Langgraph handoff issues when using prebuilt agents

[alywonder]

I am trying to build a multi-agent system. When I build it following the examples, everything works fine. I realize that in most of those examples, each agent is a prebuilt one, mostly react agent.

But when I create my own agent with different graph, I find one curious problem. I see that the handoff is happening, but it does not execute. I can see that the active agent has changed and my next conversation does get handled by that agent.

I have two agents, starter and builder. I have included the prebuilt version and modified version.

Using prebuilt agents only

transfer_to_builder = create_handoff_tool(
    agent_name="builder",
    description="Transfer user to the report builder agent to build a new report.",
)

abort_report_build = create_handoff_tool(
    agent_name="starter",
    description="Transfer user to the starter agent if user chooses to end the report build process.",
)

starter = create_react_agent(
    model,
    [list_reports, transfer_to_builder],
    prompt=make_prompt(STARTER_INSTRUCTIONS),
    name="starter",
)

builder = create_react_agent(
    model,
    [abort_report_build],
    prompt=make_prompt(BUILDER_INSTRUCTIONS),
    name="builder",
)

raph_builder = create_swarm(
    [starter, builder], default_active_agent="starter"
)
app = graph_builder.compile()

This case works fine.

1. Starts with starter agent
2. Handoff to builder agent with request to build a report
3. When asked to abort the build process, handoff to starter agent
4. starter agent continue with the user

Using modified version of builder
I modified the definition of builder as below:

def initialize_node(state: AgentState, config: RunnableConfig):
    pass

builder_graph = StateGraph(AgentState)
builder_graph.add_node("report_builder", create_react_agent(
    model,
    [abort_report_build],
    prompt=make_prompt(BUILDER_INSTRUCTIONS),
    name="builder",
))
builder_graph.add_node("initialize", initialize_node)

builder_graph.add_edge(START, "initialize")
builder_graph.add_edge("initialize", "report_builder")
builder_graph.add_edge("report_builder", END)

builder_graph.set_entry_point("initialize")

builder = builder_graph.compile(name="builder")

Here step 1-3 works as in the previous case, but when I do a handoff back to starter, it does not run.

1. Starts with starter agent
2. Handoff to builder agent with request to build a report
3. When asked to abort the build process, handoff to starter agent
4. starter agent DOES NOT RUN

The active agent has changed and if I initiate a conversation, I can continue conversation with the starter agent. Please note that the handoff is working perfectly from starter to builder and it is the builder that I changed.

Anyone encountered this problem? Any ideas as to what could be causing this?