diff --git a/setup b/setup
index c28b44a71..49f05ffc2 100755
--- a/setup
+++ b/setup
@@ -1293,7 +1293,14 @@ rm -f /tmp/gstack-latest-version
 
 # 10. Team mode: register/unregister SessionStart hook
 SETTINGS_HOOK="$SOURCE_GSTACK_DIR/bin/gstack-settings-hook"
-HOOK_CMD="$SOURCE_GSTACK_DIR/bin/gstack-session-update"
+# On Windows (Git Bash / MSYS2 / Cygwin), extensionless scripts can't be
+# launched directly by the OS — the file-association dialog appears instead.
+# Prefix with 'bash' so Claude Code's hook runner invokes Git Bash explicitly.
+if [ "$IS_WINDOWS" -eq 1 ]; then
+  HOOK_CMD="bash $SOURCE_GSTACK_DIR/bin/gstack-session-update"
+else
+  HOOK_CMD="$SOURCE_GSTACK_DIR/bin/gstack-session-update"
+fi
 
 if [ "$TEAM_MODE" -eq 1 ]; then
   "$GSTACK_CONFIG" set auto_upgrade true 2>/dev/null || true
diff --git a/test/setup-windows-fallback.test.ts b/test/setup-windows-fallback.test.ts
index cc04fbbd7..142bb7912 100644
--- a/test/setup-windows-fallback.test.ts
+++ b/test/setup-windows-fallback.test.ts
@@ -55,6 +55,14 @@ describe('setup: _link_or_copy invariant (D7)', () => {
     const fnBody = SETUP_SRC.slice(fnStart, fnEnd);
     expect(fnBody).toContain('_print_windows_copy_note_once');
   });
+
+  test('SessionStart HOOK_CMD is prefixed with bash on Windows (D7-session-hook)', () => {
+    const hookStart = SETUP_SRC.indexOf('# 10. Team mode: register/unregister SessionStart hook');
+    const hookEnd = SETUP_SRC.indexOf('\nif [ "$TEAM_MODE" -eq 1 ]', hookStart);
+    const hookSection = SETUP_SRC.slice(hookStart, hookEnd);
+    expect(hookSection).toContain('IS_WINDOWS');
+    expect(hookSection).toContain('bash $SOURCE_GSTACK_DIR/bin/gstack-session-update');
+  });
 });
 
 // Behavior matrix uses Unix `ln -snf` semantics in the IS_WINDOWS=0 cells.