Building a Code Review Bot

In this guide, you'll build a code review bot that automatically reviews pull requests using workspace skills. The bot loads coding standards from skill files and provides structured feedback. You'll learn how to create a workspace with a skills directory, define an Agent Skill with review instructions and reference files, and connect it to an agent that performs automated reviews.

Prerequisites

• Node.js v22.13.0 or later installed
• An API key from a supported Model Provider
• An existing Mastra project (Follow the installation guide to set up a new project)

Create the workspace

In your src/mastra/index.ts file, import the Workspace and LocalFilesystem classes. On the Workspace instance, configure the skills option to point to a skills directory. The skills directory will live inside the filesystem's basePath.

src/mastra/index.ts

import { Mastra } from '@mastra/core';
import { resolve } from 'node:path';
import { Workspace, LocalFilesystem } from '@mastra/core/workspace';

const workspace = new Workspace({
  filesystem: new LocalFilesystem({
    basePath: resolve(import.meta.dirname, '../../workspace'),
  }),
  skills: ['/skills'],
});

export const mastra = new Mastra({
  workspace,
});

At the root of your project, create a new folder called workspace. Inside that, create a skills folder. This is where you'll define the code standards skill in the next step.

Create the code standards skill

Skills are structured directories containing a SKILL.md file with instructions for the agent. The code standards skill defines the review process and references a style guide.

Inside workspace/skills, create a new folder called code-standards. Create a file called SKILL.md and add the review instructions.

workspace/skills/code-standards/SKILL.md

---
name: code-standards
description: Automated code review standards and checks
---

# Code Review Standards

Review code systematically using these steps:

1. **Critical Issues**: Security vulnerabilities, memory leaks, logic bugs, missing error handling
2. **Code Quality**: Functions over 50 lines, code duplication, confusing names, missing types
3. **Style Guide**: Check references/style-guide.md for naming and organization
4. **Linting**: Flag common issues like use of `var`, leftover `console.log` statements, and `debugger` statements

Provide feedback in this format:

**Summary**: One sentence overview

**Critical Issues**: List with line numbers

**Suggestions**: Improvements that would help

**Positive Notes**: What the code does well

Inside workspace/skills/code-standards, create a references folder to hold reference materials for the skill. Author a style guide file that outlines the project's coding conventions with the file name style-guide.md.

workspace/skills/code-standards/references/style-guide.md

# Style Guide

## Naming

- Variables/Functions: `camelCase`
- Constants: `UPPER_SNAKE_CASE`
- Files: `kebab-case.ts`
- Booleans: Start with `is`, `has`, `should`

## Code Organization

```typescript
// 1. Imports
import { foo } from 'bar';

// 2. Constants
const MAX_SIZE = 100;

// 3. Types
interface User { id: string; }

// 4. Functions
function doSomething() {}

// 5. Exports
export { doSomething };
```

## Error Handling

Always handle errors explicitly - never silently catch.

## Comments

Write "why" not "what".

Create the review agent

Now it's time to create the code review bot agent that uses the code-standards skill. Create a new file at src/mastra/agents/code-reviewer.ts and define the agent:

src/mastra/agents/code-reviewer.ts

import { Agent } from '@mastra/core/agent';

export const codeReviewer = new Agent({
  id: 'code-reviewer',
  name: 'Code Review Bot',
  instructions: `You are an automated code reviewer.

When asked to review code:
1. Activate the 'code-standards' skill
2. Follow the review process from the skill
3. Check against the style guide in skill references
4. Be constructive and specific with line numbers`,
  model: 'openai/gpt-4o',
});

Define the agent by importing it inside src/mastra/index.ts and registering it with the Mastra instance:

src/mastra/index.ts

import { Mastra } from '@mastra/core';
import { resolve } from 'node:path';
import { Workspace, LocalFilesystem } from '@mastra/core/workspace';
import { codeReviewer } from './agents/code-reviewer';

const workspace = new Workspace({
  filesystem: new LocalFilesystem({
    basePath: resolve(import.meta.dirname, '../../workspace'),
  }),
  skills: ['/skills'],
});

export const mastra = new Mastra({
  workspace,
  agents: { codeReviewer },
});

Test the bot

Start Mastra Studio and interact with the code review bot to see it in action.

npm

npm run dev

pnpm

pnpm run dev

Yarn

yarn dev

Bun

bun run dev

Open localhost:4111 and navigate to the code reviewer agent.

Inside the chat input, provide a code snippet for review, such as:

Review this code:

function getData(id) {
  var result = fetch('/api/data/' + id);
  console.log(result);
  return result;
}

The bot should activate the code-standards skill and provide structured feedback. Since agent responses are non-deterministic, your output may vary, but you should see something similar to:

**Summary**: Function has several issues with variable declaration,
debugging statements, and missing error handling.

**Critical Issues**:
- Missing error handling for fetch (line 2)
- No async/await for asynchronous operation (line 2)

**Suggestions**:
- Use const instead of var (line 2)
- Remove console.log before committing (line 3)
- Add TypeScript type for id parameter
- Use template literals instead of concatenation

**Positive Notes**:
- Function name is clear and descriptive

Next steps

You can extend this bot to:

• Add skills for different languages or frameworks
• Create skills for security checks and performance reviews
• Integrate with GitHub Actions for automatic PR reviews
• Build a PR comment bot that leaves inline feedback

Learn more:

• Agent Skills spec