How do you implement human-in-the-loop with LangGraph?

Human-in-the-loop (HITL) in LangGraph means pausing graph execution at a specified point so a human can inspect the current state, approve an action, or modify a value before the graph continues. This is a first-class LangGraph feature built on top of checkpointing.

Step 1: Compile the graph with interrupt_before or interrupt_after

from langgraph.checkpoint.memory import MemorySaver

graph = graph_builder.compile(
    checkpointer=MemorySaver(),
    interrupt_before=["tool_executor"],  # pause before this node runs
)

Step 2: Run until the interrupt

config = {"configurable": {"thread_id": "session-1"}}
for event in graph.stream({"messages": [HumanMessage("Search for X")]}, config):
    print(event)  # stops before tool_executor

Step 3: Inspect and optionally update state

current_state = graph.get_state(config)
print(current_state.values)  # see what the agent is about to do

# Optionally modify the state before continuing:
graph.update_state(config, {"messages": [HumanMessage("Actually search for Y")]}, as_node="agent")

Step 4: Resume execution

# Pass None as input to resume from the checkpoint
for event in graph.stream(None, config):
    print(event)

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