fix(pregel): skip scheduler writes on interrupt

fix(pregel): persist pending writes on interrupt
2026-09-06 17:57:49 +02:00 · 2026-06-23 14:13:43 -07:00 · 2026-06-23 13:43:04 -07:00
26 changed files with 858 additions and 1733 deletions
@@ -126,8 +126,8 @@ jobs:
            exit 1
          fi
          LANGCHAIN_ANTHROPIC_VERSION=$(docker run --rm --entrypoint "" langgraph-test-h python -c "import sys; from importlib.metadata import version; v = version('langchain-anthropic'); print(v);")
-          if [ "$LANGCHAIN_ANTHROPIC_VERSION" != "1.4.6" ]; then
-            echo "LANGCHAIN_ANTHROPIC_VERSION != 1.4.6; $LANGCHAIN_ANTHROPIC_VERSION"
+          if [ "$LANGCHAIN_ANTHROPIC_VERSION" != "1.0.0a5" ]; then
+            echo "LANGCHAIN_ANTHROPIC_VERSION != 1.0.0a5; $LANGCHAIN_ANTHROPIC_VERSION"
            exit 1
          fi

@@ -54,12 +54,6 @@ async def build_delta_chain(
    stored: list[RunnableConfig] = []
    parent_cfg: RunnableConfig | None = None

-    # Track supersteps since the channel's last snapshot, mirroring Pregel's
-    # `counters_since_delta_snapshot` bookkeeping: every step increments the
-    # counter; a snapshot step resets it (and carries no counter entry, so
-    # the walk locates the seed exactly `supersteps` hops back).
-    supersteps_since_snapshot = 0
-
    for step in range(total_steps):
        config: RunnableConfig = {
            "configurable": {
@@ -74,19 +68,11 @@ async def build_delta_chain(

        channel_values: dict[str, Any] = {}
        channel_versions: dict[str, int] = {}
-        supersteps_since_snapshot += 1
-        counters: dict[str, tuple[int, int]] = {}
        if step in snapshot_set:
            channel_values[channel] = _DeltaSnapshot(
                write_value_fn(step),
            )
            channel_versions[channel] = step + 1
-            supersteps_since_snapshot = 0
-        else:
-            counters[channel] = (
-                supersteps_since_snapshot,
-                supersteps_since_snapshot,
-            )

        cp = Checkpoint(
            v=1,
@@ -98,10 +84,9 @@ async def build_delta_chain(
            updated_channels=None,
        )
        new_versions = dict(channel_versions)
-        md = generate_metadata(step=step)
-        if counters:
-            md["counters_since_delta_snapshot"] = counters
-        parent_cfg = await saver.aput(config, cp, md, new_versions)
+        parent_cfg = await saver.aput(
+            config, cp, generate_metadata(step=step), new_versions
+        )
        stored.append(parent_cfg)

        # Write a pending write for non-snapshot steps so the walk has
@@ -87,10 +87,6 @@ async def test_history_multi_channel(

    from langgraph.checkpoint.conformance.test_utils import generate_metadata

-    # Per-channel supersteps since last snapshot ("a" snapshots at step 1,
-    # "b" at step 3). Drives the per-channel walk depth from the head.
-    s_a = 0
-    s_b = 0
    for step in range(5):
        config = {"configurable": {"thread_id": tid, "checkpoint_ns": ""}}
        if parent_cfg:
@@ -99,21 +95,12 @@ async def test_history_multi_channel(
            ]
        cv: dict = {}
        cvs: dict = {}
-        s_a += 1
-        s_b += 1
-        counters: dict = {}
        if step == 1:
            cv["a"] = _DeltaSnapshot("snap_a")
            cvs["a"] = step + 1
-            s_a = 0
-        else:
-            counters["a"] = (s_a, s_a)
        if step == 3:
            cv["b"] = _DeltaSnapshot("snap_b")
            cvs["b"] = step + 1
-            s_b = 0
-        else:
-            counters["b"] = (s_b, s_b)
        cp = Checkpoint(
            v=1,
            id=str(uuid6(clock_seq=-1)),
@@ -123,10 +110,7 @@ async def test_history_multi_channel(
            versions_seen={},
            updated_channels=None,
        )
-        md = generate_metadata(step=step)
-        if counters:
-            md["counters_since_delta_snapshot"] = counters
-        parent_cfg = await saver.aput(config, cp, md, cvs)
+        parent_cfg = await saver.aput(config, cp, generate_metadata(step=step), cvs)
        configs.append(parent_cfg)
        await saver.aput_writes(parent_cfg, [("a", step), ("b", step)], str(uuid4()))

@@ -164,12 +148,7 @@ async def test_history_walk_to_root_no_seed(
    )
    head = configs[-1]
    result = await saver.aget_delta_channel_history(config=head, channels=["ch"])
-    # The head's supersteps counter (4) runs one hop past the real root —
-    # the snapshot checkpoint was never persisted (implicit empty baseline).
-    # No seed, and the full chain (steps 0..2) replays from empty.
    assert "seed" not in result["ch"], f"Expected no seed, got {result['ch']}"
-    values = [w[2] for w in result["ch"]["writes"]]
-    assert values == [0, 1, 2], f"Expected full-chain writes [0,1,2], got {values}"


 async def test_history_migration_plain_value_as_seed(
@@ -191,10 +170,6 @@ async def test_history_migration_plain_value_as_seed(
    configs: list = []
    parent_cfg = None

-    # The channel was a non-delta channel through step 1 (plain value, no
-    # delta counter), then migrated to DeltaChannel at step 2. Supersteps
-    # count from the migration boundary: step 2 -> 1, step 3 -> 2, so the
-    # head (step 3) walks 2 hops back to the plain-value seed at step 1.
    for step in range(4):
        config = {"configurable": {"thread_id": tid, "checkpoint_ns": ""}}
        if parent_cfg:
@@ -207,9 +182,6 @@ async def test_history_migration_plain_value_as_seed(
        if step == 1:
            cv["ch"] = [10, 20, 30]
            cvs["ch"] = step + 1
-        md = generate_metadata(step=step)
-        if step >= 2:
-            md["counters_since_delta_snapshot"] = {"ch": (step - 1, step - 1)}
        cp = Checkpoint(
            v=1,
            id=str(uuid6(clock_seq=-1)),
@@ -219,7 +191,7 @@ async def test_history_migration_plain_value_as_seed(
            versions_seen={},
            updated_channels=None,
        )
-        parent_cfg = await saver.aput(config, cp, md, cvs)
+        parent_cfg = await saver.aput(config, cp, generate_metadata(step=step), cvs)
        configs.append(parent_cfg)
        if step != 1:
            await saver.aput_writes(parent_cfg, [("ch", step)], str(uuid4()))
@@ -236,67 +208,6 @@ async def test_history_migration_plain_value_as_seed(
    assert values == [2], f"Expected [2], got {values}"


-async def test_history_migration_skips_seed_checkpoint_writes(
-    saver: BaseCheckpointSaver,
-) -> None:
-    """A migrated plain-value seed already incorporates its own checkpoint's
-    writes, so those writes must NOT be re-emitted in the history.
-
-    This is the discriminating case for non-additive reducers (e.g. an
-    even-only filter): re-applying the seed checkpoint's writes on top of
-    the seed would corrupt the reconstructed value. The saver contract is
-    to skip them — only writes strictly after the seed are returned.
-    """
-    from langgraph.checkpoint.base import Checkpoint
-    from langgraph.checkpoint.base.id import uuid6
-
-    from langgraph.checkpoint.conformance.test_utils import generate_metadata
-
-    tid = str(uuid4())
-    configs: list = []
-    parent_cfg = None
-
-    for step in range(4):
-        config = {"configurable": {"thread_id": tid, "checkpoint_ns": ""}}
-        if parent_cfg:
-            config["configurable"]["checkpoint_id"] = parent_cfg["configurable"][
-                "checkpoint_id"
-            ]
-        cv: dict = {}
-        cvs: dict = {}
-        # Step 1: pre-delta plain accumulated value AND its own writes.
-        if step == 1:
-            cv["ch"] = [2, 4]
-            cvs["ch"] = step + 1
-        md = generate_metadata(step=step)
-        if step >= 2:
-            md["counters_since_delta_snapshot"] = {"ch": (step - 1, step - 1)}
-        cp = Checkpoint(
-            v=1,
-            id=str(uuid6(clock_seq=-1)),
-            ts="",
-            channel_values=cv,
-            channel_versions=cvs,
-            versions_seen={},
-            updated_channels=None,
-        )
-        parent_cfg = await saver.aput(config, cp, md, cvs)
-        configs.append(parent_cfg)
-        # The seed checkpoint (step 1) carries writes that the plain value
-        # already subsumes; later steps carry post-migration delta writes.
-        write_value = 99 if step == 1 else step * 10
-        await saver.aput_writes(parent_cfg, [("ch", write_value)], str(uuid4()))
-
-    head = configs[-1]
-    result = await saver.aget_delta_channel_history(config=head, channels=["ch"])
-    assert result["ch"].get("seed") == [2, 4], (
-        f"Expected plain seed [2, 4], got {result['ch'].get('seed')}"
-    )
-    values = [w[2] for w in result["ch"]["writes"]]
-    # Seed checkpoint's own write (99) is skipped; step 2's write (20) kept.
-    assert values == [20], f"Expected only post-seed writes [20], got {values}"
-
-
 ALL_DELTA_CHANNEL_HISTORY_TESTS = [
    test_history_returns_writes_oldest_first,
    test_history_seed_is_nearest_snapshot,
@@ -305,7 +216,6 @@ ALL_DELTA_CHANNEL_HISTORY_TESTS = [
    test_history_empty_channels_returns_empty,
    test_history_walk_to_root_no_seed,
    test_history_migration_plain_value_as_seed,
-    test_history_migration_skips_seed_checkpoint_writes,
 ]


@@ -370,7 +370,7 @@ test = [

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+version = "0.8.2"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
    { name = "httpx" },
@@ -380,13 +380,12 @@ dependencies = [
    { name = "requests" },
    { name = "requests-toolbelt" },
    { name = "uuid-utils" },
-    { name = "websockets" },
    { name = "xxhash" },
    { name = "zstandard" },
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+    { url = "https://files.pythonhosted.org/packages/56/3e/12ab2c357593ca1652df6dddbd763cff9ea2c7155a922910ccba6ac1530e/langsmith-0.8.2-py3-none-any.whl", hash = "sha256:4ff80d7dc1b273315401b681aef9b1fc92f4fa8a6d9d49eb65535520f8264fd4", size = 397520, upload-time = "2026-05-06T17:36:23.064Z" },
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 ]

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-
 [[package]]
 name = "xxhash"
 version = "3.7.0"
@@ -14,7 +14,6 @@ from langgraph.checkpoint.base import (
    CheckpointMetadata,
    CheckpointTuple,
    DeltaChannelHistory,
-    _parse_supersteps_since_last_snapshot_by_channel,
    get_checkpoint_id,
    get_serializable_checkpoint_metadata,
 )
@@ -29,11 +28,9 @@ from langgraph.checkpoint.postgres import _internal
 from langgraph.checkpoint.postgres.base import (
    _DELTA_PAGE_SIZE,
    BasePostgresSaver,
-    _advance_shared_chain,
-    _build_delta_fetch_sql,
-    _build_delta_walk_sql,
+    _build_delta_stage1_sql,
+    _build_delta_stage2_sql,
    _DeltaStage2Row,
-    _ingest_walk_page,
 )
 from langgraph.checkpoint.postgres.shallow import ShallowPostgresSaver

@@ -449,137 +446,107 @@ class PostgresSaver(BasePostgresSaver):
    ) -> Mapping[str, DeltaChannelHistory]:
        """Fast-path override of `BaseCheckpointSaver.get_delta_channel_history`.

-        Reconstructs each delta channel's state for the target checkpoint by
-        walking the parent chain `supersteps` hops (read from the target's
-        `counters_since_delta_snapshot` metadata) to its seed snapshot, then
-        collecting the writes between the seed and the target.
+        Two-stage query, both stages cover ALL requested channels:

-        Two passes (see the `# Multi-channel two-pass` comment in `base.py`):
+        * Stage 1 (paged): dynamic SELECT over `checkpoints` with K parallel
+          JSONB key lookups (one column pair per channel) — no subquery, no
+          aggregation. Pages newest-first by `checkpoint_id` with a cursor;
+          page size is `_DELTA_PAGE_SIZE`. Stops paging when every channel
+          has found its seed or the chain is exhausted.

-        * WALK (paged): dynamic SELECT over `checkpoints` with K parallel
-          JSONB lookups for `channel_versions[ch]` (the seed blob version),
-          following `parent_checkpoint_id` newest-first. Page size is
-          `_DELTA_PAGE_SIZE`; stops once the shared chain reaches the deepest
-          requested `supersteps`, the root is reached, or the chain is
-          exhausted.
-
-        * FETCH (per-channel UNION ALL): one branch per channel reading
-          `checkpoint_writes` for its `chain_cids`, plus one branch per
-          channel that located a seed reading `checkpoint_blobs` at the
-          seed's version.
+        * Stage 2 (per-channel UNION ALL): one branch per channel reading
+          `checkpoint_writes` filtered to that channel's specific
+          `chain_cids`, plus one branch per channel that has a seed reading
+          `checkpoint_blobs` for that channel + version. Avoids the
+          over-fetch of a single `channel = ANY(channels)` filter when
+          channels have different chain depths.
        """
        if not channels:
            return {}
        channels = list(channels)
        thread_id = config["configurable"]["thread_id"]
-        if not thread_id:
-            raise ValueError("empty thread ID")
        checkpoint_ns = config["configurable"].get("checkpoint_ns", "")
-
-        # Resolve the target checkpoint id + its metadata (for supersteps).
        checkpoint_id = get_checkpoint_id(config)
-        with self._cursor() as cur:
-            if checkpoint_id is None:
-                cur.execute(
-                    "SELECT checkpoint_id, metadata FROM checkpoints "
-                    "WHERE thread_id = %s AND checkpoint_ns = %s "
-                    "ORDER BY checkpoint_id DESC LIMIT 1",
-                    (thread_id, checkpoint_ns),
-                )
-            else:
-                cur.execute(
-                    "SELECT checkpoint_id, metadata FROM checkpoints "
-                    "WHERE thread_id = %s AND checkpoint_ns = %s "
-                    "AND checkpoint_id = %s",
-                    (thread_id, checkpoint_ns, checkpoint_id),
-                )
-            target_row = cur.fetchone()
-        if target_row is None:
-            return {ch: {"writes": []} for ch in channels}
-        target_id = cast(str, target_row["checkpoint_id"])
-        supersteps_by_ch = _parse_supersteps_since_last_snapshot_by_channel(
-            target_row["metadata"] or {}, channels
-        )
-        max_supersteps = max(supersteps_by_ch.values(), default=0)
+        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"]

