claude -p "..."

`claude -p` runs a single prompt non-interactively and prints the result to stdout. Pipe stdin in, parse stdout out — Claude as a CLI primitive. Add `--output-format json` for structured output, or `--output-format stream-json` to stream events.

git diff | claude -p "Write a conventional commit message for this diff"
claude -p "Summarize CHANGELOG.md" --output-format json