onError callback in useChat hook is not working.

[Praneeth-Pike]

Description

Error message is not being passed to onError callback in useChat hook. I'm using a custom express server for the /chat endpoint.

Code example

// useChat hook

const { messages, input, setInput, handleSubmit, isLoading, stop, error } =
		useChat({
			id,
			api:  `${EXPRESS_API_URL}/chat`,
			initialMessages: data.messages,
			onError: (error) => {
				console.error('error on chat', error)
			},
			onFinish: async (message, { finishReason, usage }) => {
				console.log(`onFinish message id: ${message.id}`, finishReason)
				console.log('usage', usage)
			},
		})

/chat endpoint

expressApp.post('/chat', async (req: Request, res: Response) => {
	try {
		const { messages, context } = req.body

		const model = openai("gpt-4o")

		const payload = {
			model,
			messages: convertToCoreMessages([
				{
					role: 'system',
					content: context
						? `<MetaPrompt>${ChatSystemPrompt}</MetaPrompt>\n<PreviousContext>${context}</PreviousContext>`
						: `<MetaPrompt>${ChatSystemPrompt}</MetaPrompt>`,
				},
				...messages,
			]),
		}

		const result = streamText({
			...payload,
		})

		return result.pipeDataStreamToResponse(res, {
			sendUsage: true,
			getErrorMessage: (error) => {
				if (error == null) {
					return 'unknown error'
				}

				if (typeof error === 'string') {
					return error
				}

				if (error instanceof Error) {
					return error.message
				}

				return JSON.stringify(error)
			},
		})
	} catch (error: any) {
		log.error('Error in /chat:', error)
		res.status(500).json({
			error: error.message ?? 'Internal server error',
		})
	}
})

AI provider

No response

Additional context

onError works with the same code for v3.4, but when I upgrade to v4 and remove the await from streamText, onError is not being passed to the client hook

[oalexdoda]

Same issue over here. Sometimes useChat errors out (server-side) and it doesn't return any error. Maybe it's missing something here?