diff --git a/bin/idstack-doctor b/bin/idstack-doctor
index 8525da3..3bb81b5 100755
--- a/bin/idstack-doctor
+++ b/bin/idstack-doctor
@@ -135,14 +135,12 @@ elif [ -d "$LEGACY_DIR" ]; then
   fi
   if [ -f "$LEGACY_DIR/VERSION" ]; then
     v=$(tr -d '[:space:]' < "$LEGACY_DIR/VERSION")
-    # Two arms: explicitly skip modern/future versions first, then flag the
-    # legacy ones. Mirrors setup. Patterns avoid literal dots and single-digit
-    # ranges so multi-digit components (2.0.10.0, 2.10.0.0, 20.x, 100.0.0,
-    # 200.0.0) work. Covered by test/test-version-classifier.sh.
-    case "$v" in
-      2.0.[1-9]*|2.[1-9]*|[3-9]*|[1-9][0-9]*) ;;
-      0.*|1.*|2.0.0.*|2.0.0) signature="${signature:+$signature, }VERSION=$v" ;;
-    esac
+    # Uses shared classifier to identify pre-v2.0.1.0 versions. Mirrors setup.
+    # Covered by test/test-version-classifier.sh.
+    class=$("$IDSTACK_DIR/bin/idstack-version-classifier" "$v")
+    if [ "$class" = "legacy" ]; then
+      signature="${signature:+$signature, }VERSION=$v"
+    fi
   fi
   if [ -n "$signature" ]; then
     echo "  PROBLEM: pre-v2.0.1.0 install at $LEGACY_DIR ($signature)"
diff --git a/bin/idstack-version-classifier b/bin/idstack-version-classifier
new file mode 100755
index 0000000..3caae47
--- /dev/null
+++ b/bin/idstack-version-classifier
@@ -0,0 +1,9 @@
+#!/usr/bin/env bash
+# Classifies an idstack version string as "skip" (modern), "legacy", or "unknown".
+# Used by setup, bin/idstack-doctor, and test/test-version-classifier.sh.
+
+case "${1:-}" in
+  2.0.[1-9]*|2.[1-9]*|[3-9]*|[1-9][0-9]*) echo "skip" ;;
+  0.*|1.*|2.0.0.*|2.0.0) echo "legacy" ;;
+  *) echo "unknown" ;;
+esac
diff --git a/setup b/setup
index ed698ee..4d61bef 100755
--- a/setup
+++ b/setup
@@ -92,14 +92,12 @@ elif [ -d "$LEGACY_SKILLS_LINK" ] && [ "$LEGACY_SKILLS_LINK" != "$IDSTACK_DIR" ]
   legacy_is_old_version=0
   if [ -f "$LEGACY_SKILLS_LINK/VERSION" ]; then
     legacy_version=$(tr -d '[:space:]' < "$LEGACY_SKILLS_LINK/VERSION")
-    # Two arms: explicitly skip modern/future versions first, then flag the
-    # legacy ones. Patterns avoid literal dots and single-digit ranges so
-    # multi-digit components (2.0.10.0, 2.10.0.0, 20.x, 100.0.0, 200.0.0)
-    # work. Covered by test/test-version-classifier.sh.
-    case "$legacy_version" in
-      2.0.[1-9]*|2.[1-9]*|[3-9]*|[1-9][0-9]*) ;;
-      0.*|1.*|2.0.0.*|2.0.0) legacy_is_old_version=1 ;;
-    esac
+    # Uses shared classifier to identify pre-v2.0.1.0 versions. Covered by
+    # test/test-version-classifier.sh.
+    class=$("$IDSTACK_DIR/bin/idstack-version-classifier" "$legacy_version")
+    if [ "$class" = "legacy" ]; then
+      legacy_is_old_version=1
+    fi
   fi
   if [ "$legacy_is_dispatcher" = "1" ] || [ "$legacy_is_old_version" = "1" ]; then
     if [[ " $* " == *" --keep-legacy "* ]]; then
diff --git a/test/test-version-classifier.sh b/test/test-version-classifier.sh
index 312d15f..4b38e50 100755
--- a/test/test-version-classifier.sh
+++ b/test/test-version-classifier.sh
@@ -16,26 +16,18 @@
 
 set -e
 
+IDSTACK_DIR="$(cd "$(dirname "$0")/.." && pwd -P)"
+
 PASS=0
 FAIL=0
 TOTAL=0
 
-# Mirror of the case statement in setup and bin/idstack-doctor. Keep these
-# patterns in lockstep with both files — if you change one, change all three.
-classify_version() {
-  case "$1" in
-    2.0.[1-9]*|2.[1-9]*|[3-9]*|[1-9][0-9]*) echo "skip" ;;
-    0.*|1.*|2.0.0.*|2.0.0) echo "legacy" ;;
-    *) echo "unknown" ;;
-  esac
-}
-
 check() {
   TOTAL=$((TOTAL + 1))
   local version="$1"
   local expected="$2"
   local got
-  got="$(classify_version "$version")"
+  got="$("$IDSTACK_DIR/bin/idstack-version-classifier" "$version")"
   if [ "$got" = "$expected" ]; then
     PASS=$((PASS + 1))
     echo "  PASS: $version -> $got"