Build an AI Pull Request Description Generator with Claude API

import os
import sys
import anthropic
import requests
 
def get_pr_diff(repo: str, pr_number: int, github_token: str) -> str:
    url = f"https://api.github.com/repos/{repo}/pulls/{pr_number}"
    headers = {
        "Authorization": f"token {github_token}",
        "Accept": "application/vnd.github.v3.diff",
    }
    response = requests.get(url, headers=headers)
    response.raise_for_status()
    # Truncate to 8000 chars to stay within token limits
    return response.text[:8000]
 
def generate_description(diff: str, pr_title: str) -> str:
    client = anthropic.Anthropic(api_key=os.environ["ANTHROPIC_API_KEY"])
 
    prompt = f"""You are a senior software engineer writing a pull request description.
 
PR Title: {pr_title}
 
Git Diff:
{diff}
 
Write a clear PR description in this exact format:
 
## Summary
[2-3 bullet points describing what changed and why]
 
## Changes Made
[Bullet list of specific files/components changed and what was done]
 
## How to Test
[Step-by-step instructions to verify this PR works correctly]
 
## Potential Risks
[Any areas that could break, edge cases to watch, or dependencies changed]
 
Be specific and technical. Use the actual file names and function names from the diff.
Do not add any commentary outside this format."""
 
    message = client.messages.create(
        model="claude-haiku-4-5-20251001",
        max_tokens=1024,
        messages=[{"role": "user", "content": prompt}],
    )
    return message.content[0].text
 
def post_pr_description(
    repo: str,
    pr_number: int,
    github_token: str,
    description: str,
) -> None:
    url = f"https://api.github.com/repos/{repo}/pulls/{pr_number}"
    headers = {
        "Authorization": f"token {github_token}",
        "Accept": "application/vnd.github.v3+json",
    }
    data = {"body": description}
    response = requests.patch(url, headers=headers, json=data)
    response.raise_for_status()
    print(f"PR description updated successfully.")
 
def main():
    repo = os.environ["GITHUB_REPOSITORY"]
    pr_number = int(os.environ["PR_NUMBER"])
    pr_title = os.environ["PR_TITLE"]
    github_token = os.environ["GITHUB_TOKEN"]
 
    print(f"Fetching diff for PR #{pr_number}...")
    diff = get_pr_diff(repo, pr_number, github_token)
 
    if not diff.strip():
        print("Empty diff. Skipping.")
        sys.exit(0)
 
    print("Generating PR description with Claude...")
    description = generate_description(diff, pr_title)
 
    print("Posting description to PR...")
    post_pr_description(repo, pr_number, github_token, description)
 
if __name__ == "__main__":
    main()

name: Generate PR Description
 
on:
  pull_request:
    types: [opened]
 
jobs:
  generate-description:
    runs-on: ubuntu-latest
    permissions:
      pull-requests: write
      contents: read
 
    steps:
      - name: Checkout repository
        uses: actions/checkout@v4
 
      - name: Set up Python
        uses: actions/setup-python@v5
        with:
          python-version: "3.12"
 
      - name: Install dependencies
        run: pip install anthropic requests
 
      - name: Generate and post PR description
        env:
          ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
          GITHUB_REPOSITORY: ${{ github.repository }}
          PR_NUMBER: ${{ github.event.pull_request.number }}
          PR_TITLE: ${{ github.event.pull_request.title }}
        run: python .github/scripts/generate_pr_description.py

on:
  pull_request:
    types: [opened, synchronize]