GitHub PR Code Review Agent

An AI-powered code review agent built with Mastra that analyzes any GitHub pull request and returns detailed feedback directly in chat. It uses a structured workflow, workspace skills for review standards, observational memory for managing context across large PRs, and adaptive review depth based on PR size.

Why we built this

This template shows how Mastra's workspace primitives enable you to build a powerful, flexible code review agent that can handle real-world PRs of any size.

Demo

This demo runs in Mastra Studio, but you can connect this workflow to your React, Next.js, or Vue app using the Mastra Client SDK or agentic UI libraries like AI SDK UI, CopilotKit, or Assistant UI.

Prerequisites

Getting started

 1npm install
 2cp .env.example .env

Fill in your .env:

GITHUB_TOKEN=ghp_...
ANTHROPIC_API_KEY=sk-ant-...

Usage

Start the dev server:

 1npm run dev

Open http://localhost:4111 to access Mastra Studio.

Agent (Chat)

Open the GitHub PR Code Reviewer agent and give it a PR URL:

Review this PR: https://github.com/owner/repo/pull/123

The agent (Claude Sonnet) will fetch the PR, adaptively page through files based on PR size, and return a structured review. Observational memory compresses tool results between turns, allowing the agent to handle large PRs without truncating data.

Workflow

Open the Workflows tab and run pr-review-workflow with owner, repo, and pullNumber inputs. The workflow runs a fixed 4-step pipeline and returns a structured JSON review.

Customization

Change review standards

Edit the files in workspace/skills/ to match your team's conventions. The SKILL.md files define the review process; the references/ files provide detailed checklists.

Adjust thresholds

Edit src/mastra/lib/review-config.ts:

  • SMALL_PR_MAX / MEDIUM_PR_MAX — PR size breakpoints for adaptive review depth.
  • SKIP_PATTERNS — Regex patterns for files to skip during review.
  • MIN_DELETION_ONLY_LINES — Threshold for skipping deletion-only files.

Batch sizes for the workflow are configured at the top of src/mastra/workflows/pr-review-workflow.ts:

  • BATCH_CHAR_BUDGET — Max total characters per agent call (default: 400k).
  • BATCH_FILE_LIMIT — Max files per agent call (default: 40).

Swap models

Change the model field in the agent files. The code-review-agent uses Sonnet for quality; the workflow-review-agent uses Haiku for speed. Any Anthropic model works, or switch to another provider supported by Mastra.

About Mastra templates

Mastra templates are ready-to-use projects that show off what you can build — clone one, poke around, and make it yours. They live in the Mastra monorepo and are automatically synced to standalone repositories for easier cloning.

Want to contribute? See CONTRIBUTING.md.