-        # WALK: page the parent chain, bounded by the deepest supersteps.
-        walk_sql = _build_delta_walk_sql(channels)
+        # Stage 1: paged K-JSONB-lookup scan, walking the parent chain in
+        # Python after each page. Stops as soon as every channel has its seed.
+        stage1_sql = _build_delta_stage1_sql(channels, paged=True)
        parent_of: dict[str, str | None] = {}
        ver_by_i_by_cid: list[dict[str, str | None]] = [{} for _ in channels]
-        shared_cpid_chain: list[str] = []
-        has_reached_root = False
+        hs_by_i_by_cid: list[dict[str, bool]] = [{} for _ in channels]
+        chain_by_ch: dict[str, list[str]] = {ch: [] for ch in channels}
+        seed_ver_by_ch: dict[str, str | None] = {ch: None for ch in channels}
+        walk_cursor_by_ch: dict[str, str | None] = {}
+        seeded: set[str] = set()
        cursor: str | None = None

-        while max_supersteps > 0:
-            walk_params: list[Any] = [
-                *channels,
-                thread_id,
-                checkpoint_ns,
-                cursor,
-                cursor,
-                _DELTA_PAGE_SIZE,
-            ]
-            with self._cursor() as cur:
-                cur.execute(walk_sql, walk_params)
+        with self._cursor() as cur:
+            while True:
+                stage1_params: list[Any] = []
+                for ch in channels:
+                    stage1_params.extend([ch, ch])
+                stage1_params.extend(
+                    [thread_id, checkpoint_ns, cursor, cursor, _DELTA_PAGE_SIZE]
+                )
+                cur.execute(stage1_sql, stage1_params)
                page = cur.fetchall()
-            if not page:
-                break
-            oldest = _ingest_walk_page(
-                cast("list[Mapping[str, Any]]", page),
-                channels,
-                parent_of,
-                ver_by_i_by_cid,
-            )
-            has_reached_root = _advance_shared_chain(
-                target_id, parent_of, shared_cpid_chain, max_supersteps
-            )
-            if (
-                has_reached_root
-                or len(shared_cpid_chain) >= max_supersteps
-                or len(page) < _DELTA_PAGE_SIZE
-            ):
-                break
-            cursor = oldest
+                if not page:
+                    break
+                oldest = self._ingest_stage1_page(
+                    cast("list[Mapping[str, Any]]", page),
+                    channels,
+                    parent_of,
+                    ver_by_i_by_cid,
+                    hs_by_i_by_cid,
+                )
+                self._try_advance_walks(
+                    checkpoint_id,
+                    channels,
+                    parent_of,
+                    ver_by_i_by_cid,
+                    hs_by_i_by_cid,
+                    chain_by_ch,
+                    seed_ver_by_ch,
+                    walk_cursor_by_ch,
+                    seeded,
+                )
+                # Stop if every channel is seeded, or the page was short
+                # (chain exhausted — no more rows to fetch).
+                if len(seeded) == len(channels) or len(page) < _DELTA_PAGE_SIZE:
+                    break
+                cursor = oldest

-        chained_cpid_by_ch, seed_cpid_by_ch, seed_ver_by_ch = (
-            self._resolve_delta_chains(
-                channels,
-                supersteps_by_ch,
-                shared_cpid_chain,
-                ver_by_i_by_cid,
-                has_reached_root,
-                thread_id,
-            )
-        )
-
-        # FETCH: per-channel UNION ALL — writes branch per chained channel,
-        # blob branch per seeded channel.
-        channels_with_chain = [ch for ch in channels if chained_cpid_by_ch[ch]]
+        # Stage 2: per-channel UNION ALL — one writes branch per channel
+        # with non-empty chain, plus one blob branch per seeded channel.
+        channels_with_chain = [ch for ch in channels if chain_by_ch[ch]]
        channels_with_seed = [ch for ch in channels if seed_ver_by_ch[ch] is not None]
-        fetch_sql = _build_delta_fetch_sql(
+        stage2_sql = _build_delta_stage2_sql(
            channels_with_chain=channels_with_chain,
            channels_with_seed=channels_with_seed,
        )
-        if fetch_sql:
-            fetch_params: list[Any] = []
-            for ch in channels_with_chain:
-                fetch_params.extend(
-                    [thread_id, checkpoint_ns, ch, chained_cpid_by_ch[ch]]
-                )
-            for ch in channels_with_seed:
-                fetch_params.extend([thread_id, checkpoint_ns, ch, seed_ver_by_ch[ch]])
-            with self._cursor() as cur:
-                cur.execute(fetch_sql, fetch_params)
-                fetch_rows = cur.fetchall()
-        else:
-            fetch_rows = []

-        return self._assemble_delta_history(
+        if stage2_sql:
+            stage2_params: list[Any] = []
+            for ch in channels_with_chain:
+                stage2_params.extend([thread_id, checkpoint_ns, ch, chain_by_ch[ch]])
+            for ch in channels_with_seed:
+                stage2_params.extend([thread_id, checkpoint_ns, ch, seed_ver_by_ch[ch]])
+            with self._cursor() as cur:
+                cur.execute(stage2_sql, stage2_params)
+                stage2_rows = cur.fetchall()
+        else:
+            stage2_rows = []
+
+        return self._build_delta_channels_writes_history(
            channels=channels,
-            chained_cpid_by_ch=chained_cpid_by_ch,
-            seed_cpid_by_ch=seed_cpid_by_ch,
+            chain_by_ch=chain_by_ch,
            seed_ver_by_ch=seed_ver_by_ch,
-            fetch_rows=cast("list[_DeltaStage2Row]", fetch_rows),
+            stage2_rows=cast("list[_DeltaStage2Row]", stage2_rows),
        )

    def _load_checkpoint_tuple(self, value: DictRow) -> CheckpointTuple:
@@ -14,7 +14,6 @@ from langgraph.checkpoint.base import (
    CheckpointMetadata,
    CheckpointTuple,
    DeltaChannelHistory,
-    _parse_supersteps_since_last_snapshot_by_channel,
    get_checkpoint_id,
    get_serializable_checkpoint_metadata,
 )
@@ -29,11 +28,9 @@ from langgraph.checkpoint.postgres import _ainternal
 from langgraph.checkpoint.postgres.base import (
    _DELTA_PAGE_SIZE,
    BasePostgresSaver,
-    _advance_shared_chain,
-    _build_delta_fetch_sql,
-    _build_delta_walk_sql,
+    _build_delta_stage1_sql,
+    _build_delta_stage2_sql,
    _DeltaStage2Row,
-    _ingest_walk_page,
 )
 from langgraph.checkpoint.postgres.shallow import AsyncShallowPostgresSaver

@@ -411,122 +408,89 @@ class AsyncPostgresSaver(BasePostgresSaver):
        """Fast-path override of `BaseCheckpointSaver.aget_delta_channel_history`.

        See `PostgresSaver.get_delta_channel_history` for design notes; this is
-        the async equivalent: paged supersteps-bounded WALK + per-channel
-        UNION ALL FETCH.
+        the async equivalent with internal stage-1 paging and per-channel
+        UNION ALL stage-2.
        """
        if not channels:
            return {}
        channels = list(channels)
        thread_id = config["configurable"]["thread_id"]
-        if not thread_id:
-            raise ValueError("empty thread ID")
        checkpoint_ns = config["configurable"].get("checkpoint_ns", "")
-
-        # Resolve the target checkpoint id + its metadata (for supersteps).
        checkpoint_id = get_checkpoint_id(config)
-        async with self._cursor() as cur:
-            if checkpoint_id is None:
-                await cur.execute(
-                    "SELECT checkpoint_id, metadata FROM checkpoints "
-                    "WHERE thread_id = %s AND checkpoint_ns = %s "
-                    "ORDER BY checkpoint_id DESC LIMIT 1",
-                    (thread_id, checkpoint_ns),
-                )
-            else:
-                await cur.execute(
-                    "SELECT checkpoint_id, metadata FROM checkpoints "
-                    "WHERE thread_id = %s AND checkpoint_ns = %s "
-                    "AND checkpoint_id = %s",
-                    (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 = cast(str, target_row["checkpoint_id"])
-        supersteps_by_ch = _parse_supersteps_since_last_snapshot_by_channel(
-            target_row["metadata"] or {}, channels
-        )
-        max_supersteps = max(supersteps_by_ch.values(), default=0)
+        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"]

-        # WALK: page the parent chain, bounded by the deepest supersteps.
-        walk_sql = _build_delta_walk_sql(channels)
+        stage1_sql = _build_delta_stage1_sql(channels, paged=True)
        parent_of: dict[str, str | None] = {}
        ver_by_i_by_cid: list[dict[str, str | None]] = [{} for _ in channels]
-        shared_cpid_chain: list[str] = []
-        has_reached_root = False
+        hs_by_i_by_cid: list[dict[str, bool]] = [{} for _ in channels]
+        chain_by_ch: dict[str, list[str]] = {ch: [] for ch in channels}
+        seed_ver_by_ch: dict[str, str | None] = {ch: None for ch in channels}
+        walk_cursor_by_ch: dict[str, str | None] = {}
+        seeded: set[str] = set()
        cursor: str | None = None

-        while max_supersteps > 0:
-            walk_params: list[Any] = [
-                *channels,
-                thread_id,
-                checkpoint_ns,
-                cursor,
-                cursor,
-                _DELTA_PAGE_SIZE,
-            ]
-            async with self._cursor() as cur:
-                await cur.execute(walk_sql, walk_params)
+        async with self._cursor() as cur:
+            while True:
+                stage1_params: list[Any] = []
+                for ch in channels:
+                    stage1_params.extend([ch, ch])
+                stage1_params.extend(
+                    [thread_id, checkpoint_ns, cursor, cursor, _DELTA_PAGE_SIZE]
+                )
+                await cur.execute(stage1_sql, stage1_params)
                page = await cur.fetchall()
-            if not page:
-                break
-            oldest = _ingest_walk_page(
-                cast("list[Mapping[str, Any]]", page),
-                channels,
-                parent_of,
-                ver_by_i_by_cid,
-            )
-            has_reached_root = _advance_shared_chain(
-                target_id, parent_of, shared_cpid_chain, max_supersteps
-            )
-            if (
-                has_reached_root
-                or len(shared_cpid_chain) >= max_supersteps
-                or len(page) < _DELTA_PAGE_SIZE
-            ):
-                break
-            cursor = oldest
+                if not page:
+                    break
+                oldest = self._ingest_stage1_page(
+                    cast("list[Mapping[str, Any]]", page),
+                    channels,
+                    parent_of,
+                    ver_by_i_by_cid,
+                    hs_by_i_by_cid,
+                )
+                self._try_advance_walks(
+                    checkpoint_id,
+                    channels,
+                    parent_of,
+                    ver_by_i_by_cid,
+                    hs_by_i_by_cid,
+                    chain_by_ch,
+                    seed_ver_by_ch,
+                    walk_cursor_by_ch,
+                    seeded,
+                )
+                if len(seeded) == len(channels) or len(page) < _DELTA_PAGE_SIZE:
+                    break
+                cursor = oldest

-        chained_cpid_by_ch, seed_cpid_by_ch, seed_ver_by_ch = (
-            self._resolve_delta_chains(
-                channels,
-                supersteps_by_ch,
-                shared_cpid_chain,
-                ver_by_i_by_cid,
-                has_reached_root,
-                thread_id,
-            )
-        )
-
-        # FETCH: per-channel UNION ALL — writes branch per chained channel,
-        # blob branch per seeded channel.
-        channels_with_chain = [ch for ch in channels if chained_cpid_by_ch[ch]]
+        channels_with_chain = [ch for ch in channels if chain_by_ch[ch]]
        channels_with_seed = [ch for ch in channels if seed_ver_by_ch[ch] is not None]
-        fetch_sql = _build_delta_fetch_sql(
+        stage2_sql = _build_delta_stage2_sql(
            channels_with_chain=channels_with_chain,
            channels_with_seed=channels_with_seed,
        )

-        if fetch_sql:
-            fetch_params: list[Any] = []
+        if stage2_sql:
+            stage2_params: list[Any] = []
            for ch in channels_with_chain:
-                fetch_params.extend(
-                    [thread_id, checkpoint_ns, ch, chained_cpid_by_ch[ch]]
-                )
+                stage2_params.extend([thread_id, checkpoint_ns, ch, chain_by_ch[ch]])
            for ch in channels_with_seed:
-                fetch_params.extend([thread_id, checkpoint_ns, ch, seed_ver_by_ch[ch]])
+                stage2_params.extend([thread_id, checkpoint_ns, ch, seed_ver_by_ch[ch]])
            async with self._cursor() as cur:
-                await cur.execute(fetch_sql, fetch_params)
-                fetch_rows = await cur.fetchall()
+                await cur.execute(stage2_sql, stage2_params)
+                stage2_rows = await cur.fetchall()
        else:
-            fetch_rows = []
+            stage2_rows = []

-        return self._assemble_delta_history(
+        return self._build_delta_channels_writes_history(
            channels=channels,
-            chained_cpid_by_ch=chained_cpid_by_ch,
-            seed_cpid_by_ch=seed_cpid_by_ch,
+            chain_by_ch=chain_by_ch,
            seed_ver_by_ch=seed_ver_by_ch,
-            fetch_rows=cast("list[_DeltaStage2Row]", fetch_rows),
+            stage2_rows=cast("list[_DeltaStage2Row]", stage2_rows),
        )

    async def _load_checkpoint_tuple(self, value: DictRow) -> CheckpointTuple:
@@ -1,6 +1,5 @@
 from __future__ import annotations

-import logging
 import random
 import warnings
 from collections.abc import Mapping, Sequence
@@ -16,12 +15,10 @@ from langgraph.checkpoint.base import (
    PendingWrite,
    get_checkpoint_id,
 )
-from langgraph.checkpoint.serde.types import TASKS, _DeltaSnapshot
+from langgraph.checkpoint.serde.types import TASKS
 from psycopg.types.json import Jsonb

-logger = logging.getLogger(__name__)
-
-# Page size for the paged WALK scan in `get_delta_channel_history`. Internal
+# Page size for stage-1 paged scan in `get_delta_channel_history`. Internal
 # constant — exposing this as a kwarg is left as a follow-up.
 _DELTA_PAGE_SIZE = 1024

@@ -163,7 +160,7 @@ INSERT_CHECKPOINT_WRITES_SQL = """


 class _DeltaStage2Row(TypedDict, total=False):
-    """One row from `_build_delta_fetch_sql` (a UNION ALL of writes and blobs)."""
+    """One row from `_build_delta_stage2_sql` (a UNION ALL of writes and blobs)."""

