diff --git a/langgraph/checkpoint/aiosqlite.py b/langgraph/checkpoint/aiosqlite.py
index b508cefb9..8a304b1a9 100644
--- a/langgraph/checkpoint/aiosqlite.py
+++ b/langgraph/checkpoint/aiosqlite.py
@@ -16,9 +16,12 @@ from langgraph.checkpoint.base import (
 )
 
 
-class AsyncSqliteSaver(BaseCheckpointSaver, AbstractAsyncContextManager):
-    serde = pickle
+# for backwards compat we continue to support loading pickled checkpoints
+def is_pickled(value: bytes) -> bool:
+    return value.startswith(b"\x80") and value.endswith(b".")
 
+
+class AsyncSqliteSaver(BaseCheckpointSaver, AbstractAsyncContextManager):
     conn: aiosqlite.Connection
 
     is_setup: bool
@@ -69,6 +72,11 @@ class AsyncSqliteSaver(BaseCheckpointSaver, AbstractAsyncContextManager):
 
         self.is_setup = True
 
+    def _loads(self, value: bytes) -> Checkpoint:
+        if is_pickled(value):
+            return pickle.loads(value)
+        return self.serde.loads(value)
+
     async def aget_tuple(self, config: RunnableConfig) -> Optional[CheckpointTuple]:
         await self.setup()
         if config["configurable"].get("thread_ts"):
@@ -82,7 +90,7 @@ class AsyncSqliteSaver(BaseCheckpointSaver, AbstractAsyncContextManager):
                 if value := await cursor.fetchone():
                     return CheckpointTuple(
                         config,
-                        self.serde.loads(value[0]),
+                        self._loads(value[0]),
                         {
                             "configurable": {
                                 "thread_id": config["configurable"]["thread_id"],
@@ -105,7 +113,7 @@ class AsyncSqliteSaver(BaseCheckpointSaver, AbstractAsyncContextManager):
                                 "thread_ts": value[1],
                             }
                         },
-                        self.serde.loads(value[3]),
+                        self._loads(value[3]),
                         {
                             "configurable": {
                                 "thread_id": value[0],
@@ -125,7 +133,7 @@ class AsyncSqliteSaver(BaseCheckpointSaver, AbstractAsyncContextManager):
             async for thread_id, thread_ts, parent_ts, value in cursor:
                 yield CheckpointTuple(
                     {"configurable": {"thread_id": thread_id, "thread_ts": thread_ts}},
-                    self.serde.loads(value),
+                    self._loads(value),
                     {"configurable": {"thread_id": thread_id, "thread_ts": parent_ts}}
                     if parent_ts
                     else None,
diff --git a/langgraph/checkpoint/base.py b/langgraph/checkpoint/base.py
index 28b485845..b8c0a9ec7 100644
--- a/langgraph/checkpoint/base.py
+++ b/langgraph/checkpoint/base.py
@@ -1,6 +1,5 @@
 from abc import ABC
 from collections import defaultdict
-from copy import deepcopy
 from datetime import datetime, timezone
 from typing import (
     Any,
@@ -14,6 +13,7 @@ from typing import (
 from langchain_core.runnables import ConfigurableFieldSpec, RunnableConfig
 
 from langgraph.serde.base import SerializerProtocol
+from langgraph.serde.jsonplus import JsonPlusSerializer
 from langgraph.utils import StrEnum
 
 
@@ -63,8 +63,11 @@ def copy_checkpoint(checkpoint: Checkpoint) -> Checkpoint:
         v=checkpoint["v"],
         ts=checkpoint["ts"],
         channel_values=checkpoint["channel_values"].copy(),
-        channel_versions=checkpoint["channel_versions"].copy(),
-        versions_seen=deepcopy(checkpoint["versions_seen"]),
+        channel_versions=defaultdict(int, checkpoint["channel_versions"]),
+        versions_seen=defaultdict(
+            _seen_dict,
+            {k: defaultdict(int, v) for k, v in checkpoint["versions_seen"].items()},
+        ),
     )
 
 
@@ -105,7 +108,7 @@ CheckpointThreadTs = ConfigurableFieldSpec(
 class BaseCheckpointSaver(ABC):
     at: CheckpointAt = CheckpointAt.END_OF_STEP
 
