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Atlas Whoff
Atlas Whoff

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GitHub Actions + Claude Code: Automated PR Review on Every Commit

#githubactions #claudeai #devops #programming

Code review is a bottleneck. Here is how to put Claude in the loop for every PR automatically.

The Workflow File 

# .github/workflows/claude-review.yml
name: Claude Code Review
on:
  pull_request:
    types: [opened, synchronize]
    paths: ["**.ts", "**.tsx", "**.py"]

jobs:
  review:
    runs-on: ubuntu-latest
    permissions:
      pull-requests: write
      contents: read
    steps:
      - uses: actions/checkout@v4
        with:
          fetch-depth: 0
      - name: Get PR diff
        run: git diff origin/${{ github.base_ref }}...HEAD > /tmp/pr.diff
      - name: Claude review
        env:
          ANTHROPIC_API_KEY: ${{ secrets.ANTHROPIC_API_KEY }}
        run: python3 scripts/claude_review.py /tmp/pr.diff > /tmp/review.md
      - name: Post comment
        uses: actions/github-script@v7
        with:
          script: |
            const review = require("fs").readFileSync("/tmp/review.md", "utf8");
            github.rest.issues.createComment({
              owner: context.repo.owner, repo: context.repo.repo,
              issue_number: context.issue.number, body: review
            });
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The Review Script 

#!/usr/bin/env python3
import sys, anthropic

def review_diff(path):
    with open(path) as f:
        diff = f.read()
    if not diff.strip():
        return "No changes to review."
    if len(diff) > 80000:
        diff = diff[:80000] + "\n[truncated]"

    client = anthropic.Anthropic()
    response = client.messages.create(
        model="claude-sonnet-4-6",
        max_tokens=2048,
        system=(
            "Senior engineer reviewing a PR diff. "
            "Identify: 1) Bugs/security (CRITICAL) 2) Performance (HIGH) 3) Code quality (MEDIUM). "
            "Skip style. Reference file paths and line numbers. "
            "Sections: ## Claude Code Review, ### Critical Issues, ### High Priority, ### Summary."
        ),
        messages=[{"role": "user", "content": "Review:\n\n" + diff}]
    )
    return response.content[0].text

if __name__ == "__main__":
    print(review_diff(sys.argv[1]))
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Quality Gate: Block Merges on Critical Issues 

- name: Check for critical issues
  run: |
    if grep -q "Critical Issues" /tmp/review.md; then
      if ! grep -A2 "Critical Issues" /tmp/review.md | grep -qi "none"; then
        echo "Critical issues found -- blocking merge"; exit 1
      fi
    fi
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Cost: About $1.50/day for 50 PRs

At $0.003/1k input tokens (Sonnet), a 10k-token diff costs ~$0.03. Use prompt caching to cut system prompt cost:

response = client.messages.create(
    model="claude-sonnet-4-6",
    max_tokens=2048,
    system=[{"type": "text", "text": SYSTEM_PROMPT, "cache_control": {"type": "ephemeral"}}],
    messages=[{"role": "user", "content": "Review:\n\n" + diff}]
)
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What Claude Catches That Linters Miss

  • Logic errors: "This condition is always true because X is set above"
  • Race conditions: "This increment is not atomic -- two concurrent requests corrupt state"
  • Missing auth checks: "This endpoint updates user data without verifying ownership"
  • N+1 queries: "This loop calls the database on every iteration"

Linters catch syntax. Claude catches semantics.

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