    _kind: str  # "w" or "b"
    checkpoint_id: str | None  # "w" rows only
@@ -175,58 +172,42 @@ class _DeltaStage2Row(TypedDict, total=False):
    version: str | None  # "b" rows only


-# Multi-channel two-pass DeltaChannel reconstruction.
+# Multi-channel two-stage DeltaChannel reconstruction.
 #
-# A `DeltaChannel` does not store its full value at every checkpoint — it
-# stores periodic full-value *snapshots* and accumulates intermediate
-# *writes* between snapshots. To rebuild a channel's value at a target
-# checkpoint we need:
+# Stage 1 scans checkpoint metadata (no blob bytes) and emits one row per
+# checkpoint with K parallel JSONB key lookups (one column pair per
+# requested delta channel: ver_i / hs_i).  No subqueries, no aggregation.
+# Python walks the parent chain once across all channels.
 #
-#   - the **seed** — the most recent snapshot at-or-before the target
-#     (a single blob row in `checkpoint_blobs`); and
-#   - the **chain writes** — every write for this channel committed
-#     between that snapshot and the target, in order
-#     (rows in `checkpoint_writes`).
+# Stage 2 fetches all writes and the seed blobs for ALL channels in a
+# single roundtrip via `channel = ANY(%s)` and chain/seed-version
+# filtering.
 #
-# Two passes, in order:
+# Empirical comparison vs an alternative "ship full channel_versions /
+# channel_values JSONB and let Python pick" form (1000 checkpoints,
+# 8 total channels in graph, 3 delta channels requested):
 #
-#   1. WALK — scan checkpoint metadata only (no blob bytes). For each
-#      requested channel we read `channel_versions[ch]` (the seed's blob
-#      version pointer). Then follow `parent_checkpoint_id` from the
-#      target backwards in pages of `_DELTA_PAGE_SIZE` rows.
+#   Postgres execution:    A=0.24ms vs B=0.38ms   (both negligible)
+#   End-to-end latency:    A=6.83ms vs B=2.28ms   (B is 3.0x faster)
+#   Wire payload:          A=836KB  vs B=330KB    (61% smaller)
+#   Buffer hits:           identical (167 blocks)
 #
-#      Walk depth is driven by the *supersteps since last snapshot*
-#      counter — `metadata.counters_since_delta_snapshot[ch][1]` — read
-#      from the target checkpoint. A channel's seed snapshot sits exactly
-#      `supersteps` hops back along the parent chain; walking by the
-#      counter (rather than scanning `channel_values` for the snapshot
-#      marker) is the only reliable way to locate seeds that aren't a
-#      `_DeltaSnapshot` sentinel — e.g. legacy plain-value blobs left by a
-#      thread that migrated from a non-delta channel, which `put` stores
-#      out of the inline `channel_values` map.
-#
-#   2. FETCH — given each channel's chained checkpoint ids and seed
-#      version from WALK, pull only the rows we need: writes for those
-#      exact checkpoint_ids and the seed blob at that exact version. One
-#      roundtrip, per-channel UNION ALL — no over-fetch.
-#
-# Walking the *parent chain* (not `list(before=...)`) matters: forked
-# threads have multiple branches, and only on-path ancestors contribute.
+# B (this dynamic-columns design) wins because it avoids JSONB
+# serialization on the wire and JSONB-to-dict deserialization in
+# psycopg.  Even at K=8 (8 delta channels = 16 dynamic columns), B
+# still beats A end-to-end (4.2ms vs 6.8ms).


-def _build_delta_walk_sql(channels: Sequence[str]) -> str:
-    """Build the paged WALK SQL — scans checkpoint metadata only.
+def _build_delta_stage1_sql(channels: Sequence[str], *, paged: bool) -> str:
+    """Build stage 1 SQL with 2K parallel JSONB key lookups.

-    Emits one row per checkpoint with K parallel JSONB key lookups (one
-    `ver_i` column per requested channel — the channel's blob version, the
-    pointer we dereference in FETCH if this checkpoint is the seed). No
-    blob bytes; the result set fits a paged `LIMIT` cleanly.
-
-    For channels=["messages", "files"] the result is::
+    For channels=["messages", "files"] (with `paged=True`) the result is::

        SELECT checkpoint_id, parent_checkpoint_id,
               checkpoint -> 'channel_versions' ->> %s AS ver_0,
-               checkpoint -> 'channel_versions' ->> %s AS ver_1
+               (checkpoint -> 'channel_values' -> %s) IS NOT NULL AS hs_0,
+               checkpoint -> 'channel_versions' ->> %s AS ver_1,
+               (checkpoint -> 'channel_values' -> %s) IS NOT NULL AS hs_1
        FROM checkpoints
        WHERE thread_id = %s AND checkpoint_ns = %s
          AND (%s::text IS NULL OR checkpoint_id < %s)
@@ -234,38 +215,45 @@ def _build_delta_walk_sql(channels: Sequence[str]) -> str:
        LIMIT %s

    Channel names are passed as `%s` parameters (safe from SQL injection).
-    Only the column aliases `ver_i` are interpolated into the SQL string
-    (i is bounded by len(channels) and uses safe identifiers).
+    Only the column aliases `ver_i` / `hs_i` are interpolated into the
+    SQL string (i is bounded by len(channels) and uses safe identifiers).

-    Caller must extend params with `[ch_0, ch_1, ..., thread_id, ns,
-    cursor, cursor, page_size]`. The `cursor` is the smallest
-    `checkpoint_id` from the previous page (or `None` on the first page);
-    `(%s::text IS NULL OR ...)` makes the first-page `WHERE` a no-op.
+    Caller must extend params with `[ch_0, ch_0, ch_1, ch_1, ...,
+    thread_id, ns, cursor, cursor, page_size]` when `paged=True`.
+
+    When `paged=False`, the WHERE has no cursor predicate and there's no
+    LIMIT/ORDER BY — kept as a non-public helper for tests/diagnostics.
    """
-    cols = [
-        f"checkpoint -> 'channel_versions' ->> %s AS ver_{i}"
-        for i in range(len(channels))
-    ]
-    return (
+    cols = []
+    for i in range(len(channels)):
+        cols.append(
+            f"checkpoint -> 'channel_versions' ->> %s AS ver_{i}, "
+            f"(checkpoint -> 'channel_values' -> %s) IS NOT NULL AS hs_{i}"
+        )
+    sql = (
        "SELECT checkpoint_id, parent_checkpoint_id, "
        + ", ".join(cols)
        + " FROM checkpoints WHERE thread_id = %s AND checkpoint_ns = %s"
-        " AND (%s::text IS NULL OR checkpoint_id < %s)"
-        " ORDER BY checkpoint_id DESC LIMIT %s"
    )
+    if paged:
+        sql += (
+            " AND (%s::text IS NULL OR checkpoint_id < %s)"
+            " ORDER BY checkpoint_id DESC LIMIT %s"
+        )
+    return sql


-def _build_delta_fetch_sql(
+def _build_delta_stage2_sql(
    *,
    channels_with_chain: Sequence[str],
    channels_with_seed: Sequence[str],
 ) -> str:
-    """Build the FETCH SQL as a per-channel UNION ALL.
+    """Build stage 2 SQL as a per-channel UNION ALL.

    For each channel with a non-empty chain, emit one branch reading
    `checkpoint_writes` for that specific channel + chain_cids. For each
    channel with a seed_version, emit one branch reading `checkpoint_blobs`
-    for that channel + version. This avoids the over-fetch of a single
+    for that channel + version. This avoids the over-fetch of the prior
    `channel = ANY(channels) AND checkpoint_id = ANY(union)` form when
    channels have different chain depths.

@@ -281,7 +269,6 @@ def _build_delta_fetch_sql(
    """
    branches: list[str] = []
    for _ in channels_with_chain:
-        # NOTE: no ORDER BY on this branch — writes are sorted in assembly.
        branches.append(
            "SELECT 'w'::text AS _kind, "
            "checkpoint_id, channel, "
@@ -301,59 +288,10 @@ def _build_delta_fetch_sql(
    return " UNION ALL ".join(branches)


-def _ingest_walk_page(
-    page_rows: Sequence[Mapping[str, Any]],
-    channels: Sequence[str],
-    parent_of: dict[str, str | None],
-    ver_by_i_by_cid: list[dict[str, str | None]],
-) -> str | None:
-    """Fold one WALK page into `parent_of` + per-channel `ver_by_cid`.
-
-    Returns the oldest checkpoint_id seen on this page (smallest, since
-    pages come back DESC). Caller uses it as the cursor for the next page
-    (`AND checkpoint_id < cursor`).
-    """
-    oldest: str | None = None
-    for r in page_rows:
-        cid = cast(str, r["checkpoint_id"])
-        parent_of[cid] = cast("str | None", r["parent_checkpoint_id"])
-        for i in range(len(channels)):
-            ver_by_i_by_cid[i][cid] = cast("str | None", r.get(f"ver_{i}"))
-        # Rows are DESC; the last one is the smallest cid in the page.
-        oldest = cid
-    return oldest
-
-
-def _advance_shared_chain(
-    target_id: str,
-    parent_of: Mapping[str, str | None],
-    shared_cpid_chain: list[str],
-    max_supersteps: int,
-) -> bool:
-    """Extend the shared parent chain as far as ingested pages allow.
-
-    The chain holds ancestors of the target, newest first: `chain[0]` is
-    the target's parent, `chain[1]` its grandparent, and so on. A single
-    chain is shared across all channels and grown to the maximum requested
-    depth; each channel later slices `chain[:supersteps]`.
-
-    Stops when:
-      - the chain reaches `max_supersteps` hops, OR
-      - the root is reached (parent is None) — returns True, OR
-      - the next ancestor's cid isn't in `parent_of` yet (waits for the
-        next page).
-
-    Returns True iff the root was reached.
-    """
-    while len(shared_cpid_chain) < max_supersteps:
-        top_of_chain = shared_cpid_chain[-1] if shared_cpid_chain else target_id
-        if top_of_chain not in parent_of:
-            return False  # wait for the next page
-        parent = parent_of[top_of_chain]
-        if parent is None:
-            return True  # hit the root
-        shared_cpid_chain.append(parent)
-    return False
+# Stage 1 rows are dynamic-shape dicts: {checkpoint_id, parent_checkpoint_id,
+# ver_0, hs_0, ver_1, hs_1, ...}.  Walking is parameterized by the channel
+# list to map indices back to channel names — no static TypedDict here.
+# `dict[str, Any]` is the practical signature.


 class BasePostgresSaver(BaseCheckpointSaver[str]):
@@ -398,92 +336,107 @@ class BasePostgresSaver(BaseCheckpointSaver[str]):
            if t.decode() != "empty"
        }

-    def _resolve_delta_chains(
-        self,
+    @staticmethod
+    def _ingest_stage1_page(
+        stage1_rows: Sequence[Mapping[str, Any]],
        channels: Sequence[str],
-        supersteps_by_ch: Mapping[str, int],
-        shared_cpid_chain: Sequence[str],
-        ver_by_i_by_cid: Sequence[Mapping[str, str | None]],
-        has_reached_root: bool,
-        thread_id: str,
-    ) -> tuple[
-        dict[str, list[str]],
-        dict[str, str | None],
-        dict[str, str | None],
-    ]:
-        """Slice the shared parent chain into per-channel chain/seed mappings.
+        parent_of: dict[str, str | None],
+        ver_by_i_by_cid: list[dict[str, str | None]],
+        hs_by_i_by_cid: list[dict[str, bool]],
+    ) -> str | None:
+        """Fold one stage-1 page into the running walk-state mappings.

-        For each channel the seed snapshot sits `supersteps` hops back, so
-        the seed checkpoint is `shared_cpid_chain[supersteps - 1]` and the
-        chain is `shared_cpid_chain[:supersteps]` (newest first).
-
-        When the chain is shorter than `supersteps` but the walk reached the
-        root, the persisted chain is "compressed" relative to the logical
-        superstep count — either because intermediate supersteps were never
-        persisted (`durability="exit"`) or because the thread never produced
-        a snapshot at all. In both cases the seed candidate is the oldest
-        persisted checkpoint (`shared_cpid_chain[-1]`): FETCH loads its blob,
-        and assembly keeps it only if non-empty (a real snapshot or migrated
-        value) — otherwise it omits `seed` and replays the full chain on an
-        empty baseline.
+        Returns the oldest checkpoint_id seen on this page (smallest, since
+        pages come back DESC). Caller uses it as the cursor for the next
+        page (`AND checkpoint_id < cursor`).
        """
-        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_ver_by_ch: dict[str, str | None] = {ch: None for ch in channels}
-        for i, ch in enumerate(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_ver_by_ch[ch] = ver_by_i_by_cid[i].get(seed_cpid_by_ch[ch])
-        return chained_cpid_by_ch, seed_cpid_by_ch, seed_ver_by_ch
+        oldest: str | None = None
+        for r in stage1_rows:
+            cid = cast(str, r["checkpoint_id"])
+            parent_of[cid] = cast("str | None", r["parent_checkpoint_id"])
+            for i in range(len(channels)):
+                ver_by_i_by_cid[i][cid] = cast("str | None", r.get(f"ver_{i}"))
+                hs_by_i_by_cid[i][cid] = bool(r.get(f"hs_{i}"))
+            # Rows are DESC; the last one is the smallest cid in the page.
+            oldest = cid
+        return oldest

-    def _assemble_delta_history(
+    @staticmethod
+    def _try_advance_walks(
+        target_id: str,
+        channels: Sequence[str],
+        parent_of: Mapping[str, str | None],
+        ver_by_i_by_cid: Sequence[Mapping[str, str | None]],
+        hs_by_i_by_cid: Sequence[Mapping[str, bool]],
+        chain_by_ch: dict[str, list[str]],
+        seed_ver_by_ch: dict[str, str | None],
+        walk_cursor_by_ch: dict[str, str | None],
+        seeded: set[str],
+    ) -> None:
+        """Advance each not-yet-seeded channel's walk as far as possible.
+
+        Uses the partial `parent_of` map accumulated so far. A walk stops
+        either because:
+          (a) it found a snapshot for its channel (channel becomes seeded),
+          (b) it reached a real root (parent_of[cid] is None — fully
+              materialized at this point), or
+          (c) the next ancestor cid isn't in `parent_of` yet (waiting for
+              a later page; the cursor stays put).
+
+        Mutates `chain_by_ch`, `seed_ver_by_ch`, `walk_cursor_by_ch`, and
+        `seeded` in place.
+        """
+        for i, ch in enumerate(channels):
+            if ch in seeded:
+                continue
+            # First-time entry: cursor starts at the target's parent.
+            if ch not in walk_cursor_by_ch:
+                walk_cursor_by_ch[ch] = parent_of.get(target_id)
+            cur_cid = walk_cursor_by_ch[ch]
+            ch_chain = chain_by_ch[ch]
+            hs_i = hs_by_i_by_cid[i]
+            ver_i = ver_by_i_by_cid[i]
+            while cur_cid is not None:
+                if cur_cid not in parent_of:
+                    # Need more pages to continue this walk.
+                    break
+                ch_chain.append(cur_cid)
+                if hs_i.get(cur_cid, False):
+                    seed_ver_by_ch[ch] = ver_i.get(cur_cid)
+                    seeded.add(ch)
+                    cur_cid = None
+                    break
+                cur_cid = parent_of[cur_cid]
+            walk_cursor_by_ch[ch] = cur_cid
+
+    def _build_delta_channels_writes_history(
        self,
        *,
        channels: Sequence[str],
-        chained_cpid_by_ch: Mapping[str, Sequence[str]],
-        seed_cpid_by_ch: Mapping[str, str | None],
+        chain_by_ch: Mapping[str, list[str]],
        seed_ver_by_ch: Mapping[str, str | None],
-        fetch_rows: Sequence[_DeltaStage2Row],
+        stage2_rows: Sequence[_DeltaStage2Row],
    ) -> dict[str, DeltaChannelHistory]:
-        """Demux FETCH rows per channel and produce per-channel histories.
+        """Demux stage 2 rows per channel; produce per-channel histories.

