If you searched "Gravity vs Fixie.ai" expecting two head-to-head agent platforms, the honest first thing to say is that Fixie.ai is not quite the product it was pitched as a couple of years ago. The team that built Fixie.ai now ships Ultravox, a real-time voice AI platform with a speech-native model and developer APIs for building fast, fluent voice agents (Ultravox, retrieved 2026). It is a genuinely strong piece of voice infrastructure. It is also a different thing from what Gravity does.

So this is less a cage match and more a map. I will explain what Fixie.ai became, what Gravity is, and the surprisingly clean line between them: one is a voice engine you build on as a developer, the other is a marketplace where you run a finished task agent without building anything. Knowing which problem you actually have saves you a month of pointing the wrong tool at it.

What Fixie.ai is in 2026

Fixie.ai was founded in 2022 by Matt Welsh, Zach Koch, and Justin Uberti, originally around the broad idea of building and connecting AI agents (Crunchbase, retrieved 2026). Over time the company concentrated on the part of that vision where it had the deepest edge: voice. Today the flagship is Ultravox, described as a real-time voice AI platform powered by a speech-native model plus developer-friendly APIs and agentic primitives for building voice agents (Ultravox, retrieved 2026).

A voice-native model and SDKs

The core of Ultravox is speed and fluency in spoken conversation: low latency, high-quality voices, and SDKs across major languages so engineers can wire voice into their own apps. The platform ships an in-house agent builder and continues to iterate on the underlying model; the team noted a default-model change in late 2025 aimed at better instruction following and tool calling (Ultravox changelog, retrieved 2026). This is infrastructure for builders, not a finished assistant for end users.

Where Fixie.ai is genuinely excellent

If you are building a phone-based support agent, a voice assistant inside a product, or any spoken interface that has to respond in real time without an awkward pause, Ultravox is exactly the kind of specialist tool you want. The whole product is organized around making speech feel immediate, which is hard and which general platforms do badly. Credit where it is due: this is a focused, well-built voice stack.

What Gravity does differently

Gravity is not a voice engine and not a developer SDK. It is a marketplace. You describe the outcome you want in plain language, and Gravity matches you with an expert-built agent that runs the task in about 60 seconds. There is no model to choose, no API to call, and no canvas to assemble. An expert already built and tested the agent; you run it and pay per use, where one dollar buys 1,000 credits, with no subscription. For the full shape of it, see how Gravity works.

The unit of value is the finished result, not the building block. Where Ultravox hands a developer the parts to build a voice agent, Gravity hands an operator a completed task agent. That is the entire difference, and it explains why almost everything else about the two products diverges.

The three-sided marketplace

Gravity has three sides. Users run agents and pay per run. Builders publish agents and earn 20% of every run as pure profit, since execution cost and platform overhead sit with Gravity. Creators earn 10% on runs from people they refer. On a developer platform like Ultravox you build for your own product; on Gravity an expert builds once and earns every time someone runs it. For the economics, see how to monetize AI agents.

Side-by-side comparison

Here is the honest mapping across the dimensions buyers actually weigh. Where I am unsure of a current Ultravox number, I say so rather than invent one.

DimensionFixie.ai (Ultravox)Gravity
CategoryReal-time voice AI infrastructureMarketplace of task agents
Primary interfaceDeveloper API and SDKsPlain-language text prompt
Who builds the agentYou do, in codeVetted expert builders
Target userEngineers building voice productsNon-technical operators
ModalitySpeech firstText-described outcomes
Pricing modelPer-minute voice usage Pay per use, $1 = 1,000 credits
Setup timeAn engineering projectDescribe outcome, run in about 60 seconds
Earning modelBuild for your own productBuilders earn 20%, creators 10%
Best fitPhone and voice assistantsRecurring back-office tasks

Notice how few rows are even contested. This is not a case where one product is better on a shared scorecard; it is two products answering different questions. The table is most useful for ruling one out fast.

Two different categories

The cleanest way to think about it: Ultravox sits at the modality layer and Gravity sits at the outcome layer. Ultravox makes a machine talk and listen well in real time. Gravity gets a defined job done. A voice call is an interface; chasing an unpaid invoice or compiling a weekly report is an outcome. If you confuse the two, you end up trying to make a voice SDK file your expense reports or trying to make a task marketplace hold a fluent phone conversation, and both go badly.

This is the same definitional discipline I push in AI agent vs chatbot vs assistant. The word "agent" is stretched across voice bots, copilots, frameworks, and autonomous task runners until it means almost nothing. Naming the layer you need, voice versus outcome, is half the purchasing decision.

