Cursor Composer Agent Test: Refactoring a Legacy App

Cursor Composer Agent: We Let the New Terminal Mode Refactor a Legacy App

Short answer: Cursor Composer Agent is a powerful accelerator for repetitive, mid‑tier tasks—but it is not autonomous enough to watch. In our 24‑hour test on a messy Next.js codebase, it cleaned up boilerplate and fixed TypeScript errors in seconds, yet it burned through $3 of API credits in three minutes when it looped into dependency hell. You must stay at the keyboard, review every terminal command, and be ready to hit Ctrl+C.

It’s 2 AM. Your build is failing. Your terminal output looks like a crime scene of outdated React peer dependencies and deprecated Webpack configs. This is usually the part of the night where you contemplate becoming a park ranger.

But yesterday, Cursor dropped a major update to its Composer feature: a fully autonomous Agent Mode. It doesn't just suggest code blocks anymore. It now reads your workspace, writes files, runs terminal commands, installs packages, and attempts to debug its own compiler errors until the build passes.

We spent the last 24 hours putting this new agent through its paces. We threw it into a messy, neglected Next.js repository to see if it could actually modernize the stack, or if it would just create a giant git-rebase headache.

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How the Agent Mode Loop Actually Works

Most AI coding assistants act like backseat drivers. They tell you what to do, but you still have to turn the wheel. Cursor’s Agent Mode wants the keys.

When you boot up the agent inside Composer, you give it a high‑level goal. For example: "Migrate our state management from Redux to Zustand and make sure the tests still pass."

Instead of dumping a wall of text for you to copy‑paste, the agent initializes a multi‑step loop:

1. Workspace Scanning: It searches your codebase for files matching the query.
2. Execution Plan: It outlines the steps it intends to take.
3. File Editing: It edits multiple files concurrently.
4. Terminal Execution: It runs commands like npm install zustand and npm run test directly in your terminal.
5. Self‑Correction: If the tests fail, it reads the error stack trace, modifies the code, and runs the tests again.

This loop continues until the task is complete or the agent hits a blocker it can't solve. It feels less like using an editor and more like managing an eager junior engineer who works at 1,000 words per minute.

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The Wins: Where the Agent Dominates

We expected a mess of merge conflicts. Instead, we found several areas where the agent performed surprisingly well.

1. Upgrading Legacy Packages

Migrating from React Router v5 to v6 is a notorious chore. We asked the agent to handle this migration for a small internal tool. It successfully parsed the deprecated <Route> syntax, replaced it with the modern <Routes> element, updated the imports, and ran the build. It caught two missing import errors on its first run, corrected them autonomously, and delivered a clean, buildable commit.

2. Boilerplate Generation and Setup

Scaffolding a new CRUD endpoint with Tailwind, Prisma, and Next.js Route Handlers used to take twenty minutes of copy‑pasting. The agent did it in ninety seconds. It created the schema file, ran prisma db push, spun up the API routes, and built the frontend form.

3. Fixing Simple but Tedious TypeScript Errors

If you have fifty files complaining about a minor change in a shared interface, the agent is a lifesaver. You can point it at the directory and say, "Fix all TypeScript errors related to the User type." It hums along, editing files and running tsc --noEmit until the compiler is happy.

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The Hard Limitations: Where It Absolutely Breaks

Let's be clear: you cannot walk away and make coffee while this tool rewrites your core architecture. It will break, and when it breaks, it gets expensive.

The "Dependency Hell" Loop

During our test, we asked the agent to resolve a series of ESLint warnings. The agent decided it needed a specific ESLint plugin. It ran npm i eslint-plugin-import. This triggered a peer dependency conflict with our older React version. The agent then tried to force install it. Then it tried to upgrade React. Then the build broke entirely.

It spent $3 worth of API credits in three minutes running in a circle before we killed the terminal process. You must watch its terminal execution closely.

Context Window Exhaustion

Because the agent reads files, runs terminal commands, and analyzes errors, its context window fills up incredibly fast. After about four or five iterations on a complex task, the agent starts to "forget" the original instructions or starts making repetitive edits to the same file. For large tasks, you have to break the goals down into tiny, bite‑sized steps.

Hallucinated CLI Arguments

The agent sometimes tries to run clever terminal commands that do not exist. It attempted to use flag options that were deprecated three years ago for a database CLI tool, resulting in a loop of terminal errors that it spent several steps trying to debug.

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The Verdict: Should You Use It?

The Cursor Composer Agent is not going to replace software engineers anytime soon, but it completely changes how you handle grunt work.

It excels at the repetitive, mid‑tier tasks that usually drain your cognitive energy—like writing test suites, converting CSS to Tailwind, or setting up API boilerplate. However, you cannot treat it as an autonomous worker. You must act as the tech lead, reviewing every terminal command it wants to run and stepping in before it spirals into an expensive package‑installation loop.

If you are already paying for Cursor, switch Composer to "Agent" and give it a spin on a non‑production branch. Just keep your hand near the Ctrl+C shortcut.