-        `fetch_rows` carry `channel` on every row. Write rows (`_kind = 'w'`)
-        are bucketed per channel per checkpoint; seed-blob rows
-        (`_kind = 'b'`) give each channel its snapshot value.
-
-        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). The `seed` key is
-        omitted when no seed was located or the blob is the "empty"
-        tombstone — the consumer treats absence as "start empty".
+        stage2_rows carry `channel` on every row. We build per-channel
+        `writes_by_cid` and per-channel `seed_blob` dicts, then assemble
+        a `DeltaChannelHistory` per requested channel. The `seed` key is omitted
+        when the walk reached root with no snapshot found, or when the
+        seed blob is sentinel "empty" — in both cases the consumer treats
+        absence as "start empty".
        """
        # writes_by_ch_by_cid[channel][cid] = list of (type, blob, task_id, idx)
        writes_by_ch_by_cid: dict[str, dict[str, list[tuple[str, bytes, str, int]]]] = {
            ch: {} for ch in channels
        }
-        seed_blob_by_ch: dict[str, tuple[str, bytes]] = {}
+        # seed_blob_by_ver[(channel, version)] = (type, blob)
+        seed_blob_by_ver: dict[tuple[str, str], tuple[str, bytes]] = {}

-        for r in fetch_rows:
+        for r in stage2_rows:
            ch = cast(str, r["channel"])
-            if r["_kind"] == "w":
+            kind = r["_kind"]
+            if kind == "w":
                cid = cast(str, r["checkpoint_id"])
                writes_by_ch_by_cid.setdefault(ch, {}).setdefault(cid, []).append(
                    cast(
@@ -491,39 +444,35 @@ class BasePostgresSaver(BaseCheckpointSaver[str]):
                        (r["type"], r["blob"], r["task_id"], r["idx"]),
                    )
                )
-            else:  # _kind == "b" — the seed blob for this channel.
-                seed_blob_by_ch[ch] = cast("tuple[str, bytes]", (r["type"], r["blob"]))
+            else:  # kind == "b"
+                ver = cast(str, r["version"])
+                seed_blob_by_ver[(ch, ver)] = cast(
+                    "tuple[str, bytes]", (r["type"], r["blob"])
+                )

-        # Within a checkpoint, writes apply oldest→newest by (task_id, idx).
+        # Sort writes per (channel, cid) newest-first by (task_id, idx)
        for cid_map in writes_by_ch_by_cid.values():
            for ws in cid_map.values():
-                ws.sort(key=lambda w: (w[2], w[3]))
+                ws.sort(key=lambda w: (w[2], w[3]), reverse=True)

        result: dict[str, DeltaChannelHistory] = {}
        for ch in channels:
-            entry: DeltaChannelHistory = {"writes": []}
-
-            skip_seed_checkpoint_writes = False
-            seed_blob = seed_blob_by_ch.get(ch)
-            if seed_blob is not None and seed_blob[0] != "empty":
-                seed_value = self.serde.loads_typed(seed_blob)
-                entry["seed"] = seed_value
-                # A migrated (non-delta) seed already includes the writes on
-                # its own checkpoint; a `_DeltaSnapshot` does not.
-                skip_seed_checkpoint_writes = not isinstance(seed_value, _DeltaSnapshot)
+            chain_cids = chain_by_ch.get(ch, [])
+            seed_version = seed_ver_by_ch.get(ch)

+            collected: list[PendingWrite] = []
            cid_writes = writes_by_ch_by_cid.get(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(chained_cpid_by_ch.get(ch, [])):
-                    if skip_seed_checkpoint_writes and cid == seed_cpid:
-                        continue
-                    for type_tag, write_blob, task_id, _idx in cid_writes.get(cid, []):
-                        val = self.serde.loads_typed((type_tag, write_blob))
-                        collected.append((task_id, ch, val))
-                entry["writes"] = collected
+            for cid in chain_cids:
+                for type_tag, write_blob, task_id, _idx in cid_writes.get(cid, []):
+                    val = self.serde.loads_typed((type_tag, write_blob))
+                    collected.append((task_id, ch, val))
+            collected.reverse()
+
+            entry: DeltaChannelHistory = {"writes": collected}
+            if seed_version is not None:
+                blob = seed_blob_by_ver.get((ch, seed_version))
+                if blob is not None and blob[0] != "empty":
+                    entry["seed"] = self.serde.loads_typed(blob)
            result[ch] = entry
        return result

@@ -399,7 +399,7 @@ test = [

 [[package]]
 name = "langsmith"
-version = "0.8.18"
+version = "0.8.0"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
    { name = "httpx" },
@@ -409,13 +409,12 @@ dependencies = [
    { name = "requests" },
    { name = "requests-toolbelt" },
    { name = "uuid-utils" },
-    { name = "websockets" },
    { name = "xxhash" },
    { name = "zstandard" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/9a/d9/a6681aa9847bbbc5ec21abe20a5e233b94e5edcfe39624db607ac7e8ccb4/langsmith-0.8.18.tar.gz", hash = "sha256:32dde9c0e67e053e0fb738921fc8ced768af7b8fa83d7a0e3fd63597cf8776dd", size = 4526988, upload-time = "2026-06-19T13:12:17.123Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/a8/64/95f1f013531395f4e8ed73caeee780f65c7c58fe028cb543f8937b45611b/langsmith-0.8.0.tar.gz", hash = "sha256:59fe5b2a56bbbe14a08aa76691f84b49e8675dd21e11b57d80c6db8c08bac2e3", size = 4432996, upload-time = "2026-04-30T22:13:07.341Z" }
 wheels = [
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+    { url = "https://files.pythonhosted.org/packages/f3/e1/a4be2e696c9473bb53298df398237da5674704d781d4b748ed35aeef592a/langsmith-0.8.0-py3-none-any.whl", hash = "sha256:12cc4bc5622b835a6d841964d6034df3617bdb912dae0c1381fd0a68a9b3a3ef", size = 393268, upload-time = "2026-04-30T22:13:05.56Z" },
 ]

 [[package]]
@@ -1168,74 +1167,6 @@ wheels = [
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 ]

-[[package]]
-name = "websockets"
-version = "16.0"
-source = { registry = "https://pypi.org/simple" }
-sdist = { url = "https://files.pythonhosted.org/packages/04/24/4b2031d72e840ce4c1ccb255f693b15c334757fc50023e4db9537080b8c4/websockets-16.0.tar.gz", hash = "sha256:5f6261a5e56e8d5c42a4497b364ea24d94d9563e8fbd44e78ac40879c60179b5", size = 179346, upload-time = "2026-01-10T09:23:47.181Z" }
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-
 [[package]]
 name = "xxhash"
 version = "3.6.0"
@@ -24,12 +24,10 @@ from langgraph.checkpoint.base import (
 from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer

 from langgraph.checkpoint.sqlite._delta import (
-    DELTA_WALK_SQL,
+    DELTA_STAGE1_SQL,
    build_delta_channels_writes_history,
-    build_delta_writes_fetch_sql,
-    parse_supersteps_since_last_snapshot_by_channel,
-    resolve_delta_chains,
-    step_walk_supersteps,
+    build_delta_stage2_sql,
+    step_walk_with_row,
 )
 from langgraph.checkpoint.sqlite.utils import search_where

@@ -507,106 +505,68 @@ class SqliteSaver(BaseCheckpointSaver[str]):
    ) -> Mapping[str, DeltaChannelHistory]:
        """Fast-path override of `BaseCheckpointSaver.get_delta_channel_history`.

-        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.
+        Two-stage query:

-        * WALK: stream a newest-first slice of `checkpoints` returning
+        * Stage 1 (paged): newest-first slice of `checkpoints` returning
          `(checkpoint_id, parent_checkpoint_id, type, checkpoint)` per
-          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.
+          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.

-        * 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.
+        * 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.
        """
        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", "")
-
-        # 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())
+        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"]

-        shared_cpid_chain: list[str] = []
+        chain_by_ch: dict[str, list[str]] = {ch: [] for ch in channels}
+        seed_val_by_ch: dict[str, Any] = {}
        walk_state: dict[str, Any] = {}
-        seed_values_by_depth: dict[int, dict[str, Any]] = {}
+        seeded: set[str] = set()

        with self.cursor(transaction=False) as cur:
-            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
+            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

-            (
-                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,
+            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],
            )
-
-            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] = []
+            if stage2_sql:
+                stage2_params: list[Any] = []
                for ch in channels_with_chain:
-                    fetch_params.extend(
-                        [thread_id, checkpoint_ns, ch, *chained_cpid_by_ch[ch]]
+                    stage2_params.extend(
+                        [thread_id, checkpoint_ns, ch, *chain_by_ch[ch]]
                    )
-                cur.execute(fetch_sql, fetch_params)
+                cur.execute(stage2_sql, stage2_params)
                stage2_rows = cast(
                    "list[tuple[str, str, str, int, str, bytes]]", cur.fetchall()
                )
@@ -615,9 +575,9 @@ class SqliteSaver(BaseCheckpointSaver[str]):

        return build_delta_channels_writes_history(
            channels=channels,
-            chained_cpid_by_ch=chained_cpid_by_ch,
-            seed_cpid_by_ch=seed_cpid_by_ch,
-            seed_value_by_ch=seed_value_by_ch,
+            chain_by_ch=chain_by_ch,
+            seed_val_by_ch=seed_val_by_ch,
+            seeded=seeded,
            stage2_rows=stage2_rows,
            serde=self.serde,
        )
@@ -1,50 +1,37 @@
 """Shared helpers for `get_delta_channel_history` on sqlite savers.

-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:
+Mirrors the two-stage shape of `BasePostgresSaver` (ancestor walk +
+per-channel UNION ALL writes fetch), but adapted for sqlite's
+constraints. The structural differences:

 * No JSONB — to inspect `channel_values` for a checkpoint we must
-  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.
+  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.
 * No separate blob table — `channel_values` lives inline in the
-  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.
+  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.

-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).
+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.
 """

 from __future__ import annotations

-import logging
 from collections.abc import Mapping, Sequence
 from typing import Any

-from langgraph.checkpoint.base import (
-    DeltaChannelHistory,
-    PendingWrite,
-    _parse_supersteps_since_last_snapshot_by_channel,
-)
-from langgraph.checkpoint.serde.types import _DeltaSnapshot
+from langgraph.checkpoint.base import DeltaChannelHistory, PendingWrite

-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 `<=`
+# Stage 1 streams ancestors of `target_cid` newest-first. The `<=`
 # predicate keeps target itself in the stream so we can read its
-# `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 = (
+# `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 = (
    "SELECT checkpoint_id, parent_checkpoint_id, type, checkpoint "
    "FROM checkpoints "
    "WHERE thread_id = ? AND checkpoint_ns = ? AND checkpoint_id <= ? "
@@ -52,12 +39,12 @@ DELTA_WALK_SQL = (
 )


-def build_delta_writes_fetch_sql(*, chain_lens: Sequence[int]) -> str:
-    """Per-channel UNION ALL fetching writes from `writes`.
+def build_delta_stage2_sql(*, chain_lens: Sequence[int]) -> str:
+    """Stage-2 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(?)`. Caller passes parameters in
+    equivalent of postgres's `= ANY(%s)`. Caller passes parameters in
    matching order: `[thread_id, checkpoint_ns, channel, *chain_cids]` per
    branch.

@@ -78,7 +65,7 @@ def build_delta_writes_fetch_sql(*, chain_lens: Sequence[int]) -> str:
    return " UNION ALL ".join(branches)


-def step_walk_supersteps(
+def step_walk_with_row(
    *,
    cid: str,
    parent_cid: str | None,
@@ -86,142 +73,75 @@ def step_walk_supersteps(
    blob: bytes,
    target_id: str,
    serde: Any,
-    shared_cpid_chain: list[str],
+    chain_by_ch: dict[str, list[str]],
+    seed_val_by_ch: dict[str, Any],
    walk_state: dict[str, Any],
-    max_supersteps: int,
-    needed_depths: set[int],
-    seed_values_by_depth: dict[int, dict[str, Any]],
+    seeded: set[str],
    channels: Sequence[str],
 ) -> bool:
-    """Process one streamed WALK row, extending the shared parent chain.
+    """Process one streamed stage-1 row in the merged ancestor walk.

-    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.
+    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).

-    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.
+    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.

-    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.
+    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.
    """
    if "started" not in walk_state:
        if cid == target_id:
            walk_state["started"] = True
            walk_state["cur_cid"] = parent_cid
-            if parent_cid is None:
-                walk_state["reached_root"] = True
-                return True
+            walk_state["active"] = {ch for ch in channels if ch not in seeded}
        # Not target yet (or target not present): keep streaming.
        return False
-    if len(shared_cpid_chain) >= max_supersteps:
+    active: set[str] = walk_state["active"]
+    if not active:
        return True
    if cid != walk_state["cur_cid"]:
        # Off-path row from a sibling branch — skip without deserializing.
        return False
-    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
+    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
    walk_state["cur_cid"] = parent_cid
-    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
+    return not active


 def build_delta_channels_writes_history(
    *,
    channels: Sequence[str],
-    chained_cpid_by_ch: Mapping[str, Sequence[str]],
-    seed_cpid_by_ch: Mapping[str, str | None],
-    seed_value_by_ch: Mapping[str, Any],
+    chain_by_ch: Mapping[str, list[str]],
+    seed_val_by_ch: Mapping[str, Any],
+    seeded: set[str],
    stage2_rows: Sequence[tuple[str, str, str, int, str, bytes]],
    serde: Any,
 ) -> dict[str, DeltaChannelHistory]:
-    """Demux writes rows per channel; produce per-channel histories.
+    """Demux stage-2 rows per channel; produce per-channel histories.

