Retrieve Diff from GitHub PR Skill

An input skill that retrieves code diff and PR metadata from GitHub via the API. This is the entry point for reviewing GitHub Pull Requests.

Role

• Fetch: Get PR details and diff via GitHub API
• Extract: Parse changed files and their diffs
• Context: Gather PR metadata (title, description, author, reviews)

Inputs

Input	Required	Description
owner	Yes	Repository owner (username or organization)
repo	Yes	Repository name
pull_number	Yes	Pull Request number

Outputs

Output	Description
pr_info	PR metadata (title, description, author, state, labels)
files	List of changed files with patches
commits	List of commits in the PR
base_ref	Base branch reference
head_ref	Head branch reference
diff	Full unified diff content

Required MCP Tools

This skill uses the GitHub MCP server with the following tools:

Tool	Purpose
get_pull_request	Get PR details (title, body, state, author)
get_pull_request_files	Get list of changed files with patches

Step 1: Parse PR Reference

Accept PR reference in various formats:

Format	Example	Parsing
Full URL	https://github.com/owner/repo/pull/123	Extract owner, repo, pull_number
Short reference	owner/repo#123	Split on / and #
Number only	123	Requires owner/repo from context

Regex for URL: github\.com/([^/]+)/([^/]+)/pull/(\d+)
Regex for short: ([^/]+)/([^#]+)#(\d+)

Step 2: Fetch PR Details

Use the GitHub MCP tool to get PR information:

{
  "tool": "get_pull_request",
  "server": "user-github",
  "arguments": {
    "owner": "<owner>",
    "repo": "<repo>",
    "pull_number": <number>
  }
}

This returns:

• PR title and description
• Author information
• Base and head branches
• State (open/closed/merged)
• Labels and reviewers
• Head commit SHA (needed for submitting review)

Step 3: Fetch Changed Files

Use the GitHub MCP tool to get file changes:

{
  "tool": "get_pull_request_files",
  "server": "user-github",
  "arguments": {
    "owner": "<owner>",
    "repo": "<repo>",
    "pull_number": <number>
  }
}

This returns for each file:

• filename: Path to the file
• status: added, removed, modified, renamed
• additions: Lines added
• deletions: Lines deleted
• patch: Unified diff patch for the file

Step 4: Format Output

Structure the output for the review pipeline:

## PR Retrieved

**Repository**: owner/repo
**PR #**: 123
**Title**: feat: add user authentication

### PR Info

| Field | Value |
|-------|-------|
| Author | @username |
| State | open |
| Base | main |
| Head | feature/auth |
| Commits | 3 |
| Changed Files | 5 |

### Description

<PR description content>

### Files Changed

| Status | File | +/- |
|--------|------|-----|
| modified | src/auth/login.ts | +50/-10 |
| added | src/utils/helper.ts | +30/-0 |
| deleted | src/deprecated.ts | +0/-25 |

### Diff Content

<unified diff for each file>

Output Format

{
  "source": "github-pr",
  "repository": {
    "owner": "owner",
    "repo": "repo"
  },
  "pull_request": {
    "number": 123,
    "title": "feat: add user authentication",
    "body": "PR description...",
    "author": "username",
    "state": "open",
    "base": {
      "ref": "main",
      "sha": "abc123"
    },
    "head": {
      "ref": "feature/auth",
      "sha": "def456"
    },
    "labels": ["enhancement"],
    "created_at": "2024-01-15T10:30:00Z",
    "updated_at": "2024-01-15T12:00:00Z"
  },
  "stats": {
    "files_changed": 5,
    "additions": 120,
    "deletions": 45,
    "commits": 3
  },
  "files": [
    {
      "path": "src/auth/login.ts",
      "status": "modified",
      "additions": 50,
      "deletions": 10,
      "patch": "@@ -1,10 +1,50 @@\n..."
    }
  ],
  "diff": "<combined unified diff>"
}

Integration with Review Pipeline

After retrieving the PR diff, pass the output to:

1. codereview-orchestrator - For triage and routing
2. Then to appropriate specialist skills based on the triage
3. Finally to submit-github-review to post the review

Quick Reference

□ Parse PR Reference
  □ Extract owner, repo, pull_number
  □ Handle URL or short format

□ Fetch PR Details
  □ Call get_pull_request MCP tool
  □ Extract metadata and head SHA

□ Fetch Changed Files
  □ Call get_pull_request_files MCP tool
  □ Get patches for all files

□ Format Output
  □ Structure for review pipeline
  □ Include commit SHA for review submission

Example Usage in Pipeline

1. retrieve-diff-from-github-pr (this skill)
   ↓
2. codereview-orchestrator (triage)
   ↓
3. Specialist skills (review)
   ↓
4. submit-github-review (post review)

Error Handling

Error	Cause	Resolution
404 Not Found	PR doesn't exist or no access	Verify PR number and permissions
403 Forbidden	Rate limited or no auth	Check GitHub token
Invalid format	Can't parse PR reference	Use format: owner/repo#number