graph.stream() skips some tool_call and ToolMessage events, but they're in MongoDB checkpoints

[sveyek]

I'm building a LangGraph JS system using the prebuilt supervisor-based architecture. I'm using MongoDBSaver for memory. I stream the graph output to the frontend using SSE, and to do that, I loop over graph.stream() like this:

for await (const chunk of await selectedAgent.stream(state, {
  configurable: { thread_id },
  streamMode: "updates",
  signal: controller.signal,
})) {
  console.log("Chunk: ", chunk);
}

The problem is that some chunks are missing in this stream — specifically intermediate tool_call and their corresponding ToolMessage. For example, when the supervisor hands off to a child agent via tool_call, that transition or its result sometimes doesn't show up in the streamed chunks.

However, when I check the checkpoints collection in MongoDB for the same thread, the full set of messages (including the missing ones) is there.

I also tried other streamMode values (like values, messages, etc.), but the result is still the same — the same chunks are missing in the stream output.

Why are some intermediate messages missing in the stream but present in the persisted memory? Is this expected behavior? If not, how can I ensure all messages (including intermediate tool interactions) are emitted in real time through the stream?