bcherny · Sep 16, 2026 · Sep 18, 2026 · Sep 18, 2026 · Sep 18, 2026 · Sep 19, 2026
diff --git a/mods/diff/.claude-plugin/plugin.json b/mods/diff/.claude-plugin/plugin.json
@@ -1,7 +1,7 @@
 {
   "name": "diff",
   "version": "0.1.0",
-  "description": "The diff pane as a plugin: /diff opens the session's uncommitted changes beside the transcript, file by file with their hunks, refreshed as Claude edits and runs commands; the first edit opens it where the terminal is wide enough, and a file's ask button rides its hunks on the next prompt.",
+  "description": "The diff pane as a plugin: /diff opens the session's uncommitted changes beside the transcript, file by file with their hunks, refreshed as Claude edits and runs commands; the first edit it has a change to list for opens it where the terminal is wide enough, and a file's ask button rides its hunks on the next prompt.",
   "author": {
     "name": "Anthropic"
   }

diff --git a/mods/diff/README.md b/mods/diff/README.md
@@ -13,19 +13,22 @@ built-in's list keys (`ctrl+up`/`ctrl+down`, `opt+up`/`opt+down`), and
 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-leaves waiting undrawn is withdrawn, so no later resize seats it, and the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.
+The main loop's first successful edit of a session the diff has a file to
+list for 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. An edit to a file outside the repository, or one
+after which the diff lists nothing or cannot be read, opens nothing and
+leaves the opening to a later edit. 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
@@ -58,10 +61,11 @@ 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.
+is open, after an edit that landed or a shell command that ran, or by the
+fetch each edit inside the repository, with room for a pane, makes until
+one lists a file and the pane opens on it. The one read the built-in has
+no counterpart for is a `git status` at the first of those fetches, 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.
 
@@ -70,14 +74,14 @@ moved file by.
 | 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 `PromptHint` | Reads the terminal's width and whether its layout docks a pane, which decide whether the first edit may open 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 `Edit`, `Write`, `NotebookEdit` | After an edit that landed (not refused, not failed), refreshes an open pane; the main loop's first such edit inside the repository whose fetch lists a file opens it on that fetch, 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. |
 

diff --git a/mods/diff/hooks/hooks.json b/mods/diff/hooks/hooks.json
@@ -1,4 +1,4 @@
 {
-  "description": "The diff pane: /diff and its Pane drawing, a refresh on Claude's edits, shell commands and finished turns, the pane's opening on the first edit, and the ask that rides a file's hunks on the next prompt",
+  "description": "The diff pane: /diff and its Pane drawing, a refresh on Claude's edits, shell commands and finished turns, the pane's opening on the first edit with a change to list, and the ask that rides a file's hunks on the next prompt",
   "modules": ["./register.ts"]
 }
diff --git a/mods/diff/hooks/index.ts b/mods/diff/hooks/index.ts
@@ -10,6 +10,7 @@ export * from './git'
 export * from './host'
 export * from './is-checkpointing'
 export * from './is-on-pane-surface'
+export * from './is-outside-working-tree'
 export * from './is-record'
 export * from './kept-of'
 export * from './limits'

diff --git a/mods/diff/hooks/is-outside-working-tree/index.ts b/mods/diff/hooks/is-outside-working-tree/index.ts
@@ -0,0 +1,4 @@
+export * from './is-outside-working-tree.js'
+export * from './normal-path-of.js'
+
+export * as default from '.'
diff --git a/mods/diff/hooks/is-outside-working-tree/is-outside-working-tree.ts b/mods/diff/hooks/is-outside-working-tree/is-outside-working-tree.ts
@@ -0,0 +1,29 @@
+import Git from '../git'
+import { normalPathOf } from './normal-path-of.js'
+
+/**
+ * Whether a file a tool edited lies outside the pinned working tree, by
+ * how the paths are spelled alone (normalPathOf): no row could show it.
+ *
+ * A relative path is read from the session's directory. Where that
+ * directory is itself not spelled under the top (a symbolic link on the
+ * way to it), spelling proves nothing and nothing is outside.
+ *
+ * @param path the path the tool was called with
+ * @param tree the session's directory and the working tree's top
+ * @returns true only for a path surely outside the tree
+ */
+export function isOutsideWorkingTree(
+  path: string,
+  tree: { cwd: string; toplevel: string },
+): boolean {
+  const top = normalPathOf(tree.toplevel)
+  const absolute = Git.isAbsolutePath(path) ? path : `${tree.cwd}/${path}`
+
+  const isUnderTop = (spelled: string) =>
+    top === '' || spelled === top || spelled.startsWith(`${top}/`)
+
+  return (
+    isUnderTop(normalPathOf(tree.cwd)) && !isUnderTop(normalPathOf(absolute))
+  )
+}
diff --git a/mods/diff/hooks/is-outside-working-tree/normal-path-of.ts b/mods/diff/hooks/is-outside-working-tree/normal-path-of.ts
@@ -0,0 +1,25 @@
+/**
+ * An absolute path spelled one way, its names joined by `/` with no
+ * leading or trailing one: either separator splits, `.` and empty names
+ * drop, `..` drops the name before it. No disk is read, no link followed.
+ *
+ * A drive-letter path is lowercased, as its file system compares names.
+ *
+ * @param path an absolute path, POSIX or drive-letter
+ * @returns the names joined, empty for the root
+ */
+export function normalPathOf(path: string): string {
+  const names: string[] = []
+
+  for (const name of path.split(/[\\/]/)) {
+    if (name === '..') {
+      names.pop()
+    } else if (name !== '' && name !== '.') {
+      names.push(name)
+    }
+  }
+
+  const spelled = names.join('/')
+
+  return /^[A-Za-z]:/.test(spelled) ? spelled.toLowerCase() : spelled
+}
diff --git a/mods/diff/hooks/pane-state/has-session-files.ts b/mods/diff/hooks/pane-state/has-session-files.ts
@@ -0,0 +1,22 @@
+import type Git from '../git'
+
+/**
+ * Whether a fetch settled with a file the header would count, so a pane
+ * opened on it lists something where emptyStateOf would find a headline.
+ *
+ * No repository, a diff git could not read, and rows that all predate the
+ * session count nothing.
+ *
+ * @param outcome how the fetch ended
+ * @returns whether the header's session file count is above zero
+ */
+export function hasSessionFiles(outcome: Git.FetchOutcome): boolean {
+  if (outcome.kind !== 'data') {
+    return false
+  }
+
+  const { stats, files } = outcome.data
+  const before = files.filter(file => file.isPreSession)
+
+  return stats.filesCount - before.length > 0
+}
diff --git a/mods/diff/hooks/pane-state/index.ts b/mods/diff/hooks/pane-state/index.ts
@@ -7,6 +7,7 @@ export * from './dialog-title'
 export * from './dialog-title-of.js'
 export * from './empty-state'
 export * from './empty-state-of.js'
+export * from './has-session-files.js'
 export * from './header-totals'
 export * from './header-totals-of.js'
 export * from './initial-model'