diff --git a/src/spikeinterface/core/sortinganalyzer.py b/src/spikeinterface/core/sortinganalyzer.py
index 94bf1ea11c..7138807e73 100644
--- a/src/spikeinterface/core/sortinganalyzer.py
+++ b/src/spikeinterface/core/sortinganalyzer.py
@@ -8,6 +8,7 @@
 import weakref
 import shutil
 import warnings
+import importlib
 
 import numpy as np
 
@@ -908,35 +909,29 @@ def compute_several_extensions(self, extensions, save=True, **job_kwargs):
 
     def get_saved_extension_names(self):
         """
-        Get extension saved in folder or zarr that can be loaded.
+        Get extension names saved in folder or zarr that can be loaded.
+        This do not load data, this only explores the directory.
         """
-        assert self.format != "memory"
-        global _possible_extensions
+        saved_extension_names = []
+        if self.format == "binary_folder":
+            ext_folder = self.folder / "extensions"
+            if ext_folder.is_dir():
+                for extension_folder in ext_folder.iterdir():
+                    is_saved = extension_folder.is_dir() and (extension_folder / "params.json").is_file()
+                    if not is_saved:
+                        continue
+                    saved_extension_names.append(extension_folder.stem)
 
-        if self.format == "zarr":
+        elif self.format == "zarr":
             zarr_root = self._get_zarr_root(mode="r")
             if "extensions" in zarr_root.keys():
                 extension_group = zarr_root["extensions"]
-            else:
-                extension_group = None
+                for extension_name in extension_group.keys():
+                    if "params" in extension_group[extension_name].attrs.keys():
+                        saved_extension_names.append(extension_name)
 
-        saved_extension_names = []
-        for extension_class in _possible_extensions:
-            extension_name = extension_class.extension_name
-
-            if self.format == "binary_folder":
-                extension_folder = self.folder / "extensions" / extension_name
-                is_saved = extension_folder.is_dir() and (extension_folder / "params.json").is_file()
-            elif self.format == "zarr":
-                if extension_group is not None:
-                    is_saved = (
-                        extension_name in extension_group.keys()
-                        and "params" in extension_group[extension_name].attrs.keys()
-                    )
-                else:
-                    is_saved = False
-            if is_saved:
-                saved_extension_names.append(extension_class.extension_name)
+        else:
+            raise ValueError("SortingAnalyzer.get_saved_extension_names() works only with binary_folder and zarr")
 
         return saved_extension_names
 
@@ -1057,7 +1052,7 @@ def register_result_extension(extension_class):
         _possible_extensions.append(extension_class)
 
 
-def get_extension_class(extension_name: str):
+def get_extension_class(extension_name: str, auto_import=True):
     """
     Get extension class from name and check if registered.
 
@@ -1065,6 +1060,8 @@ def get_extension_class(extension_name: str):
     ----------
     extension_name: str
         The extension name.
+    auto_import: bool, default True
+        Auto import the module if the extension class is not registered yet.
 
     Returns
     -------
@@ -1073,9 +1070,20 @@ def get_extension_class(extension_name: str):
     """
     global _possible_extensions
     extensions_dict = {ext.extension_name: ext for ext in _possible_extensions}
-    assert (
-        extension_name in extensions_dict
-    ), f"Extension '{extension_name}' is not registered, please import related module before use"
+
+    if extension_name not in extensions_dict:
+        if extension_name in _builtin_extensions:
+            module = _builtin_extensions[extension_name]
+            if auto_import:
+                imported_module = importlib.import_module(module)
+                extensions_dict = {ext.extension_name: ext for ext in _possible_extensions}
+            else:
+                raise ValueError(
+                    f"Extension '{extension_name}' is not registered, please import related module before use: 'import {module}'"
+                )
+        else:
+            raise ValueError(f"Extension '{extension_name}' is unknown maybe this is an external extension or a typo.")
+
     ext_class = extensions_dict[extension_name]
     return ext_class
 
@@ -1471,3 +1479,27 @@ def get_pipeline_nodes(self):
     def get_data(self, *args, **kwargs):
         assert len(self.data) > 0, f"You must run the extension {self.extension_name} before retrieving data"
         return self._get_data(*args, **kwargs)
+
+
+# this is a hardcoded list to to improve error message and auto_import mechanism
+# this is important because extension are registered when the submodule is imported
+_builtin_extensions = {
+    # from core
+    "random_spikes": "spikeinterface.core",
+    "waveforms": "spikeinterface.core",
+    "templates": "spikeinterface.core",
+    "fast_templates": "spikeinterface.core",
+    "noise_levels": "spikeinterface.core",
+    # from postprocessing
+    "amplitude_scalings": "spikeinterface.postprocessing",
+    "correlograms": "spikeinterface.postprocessing",
+    "isi_histograms": "spikeinterface.postprocessing",
+    "principal_components": "spikeinterface.postprocessing",
+    "spike_amplitudes": "spikeinterface.postprocessing",
+    "spike_locations": "spikeinterface.postprocessing",
+    "template_metrics": "spikeinterface.postprocessing",
+    "template_similarity": "spikeinterface.postprocessing",
+    "unit_locations": "spikeinterface.postprocessing",
+    # from quality metrics
+    "quality_metrics": "spikeinterface.qualitymetrics",
+}