-    `stage2_rows` are `(checkpoint_id, channel, task_id, idx, type, value)`.
+    Stage-2 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`.

-    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".
+    `seed` is omitted when the walk reached a true root with no snapshot
+    found (channel never entered `seeded`); 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
@@ -236,26 +156,17 @@ def build_delta_channels_writes_history(

    result: dict[str, DeltaChannelHistory] = {}
    for ch in channels:
-        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)
-
+        chain_cids = chain_by_ch.get(ch, [])
        cid_writes = writes_by_ch_by_cid.get(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
+        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]
        result[ch] = entry
    return result
@@ -25,12 +25,10 @@ from langgraph.checkpoint.base import (
 from langgraph.checkpoint.serde.jsonplus import JsonPlusSerializer

 from langgraph.checkpoint.sqlite._delta import (
-    DELTA_WALK_SQL,
+    DELTA_STAGE1_SQL,
    build_delta_channels_writes_history,
-    build_delta_writes_fetch_sql,
-    parse_supersteps_since_last_snapshot_by_channel,
-    resolve_delta_chains,
-    step_walk_supersteps,
+    build_delta_stage2_sql,
+    step_walk_with_row,
 )
 from langgraph.checkpoint.sqlite.utils import search_where

@@ -627,98 +625,61 @@ 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. 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.
+        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.
        """
        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", "")
-
-        # 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())
+        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"]

-        shared_cpid_chain: list[str] = []
+        chain_by_ch: dict[str, list[str]] = {ch: [] for ch in channels}
+        seed_val_by_ch: dict[str, Any] = {}
        walk_state: dict[str, Any] = {}
-        seed_values_by_depth: dict[int, dict[str, Any]] = {}
+        seeded: set[str] = set()

        async with self.lock, self.conn.cursor() as cur:
-            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
-
-            (
-                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,
+            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

-            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],
+            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],
            )
-            if fetch_sql:
-                fetch_params: list[Any] = []
+            if stage2_sql:
+                stage2_params: list[Any] = []
                for ch in channels_with_chain:
-                    fetch_params.extend(
-                        [thread_id, checkpoint_ns, ch, *chained_cpid_by_ch[ch]]
+                    stage2_params.extend(
+                        [thread_id, checkpoint_ns, ch, *chain_by_ch[ch]]
                    )
-                await cur.execute(fetch_sql, fetch_params)
+                await cur.execute(stage2_sql, stage2_params)
                stage2_rows = cast(
                    "list[tuple[str, str, str, int, str, bytes]]",
                    await cur.fetchall(),
@@ -728,9 +689,9 @@ class AsyncSqliteSaver(BaseCheckpointSaver[str]):

        return build_delta_channels_writes_history(
            channels=channels,
-            chained_cpid_by_ch=chained_cpid_by_ch,
-            seed_cpid_by_ch=seed_cpid_by_ch,
-            seed_value_by_ch=seed_value_by_ch,
+            chain_by_ch=chain_by_ch,
+            seed_val_by_ch=seed_val_by_ch,
+            seeded=seeded,
            stage2_rows=stage2_rows,
            serde=self.serde,
        )
@@ -402,7 +402,7 @@ test = [

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-version = "0.8.18"
+version = "0.8.0"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
    { name = "httpx" },
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    { name = "requests-toolbelt" },
    { name = "uuid-utils" },
-    { name = "websockets" },
    { name = "xxhash" },
    { name = "zstandard" },
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 ]

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-
 [[package]]
 name = "xxhash"
 version = "3.6.0"
@@ -34,37 +34,6 @@ PendingWrite = tuple[str, str, Any]
 logger = logging.getLogger(__name__)


-def _parse_supersteps_since_last_snapshot_by_channel(
-    metadata: Mapping[str, Any],
-    channels: Sequence[str],
-) -> dict[str, int]:
-    """Per-channel supersteps-since-last-snapshot, parsed from metadata.
-
-    Reads `metadata.counters_since_delta_snapshot[ch]`, a `(updates,
-    supersteps)` pair, and returns `{ch: supersteps}` for channels whose
-    supersteps count is positive. Channels with no counter entry (just
-    snapshotted, or never written) are omitted — their seed, if any, is the
-    target checkpoint itself and needs no ancestor walk.
-
-    Used to drive the ancestor-walk depth in `get_delta_channel_history`:
-    a channel's seed snapshot sits exactly `supersteps` hops back along the
-    parent chain.
-    """
-    counters = metadata.get("counters_since_delta_snapshot") or {}
-    result: dict[str, int] = {}
-    for ch in channels:
-        entry = counters.get(ch)
-        if not isinstance(entry, (list, tuple)) or len(entry) < 2:
-            continue
-        try:
-            supersteps = int(entry[1])
-        except (TypeError, ValueError):
-            continue
-        if supersteps > 0:
-            result[ch] = supersteps
-    return result
-
-
 # Marked as total=False to allow for future expansion.
 class CheckpointMetadata(TypedDict, total=False):
    """Metadata associated with a checkpoint."""
@@ -650,30 +619,34 @@ class BaseCheckpointSaver(Generic[V]):
        """
        if not channels:
            return {}
-        channels = list(channels)
+        collected_by_ch: dict[str, list[PendingWrite]] = {c: [] for c in channels}
+        seed_by_ch: dict[str, Any] = {}
+        remaining: set[str] = set(channels)
        target_tuple = self.get_tuple(config)
-        if target_tuple is None:
-            return {ch: {"writes": []} for ch in channels}
-        supersteps_by_ch = _parse_supersteps_since_last_snapshot_by_channel(
-            target_tuple.metadata or {}, channels
+        cursor_config: RunnableConfig | None = (
+            target_tuple.parent_config if target_tuple else None
        )
-        max_supersteps = max(supersteps_by_ch.values(), default=0)
-
-        chain: list[CheckpointTuple] = []
-        has_reached_root = False
-        cursor_config: RunnableConfig | None = target_tuple.parent_config
-        while cursor_config is not None and len(chain) < max_supersteps:
+        while cursor_config is not None and remaining:
            tup = self.get_tuple(cursor_config)
            if tup is None:
                break
-            chain.append(tup)
-            if tup.parent_config is None:
-                has_reached_root = True
-                break
+            if tup.pending_writes:
+                for write in reversed(tup.pending_writes):
+                    ch = write[1]
+                    if ch in remaining:
+                        collected_by_ch[ch].append(write)
+            for ch in list(remaining):
+                if ch in tup.checkpoint["channel_values"]:
+                    seed_by_ch[ch] = tup.checkpoint["channel_values"][ch]
+                    remaining.discard(ch)
            cursor_config = tup.parent_config
-        return self._assemble_default_delta_history(
-            channels, supersteps_by_ch, chain, has_reached_root, config
-        )
+        result: dict[str, DeltaChannelHistory] = {}
+        for ch in channels:
+            entry: DeltaChannelHistory = {"writes": list(reversed(collected_by_ch[ch]))}
+            if ch in seed_by_ch:
+                entry["seed"] = seed_by_ch[ch]
+            result[ch] = entry
+        return result

    async def aget_delta_channel_history(
        self, *, config: RunnableConfig, channels: Sequence[str]
@@ -687,106 +660,32 @@ class BaseCheckpointSaver(Generic[V]):
        """
        if not channels:
            return {}
-        channels = list(channels)
+        collected_by_ch: dict[str, list[PendingWrite]] = {c: [] for c in channels}
+        seed_by_ch: dict[str, Any] = {}
+        remaining: set[str] = set(channels)
        target_tuple = await self.aget_tuple(config)
-        if target_tuple is None:
-            return {ch: {"writes": []} for ch in channels}
-        supersteps_by_ch = _parse_supersteps_since_last_snapshot_by_channel(
-            target_tuple.metadata or {}, channels
+        cursor_config: RunnableConfig | None = (
+            target_tuple.parent_config if target_tuple else None
        )
-        max_supersteps = max(supersteps_by_ch.values(), default=0)
-
-        chain: list[CheckpointTuple] = []
-        has_reached_root = False
-        cursor_config: RunnableConfig | None = target_tuple.parent_config
-        while cursor_config is not None and len(chain) < max_supersteps:
+        while cursor_config is not None and remaining:
            tup = await self.aget_tuple(cursor_config)
            if tup is None:
                break
-            chain.append(tup)
-            if tup.parent_config is None:
-                has_reached_root = True
-                break
+            if tup.pending_writes:
+                for write in reversed(tup.pending_writes):
+                    ch = write[1]
+                    if ch in remaining:
+                        collected_by_ch[ch].append(write)
+            for ch in list(remaining):
+                if ch in tup.checkpoint["channel_values"]:
+                    seed_by_ch[ch] = tup.checkpoint["channel_values"][ch]
+                    remaining.discard(ch)
            cursor_config = tup.parent_config
-        return self._assemble_default_delta_history(
-            channels, supersteps_by_ch, chain, has_reached_root, config
-        )
-
-    def _assemble_default_delta_history(
-        self,
-        channels: Sequence[str],
-        supersteps_by_ch: Mapping[str, int],
-        chain: Sequence[CheckpointTuple],
-        has_reached_root: bool,
-        config: RunnableConfig,
-    ) -> dict[str, DeltaChannelHistory]:
-        """Slice the walked parent chain into per-channel histories.
-
-        For each channel the seed snapshot sits `supersteps` hops back along
-        `chain` (newest first): the seed checkpoint is `chain[supersteps - 1]`
-        and its `channel_values[ch]` is the seed value. The seed checkpoint's
-        own writes are replayed on top of a `_DeltaSnapshot` seed but skipped
-        for a migrated plain-value seed (the legacy non-delta blob already
-        incorporates them).
-
-        When the chain is shorter than `supersteps` but the root was reached,
-        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 checkpoint (`chain[-1]`); if its
-        `channel_values[ch]` is absent the channel has no seed and replays
-        the full chain on an empty baseline.
-        """
-        # Imported lazily to avoid a hard checkpoint→serde-types coupling at
-        # module import; only the delta surface needs the runtime check.
-        from langgraph.checkpoint.serde.types import _DeltaSnapshot
-
-        thread_id = config["configurable"].get("thread_id")
        result: dict[str, DeltaChannelHistory] = {}
        for ch in channels:
-            entry: DeltaChannelHistory = {"writes": []}
-            bound = supersteps_by_ch.get(ch, 0)
-            if bound <= 0:
-                result[ch] = entry
-                continue
-            if len(chain) >= bound:
-                seed_depth = bound
-            elif has_reached_root:
-                seed_depth = len(chain)
-            else:
-                logger.warning(
-                    "cannot find seed snapshot for delta channel "
-                    "(thread_id=%s, channel=%s)",
-                    thread_id,
-                    ch,
-                )
-                result[ch] = entry
-                continue
-            if seed_depth <= 0:
-                result[ch] = entry
-                continue
-            chain_slice: Sequence[CheckpointTuple] = chain[:seed_depth]
-            seed_tuple: CheckpointTuple | None = chain[seed_depth - 1]
-
-            skip_seed_checkpoint_writes = False
-            channel_values = seed_tuple.checkpoint["channel_values"]
-            if ch in channel_values:
-                seed_value = channel_values[ch]
-                entry["seed"] = seed_value
-                skip_seed_checkpoint_writes = not isinstance(seed_value, _DeltaSnapshot)
-            else:
-                # No stored value at the oldest checkpoint → empty baseline,
-                # so the chain's own writes are all replayed from empty.
-                seed_tuple = None
-
-            collected: list[PendingWrite] = []
-            # Chain is newest→oldest; replay oldest→newest.
-            for tup in reversed(list(chain_slice)):
-                if skip_seed_checkpoint_writes and tup is seed_tuple:
-                    continue
-                for write in tup.pending_writes or []:
-                    if write[1] == ch:
-                        collected.append(write)
-            entry["writes"] = collected
+            entry: DeltaChannelHistory = {"writes": list(reversed(collected_by_ch[ch]))}
+            if ch in seed_by_ch:
+                entry["seed"] = seed_by_ch[ch]
            result[ch] = entry
        return result

@@ -23,7 +23,6 @@ from langgraph.checkpoint.base import (
    DeltaChannelHistory,
    PendingWrite,
    SerializerProtocol,
-    _parse_supersteps_since_last_snapshot_by_channel,
    get_checkpoint_id,
    get_checkpoint_metadata,
 )
@@ -145,20 +144,14 @@ class InMemorySaver(
    ) -> Mapping[str, DeltaChannelHistory]:
        """Override: walk the parent chain ONCE for all requested channels.

-        Walk depth is driven by the target checkpoint's
-        `counters_since_delta_snapshot[ch]` supersteps counter: each
-        channel's seed snapshot sits exactly `supersteps` hops back along
-        the parent chain. This locates seeds reliably even when they aren't
-        a `_DeltaSnapshot` sentinel — e.g. a legacy plain-value blob from a
-        thread migrated off a non-delta channel.
+        Each channel terminates independently at the nearest ancestor
+        whose stored blob is non-empty. Other channels keep walking until
+        they find their own terminator or hit the root.

