Review GitHub Pull Request

Summarize a GitHub pull request diff, highlighting key design decisions and notable code snippets.

Prerequisites

• The gh CLI must be authenticated (gh auth status)
• You must be in a git repository with a GitHub remote

Step 1: Resolve the PR

The user will provide either a PR number (e.g., #123 or 123) or a branch name. Determine which input was given and resolve it to a PR.

If a PR number is provided

Strip any leading # and use the numeric ID directly. Proceed to Step 2.

If a branch name is provided

Look up the open PR whose head branch matches:

gh pr list --head "<branch>" --state open

• If exactly one PR is found, extract its number and proceed to Step 2.
• If multiple PRs are found, list them and ask the user which one to review.
• If no PR is found, skip Step 2 (no PR description to fetch) and go directly to Step 3 using the local git diff fallback.

Step 2: Fetch PR Description

Retrieve the PR metadata:

gh pr view <number> --json title,body,state,headRefName,baseRefName,labels,author

Record the title, body, headRefName, and baseRefName for use in later steps.

Step 3: Generate the Diff

Primary: gh pr diff

Fetch the diff via the GitHub CLI:

gh pr diff <number>

If this succeeds, use this diff and proceed to Step 4.

Fallback: local git diff

If no PR exists (branch-only case) or the gh diff command fails, fall back to a local diff:

# Ensure both branches are available locally
git fetch origin <target-branch> <source-branch>

# Generate the diff
git diff origin/<target-branch>...origin/<source-branch>

If the user provided a branch name and no PR was found, diff against main:

git fetch origin main <branch>
git diff origin/main...origin/<branch>

Handling large diffs

If the diff output is very large (thousands of lines), use the Task tool to process it in chunks. Summarize each chunk independently, then merge the summaries. Do not skip or truncate parts of the diff — accuracy depends on reading all of it.

Step 4: Analyze and Summarize

Read through the full diff (and the PR description if available). Produce a summary with the following sections. Keep every section as concise as possible — brevity is a priority.

Summary format

## PR Review: <title>

**PR:** [#<number>](<url>)  <- only if a PR exists
**Author:** <author>
**Branch:** `<source>` -> `<target>`

### Overview
<1-3 sentences describing what this PR does and why>

### Key Design Decisions
- <decision 1 with file:line reference>
- <decision 2 with file:line reference>
- ...

### Notable Code
<short fenced code snippets that illustrate the most important changes -- max 3 snippets>

### Potential Concerns  <- omit if none
- <risk or issue worth discussing>

Guidelines for the summary:

• Overview: State what changed and why. Pull context from the PR description if available.
• Key Design Decisions: Focus on why something was done a particular way, not what changed. Include file_path:line_number references. Examples: choice of algorithm, new abstraction introduced, API contract change, migration strategy.
• Notable Code: Include only the most instructive or surprising snippets. Keep each snippet under 15 lines. Always include the file path above the code block.
• Potential Concerns: Only include if there are genuine risks — missing error handling, breaking changes, performance implications, security issues. Do not fabricate concerns.

Step 5: Output

Print the summary directly in the chat as formatted markdown.

If the user requests it, also save the summary to a file:

# Default path
reviews/<number>-review.md

# Or for branch-only reviews
reviews/<branch-name>-review.md

Useful Commands Reference

Command	Description
gh pr list --head <branch>	Find PR by head branch
gh pr diff <number>	Get PR diff
gh pr view <number> --json ...	Get full PR metadata
git diff origin/<target>...origin/<source>	Local diff between branches

Example Usage

Review by PR number

User says: "Review PR #456"

1. Fetch PR metadata for number 456
2. Fetch diff via gh pr diff 456
3. Produce summary

Review by branch name

User says: "Review branch feature/add-pagination"

1. Look up PR with gh pr list --head "feature/add-pagination"
2. If found, fetch PR metadata and diff
3. If not found, diff against main locally
4. Produce summary