A terminal diff viewer that actually wants you to review code

What it does

Lumen is a Rust TUI that renders git diff, commits, branches, and GitHub PRs in a side-by-side terminal viewer with tree-sitter syntax highlighting. It also bundles optional AI helpers—commit message drafting, natural-language git command generation, and change explanations—via 10+ providers including local Ollama.

The interesting bit

The coding-agent integration is the quiet standout: shell-escape from Claude Code or Codex into lumen diff, annotate inline with i, then press s to pipe your review comments back to the agent as its next prompt. No plugins, no LSP hijinks—just /dev/tty routing and stdout.

Key highlights

• Stacked-commit review mode (--stacked) with per-commit file-view tracking
• Watch mode auto-refreshes on file changes; PR view syncs “viewed” status to GitHub
• Annotations at selection, hunk, or file level; exportable or copyable
• Works with Git and Jujutsu (jj)
• Single static binary, “snappy on multi-thousand-line diffs” (per README)

Caveats

• AI features require provider configuration; the diff viewer itself is fully offline
• fzf and mdcat are optional but required for specific commands (explain --list, pretty output)
• README notes the project is “in active development” and “breaking changes may occur”

Verdict

Worth a look if you review code in the terminal and want PR-level annotation without opening a browser. Skip it if you live inside VS Code’s diff view and have no patience for TUIs.