diff --git a/test/unit/wt-sh.bats b/test/unit/wt-sh.bats
new file mode 100644
index 0000000..7d120cb
--- /dev/null
+++ b/test/unit/wt-sh.bats
@@ -0,0 +1,35 @@
+#!/usr/bin/env bats
+
+# Unit tests for wt.sh
+
+setup() {
+    load '../test_helper/common'
+    setup_test_env
+}
+
+teardown() {
+    teardown_test_env
+}
+
+# =============================================================================
+# Tests for _WT_ROOT consistency
+# =============================================================================
+
+@test "_WT_ROOT default in wt.sh matches INSTALL_DIR in install.sh" {
+    # Extract the default path from wt.sh: _WT_ROOT="${_WT_ROOT:-$HOME/.wt}"
+    local wt_line
+    wt_line=$(grep '_WT_ROOT=.*HOME' "$PROJECT_ROOT/wt.sh" | head -1)
+    # Pull out the $HOME/... portion
+    local wt_path
+    wt_path=$(echo "$wt_line" | sed 's/.*\(\$HOME\/[^}"]*\).*/\1/')
+
+    # Extract INSTALL_DIR from install.sh: INSTALL_DIR="$HOME/.wt"
+    local install_line
+    install_line=$(grep 'INSTALL_DIR=.*HOME' "$PROJECT_ROOT/install.sh" | head -1)
+    local install_path
+    install_path=$(echo "$install_line" | sed 's/.*"\(\$HOME\/[^"]*\)".*/\1/')
+
+    [ -n "$wt_path" ]
+    [ -n "$install_path" ]
+    [ "$wt_path" = "$install_path" ]
+}
diff --git a/wt.sh b/wt.sh
index f1ee957..5e75d46 100755
--- a/wt.sh
+++ b/wt.sh
@@ -11,18 +11,9 @@
 # Usage: source this file, then run `wt help` for details.
 #
 
-# Capture script path at top level (works in both bash and zsh)
-# In bash, BASH_SOURCE[0] gives the sourced file path.
-# In zsh, %x prompt expansion reliably gives the current source file path,
-# even when FUNCTION_ARGZERO is unset (where $0 would return "-zsh").
-if [[ -n "${BASH_SOURCE[0]:-}" ]]; then
-  _WT_SCRIPT_PATH="${BASH_SOURCE[0]}"
-elif [[ -n "${ZSH_VERSION:-}" ]]; then
-  # %x prompt expansion reliably gives the current source file path
-  _WT_SCRIPT_PATH="$(print -P '%x')"
-else
-  _WT_SCRIPT_PATH="$0"
-fi
+# Installation root — all wt files live under this directory.
+# Override by setting _WT_ROOT before sourcing (e.g., for development).
+_WT_ROOT="${_WT_ROOT:-$HOME/.wt}"
 
 # Ensure this file is sourced, not executed; exit with error if executed directly
 _wt_ensure_sourced() {
@@ -30,54 +21,26 @@ _wt_ensure_sourced() {
     # In zsh, check if we're being sourced by examining ZSH_EVAL_CONTEXT
     # When sourced, it contains "toplevel:file" or similar patterns with "file"
     if [[ "${ZSH_EVAL_CONTEXT:-}" != *:file:* && "${ZSH_EVAL_CONTEXT:-}" != *:file ]]; then
-      # Not sourced - but this check can be unreliable, so also check $0
-      if [[ "$0" == "$_WT_SCRIPT_PATH" ]]; then
-        echo "Error: This file must be sourced, not executed." >&2
-        echo "" >&2
-        echo "Add this to your ~/.zshrc:" >&2
-        echo "  source $_WT_SCRIPT_PATH" >&2
-        echo "" >&2
-        echo "Then use: wt <command> [args]" >&2
-        exit 1
-      fi
+      echo "Error: This file must be sourced, not executed." >&2
+      echo "" >&2
+      echo "Add this to your ~/.zshrc:" >&2
+      echo "  source $_WT_ROOT/wt.sh" >&2
+      echo "" >&2
+      echo "Then use: wt <command> [args]" >&2
+      exit 1
     fi
   elif [[ "${BASH_SOURCE[0]}" == "${0}" ]]; then
     # Bash: BASH_SOURCE[0] equals $0 when executed directly
     echo "Error: This file must be sourced, not executed." >&2
     echo "" >&2
     echo "Add this to your ~/.bashrc:" >&2
-    echo "  source $_WT_SCRIPT_PATH" >&2
+    echo "  source $_WT_ROOT/wt.sh" >&2
     echo "" >&2
     echo "Then use: wt <command> [args]" >&2
     exit 1
   fi
 }
 
-# Resolve the root directory where wt.sh lives
-_wt_resolve_root() {
-  local source="$_WT_SCRIPT_PATH"
-  local root
-  # Resolve symlinks to find the real location
-  while [[ -L "$source" ]]; do
-    local dir
-    dir="$(command cd -P "$(dirname "$source")" && pwd)"
-    source="$(readlink "$source")"
-    # If source is relative, resolve it relative to the symlink's directory
-    [[ "$source" != /* ]] && source="$dir/$source"
-  done
-  root="$(command cd -P "$(dirname "$source")" && pwd)"
-
-  # Validate: root must contain bin/ and lib/ directories
-  if [[ -d "$root/bin" && -d "$root/lib" ]]; then
-    echo "$root"
-  elif [[ -d "$HOME/.wt/bin" && -d "$HOME/.wt/lib" ]]; then
-    echo "$HOME/.wt"
-  else
-    echo "wt: cannot determine installation root" >&2
-    return 1
-  fi
-}
-
 # helper for sourcing a library file from lib/ directory
 # Args: $1 = library name, $2 = "optional" to skip error if not found
 _wt_source_lib() {
@@ -86,8 +49,6 @@ _wt_source_lib() {
   
   if [[ -f "$_WT_ROOT/lib/$lib" ]]; then
     . "$_WT_ROOT/lib/$lib"
-  elif [[ -f "$HOME/.wt/lib/$lib" ]]; then
-    . "$HOME/.wt/lib/$lib"
   elif [[ "$required" != "optional" ]]; then
     echo "wt: cannot find required library: $lib" >&2
     return 1
@@ -167,7 +128,6 @@ _wt_source_shell_completion() {
 # ═══════════════════════════════════════════════════════════════════════════════
 
 _wt_ensure_sourced
-_WT_ROOT="$(_wt_resolve_root)"
 _wt_source_lib wt-common
 _wt_source_lib wt-help
 _wt_source_shell_completion        # completion for `wt` and `wt-*` commands