diff --git a/Lts/Common/CodeGen/CMakeLists.txt b/Lts/Common/CodeGen/CMakeLists.txt
index 07ae22229..2fdff8411 100755
--- a/Lts/Common/CodeGen/CMakeLists.txt
+++ b/Lts/Common/CodeGen/CMakeLists.txt
@@ -113,6 +113,21 @@ else()
   message(STATUS "Grantlee5 not found; skipping codegen_grantlee target")
 endif()
 
+# Smoke-test: parse all .grantlee templates in this directory to detect parse/render regressions
+if (TARGET Grantlee5::Templates)
+  find_package(Qt6 COMPONENTS Core QUIET)
+  if (Qt6_FOUND)
+    set(QTCORE_TARGET Qt6::Core)
+  else()
+    find_package(Qt5 COMPONENTS Core REQUIRED)
+    set(QTCORE_TARGET Qt5::Core)
+  endif()
+
+  add_executable(grantlee_parser_smoke tests/GrantleeParserSmoke.cpp)
+  target_link_libraries(grantlee_parser_smoke PRIVATE Grantlee5::Templates ${QTCORE_TARGET})
+  add_test(NAME GrantleeParserSmoke COMMAND grantlee_parser_smoke ${CMAKE_CURRENT_SOURCE_DIR})
+endif()
+
 NGL_ADD_EXECUTABLE(ALL codegen
   main_grantlee.cpp
 )
diff --git a/Lts/Common/CodeGen/tests/GrantleeParserSmoke.cpp b/Lts/Common/CodeGen/tests/GrantleeParserSmoke.cpp
new file mode 100644
index 000000000..5af6f98aa
--- /dev/null
+++ b/Lts/Common/CodeGen/tests/GrantleeParserSmoke.cpp
@@ -0,0 +1,62 @@
+#include <QCoreApplication>
+#include <QDir>
+#include <QDebug>
+
+#include <grantlee/engine.h>
+#include <grantlee/templateloader.h>
+
+int main(int argc, char **argv)
+{
+    QCoreApplication app(argc, argv);
+
+    QString templatesDir = QDir::currentPath();
+    if (argc > 1) {
+        templatesDir = QString::fromLocal8Bit(argv[1]);
+    }
+
+    Grantlee::Engine engine;
+    auto loader = QSharedPointer<Grantlee::FileSystemTemplateLoader>::create();
+    loader->setTemplateDirs(QStringList() << templatesDir);
+    engine.addTemplateLoader(loader);
+
+    QDir d(templatesDir);
+    QStringList files = d.entryList(QStringList() << "*.grantlee", QDir::Files);
+    if (files.isEmpty()) {
+        qDebug() << "No .grantlee templates found in" << templatesDir;
+        // Treat absence as successful parse (nothing to check) to avoid failing
+        // when directory is empty in different setups.
+        return 0;
+    }
+
+    bool ok = true;
+    for (const QString &f : files) {
+        Grantlee::Template t;
+        try {
+            t = engine.loadByName(f);
+        } catch (...) {
+            qDebug() << "Exception while loading" << f;
+            ok = false;
+            continue;
+        }
+        if (!t) {
+            qDebug() << "Failed to load template" << f;
+            ok = false;
+            continue;
+        }
+        if (t->error() != Grantlee::TemplateImpl::Error::NoError) {
+            qDebug() << "Parse error in" << f << ":" << t->errorString();
+            ok = false;
+            continue;
+        }
+        // Render with empty context to ensure rendering path also works.
+        Grantlee::Context ctx;
+        QString out = t->render(&ctx);
+        if (out.isNull()) {
+            qDebug() << "Rendering produced null for" << f;
+            ok = false;
+            continue;
+        }
+    }
+
+    return ok ? 0 : 2;
+}
diff --git a/app/patches/apply_grantlee_fix.sh b/app/patches/apply_grantlee_fix.sh
new file mode 100755
index 000000000..05cd16101
--- /dev/null
+++ b/app/patches/apply_grantlee_fix.sh
@@ -0,0 +1,30 @@
+#!/usr/bin/env bash
+set -euo pipefail
+
+# Apply lightweight fix to Grantlee sources in externals to avoid duplicate
+# QT_TESTCASE_BUILDDIR definition which causes compiler errors on Qt6.
+
+GRANTLEE_SRC_DIR="${1:-./external/grantlee-src}"
+
+if [ ! -d "$GRANTLEE_SRC_DIR" ]; then
+  echo "Grantlee source dir not found: $GRANTLEE_SRC_DIR" >&2
+  exit 2
+fi
+
+TARGET_FILE="$GRANTLEE_SRC_DIR/textdocument/tests/CMakeLists.txt"
+
+if [ ! -f "$TARGET_FILE" ]; then
+  echo "Target CMake file not found: $TARGET_FILE" >&2
+  exit 3
+fi
+
+# Replace the unconditional target_compile_definitions block with a commented
+# explanation to avoid duplicate -D flags.
+
+awk '
+BEGIN{inblock=0}
+/target_compile_definitions\(plainmarkupbuildertest_exec PRIVATE QT_TESTCASE_BUILDDIR/ {inblock=1; print "# QT_TESTCASE_BUILDDIR was previously defined here which could conflict with the"; print "# the definition injected by Qt\'s test configuration. Defining the macro in"; print "# multiple places leads to compiler redefinition errors. The test framework"; print "# already provides this define when needed, so omit the explicit definition"; print "# to avoid duplicate -D flags."; next}
+{ if (inblock) { if (/^endif\(\)/) { inblock=0; next } else next } print }
+' "$TARGET_FILE" > "$TARGET_FILE.tmp" && mv "$TARGET_FILE.tmp" "$TARGET_FILE"
+
+echo "Applied grantlee fix to $TARGET_FILE"