diff --git a/tests/test_extensions/test_functions.py b/tests/test_extensions/test_functions.py
index ac4610a15..81667de91 100644
--- a/tests/test_extensions/test_functions.py
+++ b/tests/test_extensions/test_functions.py
@@ -1,12 +1,14 @@
 # License: BSD 3-Clause
 from __future__ import annotations
 
-import inspect
+from collections import OrderedDict
 
+import inspect
+import numpy as np
 import pytest
 
 import openml.testing
-from openml.extensions import get_extension_by_flow, get_extension_by_model, register_extension
+from openml.extensions import Extension, get_extension_by_flow, get_extension_by_model, register_extension
 
 
 class DummyFlow:
@@ -40,23 +42,76 @@ def can_handle_model(model):
         return False
 
 
+class DummyExtension(Extension):
+    @classmethod
+    def can_handle_flow(cls, flow):
+        return isinstance(flow, DummyFlow)
+
+    @classmethod
+    def can_handle_model(cls, model):
+        return isinstance(model, DummyModel)
+
+    def flow_to_model(
+        self,
+        flow,
+        initialize_with_defaults=False,
+        strict_version=True,
+    ):
+        if not isinstance(flow, DummyFlow):
+            raise ValueError("Invalid flow")
+
+        model = DummyModel()
+        model.defaults = initialize_with_defaults
+        model.strict_version = strict_version
+        return model
+
+    def model_to_flow(self, model):
+        if not isinstance(model, DummyModel):
+            raise ValueError("Invalid model")
+        return DummyFlow()
+
+    def get_version_information(self):
+        return ["dummy==1.0"]
+
+    def create_setup_string(self, model):
+        return "DummyModel()"
+
+    def is_estimator(self, model):
+        return isinstance(model, DummyModel)
+
+    def seed_model(self, model, seed):
+        model.seed = seed
+        return model
+
+    def _run_model_on_fold(
+        self,
+        model,
+        task,
+        X_train,
+        rep_no,
+        fold_no,
+        y_train=None,
+        X_test=None,
+    ):
+        preds = np.zeros(len(X_train))
+        probs = None
+        measures = OrderedDict()
+        trace = None
+        return preds, probs, measures, trace
+
+    def obtain_parameter_values(self, flow, model=None):
+        return []
+
+    def check_if_model_fitted(self, model):
+        return False
+
+    def instantiate_model_from_hpo_class(self, model, trace_iteration):
+        return DummyModel()
+
+
 def _unregister():
     # "Un-register" the test extensions
-    while True:
-        rem_dum_ext1 = False
-        rem_dum_ext2 = False
-        try:
-            openml.extensions.extensions.remove(DummyExtension1)
-            rem_dum_ext1 = True
-        except ValueError:
-            pass
-        try:
-            openml.extensions.extensions.remove(DummyExtension2)
-            rem_dum_ext2 = True
-        except ValueError:
-            pass
-        if not rem_dum_ext1 and not rem_dum_ext2:
-            break
+    openml.extensions.extensions.clear()
 
 
 class TestInit(openml.testing.TestBase):
@@ -91,3 +146,89 @@ def test_get_extension_by_model(self):
             ValueError, match="Multiple extensions registered which can handle model:"
         ):
             get_extension_by_model(DummyModel())
+
+
+def test_flow_to_model_with_defaults():
+    """Test flow_to_model with initialize_with_defaults=True."""
+    ext = DummyExtension()
+    flow = DummyFlow()
+
+    model = ext.flow_to_model(flow, initialize_with_defaults=True)
+
+    assert isinstance(model, DummyModel)
+    assert model.defaults is True
+
+def test_flow_to_model_strict_version():
+    """Test flow_to_model with strict_version parameter."""
+    ext = DummyExtension()
+    flow = DummyFlow()
+
+    model_strict = ext.flow_to_model(flow, strict_version=True)
+    model_non_strict = ext.flow_to_model(flow, strict_version=False)
+
+    assert isinstance(model_strict, DummyModel)
+    assert model_strict.strict_version is True
+
+    assert isinstance(model_non_strict, DummyModel)
+    assert model_non_strict.strict_version is False
+
+def test_model_to_flow_conversion():
+    """Test converting a model back to flow representation."""
+    ext = DummyExtension()
+    model = DummyModel()
+
+    flow = ext.model_to_flow(model)
+
+    assert isinstance(flow, DummyFlow)
+
+
+def test_invalid_flow_raises_error():
+    """Test that invalid flow raises appropriate error."""
+    class InvalidFlow:
+        pass
+
+    ext = DummyExtension()
+    flow = InvalidFlow()
+
+    with pytest.raises(ValueError, match="Invalid flow"):
+        ext.flow_to_model(flow)
+
+
+def test_extension_not_found_error_message():
+    """Test error message contains helpful information."""
+    class UnknownModel:
+        pass
+
+    _unregister()
+
+    with pytest.raises(ValueError, match="No extension registered"):
+        get_extension_by_model(UnknownModel(), raise_if_no_extension=True)
+
+ 
+def test_register_same_extension_twice():
+    """Test behavior when registering same extension twice."""
+    register_extension(DummyExtension)
+    register_extension(DummyExtension)
+
+    matches = [
+        ext for ext in openml.extensions.extensions
+        if ext is DummyExtension
+    ]
+
+    assert len(matches) == 2
+
+
+def test_extension_priority_order():
+    """Test that extensions are checked in registration order."""
+    _unregister()
+
+    class DummyExtensionA(DummyExtension):
+        pass
+    class DummyExtensionB(DummyExtension):
+        pass
+
+    register_extension(DummyExtensionA)
+    register_extension(DummyExtensionB)
+
+    assert openml.extensions.extensions[0] is DummyExtensionA
+    assert openml.extensions.extensions[1] is DummyExtensionB
\ No newline at end of file