analyze-pr

name	analyze-pr
description	Analyze a GitHub/GitLab Pull Request by fetching its diff, reading its web description, and generating a comprehensive document. Use when the user provides a PR link and wants to understand what changes the PR introduces, or asks to review/analyze/summarize a pull request.

Analyze Pull Request

Given a PR URL, fetch the code changes and web description, then generate a comprehensive analysis document. The PR is guaranteed to belong to the current local repository.

Workflow

Step 1: Parse the PR URL

Extract from the URL:

• Platform: GitHub or GitLab
• Owner/Org, Repo, PR Number

Supported formats:

https://github.com/{owner}/{repo}/pull/{number}
https://gitlab.com/{owner}/{repo}/-/merge_requests/{number}

Step 2: Fetch PR Metadata

Use the WebFetch tool to read the PR page and extract:

• PR title, description, author, labels, linked issues
• Review comments and discussion highlights

For GitHub PRs, also fetch the API endpoint for structured data:

https://api.github.com/repos/{owner}/{repo}/pulls/{number}

This gives you base.ref (base branch name), head.ref, additions, deletions, changed_files, etc.

Step 3: Fetch the PR Branch Locally

# Record current branch for later restore
ORIGINAL_BRANCH=$(git rev-parse --abbrev-ref HEAD)

# GitHub
git fetch origin pull/{number}/head:pr-{number}

# GitLab
git fetch origin merge-requests/{number}/head:mr-{number}

Do NOT checkout the PR branch — use it as a ref for diff only, to avoid disrupting the working tree.

Step 4: Generate the Diff

Use base.ref from Step 2 as the base branch (do NOT hardcode main):

BASE_BRANCH={base.ref from step 2}

# Changed file list
git diff --name-only origin/$BASE_BRANCH...pr-{number}

# Stats summary
git diff --stat origin/$BASE_BRANCH...pr-{number}

# Full diff
git diff origin/$BASE_BRANCH...pr-{number}

# Commit history
git log --oneline origin/$BASE_BRANCH..pr-{number}

Step 5: Analyze the Changes

For each changed file, understand:

1. What changed: additions, deletions, modifications
2. Why it changed: infer from commit messages, PR description, and code context
3. Impact: which components/modules are affected

Group changes by category:

• New Features: new files, new functions, new APIs
• Bug Fixes: error handling, logic corrections
• Refactoring: code reorganization, naming changes
• Configuration: build, CI/CD, dependency changes
• Documentation: README, comments, docs
• Tests: new or modified tests

Step 6: Generate the Analysis Document

Create a markdown document:

# PR Analysis: {PR Title}

> PR Link: {url}
> Author: {author}
> Date: {date}
> Base Branch: {base} ← {head}

## Summary

{One paragraph overview of what this PR does and why}

## Change Statistics

- Files changed: {count}
- Lines added: {additions}
- Lines deleted: {deletions}

## Detailed Changes

### {Category 1: e.g., New Features}

#### {File or component name}

- **What**: Description of the change
- **Why**: Reason for the change
- **Key code changes**: Brief description of important code modifications

### {Category 2: e.g., Bug Fixes}

...

## Commit History

| Hash | Message | Author |
|------|---------|--------|
| {hash} | {message} | {author} |

## PR Discussion Highlights

{Key points from PR comments and reviews, if any}

## Impact Assessment

- **Breaking changes**: {Yes/No, details}
- **Dependencies affected**: {list}
- **Areas requiring attention**: {list}

## Reviewer Notes

{Any additional observations or recommendations}

Step 7: Save the Diff File

Save the full diff to a local file:

git diff origin/$BASE_BRANCH...pr-{number} > pr-{number}.diff

This .diff file contains the complete code changes introduced by the PR.

Step 8: Save the Analysis Document and Cleanup

Save the analysis document to the current working directory as pr-analysis-{number}.md.

Then clean up the temporary branch ref:

git branch -D pr-{number}

Final output files:

• pr-{number}.diff — raw diff of all PR changes
• pr-analysis-{number}.md — structured analysis document

Important Notes

• If gh CLI is available, prefer it over WebFetch for fetching PR data (handles auth automatically).
• If the diff is very large (>5000 lines), summarize by file/module rather than line-by-line.
• Do NOT checkout the PR branch — only use it as a ref for diffing.
• Always clean up the temporary branch ref after analysis.