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feat(channels): DeltaChannel tracks checkpoint_id; emits prev_checkpoint_id in DeltaValue

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Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
This commit is contained in:
Sydney Runkle
2026-04-22 14:03:37 -04:00
co-authored by Claude Sonnet 4.6
parent 4608af9615
commit fa83d64eff
3 changed files with 28 additions and 24 deletions
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libs/langgraph/tests/test_channels.py
+14 -14
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@@ -134,13 +134,13 @@ def test_delta_channel_basic_two_steps() -> None:
d1 = ch.checkpoint()
assert isinstance(d1, DeltaValue)
assert len(d1.delta) == 1
assert d1.prev_version is None # first ever step
ch.after_checkpoint("v1")
assert d1.prev_checkpoint_id is None # first ever step
ch.after_checkpoint("v1", checkpoint_id="cid1")
# Step 2: another message
ch.update([AIMessage(content="hello", id="a1")])
d2 = ch.checkpoint()
assert d2.prev_version == "v1"
assert d2.prev_checkpoint_id == "cid1"
assert len(d2.delta) == 1
ch.after_checkpoint("v2")
@@ -217,11 +217,11 @@ def test_delta_channel_overwrite_resets_chain() -> None:
ch.update([HumanMessage(content="old", id="h1")])
ch.after_checkpoint("v1")
# Overwrite should create a root blob (prev_version=None)
# Overwrite should create a root blob (prev_checkpoint_id=None)
ch.update([Overwrite([HumanMessage(content="new", id="h2")])])
d = ch.checkpoint()
assert isinstance(d, DeltaValue)
assert d.prev_version is None # chain root
assert d.prev_checkpoint_id is None # chain root
assert len(d.delta) == 1
assert d.delta[0].content == "new"
@@ -232,10 +232,10 @@ def test_delta_channel_unsupported_saver_raises() -> None:
from langgraph.channels.delta import DeltaChannel
from langgraph.graph.message import add_messages
# If a saver returns a raw DeltaValue (unsupported), from_checkpoint raises
# If a saver returns a raw DeltaValue, from_checkpoint raises AssertionError
spec = DeltaChannel(add_messages)
raw_delta = DeltaValue(delta=[], prev_version=None)
with pytest.raises(ValueError, match="DeltaChannel received a raw DeltaValue"):
raw_delta = DeltaValue(delta=[], prev_checkpoint_id=None)
with pytest.raises(AssertionError, match="DeltaChannel.from_checkpoint received a raw DeltaValue"):
spec.from_checkpoint(raw_delta)
@@ -256,7 +256,7 @@ def test_delta_channel_remove_message_delta_and_replay() -> None:
ch.update([AIMessage(content="hello", id="a1")])
d1 = ch.checkpoint()
assert isinstance(d1, DeltaValue)
ch.after_checkpoint("v1")
ch.after_checkpoint("v1", checkpoint_id="cid1")
assert ch.get() == [
HumanMessage(content="hi", id="h1"),
AIMessage(content="hello", id="a1"),
@@ -266,9 +266,9 @@ def test_delta_channel_remove_message_delta_and_replay() -> None:
ch.update([RemoveMessage(id="a1")])
d2 = ch.checkpoint()
assert isinstance(d2, DeltaValue)
assert d2.prev_version == "v1"
assert d2.prev_checkpoint_id == "cid1"
assert any(isinstance(w, RemoveMessage) for w in d2.delta)
ch.after_checkpoint("v2")
ch.after_checkpoint("v2", checkpoint_id="cid2")
assert ch.get() == [HumanMessage(content="hi", id="h1")]
# Replay the full chain from scratch — must reproduce the post-remove state
@@ -293,14 +293,14 @@ def test_delta_channel_update_by_id_delta_and_replay() -> None:
ch.update([HumanMessage(content="original", id="h1")])
d1 = ch.checkpoint()
assert isinstance(d1, DeltaValue)
ch.after_checkpoint("v1")
ch.after_checkpoint("v1", checkpoint_id="cid1")
# Step 2: update the same message by ID
ch.update([HumanMessage(content="updated", id="h1")])
d2 = ch.checkpoint()
assert isinstance(d2, DeltaValue)
assert d2.prev_version == "v1"
ch.after_checkpoint("v2")
assert d2.prev_checkpoint_id == "cid1"
ch.after_checkpoint("v2", checkpoint_id="cid2")
assert ch.get() == [HumanMessage(content="updated", id="h1")]
# Replay the full chain — must produce the updated message, not the original