Meet Replit Agent 3: An AI Agent to Vibe Code Custom Apps (Hands-On Guide)

For years, the process of building apps has been a manual, time-consuming process, requiring multiple developers to write, test, and debug every line of code. But what if an AI could handle the entire software development process, from initial concept idea to a fully functional, bug-free app? That's the promise of Replit's Agent 3, an autonomous software-building AI agent that promises longer hands-off runs, in-browser testing, and even the ability to spin up other agents to automate your workflow.

What is Agent 3 by Replit?

Agent 3 by Replit is a new AI-powered vibe coding tool, launched on September 10, 2025, that can build software and also test and fix it autonomously. Agent 3 is designed to keep going where most agents stall. Replit says it's 10 times more autonomous and can run up to 200 minutes on its own, continuously iterating on your app.

The system includes a proprietary testing tool that runs your app in a real browser, identifying and fixing any issues, continuing to test until the app is stable. This process is said to be 3 times faster and 10 times more cost-effective than traditional methods that only simulate clicks and keystrokes.

Here are some of the key features:

• App Testing: Agent 3 uses a proprietary testing system to test and fix the apps it builds. It can interact with the app's user interface, check buttons, forms, and APIs, and even log in to test user authentication flows.
• Agent and Automation Generation: For the first time, Agent 3 can build other agents and automations, allowing users to automate complex workflows by describing them in natural language. For example, you could create a Slack bot that answers questions about your codebase or an automation that sends you a daily email summary of your tasks.
• Extended Autonomous Runtime: Agent 3 can run for up to 200 minutes on its own, handling complex tasks with minimal supervision. This long runtime allows the AI agent to build, test, and fix applications autonomously, freeing up developers to focus on higher-level tasks.
• New App Creation Flow: Replit has introduced a new app creation flow that allows users to start with a frontend-only prototype and add a backend later. This new feature gives more flexibility for quick prototyping and design iteration.

The potential use and application of Agent 3 are wide-ranging.

1. Small business owners could use it to create custom apps without needing to hire an entire team of developers.
2. Designers could quickly prototype and iterate on their ideas.
3. Experienced developers could use it to automate tedious tasks and accelerate their workflows.

This new AI agent covers a large spectrum of users, focusing not just on developers, but can also be used by non-technical users.

Getting started with Replit Agent 3:

To get started, first visit the Agent 3 page and sign up.

Once you are done with that, you will be sent your Replit dashboard.

Now, you can enter the prompt of the web app, agent, or automation you desire. Replit will start building the software you want, laying out all the steps.

Once it is done, you can start testing the web app/ AI agent/ automation you wanted and publish it to see it in action in your real-world workflow.

I am surprised how good and polished the outcome was, and for that reason, I would give it 8.6 out of 10.

In Conclusion:

Agent 3 is a useful tool for software development, focusing on testing, fixing, and completing projects, and during my early testing, it works as advertised. It is simple to start with; you just need to write a simple, effective prompt, and Agent 3 will build, test, fix errors, and even help you publish it. This AI agent makes software building faster. It keeps the same production discipline you would use with any other automation, including versioned updates, clear project scopes, and a quick way to roll back any changes.

If you use it effectively, Agent 3 could be the rare AI tool that helps you deliver more and supervise less, as it is completely free for both free and paid users.