Cartesia Sonic 3 vs Amazon Polly: Which TTS is best for AI agents in 2025?

The voice of your AI agent is basically the voice of your brand. So, picking the right text-to-speech (TTS) engine is a pretty big deal. It’s the difference between a smooth, real-time conversation that customers don’t mind having, and a clunky, robotic experience that makes them just want to talk to a person.

Let’s look at two of the heavy hitters in this space: Cartesia Sonic 3 and Amazon Polly. We’re going to put them side-by-side to see how they really perform when it comes to customer support and other voice AI needs.

This guide will walk you through their voice quality, speed, pricing, and key features so you can make a solid choice. More than that, we’ll talk about the bigger picture, what it actually takes to build a complete AI agent that doesn’t just talk, but solves problems.

Understanding TTS for AI agents

Text-to-speech is the tech that turns words on a screen into spoken audio. For customer support, this isn’t just a nice-to-have; it’s the foundation of the entire interaction. A natural, quick voice helps build trust and makes customers feel like they’re being listened to. A slow, robotic voice does the exact opposite, it creates friction, ramps up frustration, and usually ends in an escalation.

Let’s meet our two main players.

A look at Cartesia Sonic 3

Cartesia is an AI voice platform that’s been making waves for its super-realistic and incredibly fast voice generation. It’s designed specifically for conversations that happen in real time. Their main claims to fame are top-notch performance (meaning very low wait time for the first bit of audio), impressive voice cloning from just a few seconds of a recording, and an output that’s clean of the strange errors some models spit out.

A look at Amazon Polly

Amazon Polly is the go-to, reliable TTS service from Amazon Web Services (AWS). If you’ve spent any time in the AWS world, you’ve probably heard of it. Its biggest advantages are its tight integration with other AWS services, support for a ton of languages, and different voice types (Standard, Neural, and Generative) that let you find the right balance between cost and quality for what you need.

Comparing Cartesia Sonic 3 vs Amazon Polly: The core differences

Figuring out the “best” TTS engine comes down to what you care about most. Are you after the most human-sounding voice you can get, regardless of price? Is a lightning-fast response essential for your real-time chat? Or is your focus on keeping the budget in check as you scale?

Let’s dig in.

Voice quality and naturalness

In customer support, you have to avoid that weird, slightly-off robot voice that gives everyone the creeps. A natural, warm tone can calm down a tense customer, while a robotic one just adds fuel to the fire.

• Cartesia: In a lot of head-to-head comparisons, Cartesia tends to get high marks for sounding natural and expressive. People often say its voices are hard to tell apart from a real person’s, and they can handle subtle emotional shifts. That’s a huge win for conversations that need a bit of empathy.

• Amazon Polly: Polly’s voices are clear and dependable, no question. But to get something that sounds as natural as Cartesia, you’ll need to spring for its pricier Neural and Generative tiers. The Standard voices are budget-friendly, but they can sound noticeably more robotic and probably aren’t the right fit for your main customer-facing agent.

The takeaway: Both are good, but Cartesia seems to have a leg up in creating genuinely lifelike voices right away. For navigating tricky customer problems, that extra bit of emotional nuance can really count.

Performance and real-time latency

Latency is just the little pause between your AI figuring out what to say and the customer hearing the words. For a conversation to feel natural, you want that delay, often called Time to First Audio (TTFA), to be under 300 milliseconds. Any longer than that, and you get those awkward moments where people start talking over each other.

• Cartesia: This is an area where Cartesia really pulls ahead. It has extremely low latency, with some of its models responding in as little as 40-90ms. That speed is perfect for interactive voice systems where the conversation is quick and bounces back and forth.

• Amazon Polly: Polly’s latency is generally a bit higher, usually somewhere in the 100-500ms range. It’s fast enough for a lot of situations, but that small delay can start to feel noticeable in a fast-paced chat, creating those stilted pauses that make a call feel unnatural.

The takeaway: If you absolutely need the fastest response time possible, Cartesia has a clear edge. When you’re building a voice agent yourself, you’re managing all the moving parts, and every millisecond matters.

