diff

The diff pane as a plugin: /diff opens the session's uncommitted changes beside the transcript, one row per changed file and every file's hunks beneath, and closes it again; each toggle leaves Diff panel shown or Diff panel hidden in the transcript. The header, the file list and its toggles stay put while the wheel moves the hunks under them three rows a tick, or the list a file a tick while the wheel is over a list longer than its eight rows (the plugin answers the pane's ui.scroll itself); a row's click puts that file's hunks at the top; the list also scrolls under the built-in's list keys (ctrl+up/ctrl+down, opt+up/opt+down), and ctrl+x b moves the comparison base on, as the built-in's chord does: both through Buttons that declare the engine's own actions. The pane refreshes as Claude edits and runs shell commands, and while it is open it polls the repository's HEAD so a commit or checkout made elsewhere shows too. The main loop's first successful edit of a session opens the pane by itself, as the built-in panel opens on its first checkpoint: where the layout docks it beside the transcript (the fullscreen layout, which each drawing's viewport says), the terminal is wide enough (144 columns when the person never chose, 110 when they kept it open before; a person who closed it is left alone) and file checkpointing is on; a subagent's edit opens nothing, and where the surface does not say, nothing opens by itself. A docked pane fetches before it opens, as the built-in panel primes its data, so it never lands on Loading diff…; an open the engine leaves waiting undrawn is withdrawn, so no later resize seats it, and the next edit asks again. A session resumed or continued whose transcript already holds such an edit opens the pane on the same terms as soon as the width is known, as the built-in opens on the history it restores.

Under the fullscreen layout a terminal under 110 columns gets the built-in's line asking for a wider one and nothing opens. Without that layout (CLAUDE_CODE_NO_FLICKER=0, which /diff learns from the command's presentation) the pane opens inline at any width, focused and as tall as its content (the open's rows), in the built-in dialog's shape: the title, the count, five file rows at a time round the selected one (❯, where the focus ring starts; the plugin follows the ring's walk through ui.focus and re-centres the rows as the built-in does), the key hints; Enter shows that file's hunks alone, Escape backs out to the list and then closes, leaving Diff dialog dismissed; toasts are held while it is up. A file's ask button arms that file: its hunks ride the next prompt as context, once.

The pane compares the working tree against HEAD, split at the session's start (the default; the start the engine gives in $.session.usage(), so a resumed session keeps its first and /clear begins anew), against HEAD plainly, or against the merge-base with the default branch; the base line under the header names a base other than the session's, and the choice is kept per repository in the plugin's store. A picker shows one earlier turn's edits instead of the working tree, read from the session's messages. Files that changed before the session started (by their timestamp, among the paths already dirty when the pane first read the repository), and noise (lockfiles, generated and test files), are listed apart and folded until asked for; a rename lists as git prints it. Outside a git repository /diff says so and does nothing else.

Git runs when the built-in panel's would: nothing at the session's start; one git rev-parse, in the directory the session started in, when /diff or the first edit a pane has room to open on first needs the repository (an answer of no repository is kept too, until /clear or /resume forgets it); and the working tree is read only by a fetch for a pane that is open, after an edit that landed or a shell command that ran. The one read the built-in has no counterpart for is a git status at a pane's first fetch, which stands in for the change time the built-in dates a moved file by.

hooks/register.ts is the module; everything under hooks/ is its parts.

What it hooks

event	what the hook does
session.start	Binds the engine once and registers /diff (a session where another /diff is listed leaves the plugin idle); asks nothing of the repository, which /diff or the first edit pins when it comes; off its dispatch, reads the transcript, and for a resumed session whose turns edited opens the pane as the first edit would.
ui.render of PromptHint	Reads the terminal's width and whether its layout docks a pane, which decide whether the first edit opens the pane.
ui.render of Pane	Draws the pane: docked, the header, base line, source picker, file list and toggles over the window of hunks; inline, the dialog.
command.run of diff	Pins the repository when none is, opens or closes the pane (focused and closing on Escape without the fullscreen layout), says which, and remembers the choice.
ui.close of the pane	Backs out of the dialog's detail view instead of closing; else remembers the person's close as /diff's.
ui.scroll of the pane	Docked, moves the hunks under the pinned header and list (three rows a wheel tick, a page a page key), or the list when the wheel is over it, and keeps the engine's window still.
ui.focus in the pane	In the dialog's list, selects the file the ring lands on, re-centres the five rows on it, and lands the ring where that row now sits.
command.run of clear, resume	Closes the pane and forgets the session's state, the pinned repository with it.
tool.call of Edit, Write, NotebookEdit	After an edit that landed (not refused, not failed), refreshes an open pane; the main loop's first such edit opens it, pinning the repository then if the terminal has the room and checkpointing is on.
tool.call of Bash, PowerShell	After a command that was not refused, failed and interrupted ones too, refreshes an open pane.
prompt.submit	Adds the armed file's hunks to the prompt's context and disarms.

What it calls on $

clock.after, clock.every, clock.now, command.register, env.get (CLAUDE_CODE_DISABLE_FILE_CHECKPOINTING), fs.list, fs.read, fs.stat, process.run (git, read-only), session.id, session.messages, settings.read, store.get, store.set, telemetry.log, telemetry.mark, ui.close, ui.invalidate, ui.log, ui.open, ui.resolve, ui.status.

$.telemetry is the telemetry plugin's noun; where it is absent the rows are dropped and nothing else changes.

Try it

claude --plugin-dir /path/to/diff

then /diff inside a git repository with a modified file.