---
title: "Fullstack Builder: The AI That Writes Your Code | ProductOS"
description: "The ProductOS Fullstack Builder runs a full coding agent inside your project sandbox, implementing your PRD on your design tokens: real code you own, not a prototype."
url: https://productos.dev/agents/develop-agent
---

Stage 5 · Develop

# Fullstack Builder

The implementation agent that writes your actual codebase

[All agents](https://productos.dev/agents)

## What does the Fullstack Builder do?

> “Code belongs to the Develop sub-agent.”From the agent's actual system prompt

The Fullstack Builder is the implementation agent: a full coding agent running headless inside your project's live sandbox, editing files and running commands directly against a pre-installed Next.js project. In the Design stage it appears as the Frontend Engineer; in Develop it builds the whole stack.

It consumes the design system tokens and the PRD, so the code it writes implements the product you specified, screen for screen, on the tokens your Design System Agent locked. The ProductOS Agent coordinates this work but does not write code itself: implementation belongs here.

## What tools does the Fullstack Builder use?

- `Full sandbox toolset`
Reads, writes, and edits files and runs shell commands inside the project sandbox
- `delegate_to_develop_fix`
Receives targeted auto-fix runs generated from QA findings
- `dispatch_to_design_builder`
Runs live Next.js builds in the Design Builder tab during the Design stage

Reads
- Design system tokens and DESIGN.md
- The PRD
- A concrete build task

Produces
- The application codebase in your sandbox
- A live dev preview as it builds

In the pipeline

[Before: Design System Agent](https://productos.dev/agents/design-system-agent) [After: Code Review Agent](https://productos.dev/agents/code-review-agent)

## Put the Fullstack Builder to work.

Every agent works from the same shared context. Describe your idea once and the whole team builds on it.

[Start on Free](https://beta.productos.dev/sign-up?utm%5Fsource=productos.dev/agents/develop-agent) [Meet the whole team](https://productos.dev/agents)