-        Pre-delta plain-value seeds subsume their own checkpoint's pending
-        writes (the value already includes them), so those are skipped;
-        `_DeltaSnapshot` seeds do not (the snapshot is the value AT that
-        ancestor, prior to its own pending writes), so they are replayed.
-        When the chain reaches the root short of `supersteps`, the snapshot
-        was never persisted (implicit empty baseline) — replay the full
-        chain with no seed.
+        Pre-delta plain-value blobs subsume their ancestor's pending
+        writes (the value already includes them); `_DeltaSnapshot` blobs
+        do not (snapshot is the value AT that ancestor, prior to its own
+        pending writes that produce the child).
        """
        if not channels:
            return {}
@@ -166,98 +159,72 @@ class InMemorySaver(
        # module import; only this override needs the runtime check.
        from langgraph.checkpoint.serde.types import _DeltaSnapshot

-        channels = list(channels)
        thread_id = config["configurable"]["thread_id"]
        checkpoint_ns = config["configurable"].get("checkpoint_ns", "")
+        checkpoint_id = config["configurable"].get("checkpoint_id", "")
        ns_storage = self.storage.get(thread_id, {}).get(checkpoint_ns, {})

-        # Resolve the target checkpoint id + its metadata (for supersteps).
-        checkpoint_id = get_checkpoint_id(config)
-        if checkpoint_id is None:
-            checkpoint_id = next(reversed(ns_storage), None)
-        target_entry = ns_storage.get(checkpoint_id) if checkpoint_id else None
-        if target_entry is None:
-            return {ch: {"writes": []} for ch in channels}
-        metadata = self.serde.loads_typed(target_entry[1])
-        supersteps_by_ch = _parse_supersteps_since_last_snapshot_by_channel(
-            metadata or {}, channels
-        )
-        max_supersteps = max(supersteps_by_ch.values(), default=0)
-
-        # Walk the parent chain (newest first) up to the deepest supersteps.
        chain: list[str] = []
-        has_reached_root = False
-        current: str | None = target_entry[2]
-        while current is not None and len(chain) < max_supersteps:
+        target_entry = ns_storage.get(checkpoint_id)
+        current: str | None = target_entry[2] if target_entry is not None else None
+        while current is not None:
            entry = ns_storage.get(current)
            if entry is None:
                break
            chain.append(current)
-            parent = entry[2]
-            if parent is None:
-                has_reached_root = True
-                break
+            _, _, parent = entry
            current = parent

+        collected_by_ch: dict[str, list[PendingWrite]] = {c: [] for c in channels}
+        seed_by_ch: dict[str, Any] = {}
+        remaining: set[str] = set(channels)
+
+        for cp_id in chain:
+            if not remaining:
+                break
+            entry = ns_storage.get(cp_id)
+            ckpt = self.serde.loads_typed(entry[0]) if entry is not None else None
+
+            terminated_here: set[str] = set()
+            blob_value_by_ch: dict[str, Any] = {}
+            if ckpt is not None:
+                versions = ckpt.get("channel_versions", {})
+                for ch in remaining:
+                    ver = versions.get(ch)
+                    if ver is None:
+                        continue
+                    blob_entry = self.blobs.get((thread_id, checkpoint_ns, ch, ver))
+                    if blob_entry is None or blob_entry[0] == "empty":
+                        continue
+                    blob_value_by_ch[ch] = self.serde.loads_typed(blob_entry)
+                    terminated_here.add(ch)
+
+            step_writes = self.writes.get((thread_id, checkpoint_ns, cp_id), {})
+            for (_task_id, _idx), (tid, ch, serialized, _) in sorted(
+                step_writes.items(), reverse=True
+            ):
+                if ch not in remaining:
+                    continue
+                blob_value = blob_value_by_ch.get(ch)
+                if blob_value is not None and not isinstance(
+                    blob_value, _DeltaSnapshot
+                ):
+                    continue
+                collected_by_ch[ch].append(
+                    (tid, ch, self.serde.loads_typed(serialized))
+                )
+
+            for ch in terminated_here:
+                seed_by_ch[ch] = blob_value_by_ch[ch]
+                remaining.discard(ch)
+
        result: dict[str, DeltaChannelHistory] = {}
        for ch in channels:
-            entry_h: DeltaChannelHistory = {"writes": []}
-            bound = supersteps_by_ch.get(ch, 0)
-            if bound <= 0:
-                result[ch] = entry_h
-                continue
-            if len(chain) >= bound:
-                seed_depth = bound
-            elif has_reached_root:
-                seed_depth = len(chain)
-            else:
-                logger.warning(
-                    "cannot find seed snapshot for delta channel "
-                    "(thread_id=%s, channel=%s)",
-                    thread_id,
-                    ch,
-                )
-                result[ch] = entry_h
-                continue
-            if seed_depth <= 0:
-                result[ch] = entry_h
-                continue
-            chain_slice: list[str] = chain[:seed_depth]
-            seed_cpid: str | None = chain[seed_depth - 1]
-
-            skip_seed_checkpoint_writes = False
-            seed_entry = ns_storage.get(seed_cpid)
-            if seed_entry is not None:
-                ckpt = self.serde.loads_typed(seed_entry[0])
-                ver = (ckpt.get("channel_versions") or {}).get(ch)
-                blob_entry = (
-                    self.blobs.get((thread_id, checkpoint_ns, ch, ver))
-                    if ver is not None
-                    else None
-                )
-                if blob_entry is not None and blob_entry[0] != "empty":
-                    seed_value = self.serde.loads_typed(blob_entry)
-                    entry_h["seed"] = seed_value
-                    skip_seed_checkpoint_writes = not isinstance(
-                        seed_value, _DeltaSnapshot
-                    )
-                else:
-                    # No stored value at the oldest checkpoint → empty
-                    # baseline; the chain's own writes replay from empty.
-                    seed_cpid = None
-
-            collected: list[PendingWrite] = []
-            # Chain is newest→oldest; replay oldest→newest.
-            for cp_id in reversed(chain_slice):
-                if skip_seed_checkpoint_writes and cp_id == seed_cpid:
-                    continue
-                step_writes = self.writes.get((thread_id, checkpoint_ns, cp_id), {})
-                for (_task_id, _idx), (tid, w_ch, serialized, _) in sorted(
-                    step_writes.items()
-                ):
-                    if w_ch == ch:
-                        collected.append((tid, ch, self.serde.loads_typed(serialized)))
-            entry_h["writes"] = collected
+            entry_h: DeltaChannelHistory = {
+                "writes": list(reversed(collected_by_ch[ch]))
+            }
+            if ch in seed_by_ch:
+                entry_h["seed"] = seed_by_ch[ch]
            result[ch] = entry_h
        return result

@@ -351,12 +351,9 @@ class TestInMemorySaverDeltaChannel:
        cp1["id"] = "cp1"
        cp2 = empty_checkpoint()
        cp2["id"] = "cp2"
-        # Target (cp2) carries the supersteps counter; no snapshot was ever
-        # taken, so the walk runs back to the root → no seed, empty baseline.
-        cp2_md = {"counters_since_delta_snapshot": {channel: [1, 2]}}
        saver.storage[thread_id][ns] = {
            "cp1": (serde.dumps_typed(cp1), serde.dumps_typed({}), None),
-            "cp2": (serde.dumps_typed(cp2), serde.dumps_typed(cp2_md), "cp1"),
+            "cp2": (serde.dumps_typed(cp2), serde.dumps_typed({}), "cp1"),
        }
        # Writes stored at cp1 produced the cp1 snapshot; part of history.
        saver.writes[(thread_id, ns, "cp1")][("task1", 0)] = (
@@ -459,15 +456,10 @@ class TestBaseFallbackGetChannelWrites:
        cp1["id"] = "00000000000000000000000000000002.0000000000000000"
        cp2 = empty_checkpoint()
        cp2["id"] = "00000000000000000000000000000003.0000000000000000"
-        # The target (cp2) carries `counters_since_delta_snapshot` — the
-        # supersteps-since-last-snapshot driving the ancestor-walk depth. No
-        # snapshot was ever taken, so the count runs back past the oldest
-        # persisted checkpoint (root) → implicit empty baseline, no seed.
-        cp2_md = {"counters_since_delta_snapshot": {channel: [2, 3]}}
        saver.storage[thread_id][ns] = {
            cp0["id"]: (serde.dumps_typed(cp0), serde.dumps_typed({}), None),
            cp1["id"]: (serde.dumps_typed(cp1), serde.dumps_typed({}), cp0["id"]),
-            cp2["id"]: (serde.dumps_typed(cp2), serde.dumps_typed(cp2_md), cp1["id"]),
+            cp2["id"]: (serde.dumps_typed(cp2), serde.dumps_typed({}), cp1["id"]),
        }
        # Writes under cp0 produced cp1's state; writes under cp1 produced cp2's.
        saver.writes[(thread_id, ns, cp0["id"])][("task1", 0)] = (
@@ -615,14 +607,10 @@ class TestPreDeltaBlobTerminator:
        cp3["id"] = "cp3"
        cp3["channel_versions"][channel] = v3

-        # cp1 is the pre-delta (migration) seed; the channel became a
-        # DeltaChannel at cp2, so its supersteps counter starts there: cp2 -> 1,
-        # cp3 -> 2. The target (cp3) walks 2 hops back to the cp1 seed blob.
-        cp3_md = {"counters_since_delta_snapshot": {channel: [2, 2]}}
        saver.storage[thread_id][ns] = {
            "cp1": (serde.dumps_typed(cp1), serde.dumps_typed({}), None),
            "cp2": (serde.dumps_typed(cp2), serde.dumps_typed({}), "cp1"),
-            "cp3": (serde.dumps_typed(cp3), serde.dumps_typed(cp3_md), "cp2"),
+            "cp3": (serde.dumps_typed(cp3), serde.dumps_typed({}), "cp2"),
        }
        # Write under cp1 would be from the pre-delta era and MUST be ignored
        # (the blob already captures it). We add one and assert it is not
@@ -383,7 +383,7 @@ test = [

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+version = "0.8.0"
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+    { url = "https://files.pythonhosted.org/packages/f3/e1/a4be2e696c9473bb53298df398237da5674704d781d4b748ed35aeef592a/langsmith-0.8.0-py3-none-any.whl", hash = "sha256:12cc4bc5622b835a6d841964d6034df3617bdb912dae0c1381fd0a68a9b3a3ef", size = 393268, upload-time = "2026-04-30T22:13:05.56Z" },
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 [[package]]
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 ]

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-]
-
 [[package]]
 name = "xxhash"
 version = "3.6.0"
@@ -366,42 +366,6 @@ def normalize_image_tag(value: str) -> str:
    return value


-def _validate_prebuilt_image(runner, image: str, *, verbose: bool) -> None:
-    """Ensure a prebuilt image exists locally for linux/amd64."""
-    try:
-        stdout, _ = runner.run(
-            subp_exec(
-                "docker",
-                "image",
-                "inspect",
-                "--format",
-                "{{.Os}}/{{.Architecture}}",
-                image,
-                verbose=verbose,
-                collect=True,
-            )
-        )
-    except FileNotFoundError:
-        raise click.ClickException(
-            "Docker is required but not installed.\n"
-            "Install Docker Desktop: https://docs.docker.com/get-docker/"
-        ) from None
-    except click.exceptions.Exit:
-        raise click.ClickException(
-            f"Docker image '{image}' was not found locally. Build or pull the image "
-            "before deploying with --image."
-        ) from None
-
-    image_platform = (stdout or "").strip()
-    if image_platform != "linux/amd64":
-        detected = image_platform or "unknown"
-        raise click.ClickException(
-            f"Docker image '{image}' targets {detected}, but LangSmith Deployment "
-            "requires linux/amd64. Rebuild or pull the image for linux/amd64 before "
-            "deploying with --image."
-        )
-
-
 def _extract_deployment_url(deployment: dict[str, object]) -> str:
    source_config = deployment.get("source_config")
    if isinstance(source_config, dict):
@@ -561,18 +525,12 @@ def _secrets_from_env(

 def _resolve_build_mode(
    remote_build_flag: bool | None,
-    *,
-    force_local: bool = False,
 ) -> tuple[bool, str | None]:
    """Determine whether to use a remote build.

    Returns (use_remote_build, local_build_error).  Raises UsageError when
-    --no-remote is set but the machine cannot build locally. When
-    `force_local` is set, the function short-circuits and always selects a
-    local build.
+    --no-remote is set but the machine cannot build locally.
    """
-    if force_local:
-        return False, None
    local_build_supported, local_build_error = can_build_locally()

    if remote_build_flag is True:
@@ -939,7 +897,6 @@ def _run_local_build(
    api_version: str | None,
    base_image: str | None,
    image_name: str | None,
-    prebuilt_image: str | None,
    name: str | None,
    tag: str,
    install_command: str | None,
@@ -953,57 +910,51 @@ def _run_local_build(
    # (e.g. Apple Silicon). On amd64 hosts, plain docker build is sufficient.
    needs_buildx = platform.machine() != "x86_64"
    local_tag = f"langgraph-deploy-tmp:{int(time.time())}"
-    image_to_push = prebuilt_image or local_tag

    with Runner() as runner:
-        if prebuilt_image:
-            _log_deploy_step(step, f"Validating image {prebuilt_image}")
-            _validate_prebuilt_image(runner, prebuilt_image, verbose=verbose)
-            click.secho("   Image is available for linux/amd64", fg="green")
+        # -- Step: Build image --
+        _log_deploy_step(step, "Building image")
+        if needs_buildx:
+            build_flags: list[str] = [
+                "--platform",
+                "linux/amd64",
+                "--load",
+            ]
+            if not verbose:
+                build_flags.append("--progress=quiet")
+            with Progress(message="Building...", elapsed=not verbose):
+                build_docker_image(
+                    runner,
+                    lambda _msg: None,
+                    config,
+                    config_json,
+                    base_image,
+                    api_version,
+                    pull,
+                    local_tag,
+                    docker_build_args,
+                    install_command,
+                    build_command,
+                    docker_command=("docker", "buildx", "build"),
+                    extra_flags=build_flags,
+                    verbose=verbose,
+                )
        else:
-            # -- Step: Build image --
-            _log_deploy_step(step, "Building image")
-            if needs_buildx:
-                build_flags: list[str] = [
-                    "--platform",
-                    "linux/amd64",
-                    "--load",
-                ]
-                if not verbose:
-                    build_flags.append("--progress=quiet")
-                with Progress(message="Building...", elapsed=not verbose):
-                    build_docker_image(
-                        runner,
-                        lambda _msg: None,
-                        config,
-                        config_json,
-                        base_image,
-                        api_version,
-                        pull,
-                        local_tag,
-                        docker_build_args,
-                        install_command,
-                        build_command,
-                        docker_command=("docker", "buildx", "build"),
-                        extra_flags=build_flags,
-                        verbose=verbose,
-                    )
-            else:
-                with Progress(message="Building...", elapsed=not verbose):
-                    build_docker_image(
-                        runner,
-                        lambda _msg: None,
-                        config,
-                        config_json,
-                        base_image,
-                        api_version,
-                        pull,
-                        local_tag,
-                        docker_build_args,
-                        install_command,
-                        build_command,
-                        verbose=verbose,
-                    )
+            with Progress(message="Building...", elapsed=not verbose):
+                build_docker_image(
+                    runner,
+                    lambda _msg: None,
+                    config,
+                    config_json,
+                    base_image,
+                    api_version,
+                    pull,
+                    local_tag,
+                    docker_build_args,
+                    install_command,
+                    build_command,
+                    verbose=verbose,
+                )
        step += 1

        # -- Step: Get push token and authenticate --
@@ -1072,7 +1023,7 @@ def _run_local_build(
                subp_exec(
                    "docker",
                    "tag",
-                    image_to_push,
+                    local_tag,
                    remote_image,
                    verbose=verbose,
                )
@@ -1472,13 +1423,6 @@ def _deploy_base_options(
                show_default=True,
                help="Tag to use for the pushed deployment image.",
            ),
-            click.option(
-                "--image",
-                help=(
-                    "Use an existing local image reference (e.g. repo:tag) and "
-                    "skip building. The image must target linux/amd64."
-                ),
-            ),
            click.option(
                "--config",
                "-c",
@@ -1579,7 +1523,6 @@ def _deploy_cmd(
    deployment_type: str,
    name: str | None,
    image_name: str | None,
-    image: str | None,
    tag: str,
    base_image: str | None,
    install_command: str | None,
@@ -1626,12 +1569,7 @@ def _deploy_cmd(

    secrets = _secrets_from_env(_env_without_deployment_name(env_vars))

