diff --git a/README.md b/README.md
index 0c49e0f..9539620 100644
--- a/README.md
+++ b/README.md
@@ -88,12 +88,13 @@ from asdf.extension import Extension
 from asdf_pydantic.converter import AsdfPydanticConverter
 from mypackage.shapes import Rectangle
 
-AsdfPydanticConverter.add_models(Rectangle)
+converter = AsdfPydanticConverter()
+converter.add_models(Rectangle)
 
 class ShapesExtension(Extension):
     extension_uri = "asdf://asdf-pydantic/examples/extensions/shapes-1.0.0"
-    converters = [AsdfPydanticConverter()]
-    tags = [*AsdfPydanticConverter().tags]
+    converters = [converter]
+    tags = [*converter.tags]
 ```
 
 Install the extension either by entry point specification or add it to
diff --git a/tests/examples/test_astropy_tables.py b/tests/examples/test_astropy_tables.py
index d90bd4a..c64ae05 100644
--- a/tests/examples/test_astropy_tables.py
+++ b/tests/examples/test_astropy_tables.py
@@ -24,13 +24,14 @@ class Database(AsdfPydanticModel):
 @pytest.fixture()
 def asdf_extension():
     """Registers an ASDF extension containing models for this test."""
-    AsdfPydanticConverter.add_models(Database)
+    converter = AsdfPydanticConverter()
+    converter.add_models(Database)
 
     class TestExtension(Extension):
         extension_uri = "asdf://asdf-pydantic/examples/extensions/test-1.0.0"
 
-        converters = [AsdfPydanticConverter()]  # type: ignore
-        tags = [*AsdfPydanticConverter().tags]  # type: ignore
+        converters = [converter]  # type: ignore
+        tags = [*converter.tags]  # type: ignore
 
     asdf.get_config().add_extension(TestExtension())
 
diff --git a/tests/examples/test_node.py b/tests/examples/test_node.py
index 936c95b..5408af8 100644
--- a/tests/examples/test_node.py
+++ b/tests/examples/test_node.py
@@ -3,7 +3,6 @@
 from __future__ import annotations
 
 import textwrap
-from typing import Annotated
 from unittest.mock import MagicMock, patch
 
 import asdf
@@ -15,8 +14,6 @@
 
 from asdf_pydantic import AsdfPydanticConverter, AsdfPydanticModel
 
-SelfAnnotatedAsdfNode = Annotated[AsdfTag]
-
 
 class AsdfNode(AsdfPydanticModel):
     """Model for a node in a graph/tree.
@@ -57,12 +54,13 @@ class AsdfNode(AsdfPydanticModel):
 @pytest.fixture()
 def asdf_extension():
     """Registers an ASDF extension containing models for this test."""
-    AsdfPydanticConverter.add_models(AsdfNode)
+    converter = AsdfPydanticConverter()
+    converter.add_models(AsdfNode)
 
     class TestExtension(Extension):
         extension_uri = "asdf://asdf-pydantic/examples/extensions/test-1.0.0"
 
-        converters = [AsdfPydanticConverter()]  # type: ignore
+        converters = [converter]  # type: ignore
         tags = [AsdfNode.get_tag_definition()]  # type: ignore
 
     with asdf.config_context() as asdf_config:
diff --git a/tests/examples/test_rectangle.py b/tests/examples/test_rectangle.py
index 9d414d7..793a9fe 100644
--- a/tests/examples/test_rectangle.py
+++ b/tests/examples/test_rectangle.py
@@ -14,12 +14,13 @@
 @pytest.fixture()
 def asdf_extension():
     """Registers an ASDF extension containing models for this test."""
-    AsdfPydanticConverter.add_models(AsdfRectangle)
+    converter = AsdfPydanticConverter()
+    converter.add_models(AsdfRectangle)
 
     class TestExtension(Extension):
         extension_uri = "asdf://asdf-pydantic/examples/extensions/test-1.0.0"
 
-        converters = [AsdfPydanticConverter()]  # type: ignore
+        converters = [converter]  # type: ignore
         tags = [AsdfRectangle.get_tag_definition()]  # type: ignore
 
     with asdf.config_context() as asdf_config:
diff --git a/tests/patterns/astropy_types_test.py b/tests/patterns/astropy_types_test.py
index 4c35af4..4106ee4 100644
--- a/tests/patterns/astropy_types_test.py
+++ b/tests/patterns/astropy_types_test.py
@@ -23,13 +23,14 @@ class DataContainer(AsdfPydanticModel):
 
 
 def setup_module():
-    AsdfPydanticConverter.add_models(DataPoint, DataContainer)
+    converter = AsdfPydanticConverter()
+    converter.add_models(DataPoint, DataContainer)
 
     class TestExtension(Extension):
         extension_uri = "asdf://asdf-pydantic/examples/extensions/test-1.0.0"
 
-        converters = [AsdfPydanticConverter()]  # type: ignore
-        tags = [*AsdfPydanticConverter().tags]  # type: ignore
+        converters = [converter]  # type: ignore
+        tags = [*converter.tags]  # type: ignore
 
     asdf.get_config().add_extension(TestExtension())
 
diff --git a/tests/patterns/union_type_test.py b/tests/patterns/union_type_test.py
index f10d27c..98f1da5 100644
--- a/tests/patterns/union_type_test.py
+++ b/tests/patterns/union_type_test.py
@@ -20,13 +20,14 @@ class UnionObject(AsdfPydanticModel):
 
 
 def setup_module():
-    AsdfPydanticConverter.add_models(UnionObject)
+    converter = AsdfPydanticConverter()
+    converter.add_models(UnionObject)
 
     class TestExtension(Extension):
         extension_uri = "asdf://asdf-pydantic/examples/extensions/test-1.0.0"
 
-        converters = [AsdfPydanticConverter()]  # type: ignore
-        tags = [*AsdfPydanticConverter().tags]  # type: ignore
+        converters = [converter]  # type: ignore
+        tags = [*converter.tags]  # type: ignore
 
     asdf.get_config().add_extension(TestExtension())
 
diff --git a/tests/schema_validation_test.py b/tests/schema_validation_test.py
index 60a3f09..2508462 100644
--- a/tests/schema_validation_test.py
+++ b/tests/schema_validation_test.py
@@ -14,13 +14,14 @@
 
 
 def setup_module():
-    AsdfPydanticConverter.add_models(AsdfRectangle)
+    converter = AsdfPydanticConverter()
+    converter.add_models(AsdfRectangle)
 
     class TestExtension(Extension):
         extension_uri = "asdf://asdf-pydantic/examples/extensions/test-1.0.0"  # type: ignore
 
-        tags = [*AsdfPydanticConverter().tags]  # type: ignore
-        converters = [AsdfPydanticConverter()]  # type: ignore
+        tags = [*converter.tags]  # type: ignore
+        converters = [converter]  # type: ignore
 
     # HACK: The schema URI should be referenced from `AsdfRectangle._schema`.
     # Then there should be a way to automatically add the schema to ASDF