-    serde: SerializerProtocol
+    serde: SerializerProtocol = JsonPlusSerializer()
 
     def __init__(
         self,
diff --git a/langgraph/checkpoint/memory.py b/langgraph/checkpoint/memory.py
index c3348ed29..3f3ba5e63 100644
--- a/langgraph/checkpoint/memory.py
+++ b/langgraph/checkpoint/memory.py
@@ -1,5 +1,4 @@
 import asyncio
-import pickle
 from collections import defaultdict
 from typing import AsyncIterator, Iterator, Optional
 
@@ -15,8 +14,6 @@ from langgraph.checkpoint.base import (
 
 
 class MemorySaver(BaseCheckpointSaver):
-    serde = pickle
-
     storage: defaultdict[str, dict[str, Checkpoint]]
 
     def __init__(
diff --git a/langgraph/checkpoint/sqlite.py b/langgraph/checkpoint/sqlite.py
index 829874646..96e2bafcb 100644
--- a/langgraph/checkpoint/sqlite.py
+++ b/langgraph/checkpoint/sqlite.py
@@ -16,9 +16,13 @@ from langgraph.checkpoint.base import (
 )
 
 
-class SqliteSaver(BaseCheckpointSaver, AbstractContextManager):
-    serde = pickle
+# for backwards compat we continue to support loading pickled checkpoints
+def is_pickled(value: bytes) -> bool:
+    print(value, type(value))
+    return value.startswith(b"\x80") and value.endswith(b".")
 
+
+class SqliteSaver(BaseCheckpointSaver, AbstractContextManager):
     conn: sqlite3.Connection
 
     is_setup: bool
@@ -78,6 +82,11 @@ class SqliteSaver(BaseCheckpointSaver, AbstractContextManager):
                 self.conn.commit()
             cur.close()
 
+    def _loads(self, value: bytes) -> Checkpoint:
+        if is_pickled(value):
+            return pickle.loads(value)
+        return self.serde.loads(value)
+
     def get_tuple(self, config: RunnableConfig) -> Optional[CheckpointTuple]:
         with self.cursor(transaction=False) as cur:
             if config["configurable"].get("thread_ts"):
@@ -91,7 +100,7 @@ class SqliteSaver(BaseCheckpointSaver, AbstractContextManager):
                 if value := cur.fetchone():
                     return CheckpointTuple(
                         config,
-                        self.serde.loads(value[0]),
+                        self._loads(value[0]),
                         {
                             "configurable": {
                                 "thread_id": config["configurable"]["thread_id"],
@@ -114,7 +123,7 @@ class SqliteSaver(BaseCheckpointSaver, AbstractContextManager):
                                 "thread_ts": value[1],
                             }
                         },
-                        self.serde.loads(value[3]),
+                        self._loads(value[3]),
                         {
                             "configurable": {
                                 "thread_id": value[0],
@@ -134,7 +143,7 @@ class SqliteSaver(BaseCheckpointSaver, AbstractContextManager):
             for thread_id, thread_ts, parent_ts, value in cur:
                 yield CheckpointTuple(
                     {"configurable": {"thread_id": thread_id, "thread_ts": thread_ts}},
-                    self.serde.loads(value),
+                    self._loads(value),
                     {
                         "configurable": {
                             "thread_id": thread_id,
diff --git a/langgraph/serde/jsonplus.py b/langgraph/serde/jsonplus.py
index 1428bfb3a..9fe4b9d3c 100644
--- a/langgraph/serde/jsonplus.py
+++ b/langgraph/serde/jsonplus.py
@@ -90,7 +90,7 @@ class JsonPlusSerializer(SerializerProtocol):
         return LC_REVIVER(value)
 
     def dumps(self, obj: Any) -> bytes:
-        return json.dumps(obj, default=self._default, sort_keys=True)
+        return json.dumps(obj, default=self._default, sort_keys=True).encode()
 
     def loads(self, data: bytes) -> Any:
         return json.loads(data, object_hook=self._reviver)
diff --git a/tests/test_jsonplus.py b/tests/test_jsonplus.py
index 4dacfafb4..df4956a15 100644
--- a/tests/test_jsonplus.py
+++ b/tests/test_jsonplus.py
@@ -5,7 +5,6 @@ from datetime import datetime, timezone
 from enum import Enum
 
 import dataclasses_json
-import pytest
 from langchain_core.pydantic_v1 import BaseModel as LcBaseModel
 from langchain_core.runnables import RunnableMap
 from pydantic import BaseModel
@@ -83,7 +82,7 @@ def test_serde_jsonplus() -> None:
 
