Send objects are not serializable, causing checkpointer failures

Send objects are not serializable, causing checkpointer failures

TL;DR: if your map-reduce graph (the one using Send()) crashes when writing checkpoints, upgrade langgraph. Older versions could not serialize Send objects into checkpoints; current versions can. Your graph code does not need to change.

TypeError: ... is not serializable
# raised from the checkpointer (ormsgpack / JsonPlusSerializer path)
# when a Send object lands in pending_sends during a checkpoint write.

When this applies

• You use Send() for map-reduce fan-out (return [Send("worker", {...}) for ...]).
• The graph runs fine WITHOUT a checkpointer but fails WITH one.
• The traceback points at serialization (ormsgpack, msgpack, JsonPlusSerializer) during a checkpoint write.

When it does not

• TypeError: cannot pickle 'X' object about YOUR state values (an LLM client, a lock) means your state holds something unserializable. Remove it from state instead.
• If Send fails without any checkpointer, the bug is in your Send usage.

Fix it

1. Upgrade langgraph

# terminal
pip install -U langgraph

Expected: the same map-reduce graph now checkpoints cleanly. The issue reporter confirmed the failure was gone on the latest version with no code changes.

2. Confirm the version actually moved

# terminal
pip show langgraph | grep Version

If you pin langgraph in a lockfile, bump the pin and redeploy. Editable installs and stale .pyc caches can shadow the upgrade: if the error persists after upgrading, reinstall cleanly.

3. Sanity-check the Send pattern itself

from langgraph.types import Send

def fan_out(state):
    # Send takes a node name and the arg dict for that invocation
    return [Send("worker", {"item": item}) for item in state["items"]]

Expected: each Send routes one worker invocation. The serialization fix handles the rest.

Why it happens

When a node returns Send objects, Pregel holds them in pending_sends until the target nodes run. With a checkpointer attached, that pending state gets serialized into the checkpoint. Older langgraph versions had no serializer registered for the Send type, so the write blew up. Newer versions serialize it like any other checkpoint payload.

Edge cases

• The args you pass inside Send("worker", {...}) must themselves be serializable. A dict of strings is fine; an open file handle is not.
• If you implement a CUSTOM serializer/serde, register Send handling there too, or writes fail the same way on your custom path.
• Deep-agent style frameworks built on langgraph hit this through the same code path; upgrading the underlying langgraph fixes them as well.

Compatibility

langgraph Python. Broken: older 0.x/1.x versions (fixed in later releases). If you are current and still see it, check for a custom serde as above.