How to Build and Deploy an App with Replit Agent 3
Replit Agent 3 is the third generation of Replit’s app-building agent. It can run autonomously for long stretches, test its own work in a real browser, and deploy to a public URL — which is also why credit spend can surprise you.
Short answer: start a new Replit workspace, describe an app in one scoped paragraph, let the agent scaffold it, test in the built-in preview, then hit Deploy. Keep tasks narrow and check the credit meter after every run.
What is Replit Agent 3?
It is an autonomous coding agent inside Replit. You describe an app; it plans, writes code, installs dependencies, runs the app, and browses its own output to verify it works before handing back.
Agent 3 can work on one task for up to about 200 minutes, which is a large jump over earlier versions that stopped after a few steps. It supports common stacks — React, Node, Python, and Postgres — and handles hosting and a database for you.
How do you start a project the right way?
- Create a new Replit and choose Agent as the starting point.
- Write one paragraph: what the app does, who uses it, and the three screens it needs.
- Let the agent present a plan, then correct it before approving.
- Watch the preview pane; test each screen yourself as it appears.
- Add features one at a time through follow-up messages.
Vague starting prompts produce sprawling apps. “A task manager” gets you menus you did not ask for; “a single-page app where I add tasks with a due date and filter by today” gets you the thing you wanted.
How do you control credit spend?
This is the main practical concern. Agent 3 bills against credits, and a long autonomous run consumes them steadily.
| Practice | Effect |
|---|---|
| Small scoped prompts | Fewer wasted iterations |
| Stop runaway loops early | Prevents hours of dead work |
| Test manually between steps | Catches drift before it compounds |
Check the usage meter after each session. If you see the agent repeating the same fix three times, interrupt it — that is a stuck loop, and it will not resolve on its own.
How does deployment work?
Once the preview looks right, use the Deploy button. Replit provisions hosting and gives you a live URL, usually in a couple of minutes. You can attach a custom domain from the deployment settings.
Databases and secrets are configured in the workspace settings rather than hardcoded, which is what you want for anything beyond a toy. For a demo, the default setup is fine; for real traffic, check the deployment tier limits before launch.
What are the real limitations?
Agent 3 is strong at small full-stack apps and weak at large codebases. Past a few thousand lines it starts losing track of its own structure and duplicating logic. It also has no testing discipline unless you ask — generated code often ships without meaningful tests.
Cost is the other constraint. Heavy agent use on a complex app can consume a large share of a monthly credit allowance in a day or two.
It suits founders and hobbyists who want a working app fast and do not want to manage infrastructure. It is a poor fit for teams with strict security requirements or large existing repos they need to integrate with.
Quick Summary
- Replit Agent 3 runs autonomously for up to roughly 200 minutes per task.
- Scope the first prompt to one paragraph with three clear screens.
- It tests its own output in a real browser, but review every screen yourself.
- Credits burn fast on long runs — interrupt repeating fix loops.
- Best for small full-stack apps; it degrades past a few thousand lines.
