diff --git a/bin/setup b/bin/setup
index 28c2075..e6d77b1 100755
--- a/bin/setup
+++ b/bin/setup
@@ -8,11 +8,11 @@
 #
 # What this script does:
 #   1. Re-execs itself with bash (curl|sh consumes stdin, so bash -c re-fetch is required)
-#   2. Detects OS (Debian/Ubuntu, macOS, other Linux)
-#   3. Installs git if missing — apt-get on Debian/Ubuntu, xcode-select on macOS
+#   2. Detects OS, installs git if missing — apt-get on Debian/Ubuntu, xcode-select on macOS
+#   3. Installs zsh if missing, sets it as the default shell
 #   4. Clones this dotfiles repo into ~ via HTTPS (or pulls if already present)
 #   5. Runs arty deps — oh-my-zsh, p10k, nvm, rbenv, pyenv, zsh plugins
-#   6. Installs arty zsh completion to ~/.zsh/completions/_arty
+#   6. Installs arty zsh completion via: arty completion zsh --install
 #   7. Prints next steps
 
 # ─── Step 0: Re-exec with bash ────────────────────────────────────────────────
@@ -116,7 +116,7 @@ check_prerequisites() {
 
 # ─── Step 2: Ensure git ───────────────────────────────────────────────────────
 ensure_git() {
-  step_header "Step 2 of 6 — Git"
+  step_header "Step 2 of 7 — Git"
 
   if command -v git >/dev/null 2>&1; then
     log_success "git $(git --version | awk '{print $3}')"
@@ -155,9 +155,66 @@ ensure_git() {
   esac
 }
 
-# ─── Step 3: Clone or update dotfiles ─────────────────────────────────────────
+# ─── Step 3: Ensure zsh ───────────────────────────────────────────────────────
+ensure_zsh() {
+  step_header "Step 3 of 7 — Zsh"
+
+  if command -v zsh >/dev/null 2>&1; then
+    log_success "zsh $(zsh --version | awk '{print $2}')"
+  else
+    log_warn "zsh not found — installing for OS: $OS"
+    case "$OS" in
+      debian)
+        if ! command -v sudo >/dev/null 2>&1; then
+          log_error "sudo is required to install zsh on Debian/Ubuntu."
+          log_error "Run as root: apt-get install -y zsh — then re-run this script."
+          exit 1
+        fi
+        sudo apt-get update -qq
+        sudo apt-get install -y zsh
+        log_success "zsh installed: $(zsh --version | awk '{print $2}')"
+        ;;
+      macos)
+        if command -v brew >/dev/null 2>&1; then
+          brew install zsh
+          log_success "zsh installed: $(zsh --version | awk '{print $2}')"
+        else
+          log_warn "zsh not found and Homebrew is not available."
+          log_warn "Install zsh manually: https://www.zsh.org"
+        fi
+        ;;
+      *)
+        log_warn "Cannot install zsh automatically on OS: $OS"
+        log_warn "Please install zsh manually, then re-run this script."
+        ;;
+    esac
+  fi
+
+  # Set zsh as the default shell if it isn't already.
+  local zsh_path
+  zsh_path="$(command -v zsh 2>/dev/null || true)"
+  if [[ -n "$zsh_path" && "$SHELL" != "$zsh_path" ]]; then
+    log_info "Setting zsh as default shell..."
+    if command -v chsh >/dev/null 2>&1; then
+      # Add zsh to /etc/shells if missing (required by chsh).
+      if ! grep -qx "$zsh_path" /etc/shells 2>/dev/null; then
+        echo "$zsh_path" | sudo tee -a /etc/shells >/dev/null
+      fi
+      if sudo chsh -s "$zsh_path" "$USER" 2>/dev/null || chsh -s "$zsh_path" 2>/dev/null; then
+        log_success "Default shell set to zsh (takes effect on next login)"
+      else
+        log_warn "Could not set default shell automatically."
+        log_warn "Run manually: chsh -s $zsh_path"
+      fi
+    else
+      log_warn "chsh not available — set your default shell manually: chsh -s $zsh_path"
+    fi
+  fi
+}
+
+# ─── Step 4: Clone or update dotfiles ─────────────────────────────────────────
 setup_dotfiles() {
-  step_header "Step 3 of 6 — Dotfiles"
+  step_header "Step 4 of 7 — Dotfiles"
 
