src/App/Document.cpp

@@ -1680,7 +1680,7 @@ static std::string checkFileName(const char* file)
             .GetParameterGroupByPath("User parameter:BaseApp/Preferences/Document")
             ->GetBool("CheckExtension", true)) {
         const char* ext = strrchr(file, '.');
-        if ((ext == nullptr) || !boost::iequals(ext + 1, "fcstd")) {
+        if ((ext == nullptr) || (!boost::iequals(ext + 1, "fcstd") && !boost::iequals(ext + 1, "kc"))) {
             if (ext && ext[1] == 0) {
                 fn += "FCStd";
             }

src/App/FreeCADInit.py

@@ -420,7 +420,7 @@ App.__cmake__ = globals().get("cmake", [])
 # store unit test names
 App.__unit_test__ = []
 
-App.addImportType("FreeCAD document (*.FCStd)", "FreeCAD")
+App.addImportType("FreeCAD document (*.FCStd *.kc)", "FreeCAD")
 
 # set to no gui, is overwritten by InitGui
 App.GuiUp = 0

src/Gui/CommandDoc.cpp

@@ -529,7 +529,7 @@ void StdCmdMergeProjects::activated(int iMsg)
         Gui::getMainWindow(),
         QString::fromUtf8(QT_TR_NOOP("Merge Document")),
         FileDialog::getWorkingDirectory(),
-        QString::fromUtf8(QT_TR_NOOP("%1 document (*.FCStd)")).arg(exe)
+        QString::fromUtf8(QT_TR_NOOP("%1 document (*.FCStd *.kc)")).arg(exe)
     );
     if (!project.isEmpty()) {
         FileDialog::setWorkingDirectory(project);

src/Gui/Dialogs/DlgProjectUtility.cpp

@@ -46,7 +46,7 @@ DlgProjectUtility::DlgProjectUtility(QWidget* parent, Qt::WindowFlags fl)
     ui->setupUi(this);
     connect(ui->extractButton, &QPushButton::clicked, this, &DlgProjectUtility::extractButton);
     connect(ui->createButton, &QPushButton::clicked, this, &DlgProjectUtility::createButton);
-    ui->extractSource->setFilter(QStringLiteral("%1 (*.FCStd)").arg(tr("Project file")));
+    ui->extractSource->setFilter(QStringLiteral("%1 (*.FCStd *.kc)").arg(tr("Project file")));
 }
 
 /**

src/Gui/Document.cpp

@@ -1616,7 +1616,7 @@ bool Document::saveAs()
         getMainWindow(),
         QObject::tr("Save %1 Document").arg(exe),
         name,
-        QStringLiteral("%1 %2 (*.FCStd)").arg(exe, QObject::tr("Document"))
+        QStringLiteral("%1 %2 (*.FCStd *.kc)").arg(exe, QObject::tr("Document"))
     );
 
     if (!fn.isEmpty()) {
@@ -1739,7 +1739,7 @@ bool Document::saveCopy()
         getMainWindow(),
         QObject::tr("Save %1 Document").arg(exe),
         QString::fromUtf8(getDocument()->FileName.getValue()),
-        QObject::tr("%1 document (*.FCStd)").arg(exe)
+        QObject::tr("%1 document (*.FCStd *.kc)").arg(exe)
     );
     if (!fn.isEmpty()) {
         const char* DocName = App::GetApplication().getDocumentName(getDocument());

src/Mod/Create/InitGui.py

@@ -48,6 +48,16 @@ setup_kindred_workbenches()
 FreeCAD.Console.PrintLog("Create GUI module initialized\n")
 
 
+def _register_kc_format():
+    """Register .kc file format round-trip preservation."""
+    try:
+        import kc_format
+
+        kc_format.register()
+    except Exception as e:
+        FreeCAD.Console.PrintLog(f"Create: kc_format registration skipped: {e}\n")
+
+
 # ---------------------------------------------------------------------------
 # Silo integration enhancements
 # ---------------------------------------------------------------------------
@@ -162,6 +172,7 @@ def _check_for_updates():
 try:
     from PySide.QtCore import QTimer
 
+    QTimer.singleShot(500, _register_kc_format)
     QTimer.singleShot(1500, _register_silo_origin)
     QTimer.singleShot(2000, _setup_silo_auth_panel)
     QTimer.singleShot(3000, _check_silo_first_start)

src/Mod/Create/kc_format.py

@@ -0,0 +1,59 @@
+"""
+kc_format.py — .kc file format round-trip preservation.
+
+Caches silo/ ZIP entries before FreeCAD's C++ save rewrites the ZIP
+from scratch, then re-injects them after save completes.
+"""
+
+import os
+import zipfile
+
+import FreeCAD
+
+# Cache: filepath -> {entry_name: bytes}
+_silo_cache = {}
+
+
+class _KcFormatObserver:
+    """Document observer that preserves silo/ entries across saves."""
+
+    def slotStartSaveDocument(self, doc, filename):
+        """Before save: cache silo/ entries from the existing file."""
+        if not filename.lower().endswith(".kc"):
+            return
+        if not os.path.isfile(filename):
+            return
+        try:
+            with zipfile.ZipFile(filename, "r") as zf:
+                entries = {}
+                for name in zf.namelist():
+                    if name.startswith("silo/"):
+                        entries[name] = zf.read(name)
+                if entries:
+                    _silo_cache[filename] = entries
+        except Exception:
+            pass
+
+    def slotFinishSaveDocument(self, doc, filename):
+        """After save: re-inject cached silo/ entries into the .kc ZIP."""
+        if not filename.lower().endswith(".kc"):
+            _silo_cache.pop(filename, None)
+            return
+        entries = _silo_cache.pop(filename, None)
+        if not entries:
+            return
+        try:
+            with zipfile.ZipFile(filename, "a") as zf:
+                existing = set(zf.namelist())
+                for name, data in entries.items():
+                    if name not in existing:
+                        zf.writestr(name, data)
+        except Exception as e:
+            FreeCAD.Console.PrintWarning(
+                f"kc_format: failed to preserve silo/ entries: {e}\n"
+            )
+
+
+def register():
+    """Connect to application-level save signals."""
+    FreeCAD.addDocumentObserver(_KcFormatObserver())