Vercel AI SDK & LangChain streaming

If you got stuck like me for hours on how to use the new Vercel AI SDK with Langchain to stream, the trick is not to use the LangChainStream from Vercel's here https://sdk.vercel.ai/docs/api-reference/providers/langchain-stream

But to use the .stream with an outputparser here

https://sdk.vercel.ai/docs/guides/providers/langchain

let completion = ''

const transformStream = new TransformStream({
  transform(chunk, controller) {
    // accumulate the assistant answer aka completion
    completion += new TextDecoder('utf-8').decode(chunk)
    controller.enqueue(chunk)
  },
  flush(controller) {
    console.log('completion', completion)
    // do something like save history.. similar
    // …

assistant chunk decode history langchain llm save something utf-8 vercel