Claude Skills Guide

Claude Code for PR Diff Analysis Workflow Tutorial

Pull request diff analysis is one of the most time-consuming aspects of code review. Manually scanning through dozens of changed files, identifying potential issues, and providing constructive feedback takes hours that could be spent on feature development. In this tutorial, you’ll learn how to build a Claude Code skill that automates PR diff analysis, helping you review code faster and more consistently.

Understanding PR Diff Analysis in Claude Code

Before diving into the skill implementation, it’s important to understand how Claude Code handles diffs. When working with pull requests, you typically fetch the diff content and analyze changed files for patterns, potential bugs, security issues, and code quality concerns.

A well-designed PR diff analysis skill should:

  1. Parse diff output to understand what changed
  2. Identify common issues like security vulnerabilities
  3. Check for code style consistency
  4. Suggest improvements based on best practices
  5. Generate a comprehensive review summary

Building the PR Diff Analysis Skill

Let’s create a skill that can analyze diff output and provide actionable feedback. Save this as skills/pr-diff-analyzer.md:

---
name: pr-diff-analyzer
description: Analyzes pull request diffs for potential issues, security vulnerabilities, and code quality improvements
tools: [Read, Bash, Glob, edit_file]
---

# PR Diff Analyzer

You are an expert code reviewer specializing in analyzing pull request diffs. Your role is to identify issues, suggest improvements, and provide constructive feedback.

## Analysis Guidelines

When analyzing a diff, follow these steps:

1. **Parse the diff structure** - Identify files changed, lines added/removed, and the nature of modifications
2. **Check for security issues** - Look for hardcoded credentials, SQL injection vulnerabilities, and exposed secrets
3. **Verify code quality** - Ensure consistent formatting, proper error handling, and adherence to project conventions
4. **Identify potential bugs** - Look for null pointer risks, race conditions, and logic errors
5. **Generate a summary** - Organize findings by severity and provide actionable recommendations

## Security Checklist

Always check for these critical issues:

- Hardcoded API keys, tokens, or passwords
- SQL injection vulnerabilities (unescaped user input in queries)
- Command injection risks (unsanitized input in shell commands)
- Insecure cryptographic implementations
- Exposed sensitive data in logs or error messages

## Code Quality Standards

Evaluate the diff against these quality metrics:

- Consistent naming conventions
- Proper error handling and logging
- Code duplication (repeated patterns)
- Function complexity (too many nested conditions)
- Missing documentation for public APIs
- Test coverage for new functionality

## Output Format

After analysis, present your findings in this structure:

### Summary
[Overall assessment of the PR]

### Critical Issues
- [List any critical security or bug issues]

### Suggestions
- [List improvement recommendations by file]

### Questions
- [Any clarifying questions for the author]

## Tool Usage

Use the following tools to perform thorough analysis:

1. First, read the diff file or fetch it from the PR
2. Identify the programming language and applicable conventions
3. Run static analysis tools if available
4. Cross-reference with project coding standards
5. Compile your findings into a structured review

This skill provides a comprehensive framework for PR analysis. However, the real power comes from extending it with specific checks for your tech stack.

Practical Example: Analyzing a Real Diff

Let’s walk through how this skill works in practice. Suppose you’ve fetched a diff from a PR:

--- a/src/auth.js
+++ b/src/auth.js
@@ -15,7 +15,7 @@ function authenticateUser(username, password) {
-  const query = `SELECT * FROM users WHERE username = '${username}'`;
+  const query = `SELECT * FROM users WHERE username = ?`;
   const result = await db.execute(query, [username]);
   return result.rows[0];
 }

When you invoke the pr-diff-analyzer skill on this diff, it will identify the critical security improvement (parameterized query instead of string concatenation) and highlight it as a positive change while checking for other issues in the same file.

Extending the Skill for Specific Languages

Different programming languages have different patterns and anti-patterns. You can create specialized skills or extend the base skill with language-specific checks. Here’s an example extension for JavaScript/TypeScript:

## JavaScript/TypeScript Specific Checks

When analyzing JavaScript diffs, additionally check for:

- `console.log` statements left in production code
- `any` type usage in TypeScript files
- Missing async/await error handling
- Use of `var` instead of `let`/`const`
- Uncontrolled DOM manipulation
- Memory leaks in useEffect hooks (React)

Automating Diff Fetching

To make your workflow truly efficient, combine the analyzer skill with diff fetching capabilities. Create a helper skill that fetches diffs from GitHub:

---
name: github-diff-fetcher
description: Fetches diff from GitHub pull requests
tools: [Bash, WebFetch]
---

# GitHub Diff Fetcher

You help fetch diff content from GitHub pull requests.

## Usage

When asked to fetch a PR diff:

1. Construct the GitHub API URL: `https://api.github.com/repos/{owner}/{repo}/pulls/{number}/files`
2. Use WebFetch to retrieve the diff content
3. Parse the JSON response to extract file changes
4. Present the diff in a readable format for analysis
5. Offer to run the pr-diff-analyzer skill on the fetched diff

Example command to fetch diff:
```bash
curl -H "Authorization: token $GITHUB_TOKEN" \
  https://api.github.com/repos/OWNER/REPO/pulls/123/files


## Creating a Complete Workflow

The most powerful approach combines multiple skills into a cohesive workflow. Here's how to chain them:

1. **Fetch the PR** - Use github-diff-fetcher to get the diff content
2. **Analyze with pr-diff-analyzer** - Process the diff through the analyzer
3. **Run additional checks** - Execute linting or static analysis tools
4. **Generate report** - Compile findings into a structured review

You can create a meta-skill that orchestrates this workflow:

```markdown
---
name: pr-review-workflow
description: Complete PR review workflow from fetch to final report
tools: [Bash]
---

# PR Review Workflow

This skill orchestrates the complete PR review process.

## Workflow Steps

1. Ask user for PR URL or repository details
2. Invoke github-diff-fetcher to get the diff
3. Pass the diff to pr-diff-analyzer for analysis
4. Run project-specific linting if available
5. Combine all findings into a comprehensive report

## Output

Generate a final report with:
- Executive summary
- Security findings
- Code quality issues
- Suggestions for improvement
- Automated vs manual review checklist

Best Practices for PR Analysis Skills

When building and using PR diff analysis skills, keep these best practices in mind:

Start with the basics. Don’t try to catch every possible issue initially. Focus on high-impact problems like security vulnerabilities and critical bugs, then expand your checks over time.

Customize for your stack. Generic analysis is useful, but language and framework-specific checks provide much more value. Add rules specific to your project’s technology choices.

Avoid false positives. Nothing destroys trust in automated reviews faster than constant false alarms. When in doubt, prefer suggesting improvements over flagging issues.

Keep humans in the loop. Automated analysis should augment human review, not replace it. Use skills to surface issues and prioritize, but let developers make the final decisions.

Iterate and improve. Track which issues your skills find most valuable and refine your rules over time. Regular updates keep your analysis relevant and useful.

Conclusion

Building a PR diff analysis skill for Claude Code transforms how you approach code reviews. By automating the initial scan for common issues, you free up mental energy for higher-level architectural and design decisions. The skills outlined in this tutorial provide a foundation you can customize to your project’s specific needs.

Start with the basic pr-diff-analyzer skill, extend it with language-specific checks, and gradually build a comprehensive review workflow that fits your team’s style. With Claude Code handling the initial legwork, your code reviews will become faster, more consistent, and more thorough.

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