-    if image and remote_build_flag is True:
-        raise click.UsageError("--image cannot be combined with --remote builds.")
-
-    use_remote_build, local_build_error = _resolve_build_mode(
-        remote_build_flag, force_local=image is not None
-    )
+    use_remote_build, local_build_error = _resolve_build_mode(remote_build_flag)
    if use_remote_build and remote_build_flag is None and local_build_error:
        em.note(f"{local_build_error}\nUsing remote build instead.")
        if not json_output:
@@ -1701,7 +1639,6 @@ def _deploy_cmd(
            api_version=api_version,
            base_image=base_image,
            image_name=image_name,
-            prebuilt_image=image,
            name=name,
            tag=tag,
            install_command=install_command,
@@ -1,4 +1,3 @@
-import asyncio
 import base64
 import io
 import json
@@ -7,11 +6,9 @@ import sys
 from unittest.mock import MagicMock

 import click
-import click.exceptions
 import httpx
 import pytest

-import langgraph_cli.deploy as deploy_mod
 from langgraph_cli.deploy import (
    _call_host_backend_with_optional_tenant,
    _create_host_backend_client,
@@ -22,7 +19,6 @@ from langgraph_cli.deploy import (
    _resolve_env_path,
    _resolve_pushed_image_digest,
    _smith_dashboard_base_url,
-    _validate_prebuilt_image,
    normalize_image_tag,
    normalize_name,
 )
@@ -99,65 +95,6 @@ class TestNormalizeImageTag:
            normalize_image_tag("has space")


-class _FakeRunner:
-    def run(self, coro):
-        return asyncio.run(coro)
-
-
-class TestValidatePrebuiltImage:
-    def test_accepts_linux_amd64(self, monkeypatch):
-        calls = []
-
-        async def fake_subp_exec(*args, **kwargs):
-            calls.append((args, kwargs))
-            return "linux/amd64\n", None
-
-        monkeypatch.setattr(deploy_mod, "subp_exec", fake_subp_exec)
-
-        _validate_prebuilt_image(_FakeRunner(), "repo/app:tag", verbose=False)
-
-        assert calls == [
-            (
-                (
-                    "docker",
-                    "image",
-                    "inspect",
-                    "--format",
-                    "{{.Os}}/{{.Architecture}}",
-                    "repo/app:tag",
-                ),
-                {"verbose": False, "collect": True},
-            )
-        ]
-
-    def test_missing_docker_binary_raises_actionable_error(self, monkeypatch):
-        async def fake_subp_exec(*args, **kwargs):
-            raise FileNotFoundError("docker")
-
-        monkeypatch.setattr(deploy_mod, "subp_exec", fake_subp_exec)
-
-        with pytest.raises(click.ClickException, match="Docker is required"):
-            _validate_prebuilt_image(_FakeRunner(), "repo/app:tag", verbose=False)
-
-    def test_missing_image_raises_actionable_error(self, monkeypatch):
-        async def fake_subp_exec(*args, **kwargs):
-            raise click.exceptions.Exit(1)
-
-        monkeypatch.setattr(deploy_mod, "subp_exec", fake_subp_exec)
-
-        with pytest.raises(click.ClickException, match="not found locally"):
-            _validate_prebuilt_image(_FakeRunner(), "missing:tag", verbose=False)
-
-    def test_rejects_non_amd64_platform(self, monkeypatch):
-        async def fake_subp_exec(*args, **kwargs):
-            return "linux/arm64\n", None
-
-        monkeypatch.setattr(deploy_mod, "subp_exec", fake_subp_exec)
-
-        with pytest.raises(click.ClickException, match="requires linux/amd64"):
-            _validate_prebuilt_image(_FakeRunner(), "repo/app:arm", verbose=False)
-
-
 class TestParseEnvFromConfig:
    def test_env_dict(self, tmp_path):
        config_path = tmp_path / "langgraph.json"
@@ -1179,7 +1179,7 @@ wheels = [

 [[package]]
 name = "langsmith"
-version = "0.8.18"
+version = "0.8.0"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
    { name = "httpx", marker = "python_full_version >= '3.11'" },
@@ -1189,13 +1189,12 @@ dependencies = [
    { name = "requests", marker = "python_full_version >= '3.11'" },
    { name = "requests-toolbelt", marker = "python_full_version >= '3.11'" },
    { name = "uuid-utils", marker = "python_full_version >= '3.11'" },
-    { name = "websockets", marker = "python_full_version >= '3.11'" },
    { name = "xxhash", marker = "python_full_version >= '3.11'" },
    { name = "zstandard", marker = "python_full_version >= '3.11'" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/9a/d9/a6681aa9847bbbc5ec21abe20a5e233b94e5edcfe39624db607ac7e8ccb4/langsmith-0.8.18.tar.gz", hash = "sha256:32dde9c0e67e053e0fb738921fc8ced768af7b8fa83d7a0e3fd63597cf8776dd", size = 4526988, upload-time = "2026-06-19T13:12:17.123Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/a8/64/95f1f013531395f4e8ed73caeee780f65c7c58fe028cb543f8937b45611b/langsmith-0.8.0.tar.gz", hash = "sha256:59fe5b2a56bbbe14a08aa76691f84b49e8675dd21e11b57d80c6db8c08bac2e3", size = 4432996, upload-time = "2026-04-30T22:13:07.341Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/03/70/0e0cc80a3b064c8d6c8d697c3125ed86e39d5a7393ec6dc8b07cb1cf13c4/langsmith-0.8.18-py3-none-any.whl", hash = "sha256:3940183349993faef48e6c7d08e4822ee9cefd906b362d0e3c2d650314d2f282", size = 508108, upload-time = "2026-06-19T13:12:15.348Z" },
+    { url = "https://files.pythonhosted.org/packages/f3/e1/a4be2e696c9473bb53298df398237da5674704d781d4b748ed35aeef592a/langsmith-0.8.0-py3-none-any.whl", hash = "sha256:12cc4bc5622b835a6d841964d6034df3617bdb912dae0c1381fd0a68a9b3a3ef", size = 393268, upload-time = "2026-04-30T22:13:05.56Z" },
 ]

 [package.optional-dependencies]
@@ -2032,15 +2031,15 @@ wheels = [

 [[package]]
 name = "starlette"
-version = "1.3.1"
+version = "1.0.1"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
    { name = "anyio", marker = "python_full_version >= '3.11'" },
    { name = "typing-extensions", marker = "python_full_version >= '3.11' and python_full_version < '3.13'" },
 ]
-sdist = { url = "https://files.pythonhosted.org/packages/eb/e3/7c1dc7381d9f8ab7d854328ebfa884e62cb3f3d8549ddfd37c7814f42afa/starlette-1.3.1.tar.gz", hash = "sha256:05d0213193f2fbaae60e2ecb593b4add4262ad4e46536b54abe36f11a71724e0", size = 2703240, upload-time = "2026-06-12T09:23:11.602Z" }
+sdist = { url = "https://files.pythonhosted.org/packages/08/a3/84e821cc54b4ab50ae6dbc6ac3800a651b65ec35f045cc73785380654057/starlette-1.0.1.tar.gz", hash = "sha256:512399c5f1de7fac99c88572212ded9ddeddef2fb32afa82d724000e88b38f4f", size = 2659596, upload-time = "2026-05-21T21:58:58.433Z" }
 wheels = [
-    { url = "https://files.pythonhosted.org/packages/ec/bb/2799cc2ede3ed41131f8975621e7213dfc7ef4acbbaadfa440f32500c370/starlette-1.3.1-py3-none-any.whl", hash = "sha256:c7372aae11c3c3f26a42df7bd626cec2f47d03483d261d369516a615a53714c6", size = 73632, upload-time = "2026-06-12T09:23:10.017Z" },
+    { url = "https://files.pythonhosted.org/packages/ec/e1/b2df4bc09a1e51ff664c1e17018a4274b42e5e9352e4a478ea540512dc88/starlette-1.0.1-py3-none-any.whl", hash = "sha256:7c0e69b2ee1c848bd54669d908500117a3ee13de603a21427e5c6fc1adf98dcd", size = 72802, upload-time = "2026-05-21T21:58:56.551Z" },
 ]

 [[package]]
@@ -70,42 +70,6 @@ def delta_channels_to_snapshot(
    return result


-def advance_delta_counters(
-    channels: Mapping[str, BaseChannel],
-    prev_metadata: Mapping[str, Any] | None,
-    updated_channels: set[str] | None,
-) -> tuple[dict[str, tuple[int, int]], set[str]]:
-    """Advance per-delta-channel `(updates, supersteps)` counters one step.
-
-    Mirrors the counter bookkeeping in `PregelLoop._put_checkpoint`: every
-    superstep bumps `supersteps` for all delta channels and `updates` for
-    those written this step, then channels that hit their snapshot cadence
-    are reset to `(0, 0)`.
-
-    Used by `update_state` (which creates a delta checkpoint outside the
-    main loop) so the written checkpoint carries a correct
-    `counters_since_delta_snapshot` — without it, the supersteps-based
-    `get_delta_channel_history` walk can't locate the seed. Returns
-    `(new_counters, channels_to_snapshot)`; callers drop `(0, 0)` entries
-    before writing the metadata field.
-    """
-    prev = dict((prev_metadata or {}).get("counters_since_delta_snapshot") or {})
-    updated = updated_channels or set()
-    new_counters: dict[str, tuple[int, int]] = {}
-    for name, ch in channels.items():
-        if not isinstance(ch, DeltaChannel):
-            continue
-        u, s = prev.get(name, (0, 0))
-        s += 1
-        if name in updated:
-            u += 1
-        new_counters[name] = (u, s)
-    channels_to_snapshot = delta_channels_to_snapshot(channels, new_counters)
-    for k in channels_to_snapshot:
-        new_counters[k] = (0, 0)
-    return new_counters, channels_to_snapshot
-
-
 def create_checkpoint(
    checkpoint: Checkpoint,
    channels: Mapping[str, BaseChannel] | None,
@@ -32,10 +32,14 @@ from langgraph._internal._constants import (
    CONFIG_KEY_SCRATCHPAD,
    ERROR,
    ERROR_SOURCE_NODE,
+    INPUT,
    INTERRUPT,
    NO_WRITES,
+    NULL_TASK_ID,
+    PUSH,
    RESUME,
    RETURN,
+    TASKS,
 )
 from langgraph._internal._future import chain_future, run_coroutine_threadsafe
 from langgraph._internal._scratchpad import PregelScratchpad
@@ -585,6 +589,12 @@ class PregelRunner:
            if isinstance(exception, GraphInterrupt):
                # save interrupt to checkpointer
                if exception.args[0]:
+                    if pending_writes := _writes_to_persist_on_interrupt(task.writes):
+                        # GraphInterrupt is a controlled suspension point, not a
+                        # task failure. Persist writes emitted before the
+                        # suspension without associating them with task
+                        # completion, so the interrupted task remains resumable.
+                        self.put_writes()(NULL_TASK_ID, pending_writes)  # type: ignore[misc]
                    writes = [(INTERRUPT, exception.args[0])]
                    if resumes := [w for w in task.writes if w[0] == RESUME]:
                        writes.extend(resumes)
@@ -939,3 +949,24 @@ async def _acall_impl(
            destination.set_exception(RuntimeError("Task not scheduled"))
    except Exception as exc:
        destination.set_exception(exc)
+
+
+def _writes_to_persist_on_interrupt(
+    writes: Iterable[tuple[str, Any]],
+) -> list[tuple[str, Any]]:
+    return [
+        write
+        for write in writes
+        if write[0]
+        not in (
+            ERROR,
+            ERROR_SOURCE_NODE,
+            INPUT,
+            INTERRUPT,
+            NO_WRITES,
+            PUSH,
+            RESUME,
+            RETURN,
+            TASKS,
+        )
+    ]
@@ -130,7 +130,6 @@ from langgraph.pregel._algo import (
 from langgraph.pregel._call import identifier
 from langgraph.pregel._checkpoint import (
    achannels_from_checkpoint,
-    advance_delta_counters,
    channels_from_checkpoint,
    copy_checkpoint,
    create_checkpoint,
@@ -2033,40 +2032,15 @@ class Pregel(
                checkpointer.get_next_version,
                self.trigger_to_nodes,
            )
-            # Advance delta-channel snapshot counters for this manual super-
-            # step so the written checkpoint carries a correct
-            # `counters_since_delta_snapshot` (mirrors the main loop). Without
-            # it, the supersteps-based delta history walk can't find the seed.
-            updated_channel_names = {
-                c for task in run_tasks for c, _ in task.writes if c != PUSH
-            }
-            new_counters, channels_to_snapshot = advance_delta_counters(
-                channels,
-                saved.metadata if saved else None,
-                updated_channel_names,
-            )
-            # Keep `updated_channels` at its default (None) — matching the
-            # historical checkpoint shape here — so resume/trigger semantics
-            # (e.g. deferred nodes) are unaffected. `get_next_version` /
-            # `channels_to_snapshot` are no-ops unless a delta channel snapshots.
-            checkpoint = create_checkpoint(
-                checkpoint,
-                channels,
-                step + 1,
-                get_next_version=checkpointer.get_next_version,
-                channels_to_snapshot=channels_to_snapshot,
-            )
-            update_metadata: dict[str, Any] = {
-                "source": "update",
-                "step": step + 1,
-                "parents": saved.metadata.get("parents", {}) if saved else {},
-            }
-            if non_zero := {k: v for k, v in new_counters.items() if v != (0, 0)}:
-                update_metadata["counters_since_delta_snapshot"] = non_zero
+            checkpoint = create_checkpoint(checkpoint, channels, step + 1)
            next_config = checkpointer.put(
                checkpoint_config,
                checkpoint,
-                update_metadata,
+                {
+                    "source": "update",
+                    "step": step + 1,
+                    "parents": saved.metadata.get("parents", {}) if saved else {},
+                },
                get_new_channel_versions(
                    checkpoint_previous_versions, checkpoint["channel_versions"]
                ),
@@ -2526,41 +2500,16 @@ class Pregel(
                checkpointer.get_next_version,
                self.trigger_to_nodes,
            )
-            # Advance delta-channel snapshot counters for this manual super-
-            # step so the written checkpoint carries a correct
-            # `counters_since_delta_snapshot` (mirrors the main loop). Without
-            # it, the supersteps-based delta history walk can't find the seed.
-            updated_channel_names = {
-                c for task in run_tasks for c, _ in task.writes if c != PUSH
-            }
-            new_counters, channels_to_snapshot = advance_delta_counters(
-                channels,
-                saved.metadata if saved else None,
-                updated_channel_names,
-            )
-            # Keep `updated_channels` at its default (None) — matching the
-            # historical checkpoint shape here — so resume/trigger semantics
-            # (e.g. deferred nodes) are unaffected. `get_next_version` /
-            # `channels_to_snapshot` are no-ops unless a delta channel snapshots.
-            checkpoint = create_checkpoint(
-                checkpoint,
-                channels,
-                step + 1,
-                get_next_version=checkpointer.get_next_version,
-                channels_to_snapshot=channels_to_snapshot,
-            )
-            update_metadata: dict[str, Any] = {
-                "source": "update",
-                "step": step + 1,
-                "parents": saved.metadata.get("parents", {}) if saved else {},
-            }
-            if non_zero := {k: v for k, v in new_counters.items() if v != (0, 0)}:
-                update_metadata["counters_since_delta_snapshot"] = non_zero
+            checkpoint = create_checkpoint(checkpoint, channels, step + 1)
            # save checkpoint, after applying writes
            next_config = await checkpointer.aput(
                checkpoint_config,
                checkpoint,
-                update_metadata,
+                {
+                    "source": "update",
+                    "step": step + 1,
+                    "parents": saved.metadata.get("parents", {}) if saved else {},
+                },
                get_new_channel_versions(
                    checkpoint_previous_versions, checkpoint["channel_versions"]
                ),
@@ -40,15 +40,25 @@ from pytest_mock import MockerFixture
 from syrupy import SnapshotAssertion
 from typing_extensions import NotRequired, TypedDict

