fix(langgraph): raise when a written DeltaChannel is hydrated without a saver

A missing saver used to hydrate the channel as empty, silently. Raise when a
DeltaChannel has a version but no stored value and no saver was passed.
Keying on the version keeps a never-written channel, which has no value
either, reading as empty without a saver.

libs/langgraph/langgraph/pregel/_checkpoint.py

@@ -226,6 +226,19 @@ def _needs_replay(spec: BaseChannel, stored: object) -> bool:
     return stored is MISSING
 
 
+def _require_saver_for_history(
+    checkpoint: Checkpoint,
+    delta_channels: list[str],
+    saver: BaseCheckpointSaver | None,
+) -> None:
+    written = [k for k in delta_channels if k in checkpoint["channel_versions"]]
+    if written and saver is None:
+        raise ValueError(
+            f"DeltaChannel {written} has history to replay but no checkpointer "
+            "was passed to read it"
+        )
+
+
 def channels_from_checkpoint(
     specs: Mapping[str, BaseChannel | ManagedValueSpec],
     checkpoint: Checkpoint,
@@ -256,6 +269,7 @@ def channels_from_checkpoint(
         for k, spec in channel_specs.items()
         if _needs_replay(spec, checkpoint["channel_values"].get(k, MISSING))
     ]
+    _require_saver_for_history(checkpoint, delta_channels, saver)
     histories: Mapping[str, Any] = {}
     if delta_channels and saver is not None and config is not None:
         histories = saver.get_delta_channel_history(
@@ -298,6 +312,7 @@ async def achannels_from_checkpoint(
         for k, spec in channel_specs.items()
         if _needs_replay(spec, checkpoint["channel_values"].get(k, MISSING))
     ]
+    _require_saver_for_history(checkpoint, delta_channels, saver)
     histories: Mapping[str, Any] = {}
     if delta_channels and saver is not None and config is not None:
         histories = await saver.aget_delta_channel_history(

libs/langgraph/tests/test_delta_channel_subgraph.py

@@ -7,6 +7,11 @@ from typing_extensions import TypedDict
 
 from langgraph.channels.delta import DeltaChannel
 from langgraph.graph import END, START, StateGraph
+from langgraph.pregel._checkpoint import (
+    achannels_from_checkpoint,
+    channels_from_checkpoint,
+    empty_checkpoint,
+)
 
 pytestmark = pytest.mark.anyio
 
@@ -251,3 +256,30 @@ def test_completed_subgraph_exposes_no_task_state(
     app.invoke({}, config)
 
     assert app.get_state(config, subgraphs=True).tasks == ()
+
+
+def _written_delta_checkpoint() -> Any:
+    checkpoint = empty_checkpoint()
+    checkpoint["channel_versions"]["delta"] = 1
+    return checkpoint
+
+
+def test_hydrating_written_delta_channel_without_saver_raises() -> None:
+    with pytest.raises(ValueError, match="no checkpointer"):
+        channels_from_checkpoint(
+            {"delta": DeltaChannel(_extend)}, _written_delta_checkpoint()
+        )
+
+
+async def test_ahydrating_written_delta_channel_without_saver_raises() -> None:
+    with pytest.raises(ValueError, match="no checkpointer"):
+        await achannels_from_checkpoint(
+            {"delta": DeltaChannel(_extend)}, _written_delta_checkpoint()
+        )
+
+
+def test_hydrating_unwritten_delta_channel_without_saver_is_empty() -> None:
+    channels, _ = channels_from_checkpoint(
+        {"delta": DeltaChannel(_extend)}, empty_checkpoint()
+    )
+    assert channels["delta"].get() == []