diff --git a/invokeai/backend/stable_diffusion/extensions_manager.py b/invokeai/backend/stable_diffusion/extensions_manager.py
index 1552fb5dd7..f42a065e82 100644
--- a/invokeai/backend/stable_diffusion/extensions_manager.py
+++ b/invokeai/backend/stable_diffusion/extensions_manager.py
@@ -18,6 +18,7 @@ class ExtensionsManager:
     def __init__(self, is_canceled: Optional[Callable[[], bool]] = None):
         self._is_canceled = is_canceled
 
+        # A list of extensions in the order that they were added to the ExtensionsManager.
         self._extensions: List[ExtensionBase] = []
         self._ordered_callbacks: Dict[ExtensionCallbackType, List[CallbackFunctionWithMetadata]] = {}
 
@@ -38,6 +39,8 @@ class ExtensionsManager:
 
         # Sort each callback list.
         for callback_type, callbacks in self._ordered_callbacks.items():
+            # Note that sorted() is stable, so if two callbacks have the same order, the order that they extensions were
+            # added will be preserved.
             self._ordered_callbacks[callback_type] = sorted(callbacks, key=lambda x: x.metadata.order)
 
     def run_callback(self, callback_type: ExtensionCallbackType, ctx: DenoiseContext):
diff --git a/tests/backend/stable_diffusion/extensions/test_base.py b/tests/backend/stable_diffusion/extensions/test_base.py
new file mode 100644
index 0000000000..d024c551a2
--- /dev/null
+++ b/tests/backend/stable_diffusion/extensions/test_base.py
@@ -0,0 +1,46 @@
+from unittest import mock
+
+from invokeai.backend.stable_diffusion.denoise_context import DenoiseContext
+from invokeai.backend.stable_diffusion.extension_callback_type import ExtensionCallbackType
+from invokeai.backend.stable_diffusion.extensions.base import ExtensionBase, callback
+
+
+class MockExtension(ExtensionBase):
+    """A mock ExtensionBase subclass for testing purposes."""
+
+    def __init__(self, x: int):
+        super().__init__()
+        self._x = x
+
+    @callback(ExtensionCallbackType.PRE_DENOISE_LOOP)
+    def set_step_index(self, ctx: DenoiseContext):
+        ctx.step_index = self._x
+
+
+def test_extension_base_callback_registration():
+    """Test that a callback can be successfully registered with an extension."""
+    val = 5
+    mock_extension = MockExtension(val)
+
+    mock_ctx = mock.MagicMock()
+
+    callbacks = mock_extension.get_callbacks()
+    pre_denoise_loop_cbs = callbacks.get(ExtensionCallbackType.PRE_DENOISE_LOOP, [])
+    assert len(pre_denoise_loop_cbs) == 1
+
+    # Call the mock callback.
+    pre_denoise_loop_cbs[0].function(mock_ctx)
+
+    # Confirm that the callback ran.
+    assert mock_ctx.step_index == val
+
+
+def test_extension_base_empty_callback_type():
+    """Test that an empty list is returned when no callbacks are registered for a given callback type."""
+    mock_extension = MockExtension(5)
+
+    # There should be no callbacks registered for POST_DENOISE_LOOP.
+    callbacks = mock_extension.get_callbacks()
+
+    post_denoise_loop_cbs = callbacks.get(ExtensionCallbackType.POST_DENOISE_LOOP, [])
+    assert len(post_denoise_loop_cbs) == 0
diff --git a/tests/backend/stable_diffusion/test_extension_manager.py b/tests/backend/stable_diffusion/test_extension_manager.py
new file mode 100644
index 0000000000..889f8316e5
--- /dev/null
+++ b/tests/backend/stable_diffusion/test_extension_manager.py
@@ -0,0 +1,112 @@
+from unittest import mock
+
+import pytest
+
+from invokeai.backend.stable_diffusion.denoise_context import DenoiseContext
+from invokeai.backend.stable_diffusion.extension_callback_type import ExtensionCallbackType
+from invokeai.backend.stable_diffusion.extensions.base import ExtensionBase, callback
