fix(langgraph): share one saver resolution between runs and state methods

`_defaults` picked the run's saver on its own, and disagreed with the state
methods on `checkpointer=False`: a run ignored a lent saver while
`update_state` wrote a checkpoint with it that no run reads. A
`checkpointer=True` root also got `True` back from `_state_checkpointer`
and failed with an AttributeError instead of the run's error. Both now go
through `_resolve_checkpointer`, and the state methods require an actual
saver.

libs/langgraph/langgraph/pregel/_checkpoint.py

@@ -236,7 +236,7 @@ def _require_saver_for_history(
     if written and (saver is None or config is None):
         raise ValueError(
             f"DeltaChannel {written} has history to replay but no checkpointer "
-            "and config were passed to read it"
+            "or config was passed to read it"
         )
 
 

libs/langgraph/langgraph/pregel/main.py

@@ -835,15 +835,28 @@ class Pregel(
         if auto_validate:
             self.validate()
 
-    def _state_checkpointer(self, config: RunnableConfig) -> BaseCheckpointSaver:
-        """The checkpointer state reads and writes use: the one a parent lends a
-        subgraph through the config, else this graph's own."""
-        checkpointer = ensure_config(config)[CONF].get(
-            CONFIG_KEY_CHECKPOINTER, self.checkpointer
-        )
+    def _resolve_checkpointer(
+        self, config: RunnableConfig
+    ) -> BaseCheckpointSaver | None:
+        """The saver runs and state methods use: none for `checkpointer=False`,
+        else the one a parent lends a subgraph through the config, else this
+        graph's own."""
+        if self.checkpointer is False:
+            return None
+        conf = config.get(CONF, {})
+        if CONFIG_KEY_CHECKPOINTER in conf:
+            checkpointer = conf[CONFIG_KEY_CHECKPOINTER]
+        elif self.checkpointer is True:
+            raise RuntimeError("checkpointer=True cannot be used for root graphs.")
+        else:
+            checkpointer = self.checkpointer
         if isinstance(checkpointer, BaseCheckpointSaver):
             checkpointer = self._apply_checkpointer_allowlist(checkpointer)
-        if not checkpointer:
+        return checkpointer
+
+    def _state_checkpointer(self, config: RunnableConfig) -> BaseCheckpointSaver:
+        checkpointer = self._resolve_checkpointer(ensure_config(config))
+        if not isinstance(checkpointer, BaseCheckpointSaver):
             raise ValueError("No checkpointer set")
         return checkpointer
 
@@ -1648,7 +1661,7 @@ class Pregel(
             channels, managed = channels_from_checkpoint(
                 self.channels,
                 checkpoint,
-                saver=checkpointer if saved is not None else None,
+                saver=checkpointer,
                 config=saved.config if saved is not None else None,
             )
             values, as_node = updates[0][:2]
@@ -2107,7 +2120,7 @@ class Pregel(
             channels, managed = await achannels_from_checkpoint(
                 self.channels,
                 checkpoint,
-                saver=checkpointer if saved is not None else None,
+                saver=checkpointer,
                 config=saved.config if saved is not None else None,
             )
             values, as_node = updates[0][:2]
@@ -2548,16 +2561,7 @@ class Pregel(
             stream_modes.add(print_mode)
         else:
             stream_modes.update(print_mode)
-        if self.checkpointer is False:
-            checkpointer: BaseCheckpointSaver | None = None
-        elif CONFIG_KEY_CHECKPOINTER in config.get(CONF, {}):
-            checkpointer = config[CONF][CONFIG_KEY_CHECKPOINTER]
-        elif self.checkpointer is True:
-            raise RuntimeError("checkpointer=True cannot be used for root graphs.")
-        else:
-            checkpointer = self.checkpointer
-        if isinstance(checkpointer, BaseCheckpointSaver):
-            checkpointer = self._apply_checkpointer_allowlist(checkpointer)
+        checkpointer = self._resolve_checkpointer(config)
         if checkpointer and not config.get(CONF):
             raise ValueError(
                 "Checkpointer requires one or more of the following 'configurable' "

libs/langgraph/tests/test_delta_channel_subgraph.py

@@ -6,6 +6,7 @@ from langgraph.checkpoint.base import BaseCheckpointSaver
 from langgraph.checkpoint.memory import InMemorySaver
 from typing_extensions import TypedDict
 
+from langgraph._internal._constants import CONFIG_KEY_CHECKPOINTER
 from langgraph.channels.delta import DeltaChannel
 from langgraph.graph import END, START, StateGraph
 from langgraph.pregel._checkpoint import (
@@ -371,3 +372,24 @@ def test_hydrating_unwritten_delta_channel_without_saver_is_empty() -> None:
         {"delta": DeltaChannel(_extend)}, empty_checkpoint()
     )
     assert channels["delta"].get() == []
+
+
+def test_root_checkpointer_true_graph_state_read_raises() -> None:
+    app = _child_builder().compile(checkpointer=True)
+    with pytest.raises(RuntimeError, match="checkpointer=True cannot be used"):
+        app.get_state({"configurable": {"thread_id": "1"}})
+
+
+def test_stateless_graph_update_state_ignores_lent_saver() -> None:
+    saver = InMemorySaver()
+    app = _child_builder().compile(checkpointer=False)
+    config = {
+        "configurable": {
+            "thread_id": "1",
+            "checkpoint_ns": "child:1",
+            CONFIG_KEY_CHECKPOINTER: saver,
+        }
+    }
+    with pytest.raises(ValueError, match="No checkpointer set"):
+        app.update_state(config, _both("x"), as_node="a")
+    assert list(saver.list(None)) == []