Build an Automated GitHub PR Review Agent with Claude

What Does This PR Review Agent Do?

This project builds a GitHub PR review agent that fetches pull request diffs via the GitHub API, sends each changed file to Claude with a structured review prompt, receives JSON-formatted findings categorised by severity, and posts a formatted review comment directly to the pull request — automatically triggered by GitHub Actions on every PR open or update.

Code review is one of the highest-leverage activities in software development — and one of the most time-consuming. Every PR that sits unreviewed blocks a developer. Every review that misses a security issue costs tenfold later. Most teams are perpetually behind on review.

A PR review agent does not replace your senior engineer's judgment on architecture and design. But it can reliably handle the first pass: checking for common bugs, security issues, missing tests, style violations, and documentation gaps — before a human even opens the diff. That frees human reviewers to focus on the things that cannot be automated.

In this project you will build a complete GitHub PR review agent that:

• Reads a pull request diff from GitHub
• Analyses it across five dimensions: functionality, security, test coverage, code quality, and documentation
• Posts structured review comments back to the PR via the GitHub API
• Runs automatically on every new PR via GitHub Actions

Prerequisites

You need:

• An Anthropic API key
• A GitHub personal access token with repo scope (or a GitHub App for production)
• A GitHub repository to test against

How GitHub PR Reviews Work via API

Before building the agent, understand the GitHub objects involved:

• Pull Request: has a title, description, base branch, and head branch
• Files changed: each file has a patch (unified diff) and status (added/modified/removed)
• Review: a top-level review object with overall verdict: APPROVE, REQUEST_CHANGES, or COMMENT
• Review comments: inline comments attached to specific lines in the diff
• Issue comments: general comments on the PR conversation thread

The agent reads the diff, passes it to Claude, Claude returns structured feedback, and the agent posts that feedback as a PR review with inline comments where appropriate.

Step 1: GitHub Client

Step 2: Build the Review Prompt

The key to a good review agent is a well-structured prompt. You want Claude to produce consistent, actionable output that maps cleanly to GitHub's review format.

Step 3: The Review Agent

Step 4: GitHub Actions Integration

Create .github/workflows/ai-pr-review.yml in your repository to run the agent automatically on every PR:

Add your secrets in GitHub repository settings:

• ANTHROPIC_API_KEY: your Anthropic API key
• GITHUB_TOKEN: this is automatically provided by GitHub Actions — no manual setup needed

Step 5: Local Testing

Test the agent locally before deploying to GitHub Actions:

Customisation: Adding Review Focus Areas

You can extend the agent to focus on your team's specific concerns by modifying the prompt. Common additions:

Framework-specific checks (e.g., Django):

Dependency security:

Performance:

Project File Structure

Key Takeaways

• A PR review agent reads the unified diff from GitHub's API and passes it to Claude with a structured review prompt
• Using a JSON output format from Claude makes it straightforward to parse the review and format it for different output targets
• Always truncate large diffs — PR review quality drops on diffs over 5,000 lines and costs escalate fast
• GitHub Actions integration makes this zero-maintenance — the agent runs on every PR automatically
• Block fork PRs from triggering the action to prevent API key abuse
• The agent is most valuable on security, tests, and code quality — invest in customising the prompt for your stack and team conventions

What's Next in the AI Coding Agents Series

1. What Are AI Coding Agents?
2. AI Coding Agents Compared: GitHub Copilot vs Cursor vs Devin vs Claude Code
3. Build Your First AI Coding Agent with the Claude API
4. Build an Automated GitHub PR Review Agent ← you are here
5. Build an Autonomous Bug Fixer Agent
6. AI Coding Agents in CI/CD: Automate Code Reviews and Fixes in Production

This post is part of the AI Coding Agents Series. Previous post: Build Your First AI Coding Agent with the Claude API.

To deploy this agent in a full CI/CD pipeline with cost controls and human approval gates, see AI Coding Agents in CI/CD. For a simplified standalone code review assistant (without the full agentic loop), see Build a Code Review Assistant for GitHub PRs.

External Resources

• GitHub REST API: Pull Requests — official reference for the endpoints used to fetch PR diffs and post review comments.
• PyGithub documentation — the Python library used to interact with the GitHub API in this project.