Features and customization

Besides just talking, what else can these platforms do? Things like cloning voices, tweaking the delivery, and deploying the tech in different ways can be deal-breakers.

Feature	Cartesia Sonic	Amazon Polly
Voice Cloning	Yes, instant cloning from 3 seconds of audio	No native support (Brand Voice program for enterprise)
Voice Customization	Slider controls for speed and emotion	SSML tags for pitch, rate, emphasis
Languages Supported	~15 languages with dialect coverage	29+ languages
On-premise Deployment	Yes, supported for enterprise	No, cloud-only
Character Limits	Infinite request length	Limited character count per request

The takeaway: Cartesia offers some more advanced, developer-friendly tools like instant voice cloning and the option for on-premise deployment, which gives you more creative freedom. Amazon Polly, meanwhile, is all about providing wide language support and fitting perfectly within the AWS cloud environment.

Pricing breakdown: Cartesia Sonic 3 vs Amazon Polly

Just remember, the TTS cost is only one part of the overall bill. A fully working voice agent also needs a speech-to-text (STT) service to understand the user and a large language model (LLM) to come up with responses. Those costs can add up fast.

Cartesia's pricing

Cartesia uses a credit system, which can be pretty flexible.

• Free: $0/month (10k credits)

• Pro: $5/month (100k credits)

• Startup: $49/month (1.25M credits)

• Scale: $299/month (8M credits)

• Enterprise: Custom

This setup is great for trying things out, but it can be a little harder to predict your monthly costs compared to a per-character model, especially if your usage volume goes up and down.

Amazon Polly's pricing

Amazon Polly has a simple pay-as-you-go model based on how many characters you process.

• Standard voices: $4.00 per 1 million characters

• Neural voices: $16.00 per 1 million characters

• Long-Form voices: $100.00 per 1 million characters

• Generative voices: $30.00 per 1 million characters

This is very predictable, but the bill can climb quickly if you’re using the higher-quality neural or generative voices to get that natural sound.

The bigger picture: A TTS engine is not an AI agent

Okay, let’s be real for a second: picking a great TTS provider is just the first step, and it might be the easiest one. A voice agent that’s ready for real customers needs a lot more under the hood. You have to wire together a speech-to-text service, an LLM, your own business logic, and connections to your helpdesk (like Zendesk or Freshdesk) and all your knowledge bases.

This is where the real work, cost, and headaches are hiding. Building this kind of system from the ground up takes a dedicated engineering team, months of development, and a ton of ongoing upkeep.

That’s where a more complete platform like eesel AI comes into the picture. Instead of you having to become an expert in five different AI fields, eesel AI handles the whole process by plugging directly into the tools you already have.

• Go live in minutes, not months: You don’t have to spend a quarter building a custom system. With eesel AI, you can connect your helpdesk and knowledge sources in one click and have a working AI agent ready to go in minutes.

• Unify all your knowledge: eesel AI learns from your past tickets, your help center, and internal docs in places like Confluence or Google Docs. That means it gives answers based on your company’s info, not generic stuff from the web.

• Test with confidence: The simulation mode is a lifesaver. You can safely test your AI agent on thousands of your past tickets to see exactly how it will behave before it talks to a single customer. This takes all the guesswork out of launching an AI system.

• Transparent pricing: eesel AI has predictable plans without confusing per-resolution fees. Your costs won’t suddenly jump just because you had a busy support month.

Cartesia Sonic 3 vs Amazon Polly: Make the right choice for your strategy

So, who wins the Cartesia Sonic 3 vs Amazon Polly matchup? It really depends on your priorities.

• Cartesia Sonic 3 is your best bet if you’re aiming for top-tier voice realism and super-low latency, and you have the engineering team to build and manage the rest of the tech stack around it.

• Amazon Polly is a solid, dependable choice for teams that are already using AWS and need broad language support with predictable, usage-based pricing.

But if there’s one thing to take away, it’s this: the best TTS engine on the planet won’t do you any good without a smart, integrated AI agent platform behind it.

Instead of getting bogged down trying to piece together a dozen different components, you might want to see how eesel AI can give you a complete, ready-to-go AI support agent that you can launch in minutes, not months.