Next.js + Vercel AI SDK + Langfuse: How to get observationId (responseId) for langfuse scores?

[natterstefan]

Dear vercel/ai-Community,

I need your help please, as I do not know how to obtain the observationId and attach it to a score in langfuse.

Note: originally asked in the langfuse community, but they sent me here with the following hint: "(...) you'd need the SDK to expose the span ids of each of the requests." (src)

---

Context: this is how my experimental_telemetry looks like:

// src: https://langfuse.com/docs/integrations/vercel-ai-sdk#group-multiple-executions-in-one-trace

const parentTraceId = getRandomId()

export async function POST(req: Request) {
	// ... other code

	const trace = langfuse.trace({
	  id: parentTraceId,
	  name: 'cd-chat-messages',
	})

	console.log('trace', trace.observationId) // <<-- this returns `null`
	
	const result = streamText({
		// ....
		experimental_telemetry: {
		    isEnabled: true,
		    functionId: 'cd-chat-message-' + messages.length,
		    metadata: {
		      langfuseTraceId: parentTraceId,
			  // Do not update the parent trace with execution results
		      langfuseUpdateParent: false,
		    },
		  },
	})

	// ... other code
}

The docs state

The id of the observation to which the score should be attached. Pass from backend trace. E.g. {generation, event, span}.id

But how and where do I get the observationId from, taking into account that I use streamText (https://sdk.vercel.ai/docs/reference/ai-sdk-core/stream-text)

// source: https://langfuse.com/docs/sdk/typescript/guide-web#create-scores

// pass traceId and observationId to front end
await langfuseWeb.score({
  traceId: message.traceId,
  observationId: message.observationId, <<<-- where do I get this?
  name: "user-feedback",
  value: 1,
  comment: "I like how personalized the response is",
});

Thank you very much for your time and help!