Every time I finished a batch of changes and wanted Claude Code to help review or improve them, I ran into the same friction. The diff was right there in my terminal, but there was no clean way to annotate specific lines and hand that context to Claude. I could paste the whole diff and add notes inline, but that was messy. I wanted something purpose-built for this loop.

So I built diffai. It’s a TUI diff viewer with inline commenting, designed to sit in one pane while Claude Code sits in the other.

The Workflow

The core idea is simple. You make changes, open diffai, annotate the lines that need attention, and pass the saved notes to Claude with a single @ reference.

Here’s how that plays out in practice:

# Open the diff viewer for the current repo
diffai review

diffai opens a split view: a file tree on the left and the diff on the right. You navigate with arrow keys (or vim keys if you prefer), press c to drop a comment on any line, and quit with q. When you exit, diffai saves your annotations to a timestamped file in /tmp.

In Claude Code, you just reference that file:

@/tmp/diffai-1234.md

Claude gets your comments alongside the relevant line numbers and file paths. No pasting, no reformatting.

The saved file is intentionally minimal. It contains only your comments, grouped by file, with the actual source line number attached to each one. Here’s what it looks like:

<!-- diffai | @/tmp/diffai-1234.md in Claude Code -->

## Comments

### crates/cli/src/main.rs

**Line 47** (`self.comments.insert(...);`)
should this also clear comment_input?

That format gives Claude exactly what it needs without flooding the context with the full diff.

Installing diffai

The fastest way to install is the one-liner:

curl -fsSL https://joshfinnie.com/diffai/install.sh | sh

That script pulls the right binary for your platform, drops it in ~/.local/bin, and you’re done. No API keys. No network calls at runtime. The tool is purely local.

The TUI

The viewer has two panels. The left panel shows all changed files; a small indicator marks files where you’ve already left comments. The right panel shows the diff for the selected file, with syntax highlighting and line numbers.

You can toggle between unified and split diff modes with s. Split mode puts the old and new versions side by side, which I find much easier to read for complex hunks. Unified is the default because it uses less width, but I keep split as my personal setting.

The full keybindings are on the diffai project page, but the ones you’ll reach for every session are Tab to switch panels, c to comment, d to delete a comment, [ and ] to resize the file tree, and q to quit.

Vim Mode and Configuration

diffai reads from ~/.config/diffai/settings.toml on startup. The file is entirely optional, but this is the configuration I run with:

[view]
default_mode = "split"

[keys]
vim = true

With vim mode enabled, j/k move up and down, h returns to the file tree, l opens the selected file, g/G jump to the top and bottom, and Ctrl+d/Ctrl+u scroll by half a page. If you spend most of your day in Neovim, enabling this makes diffai feel completely natural.

The Pre-commit Hook

One more thing worth knowing: you can wire diffai into your git workflow directly.

diffai install-hook

That installs a pre-commit hook that runs diffai review automatically before every commit. You get a chance to annotate your changes and pass notes to Claude before the commit lands, which is a nice forcing function for actually reviewing what you’re about to ship.

If a hook already exists in that repo, diffai prints the line to add manually so you don’t lose anything.

Putting It Together

diffai fills a specific gap: it gives you a clean, local way to annotate a diff and hand structured context to Claude Code. The TUI is fast, the output format is predictable, and the whole thing stays off the network. I use it daily when working on larger refactors or anything where I want Claude to understand which parts of a diff matter most.

Give it a try and let me know what you think. Find me on Bluesky.