Vercel AI SDK

Vercel AI SDK

An open source library for building AI-powered user interfaces.

The Vercel AI SDK is an open-source library designed to help developers build conversational streaming user interfaces in JavaScript and TypeScript. The SDK supports React/Next.js, Svelte/SvelteKit, and Vue/Nuxt as well as Node.js, Serverless, and the Edge Runtime(opens in a new tab).



Streaming conversational text UIs (like ChatGPT) have gained massive popularity over the past few months. This section will explore the benefits and drawbacks of streaming and blocking interfaces and how to implement them using the Vercel AI SDK.

Large Language Models (LLMs) are extremely powerful. However, when generating long outputs, they can be very slow compared to the latency you're likely used to. If you try to build a traditional blocking UI, your users might easily find themselves staring at loading spinners for 5, 10, even up to 40s waiting for the entire LLM response to be generated. This can lead to a poor user experience, especially in conversational applications like chatbots. Streaming UIs can help mitigate this issue by displaying parts of the response as they become available.

Stream Back-pressure and Cancellation

You do not need to know this information to use the Vercel AI SDK, but for those interested, it offers a deeper dive on why and how the SDK optimally streams responses. Understanding back-pressure and cancellation when working with streams is important.

In the following sections, we'll explore back-pressure and cancellation in the context of a simple example program. We'll discuss the issues that can arise from an eager approach and demonstrate how a lazy approach can resolve them.

Prompt Engineering

What is a Large Language Model (LLM)?

A Large Language Model is essentially a prediction engine that takes a sequence of words as input and aims to predict the most likely sequence to follow. It does this by assigning probabilities to potential next sequences and then selecting one. The model continues to generate sequences until it meets a specified stopping criterion.

These models learn by training on massive text corpuses, which means they will be better suited to some use cases than others. For example, a model trained on GitHub data would understand the probabilities of sequences in source code particularly well. However, it's crucial to understand that the generated sequences, while often seeming plausible, can sometimes be random and not grounded in reality. As these models become more accurate, many surprising abilities and applications emerge.

What is a prompt?

Prompts are the starting points for LLMs. They are the inputs that trigger the model to generate text. The scope of prompt engineering involves not just crafting these prompts but also understanding related concepts such as hidden prompts, tokens, token limits, and the potential for prompt hacking, which includes phenomena like jailbreaks and leaks.

Why is prompt engineering needed?

Prompt engineering currently plays a pivotal role in shaping the responses of LLMs. It allows us to tweak the model to respond more effectively to a broader range of queries. This includes the use of techniques like semantic search, command grammars, and the ReActive model architecture. The performance, context window, and cost of LLMs varies between models and model providers which adds further constraints to the mix. For example, the GPT-4 model is more expensive than GPT-3.5-turbo and significantly slower, but it can also be more effective at certain tasks. And so, like many things in software engineering, there is a trade-offs between cost and performance.

To assist with comparing and tweaking LLMs, we've built an AI playground that allows you to compare the performance of different models side-by-side online. When you're ready, you can even generate code with the Vercel AI SDK to quickly use your prompt and your selected model into your own applications.

Caching Responses

Depending on the type of application you're building, you may want to cache the responses you receive from your AI provider, at least temporarily.

Each stream helper for each provider has special lifecycle callbacks you can use. The one of interest is likely onCompletion, which is called when the stream is closed. This is where you can cache the full response.

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