By the end of this article you will have a GitHub Actions workflow that, on every pull_request, runs the Claude Code SDK headlessly, reads only the diff, and posts inline review comments via the GitHub API. I'll show the exact YAML and Python that run in my own repos, the token math that keeps each review at roughly $0.03, and the three failures that cost me a weekend before it worked.
Why I stopped piping the full repo into Claude on GitHub Actions
My first version did the obvious thing: clone the repo, concatenate every changed file in full, and ask Claude to "review this PR." It worked on toy PRs and exploded on real ones. A 9-file refactor sent ~48,000 input tokens and the review drifted into commentary about code the PR didn't touch.
The fix that changed the economics: feed Claude the unified diff with 3 lines of context, not the files. A git diff against the merge base is typically 5–15x smaller than the files it touches. On claude-haiku-4-5, a median PR in my projects now costs about $0.028 per review (measured across 60 PRs: 4,100 input tokens + 900 output tokens average). The expensive version was hitting $0.40+ on Sonnet because file context dominated.
The other lesson: the diff alone is not enough context to judge correctness, but it is enough to catch the 80% of review nits that humans waste time on — unhandled errors, missing null checks, off-by-one, leftover debug prints, secrets in code. So I scoped the prompt to exactly that, and told it to stay silent when unsure. Silence is a feature; a reviewer that comments on everything gets muted by the team within a week.
