Better error message on invalid use lf lastvalue channel

langgraph/channels/last_value.py

@@ -44,7 +44,7 @@ class LastValue(Generic[Value], BaseChannel[Value, Value, Value]):
         if len(values) == 0:
             return
         if len(values) != 1:
-            raise InvalidUpdateError()
+            raise InvalidUpdateError("LastValue can only receive one value per step.")
 
         self.value = values[-1]
 

langgraph/pregel/__init__.py

@@ -46,6 +46,7 @@ from langgraph.channels.base import (
     BaseChannel,
     ChannelsManager,
     EmptyChannelError,
+    InvalidUpdateError,
     create_checkpoint,
 )
 from langgraph.channels.last_value import LastValue
@@ -684,7 +685,12 @@ def _apply_writes(
     # Apply writes to channels
     for chan, vals in pending_writes_by_channel.items():
         if chan in channels:
-            channels[chan].update(vals)
+            try:
+                channels[chan].update(vals)
+            except InvalidUpdateError as e:
+                raise InvalidUpdateError(
+                    f"Invalid update for channel {chan}: {e}"
+                ) from e
             checkpoint["channel_versions"][chan] += 1
             updated_channels.add(chan)
         else: