Over the past 12 months, tens of thousands of developers have built and productionized industry-leading voice agents using our frontier speech models, Sonic and Ink. In serving these developers, we found that even the most sophisticated teams struggled to build great voice agents with the tools available today.

Voice agents are uniquely hard to bring to production: they require maintaining natural, low-latency conversations with advanced reasoning across millions of calls. A few challenges come up repeatedly—more reasoning introduces higher latency, small changes to one component can cause unpredictable quality swings, and fragile infrastructure across multiple providers make scaling unpredictable.

That’s why we built Line. Line is the modern voice agent development platform, making it easy for developers and businesses everywhere to build best-in-class voice agents.

Line is code-first

The best products are built with code. We realized this early as we reflected on the challenges developers faced. Great businesses transform industries by putting code at the center – Stripe reimagined payments, Twilio transformed communications, and Vercel redefined web.

Intelligent agents and great conversations are impossible to express in rigid conversational builders, which put limits on reasoning ability. Instead, the best voice agents should be like the best humans, using fluid context, advanced logic, and background reasoning.

Line uses code to give you the flexibility to perfect every part of your voice agent. The Line SDK makes it intuitive to build advanced voice agents with background reasoning, and is designed to cover any use case. Edge cases are easy to handle directly in code, and you can integrate your favorite packages alongside our SDK, no compromises necessary.

With AI, code is the fastest way to build and more accessible than ever. Developers are writing more lines of code than ever, using AI to build complex applications quickly. Our code-first approach and SDK roadmap is designed with this future in mind.

Line helps you iterate fast

Building great voice agents is an iterative process. Progress comes from making changes, testing them in live conversations, learning what works, and adjusting based on feedback. Line helps you move through that cycle quickly.

Line gets you to your first agent in minutes, starting from a text prompt or template. With our CLI and GitHub integrations, you can develop agents locally, deploy with one command to our platform, and talk to your agents in seconds. You can even share your agent with others.

Every iteration requires comprehensive evals and manual testing to get right. Developers constantly refine prompts, business logic, and background reasoning. They need to measure progress on their agents over time.

Line records all calls to deployed agents, saves the audio and transcripts, reports system metrics like latency, and provides comprehensive logs for auditing and debugging. Developers can use custom LLM-as-a-judge metrics to rate calls along any axis, including user satisfaction and call success.

Line brings you our frontier models and infrastructure

Best-in-class voice agents require frontier speech models. At Cartesia, our research team is pioneering the world’s best speech models using breakthrough advances in AI architectures.

Sonic is the world’s lowest latency text-to-speech model, with ultra-realistic conversational voices, and reliable transcript following. Ink is the fastest streaming speech-to-text model for speech understanding. Collectively, Sonic and Ink have powered millions of calls for businesses, from the fastest growing companies in voice AI to the Fortune 500.

Deeply integrating models and infrastructure creates new opportunities to optimize the end-to-end voice agent experience. Line voice agents are deployed alongside our Sonic and Ink models for the fastest latencies, and they'll be the first to access our latest research advances in speech modeling.

For enterprises, this integration extends further: Line can be deployed entirely on-prem, from voice agents to models, and models can be customized with fine-tuning.

Line, Sonic, and Ink run on Cartesia’s reliable, globally distributed infrastructure, allowing every deployed voice agent to scale to thousands of calls while maintaining low latency. 

Line is now available

Line is available today to all developers. As part of our launch, all subscription tiers will get their equivalent monthly plan dollars prepaid towards agents—at no extra cost.

Our north-star is simple. No matter how complex the code you write, when you deploy to the Line platform, every call should be a flawless experience.

With Line, we hope to make frontier voice AI accessible to everyone, and in turn, to make AI more accessible to the world.

We’re excited to see what you build. Sign up and check out the docs now to create your first voice agent. Contact us to start enterprise plan discussions.