   local is_git_repo=0
   local is_this_repo=0
@@ -215,9 +272,9 @@ setup_dotfiles() {
   log_success "Dotfiles cloned into $HOME"
 }
 
-# ─── Step 4: Install dependencies via arty ────────────────────────────────────
+# ─── Step 5: Install dependencies via arty ────────────────────────────────────
 install_deps() {
-  step_header "Step 4 of 6 — Dependencies (arty deps)"
+  step_header "Step 5 of 7 — Dependencies (arty deps)"
 
   local arty_bin="$HOME/bin/arty"
 
@@ -241,38 +298,26 @@ install_deps() {
   log_success "All dependencies installed"
 }
 
-# ─── Step 5: Install arty zsh completion ──────────────────────────────────────
+# ─── Step 6: Install arty zsh completion ──────────────────────────────────────
 install_completion() {
-  step_header "Step 5 of 6 — Zsh completion"
+  step_header "Step 6 of 7 — Zsh completion"
 
   local arty_bin="$HOME/bin/arty"
-  local completion_content
 
-  if ! completion_content="$(cd "$HOME" && "$arty_bin" completion zsh 2>/dev/null)"; then
-    log_warn "arty completion zsh failed — skipping."
-    return 0
+  # Ensure tools that arty deps may have installed (yq, etc.) are on PATH.
+  # arty deps runs in a subshell — its PATH exports don't survive back here.
+  export PATH="$HOME/.local/bin:$HOME/go/bin:$PATH"
+
+  # arty completion zsh --install writes to the first writable fpath dir,
+  # defaulting to ~/.zsh/completions/_arty (which .zshenv/oh-my-zsh picks up).
+  if ! (cd "$HOME" && "$arty_bin" completion zsh --install); then
+    log_warn "arty completion zsh --install failed — skipping."
   fi
-
-  # Sanity-check: a valid zsh completion file starts with #compdef.
-  if ! echo "$completion_content" | head -1 | grep -q "#compdef"; then
-    log_warn "arty completion output is not a valid zsh completion — skipping."
-    return 0
-  fi
-
-  # Target: ~/.zsh/completions/_arty
-  # .zshenv sets ZSH_CUSTOM="$HOME/.zsh"; oh-my-zsh adds $ZSH_CUSTOM/completions
-  # to fpath automatically — this is the correct path for this configuration.
-  local comp_dir="$HOME/.zsh/completions"
-  local comp_dest="$comp_dir/_arty"
-
-  mkdir -p "$comp_dir"
-  printf '%s\n' "$completion_content" > "$comp_dest"
-  log_success "Completion installed: $comp_dest"
 }
 
-# ─── Step 6: Post-install summary ─────────────────────────────────────────────
+# ─── Step 7: Post-install summary ─────────────────────────────────────────────
 post_install() {
-  step_header "Step 6 of 6 — Done"
+  step_header "Step 7 of 7 — Done"
 
   echo
   echo -e "${GREEN}${BOLD}Setup complete.${NC} Next steps:"
@@ -300,10 +345,11 @@ main() {
 
   check_prerequisites  # Step 1
   ensure_git           # Step 2
-  setup_dotfiles       # Step 3
-  install_deps         # Step 4
-  install_completion   # Step 5
-  post_install         # Step 6
+  ensure_zsh           # Step 3
+  setup_dotfiles       # Step 4
+  install_deps         # Step 5
+  install_completion   # Step 6
+  post_install         # Step 7
 }
 
 main "$@"