-from langgraph._internal._constants import CONFIG_KEY_NODE_FINISHED, ERROR, PULL
+from langgraph._internal._constants import (
+    CONFIG_KEY_NODE_FINISHED,
+    CONFIG_KEY_SEND,
+    ERROR,
+    PULL,
+)
 from langgraph.channels.binop import BinaryOperatorAggregate
 from langgraph.channels.delta import DeltaChannel
 from langgraph.channels.ephemeral_value import EphemeralValue
 from langgraph.channels.last_value import LastValue
 from langgraph.channels.topic import Topic
 from langgraph.channels.untracked_value import UntrackedValue
-from langgraph.config import get_stream_writer
-from langgraph.errors import GraphRecursionError, InvalidUpdateError, ParentCommand
+from langgraph.config import get_config, get_stream_writer
+from langgraph.errors import (
+    GraphInterrupt,
+    GraphRecursionError,
+    InvalidUpdateError,
+    ParentCommand,
+)
 from langgraph.func import entrypoint, task
 from langgraph.graph import END, START, StateGraph
 from langgraph.graph.message import MessagesState, _messages_delta_reducer, add_messages
@@ -2755,26 +2765,23 @@ def test_in_one_fan_out_state_graph_waiting_edge_plus_regular(
        "answer": "doc1,doc2,doc3,doc4",
    }

-    rewrite = {"rewrite_query": {"query": "query: what is weather in sf"}}
-    analyzer = {"analyzer_one": {"query": "analyzed: query: what is weather in sf"}}
-    empty_qa = {"qa": {"answer": ""}}
-    retriever_one_result = {"retriever_one": {"docs": ["doc1", "doc2"]}}
-    retriever_two_result = {"retriever_two": {"docs": ["doc3", "doc4"]}}
-
-    def assert_valid_stream_order(
-        chunks: list[dict[str, Any]], terminal: dict[str, Any]
-    ) -> None:
-        assert chunks[0] == rewrite
-        assert chunks[-1] == terminal
-        middle = chunks[1:-1]
-        assert sorted(middle, key=repr) == sorted(
-            [empty_qa, analyzer, retriever_one_result, retriever_two_result], key=repr
-        )
-        assert middle.index(analyzer) < middle.index(retriever_one_result)
-
-    assert_valid_stream_order(
-        [*app.stream({"query": "what is weather in sf"})],
-        {"qa": {"answer": "doc1,doc2,doc3,doc4"}},
+    assert [*app.stream({"query": "what is weather in sf"})] in (
+        [
+            {"rewrite_query": {"query": "query: what is weather in sf"}},
+            {"qa": {"answer": ""}},
+            {"analyzer_one": {"query": "analyzed: query: what is weather in sf"}},
+            {"retriever_two": {"docs": ["doc3", "doc4"]}},
+            {"retriever_one": {"docs": ["doc1", "doc2"]}},
+            {"qa": {"answer": "doc1,doc2,doc3,doc4"}},
+        ],
+        [
+            {"rewrite_query": {"query": "query: what is weather in sf"}},
+            {"analyzer_one": {"query": "analyzed: query: what is weather in sf"}},
+            {"qa": {"answer": ""}},
+            {"retriever_two": {"docs": ["doc3", "doc4"]}},
+            {"retriever_one": {"docs": ["doc1", "doc2"]}},
+            {"qa": {"answer": "doc1,doc2,doc3,doc4"}},
+        ],
    )

    app_w_interrupt = workflow.compile(
@@ -2783,9 +2790,25 @@ def test_in_one_fan_out_state_graph_waiting_edge_plus_regular(
    )
    config = {"configurable": {"thread_id": "1"}}

-    assert_valid_stream_order(
-        [c for c in app_w_interrupt.stream({"query": "what is weather in sf"}, config)],
-        {"__interrupt__": ()},
+    assert [
+        c for c in app_w_interrupt.stream({"query": "what is weather in sf"}, config)
+    ] in (
+        [
+            {"rewrite_query": {"query": "query: what is weather in sf"}},
+            {"qa": {"answer": ""}},
+            {"analyzer_one": {"query": "analyzed: query: what is weather in sf"}},
+            {"retriever_two": {"docs": ["doc3", "doc4"]}},
+            {"retriever_one": {"docs": ["doc1", "doc2"]}},
+            {"__interrupt__": ()},
+        ],
+        [
+            {"rewrite_query": {"query": "query: what is weather in sf"}},
+            {"analyzer_one": {"query": "analyzed: query: what is weather in sf"}},
+            {"qa": {"answer": ""}},
+            {"retriever_two": {"docs": ["doc3", "doc4"]}},
+            {"retriever_one": {"docs": ["doc1", "doc2"]}},
+            {"__interrupt__": ()},
+        ],
    )

    assert [c for c in app_w_interrupt.stream(None, config)] == [
@@ -4807,6 +4830,111 @@ def test_parent_command(
    )


+def test_pending_send_write_persists_on_interrupt(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    class State(TypedDict):
+        quickjs_checkpoint: dict
+        result: str
+
+    def node(state: State) -> dict[str, Any]:
+        send = get_config()["configurable"][CONFIG_KEY_SEND]
+        send([("quickjs_checkpoint", {"snapshot": "abc"})])
+        value = interrupt("trip")
+        return {"result": value}
+
+    builder = StateGraph(State)
+    builder.add_node("node", node)
+    builder.add_edge(START, "node")
+    builder.add_edge("node", END)
+    graph = builder.compile(checkpointer=sync_checkpointer)
+    config = {"configurable": {"thread_id": "1"}}
+
+    first = graph.invoke({"quickjs_checkpoint": {}, "result": ""}, config)
+    assert "__interrupt__" in first
+
+    snapshot = graph.get_state(config)
+    assert snapshot.values["quickjs_checkpoint"] == {"snapshot": "abc"}
+    assert snapshot.next == ("node",)
+    assert snapshot.tasks[0].interrupts
+
+    second = graph.invoke(Command(resume="ok"), config)
+    assert second["quickjs_checkpoint"] == {"snapshot": "abc"}
+    assert second["result"] == "ok"
+    assert graph.get_state(config).values["quickjs_checkpoint"] == {"snapshot": "abc"}
+
+
+def test_pending_send_write_persists_on_tool_subgraph_interrupt(
+    sync_checkpointer: BaseCheckpointSaver,
+) -> None:
+    from langchain_core.tools import tool
+
+    class SubgraphState(TypedDict):
+        value: str
+
+    def subgraph_node(state: SubgraphState) -> dict[str, Any]:
+        value = interrupt("subgraph trip")
+        return {"value": value}
+
+    subgraph_builder = StateGraph(SubgraphState)
+    subgraph_builder.add_node("subgraph_node", subgraph_node)
+    subgraph_builder.add_edge(START, "subgraph_node")
+    subgraph = subgraph_builder.compile(checkpointer=True)
+
+    class ParentState(TypedDict):
+        messages: Annotated[list[AnyMessage], add_messages]
+        quickjs_checkpoint: dict
+
+    @tool
+    def task_tool() -> str:
+        """Run the interrupting subgraph task."""
+        send = get_config()["configurable"][CONFIG_KEY_SEND]
+        try:
+            response = subgraph.invoke({"value": ""})
+        except GraphInterrupt:
+            send([("quickjs_checkpoint", {"snapshot": "tool"})])
+            raise
+        return response["value"]
+
+    def call_tool(state: ParentState) -> dict[str, Any]:
+        return {
+            "messages": AIMessage(
+                content="",
+                tool_calls=[
+                    {
+                        "name": "task_tool",
+                        "args": {},
+                        "id": "tool_call_1",
+                    }
+                ],
+            )
+        }
+
+    builder = StateGraph(ParentState)
+    builder.add_node("agent", call_tool)
+    builder.add_node("tools", ToolNode([task_tool]))
+    builder.add_edge(START, "agent")
+    builder.add_edge("agent", "tools")
+    builder.add_edge("tools", END)
+    graph = builder.compile(checkpointer=sync_checkpointer)
+    config = {"configurable": {"thread_id": "1"}}
+
+    first = graph.invoke(
+        {"messages": [HumanMessage(content="start")], "quickjs_checkpoint": {}}, config
+    )
+    assert "__interrupt__" in first
+
+    snapshot = graph.get_state(config)
+    assert snapshot.values["quickjs_checkpoint"] == {"snapshot": "tool"}
+    assert snapshot.next == ("tools",)
+    assert snapshot.tasks[0].interrupts
+
+    second = graph.invoke(Command(resume="ok"), config)
+    assert second["quickjs_checkpoint"] == {"snapshot": "tool"}
+    assert second["messages"][-1].content == "ok"
+    assert graph.get_state(config).values["quickjs_checkpoint"] == {"snapshot": "tool"}
+
+
 def test_interrupt_subgraph(sync_checkpointer: BaseCheckpointSaver):
    class State(TypedDict):
        baz: str
@@ -6278,15 +6406,14 @@ def test_sync_streaming_with_functional_api() -> None:
    rather than have all the results arrive at once after the graph has completed.

    The time of arrival between the two updates corresponding to the two `slow` tasks
-    should be roughly the task delay. If results are buffered until graph completion,
-    the two updates arrive back-to-back instead.
+    should be greater than the time delay between the two tasks.
    """

    time_delay = 0.05

    @task()
    def slow() -> dict:
-        time.sleep(time_delay)
+        time.sleep(time_delay)  # Simulate a delay of 10 ms
        return {"tic": time.monotonic()}

    @entrypoint()
@@ -6304,9 +6431,8 @@ def test_sync_streaming_with_functional_api() -> None:

    assert len(arrival_times) == 2
    delta = arrival_times[1] - arrival_times[0]
-    # Allow a small amount of scheduler jitter while still verifying the chunks
-    # arrived separately rather than back-to-back after graph completion.
-    assert delta > time_delay * 0.8
+    # Delta cannot be less than 10 ms if it is streaming as results are generated.
+    assert delta > time_delay


 def test_entrypoint_without_checkpointer() -> None:
@@ -1345,7 +1345,7 @@ wheels = [

 [[package]]
 name = "jupyterlab"
-version = "4.5.9"
+version = "4.5.7"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
    { name = "async-lru" },
@@ -1363,9 +1363,9 @@ dependencies = [
    { name = "tornado" },
    { name = "traitlets" },
 ]
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+sdist = { url = "https://files.pythonhosted.org/packages/2b/22/8440ec827762146e7cdecf04335bd348795899d29dc6ae82238707353a2c/jupyterlab-4.5.7.tar.gz", hash = "sha256:55a9822c4754da305f41e113452c68383e214dcf96de760146af89ce5d5117b0", size = 23992763, upload-time = "2026-04-29T16:43:51.328Z" }
 wheels = [
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+    { url = "https://files.pythonhosted.org/packages/3d/aa/537b8f7d80e799af19af35fb3ddfc970b951088a13c57dd9387dcfbb7f61/jupyterlab-4.5.7-py3-none-any.whl", hash = "sha256:fba4cb0e2c44a52859669d8c98b45de029d5e515f8407bf8534d2a8fc5f0964d", size = 12450123, upload-time = "2026-04-29T16:43:46.639Z" },
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 [[package]]
@@ -1933,7 +1933,7 @@ test = [

 [[package]]
 name = "langsmith"
-version = "0.8.18"
+version = "0.8.0"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
    { name = "httpx" },
@@ -1943,13 +1943,12 @@ dependencies = [
    { name = "requests" },
    { name = "requests-toolbelt" },
    { name = "uuid-utils" },
-    { name = "websockets" },
    { name = "xxhash" },
    { name = "zstandard" },
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 [package.optional-dependencies]
@@ -3141,14 +3140,14 @@ wheels = [

 [[package]]
 name = "redis"
-version = "8.0.1"
+version = "7.4.0"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
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 [[package]]
@@ -640,7 +640,7 @@ test = [

 [[package]]
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-version = "0.8.18"
+version = "0.8.0"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
    { name = "httpx" },
@@ -650,13 +650,12 @@ dependencies = [
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    { name = "requests-toolbelt" },
    { name = "uuid-utils" },
-    { name = "websockets" },
    { name = "xxhash" },
    { name = "zstandard" },
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 [[package]]
@@ -553,7 +553,7 @@ test = [

 [[package]]
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-version = "0.8.18"
+version = "0.8.0"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
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@@ -563,13 +563,12 @@ dependencies = [
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-    { name = "websockets" },
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 [[package]]
@@ -1041,15 +1040,15 @@ wheels = [

 [[package]]
 name = "starlette"
-version = "1.3.1"
+version = "1.0.1"
 source = { registry = "https://pypi.org/simple" }
 dependencies = [
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    { name = "typing-extensions", marker = "python_full_version < '3.13'" },
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 [[package]]
Author	SHA1	Message	Date
Hunter Lovell	a07be124e8	fix(pregel): skip scheduler writes on interrupt	2026-06-23 14:13:43 -07:00
Hunter Lovell	6d9f9f5326	fix(pregel): persist pending writes on interrupt	2026-06-23 13:43:04 -07:00