+from invokeai.backend.stable_diffusion.extensions_manager import ExtensionsManager
+
+
+class MockExtension(ExtensionBase):
+    """A mock ExtensionBase subclass for testing purposes."""
+
+    def __init__(self, x: int):
+        super().__init__()
+        self._x = x
+
+    # Note that order is not specified. It should default to 0.
+    @callback(ExtensionCallbackType.PRE_DENOISE_LOOP)
+    def set_step_index(self, ctx: DenoiseContext):
+        ctx.step_index = self._x
+
+
+class MockExtensionLate(ExtensionBase):
+    """A mock ExtensionBase subclass with a high order value on its PRE_DENOISE_LOOP callback."""
+
+    def __init__(self, x: int):
+        super().__init__()
+        self._x = x
+
+    @callback(ExtensionCallbackType.PRE_DENOISE_LOOP, order=1000)
+    def set_step_index(self, ctx: DenoiseContext):
+        ctx.step_index = self._x
+
+
+def test_extension_manager_run_callback():
+    """Test that run_callback runs all callbacks for the given callback type."""
+
+    em = ExtensionsManager()
+    mock_extension_1 = MockExtension(1)
+    em.add_extension(mock_extension_1)
+
+    mock_ctx = mock.MagicMock()
+    em.run_callback(ExtensionCallbackType.PRE_DENOISE_LOOP, mock_ctx)
+
+    assert mock_ctx.step_index == 1
+
+
+def test_extension_manager_run_callback_no_callbacks():
+    """Test that run_callback does not raise an error when there are no callbacks for the given callback type."""
+    em = ExtensionsManager()
+    mock_ctx = mock.MagicMock()
+    em.run_callback(ExtensionCallbackType.PRE_DENOISE_LOOP, mock_ctx)
+
+
+@pytest.mark.parametrize(
+    ["extension_1", "extension_2"],
+    # Regardless of initialization order, we expect MockExtensionLate to run last.
+    [(MockExtension(1), MockExtensionLate(2)), (MockExtensionLate(2), MockExtension(1))],
+)
+def test_extension_manager_order_callbacks(extension_1: ExtensionBase, extension_2: ExtensionBase):
+    """Test that run_callback runs callbacks in the correct order."""
+    em = ExtensionsManager()
+    em.add_extension(extension_1)
+    em.add_extension(extension_2)
+
+    mock_ctx = mock.MagicMock()
+    em.run_callback(ExtensionCallbackType.PRE_DENOISE_LOOP, mock_ctx)
+
+    assert mock_ctx.step_index == 2
+
+
+class MockExtensionStableSort(ExtensionBase):
+    """A mock extension with three PRE_DENOISE_LOOP callbacks, each with a different order value."""
+
+    @callback(ExtensionCallbackType.PRE_DENOISE_LOOP, order=-1000)
+    def early(self, ctx: DenoiseContext):
+        pass
+
+    @callback(ExtensionCallbackType.PRE_DENOISE_LOOP)
+    def middle(self, ctx: DenoiseContext):
+        pass
+
+    @callback(ExtensionCallbackType.PRE_DENOISE_LOOP, order=1000)
+    def late(self, ctx: DenoiseContext):
+        pass
+
+
+def test_extension_manager_stable_sort():
+    """Test that when two callbacks have the same 'order' value, they are sorted based on the order they were added to
+    the ExtensionsManager."""
+
+    em = ExtensionsManager()
+
+    mock_extension_1 = MockExtensionStableSort()
+    mock_extension_2 = MockExtensionStableSort()
+
+    em.add_extension(mock_extension_1)
+    em.add_extension(mock_extension_2)
+
+    expected_order = [
+        mock_extension_1.early,
+        mock_extension_2.early,
+        mock_extension_1.middle,
+        mock_extension_2.middle,
+        mock_extension_1.late,
+        mock_extension_2.late,
+    ]
+
+    # It's not ideal that we are accessing a private attribute here, but this was the most direct way to assert the
+    # desired behaviour.
+    assert [cb.function for cb in em._ordered_callbacks[ExtensionCallbackType.PRE_DENOISE_LOOP]] == expected_order