Build the engine vs run the agent

Even within their own categories, the two products sit on opposite sides of the build-versus-buy line. Ultravox is a build tool. It gives capable engineers fast, flexible primitives and gets out of the way. The payoff is control and differentiation; the cost is that you own the build, the integration, and the reliability when something breaks at 2am.

Gravity removes the build entirely. The agent is already made, tested, and published by someone with domain expertise. The payoff is speed and not owning reliability; the cost is that you do not control the internal wiring. This is the same trade-off I lay out in build vs buy AI agent: building buys control at the price of time and maintenance, buying buys speed at the price of some control. Ultravox is build, Gravity is buy, and your team's appetite for engineering work decides which side you belong on.

Pricing reality

Ultravox has published voice pricing on a per-minute basis, in the range of a few cents per minute, alongside its SDKs (Ultravox, retrieved 2026). That model fits voice well, where usage maps naturally to minutes of conversation. Per-minute rates and plan details change, so confirm the live numbers on Ultravox's own site before you model costs.

Gravity is in pre-launch waitlist in 2026 and prices per use rather than per seat: one dollar buys 1,000 credits, and you spend them only on runs you actually trigger, with recurring automations available. Public per-agent pricing publishes when the waitlist opens. For how this compares against subscription-heavy rivals, see AI agent pricing explained and our cheapest AI agent platforms roundup.

When Fixie.ai is the right choice

Three signals point to Ultravox. First, your problem is fundamentally about speech: a phone agent, an in-product voice assistant, a spoken interface where latency ruins the experience. Second, you have engineers who want to build against an API and own the result. Third, you value a focused specialist over a generalist; for voice, a tool that does only voice is usually better than one that does voice as a side feature.

If those are true, do not waste time on a task marketplace. Pick the specialist, and lean on the SDKs.

When Gravity is the right choice

Three opposite signals point to Gravity. First, the job is a text-described outcome, not a conversation: triage this inbox, chase these invoices, compile this report. Second, you would rather run an expert's tested agent than build and maintain your own. Third, you want to pay only for the runs you use, with no engineering team and no seat you are stuck paying for in slow months.

The deeper bet is the marketplace one. As experts publish and test more agents and earn per run, the catalogue of finished, trustworthy agents grows, and building each one yourself looks less and less worth it. For why I made that bet, see about Gravity, and for head-to-heads with autonomous-agent products closer to Gravity's category, our Gravity vs Lindy and Gravity vs Manus breakdowns go deeper.

Using both together

Because they sit at different layers, Ultravox and Gravity can coexist in one workflow. Picture a support flow where Ultravox handles the live call and, the moment it ends, a marketplace task agent logs the outcome, updates the CRM, and drafts the follow-up email. The voice layer and the back-office task layer rarely compete; they hand off to each other. If you are evaluating both, you probably have two problems, not one, and that is fine.

Frequently asked questions

Is Fixie.ai still a company in 2026?

Yes, but the product changed. The team behind Fixie.ai now ships Ultravox, a real-time voice AI platform with a speech-native model and developer APIs for building voice agents. If you came looking for the original general-purpose agent vision, the company's energy in 2026 is squarely on voice. Verify the current product scope on ultravox.ai before you plan around it.

What is the main difference between Fixie.ai and Gravity?

Fixie.ai, now Ultravox, is voice infrastructure for developers: you write code against an API to build fast, fluent voice agents. Gravity is a marketplace where a non-developer describes an outcome in text and runs an expert-built agent that executes the task. One gives you a voice engine to build on; the other gives you a finished task agent to run.

Can Gravity do voice like Ultravox?

No, and it should not try to. Ultravox is purpose-built for real-time speech, with low latency and a speech-native model. Gravity is built for getting text-described outcomes done by expert agents. If your core need is a phone or voice assistant that talks to people in real time, Ultravox is the specialist tool and Gravity is the wrong category.

How much does Fixie.ai (Ultravox) cost?

Ultravox has published per-minute voice pricing in the range of a few cents per minute, plus developer SDKs. Pricing changes over time, so confirm the live rate on ultravox.ai. Gravity is in pre-launch waitlist in 2026 and uses pay per use, where one dollar buys 1,000 credits and you spend only on runs you use.

When should I choose Fixie.ai over Gravity?

Choose Fixie.ai and Ultravox when you are a developer building a real-time voice experience, such as a phone agent, a voice assistant, or a spoken interface, and you want low latency and SDK control. Choose Gravity when you want a text-described task done by a tested agent and you would rather not write or maintain code at all.

Can Fixie.ai and Gravity be used together?

Conceptually yes. A team could use Ultravox for the live voice layer of a product and a marketplace agent for the back-office task that follows the call, like logging the outcome or drafting a follow-up. They solve different parts of a workflow, so they are complements rather than competitors for most buyers.

Three takeaways before you close this tab

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