diff --git a/scripts/jj_commit_msg.sh b/scripts/jj_commit_msg.sh
index ce956c6..5ef8a51 100755
--- a/scripts/jj_commit_msg.sh
+++ b/scripts/jj_commit_msg.sh
@@ -1,15 +1,22 @@
 #!/usr/bin/env bash
 # jj_commit_msg.sh — generate a commit message from the current jj change using aichat
 #
-# Usage: jj_commit_msg.sh
+# Usage: jj_commit_msg.sh [REV]
 #   Summarises each changed file's diff individually, then combines all
 #   summaries into a single commit message via aichat.
+#   REV defaults to `@` (current working copy). Accepts any jj revision:
+#   `@-`, `lk`, a commit ID, a branch name, etc.
 #
 # Typical use:
 #   jj describe -m "$(jj_commit_msg.sh)"
+#   jj describe -m "$(jj_commit_msg.sh @-)"
+#   jj describe -m "$(jj_commit_msg.sh lk)"
 
 set -euo pipefail
 
+# Optional revision to diff (default: @ = current working copy)
+REV="${1:-@}"
+
 # Log to stderr so progress doesn't pollute the commit message on stdout
 log()  { printf '\033[1;34m==>\033[0m %s\n' "$*" >&2; }
 info() { printf '    \033[0;37m%s\033[0m\n' "$*" >&2; }
@@ -23,7 +30,7 @@ ok()   { printf '    \033[0;32m✓\033[0m %s\n' "$*" >&2; }
 _readable_diff() {
     local file="$1"
     local raw_diff
-    raw_diff=$(jj diff -- "$file")
+    raw_diff=$(jj diff -r "$REV" -- "$file")
     [[ -z "$raw_diff" ]] && return 0
 
     # Detect pathological diff: any +/- content line longer than 500 chars
@@ -40,8 +47,8 @@ _readable_diff() {
     local pretty_old pretty_new
     case "$ext" in
         json)
-            pretty_old=$(jj file show -r @- -- "$file" 2>/dev/null | python3 -m json.tool 2>/dev/null || true)
-            pretty_new=$(jj file show          -- "$file" 2>/dev/null | python3 -m json.tool 2>/dev/null || true)
+            pretty_old=$(jj file show -r "$REV@-" -- "$file" 2>/dev/null | python3 -m json.tool 2>/dev/null || true)
+            pretty_new=$(jj file show -r "$REV" -- "$file" 2>/dev/null | python3 -m json.tool 2>/dev/null || true)
             ;;
         js|mjs|cjs|css|ts)
             local node_fmt='
@@ -56,13 +63,13 @@ _readable_diff() {
                         .replace(/,\s*/g,    ",\n  ");
                     process.stdout.write(out);
                 });'
-            pretty_old=$(jj file show -r @- -- "$file" 2>/dev/null | node -e "$node_fmt" 2>/dev/null || true)
-            pretty_new=$(jj file show          -- "$file" 2>/dev/null | node -e "$node_fmt" 2>/dev/null || true)
+            pretty_old=$(jj file show -r "$REV@-" -- "$file" 2>/dev/null | node -e "$node_fmt" 2>/dev/null || true)
+            pretty_new=$(jj file show -r "$REV" -- "$file" 2>/dev/null | node -e "$node_fmt" 2>/dev/null || true)
             ;;
         *)
             # Generic fallback: fold long lines at 120 chars
-            pretty_old=$(jj file show -r @- -- "$file" 2>/dev/null | fold -s -w 120 || true)
-            pretty_new=$(jj file show          -- "$file" 2>/dev/null | fold -s -w 120 || true)
+            pretty_old=$(jj file show -r "$REV@-" -- "$file" 2>/dev/null | fold -s -w 120 || true)
+            pretty_new=$(jj file show -r "$REV" -- "$file" 2>/dev/null | fold -s -w 120 || true)
             ;;
     esac
 
@@ -75,7 +82,7 @@ _readable_diff() {
 }
 
 # Collect changed files in the current working copy change
-changed_files=$(jj diff --name-only)
+changed_files=$(jj diff -r "$REV" --name-only)
 
 if [[ -z "$changed_files" ]]; then
     echo "No changed files." >&2