LangChain LangGraph Human-in-the-Loop (Python)

Overview

A team adds interrupt_before=["send_email"] to require a human approval before the email goes out. First integration test crashes at the interrupt boundary with:

TypeError: Object of type datetime is not JSON serializable

The culprit is two nodes upstream: a classify node stashed "received_at": datetime.utcnow() into state. Every node-level unit test passed because node completion does not serialize state — only the checkpointer does, and only at supersteps that include an interrupt. The failure is invisible until interrupt time (P17).

A week later the resume path ships. The human reviews the draft, clicks "approve with edits," and the backend runs: