diff --git a/.github/workflows/lint.yml b/.github/workflows/lint.yml
index 84074b4791..8ec68a6e98 100644
--- a/.github/workflows/lint.yml
+++ b/.github/workflows/lint.yml
@@ -54,3 +54,16 @@ jobs:
       # (notably SC2155). Tighten in a follow-up after cleanup.
       - name: Run shellcheck on shell scripts
         run: git ls-files -z -- '*.sh' | xargs -0 shellcheck --severity=error
+
+      # macOS ships bash 3.2, where bash 4+ case-modification parameter
+      # expansions error with "bad substitution". shellcheck assumes bash 4+
+      # from the shebang and cannot flag these, so guard explicitly; use tr
+      # for portable case conversion.
+      - name: Reject bash 4+ case-modification expansions
+        run: |
+          matches=$(git ls-files -z -- '*.sh' | xargs -0 grep -nE '\$\{[A-Za-z_][A-Za-z0-9_]*(\[[^]]*\])?(\^\^?|,,?|~~?|@[UuLl])[^}]*\}' || true)
+          if [ -n "$matches" ]; then
+            echo "Found bash 4+ case-modification expansion(s); use tr for portability (macOS ships bash 3.2):"
+            echo "$matches"
+            exit 1
+          fi
diff --git a/extensions/git/scripts/bash/create-new-feature-branch.sh b/extensions/git/scripts/bash/create-new-feature-branch.sh
index a13e49812c..c6e4e0668f 100755
--- a/extensions/git/scripts/bash/create-new-feature-branch.sh
+++ b/extensions/git/scripts/bash/create-new-feature-branch.sh
@@ -280,7 +280,7 @@ generate_branch_name() {
 
     local stop_words="^(i|a|an|the|to|for|of|in|on|at|by|with|from|is|are|was|were|be|been|being|have|has|had|do|does|did|will|would|should|could|can|may|might|must|shall|this|that|these|those|my|your|our|their|want|need|add|get|set)$"
 
-    local clean_name=$(echo "$description" | tr '[:upper:]' '[:lower:]' | sed 's/[^a-z0-9]/ /g')
+    local clean_name=$(printf '%s' "$description" | tr '[:upper:]' '[:lower:]' | sed 's/[^a-z0-9]/ /g')
 
     local meaningful_words=()
     for word in $clean_name; do
@@ -288,6 +288,8 @@ generate_branch_name() {
         if ! echo "$word" | grep -qiE "$stop_words"; then
             if [ ${#word} -ge 3 ]; then
                 meaningful_words+=("$word")
+            # Uppercase via tr (portable) rather than bash's 4+ "^^" case
+            # expansion, which breaks on macOS's default bash 3.2 (bad substitution).
             elif printf '%s' "$description" | grep -qw -- "$(printf '%s' "$word" | tr '[:lower:]' '[:upper:]')"; then
                 meaningful_words+=("$word")
             fi
diff --git a/extensions/git/scripts/powershell/create-new-feature-branch.ps1 b/extensions/git/scripts/powershell/create-new-feature-branch.ps1
index 597bdf40d2..0439ec80ad 100644
--- a/extensions/git/scripts/powershell/create-new-feature-branch.ps1
+++ b/extensions/git/scripts/powershell/create-new-feature-branch.ps1
@@ -253,9 +253,10 @@ function Get-BranchName {
         if ($word.Length -ge 3) {
             $meaningfulWords += $word
         } elseif ($Description -cmatch "\b$($word.ToUpper())\b") {
-            # Case-sensitive (-cmatch) to mirror the bash twin's `grep -qw -- "${word^^}"`:
-            # keep a short word only when its UPPERCASE form appears in the original
-            # (an acronym). -match is case-insensitive and would keep every short word.
+            # Case-sensitive (-cmatch) to mirror the bash twin's case-sensitive
+            # whole-word acronym match: keep a short word only when its UPPERCASE
+            # form appears in the original (an acronym). -match is case-insensitive
+            # and would keep every short word.
             $meaningfulWords += $word
         }
     }
diff --git a/scripts/bash/create-new-feature.sh b/scripts/bash/create-new-feature.sh
index 746e88a2dc..3cffce8602 100644
--- a/scripts/bash/create-new-feature.sh
+++ b/scripts/bash/create-new-feature.sh
@@ -140,7 +140,7 @@ generate_branch_name() {
     local stop_words="^(i|a|an|the|to|for|of|in|on|at|by|with|from|is|are|was|were|be|been|being|have|has|had|do|does|did|will|would|should|could|can|may|might|must|shall|this|that|these|those|my|your|our|their|want|need|add|get|set)$"
     
     # Convert to lowercase and split into words
-    local clean_name=$(echo "$description" | tr '[:upper:]' '[:lower:]' | sed 's/[^a-z0-9]/ /g')
+    local clean_name=$(printf '%s' "$description" | tr '[:upper:]' '[:lower:]' | sed 's/[^a-z0-9]/ /g')
     
     # Filter words: remove stop words and words shorter than 3 chars (unless they're uppercase acronyms in original)
     local meaningful_words=()
@@ -152,8 +152,10 @@ generate_branch_name() {
         if ! echo "$word" | grep -qiE "$stop_words"; then
             if [ ${#word} -ge 3 ]; then
                 meaningful_words+=("$word")
+            # Keep short words that appear as an uppercase acronym in the original.
+            # Uppercase via tr and match with grep -w (both portable) rather than
+            # bash's 4+ "^^" case expansion (breaks on macOS bash 3.2) and \b (non-POSIX).
             elif printf '%s' "$description" | grep -qw -- "$(printf '%s' "$word" | tr '[:lower:]' '[:upper:]')"; then
-                # Keep short words if they appear as uppercase in original (likely acronyms)
                 meaningful_words+=("$word")
             fi
         fi