     assert (
         dumped
-        == """{"a_bool": true, "a_float": 1.1, "a_none": null, "a_str": "foo", "an_int": 1, "my_dataclass": {"args": [], "id": ["tests", "test_jsonplus", "MyDataclass"], "kwargs": {"bar": 1, "foo": "foo"}, "lc": 2, "method": null, "type": "constructor"}, "my_enum": {"args": ["foo"], "id": ["tests", "test_jsonplus", "MyEnum"], "kwargs": {}, "lc": 2, "method": null, "type": "constructor"}, "my_funny_pydantic": {"args": [], "id": ["tests", "test_jsonplus", "MyFunnyPydantic"], "kwargs": {"bar": 1, "foo": "foo"}, "lc": 2, "method": null, "type": "constructor"}, "my_pydantic": {"args": [], "id": ["tests", "test_jsonplus", "MyPydantic"], "kwargs": {"bar": 1, "foo": "foo"}, "lc": 2, "method": null, "type": "constructor"}, "my_slotted_class": {"args": [], "id": ["tests", "test_jsonplus", "MyDataclassWSlots"], "kwargs": {"bar": 2, "foo": "bar"}, "lc": 2, "method": null, "type": "constructor"}, "person": {"args": [], "id": ["tests", "test_jsonplus", "Person"], "kwargs": {"name": "foo"}, "lc": 2, "method": null, "type": "constructor"}, "runnable_map": {"graph": {"edges": [], "nodes": [{"data": "Parallel<>Input", "id": 0, "type": "schema"}, {"data": "Parallel<>Output", "id": 1, "type": "schema"}]}, "id": ["langchain", "schema", "runnable", "RunnableParallel"], "kwargs": {"steps__": {}}, "lc": 1, "name": "RunnableParallel<>", "type": "constructor"}, "time": {"args": ["2024-04-19T23:04:57.051022+23:59"], "id": ["datetime", "datetime"], "kwargs": {}, "lc": 2, "method": "fromisoformat", "type": "constructor"}, "uid": {"args": ["00000000000000000000000000000001"], "id": ["uuid", "UUID"], "kwargs": {}, "lc": 2, "method": null, "type": "constructor"}}"""
+        == b"""{"a_bool": true, "a_float": 1.1, "a_none": null, "a_str": "foo", "an_int": 1, "my_dataclass": {"args": [], "id": ["tests", "test_jsonplus", "MyDataclass"], "kwargs": {"bar": 1, "foo": "foo"}, "lc": 2, "method": null, "type": "constructor"}, "my_enum": {"args": ["foo"], "id": ["tests", "test_jsonplus", "MyEnum"], "kwargs": {}, "lc": 2, "method": null, "type": "constructor"}, "my_funny_pydantic": {"args": [], "id": ["tests", "test_jsonplus", "MyFunnyPydantic"], "kwargs": {"bar": 1, "foo": "foo"}, "lc": 2, "method": null, "type": "constructor"}, "my_pydantic": {"args": [], "id": ["tests", "test_jsonplus", "MyPydantic"], "kwargs": {"bar": 1, "foo": "foo"}, "lc": 2, "method": null, "type": "constructor"}, "my_slotted_class": {"args": [], "id": ["tests", "test_jsonplus", "MyDataclassWSlots"], "kwargs": {"bar": 2, "foo": "bar"}, "lc": 2, "method": null, "type": "constructor"}, "person": {"args": [], "id": ["tests", "test_jsonplus", "Person"], "kwargs": {"name": "foo"}, "lc": 2, "method": null, "type": "constructor"}, "runnable_map": {"graph": {"edges": [], "nodes": [{"data": "Parallel<>Input", "id": 0, "type": "schema"}, {"data": "Parallel<>Output", "id": 1, "type": "schema"}]}, "id": ["langchain", "schema", "runnable", "RunnableParallel"], "kwargs": {"steps__": {}}, "lc": 1, "name": "RunnableParallel<>", "type": "constructor"}, "time": {"args": ["2024-04-19T23:04:57.051022+23:59"], "id": ["datetime", "datetime"], "kwargs": {}, "lc": 2, "method": "fromisoformat", "type": "constructor"}, "uid": {"args": ["00000000000000000000000000000001"], "id": ["uuid", "UUID"], "kwargs": {}, "lc": 2, "method": null, "type": "constructor"}}"""
     )
 
     assert serde.loads(dumped) == to_serialize