From aa2eb4606d73a616b50b1ea117e51de453d54baa Mon Sep 17 00:00:00 2001
From: Nuno Campos <nuno@langchain.dev>
Date: Wed, 17 Jan 2024 10:32:10 -0800
Subject: [PATCH] Better error message on invalid use lf lastvalue channel

---
 langgraph/channels/last_value.py | 2 +-
 langgraph/pregel/__init__.py     | 8 +++++++-
 2 files changed, 8 insertions(+), 2 deletions(-)

diff --git a/langgraph/channels/last_value.py b/langgraph/channels/last_value.py
index c585ceb86..858943c91 100644
--- a/langgraph/channels/last_value.py
+++ b/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]
 
diff --git a/langgraph/pregel/__init__.py b/langgraph/pregel/__init__.py
index b557b9c5d..6a714f271 100644
--- a/langgraph/pregel/__init__.py
+++ b/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: