is not None else {}
+        supersteps_by_ch = parse_supersteps_since_last_snapshot_by_channel(
+            metadata, channels
+        )
+        max_supersteps = max(supersteps_by_ch.values(), default=0)
+        needed_depths = set(supersteps_by_ch.values())
+
+        shared_cpid_chain: list[str] = []
         walk_state: dict[str, Any] = {}
-        seeded: set[str] = set()
+        seed_values_by_depth: dict[int, dict[str, Any]] = {}
 
         async with self.lock, self.conn.cursor() as cur:
-            await cur.execute(
-                DELTA_STAGE1_SQL, (thread_id, checkpoint_ns, checkpoint_id)
-            )
-            async for row in cur:
-                cid, parent_cid, type_tag, blob = row
-                if step_walk_with_row(
-                    cid=cid,
-                    parent_cid=parent_cid,
-                    type_tag=type_tag,
-                    blob=blob,
-                    target_id=checkpoint_id,
-                    serde=self.serde,
-                    chain_by_ch=chain_by_ch,
-                    seed_val_by_ch=seed_val_by_ch,
-                    walk_state=walk_state,
-                    seeded=seeded,
-                    channels=channels,
-                ):
-                    break
+            if max_supersteps > 0:
+                await cur.execute(DELTA_WALK_SQL, (thread_id, checkpoint_ns, target_id))
+                async for row in cur:
+                    cid, parent_cid, type_tag, blob = row
+                    if step_walk_supersteps(
+                        cid=cid,
+                        parent_cid=parent_cid,
+                        type_tag=type_tag,
+                        blob=blob,
+                        target_id=target_id,
+                        serde=self.serde,
+                        shared_cpid_chain=shared_cpid_chain,
+                        walk_state=walk_state,
+                        max_supersteps=max_supersteps,
+                        needed_depths=needed_depths,
+                        seed_values_by_depth=seed_values_by_depth,
+                        channels=channels,
+                    ):
+                        break
 
-            channels_with_chain = [ch for ch in channels if chain_by_ch[ch]]
-            stage2_sql = build_delta_stage2_sql(
-                chain_lens=[len(chain_by_ch[ch]) for ch in channels_with_chain],
+            (
+                chained_cpid_by_ch,
+                seed_cpid_by_ch,
+                seed_value_by_ch,
+            ) = resolve_delta_chains(
+                channels=channels,
+                supersteps_by_ch=supersteps_by_ch,
+                shared_cpid_chain=shared_cpid_chain,
+                seed_values_by_depth=seed_values_by_depth,
+                has_reached_root=bool(walk_state.get("reached_root")),
+                thread_id=thread_id,
             )
-            if stage2_sql:
-                stage2_params: list[Any] = []
+
+            channels_with_chain = [ch for ch in channels if chained_cpid_by_ch[ch]]
+            fetch_sql = build_delta_writes_fetch_sql(
+                chain_lens=[len(chained_cpid_by_ch[ch]) for ch in channels_with_chain],
+            )
+            if fetch_sql:
+                fetch_params: list[Any] = []
                 for ch in channels_with_chain:
-                    stage2_params.extend(
-                        [thread_id, checkpoint_ns, ch, *chain_by_ch[ch]]
+                    fetch_params.extend(
+                        [thread_id, checkpoint_ns, ch, *chained_cpid_by_ch[ch]]
                     )
-                await cur.execute(stage2_sql, stage2_params)
+                await cur.execute(fetch_sql, fetch_params)
                 stage2_rows = cast(
                     "list[tuple[str, str, str, int, str, bytes]]",
                     await cur.fetchall(),
@@ -689,9 +728,9 @@ class AsyncSqliteSaver(BaseCheckpointSaver[str]):
 
         return build_delta_channels_writes_history(
             channels=channels,
-            chain_by_ch=chain_by_ch,
-            seed_val_by_ch=seed_val_by_ch,
-            seeded=seeded,
+            chained_cpid_by_ch=chained_cpid_by_ch,
+            seed_cpid_by_ch=seed_cpid_by_ch,
+            seed_value_by_ch=seed_value_by_ch,
             stage2_rows=stage2_rows,
             serde=self.serde,
         )