diff --git a/packages/jupyter-ai/jupyter_ai/config_manager.py b/packages/jupyter-ai/jupyter_ai/config_manager.py
index e8fea6306..608bea77f 100644
--- a/packages/jupyter-ai/jupyter_ai/config_manager.py
+++ b/packages/jupyter-ai/jupyter_ai/config_manager.py
@@ -107,11 +107,11 @@ def __init__(
         log: Logger,
         lm_providers: LmProvidersDict,
         em_providers: EmProvidersDict,
-        allowed_providers: Optional[List[str]],
-        blocked_providers: Optional[List[str]],
-        allowed_models: Optional[List[str]],
-        blocked_models: Optional[List[str]],
         defaults: dict,
+        allowed_providers: Optional[List[str]] = None,
+        blocked_providers: Optional[List[str]] = None,
+        allowed_models: Optional[List[str]] = None,
+        blocked_models: Optional[List[str]] = None,
         *args,
         **kwargs,
     ):
diff --git a/packages/jupyter-ai/jupyter_ai/tests/test_config_manager.py b/packages/jupyter-ai/jupyter_ai/tests/test_config_manager.py
index 437333960..c807cf91b 100644
--- a/packages/jupyter-ai/jupyter_ai/tests/test_config_manager.py
+++ b/packages/jupyter-ai/jupyter_ai/tests/test_config_manager.py
@@ -24,6 +24,29 @@ def config_path(jp_data_dir):
 def schema_path(jp_data_dir):
     return str(jp_data_dir / "config_schema.json")
 
+@pytest.fixture
+def config_file_with_model_fields(jp_data_dir):
+    """
+    Fixture that creates a `config.json` file with the chat model set to
+    `openai-chat:gpt-4o` and fields for that model. Returns path to the file.
+    """
+    config_data = {
+        "model_provider_id:": "openai-chat:gpt-4o",
+        "embeddings_provider_id": None,
+        "api_keys": {
+            "openai_api_key": "foobar"
+        },
+        "send_with_shift_enter": False,
+        "fields": {
+            "openai-chat:gpt-4o": {
+                "openai_api_base": "https://example.com"
+            }
+        },
+    }
+    config_path = jp_data_dir / "config.json"
+    with open(config_path, "w") as file:
+        json.dump(config_data, file)
+    return str(config_path)
 
 @pytest.fixture
 def common_cm_kwargs(config_path, schema_path):
@@ -426,5 +449,39 @@ def test_handle_bad_provider_ids(cm_with_bad_provider_ids):
     assert config_desc.embeddings_provider_id is None
 
 def test_config_manager_returns_fields(cm):
+    """
+    Asserts that `ConfigManager.lm_provider_params` returns model fields set by
+    the user.
+    """
     expected_model_args = configure_with_fields(cm)
     assert cm.lm_provider_params == expected_model_args
+
+def test_config_manager_does_not_write_to_defaults(config_file_with_model_fields, schema_path):
+    """
+    Asserts that `ConfigManager` does not write to the `defaults` argument when
+    the configured chat model differs from the one specified in `defaults`.
+    """
+    from copy import deepcopy
+    config_path = config_file_with_model_fields
+    log = logging.getLogger()
+    lm_providers = get_lm_providers()
+    em_providers = get_em_providers()
+
+    defaults = {
+        "model_provider_id": None,
+        "embeddings_provider_id": None,
+        "api_keys": {},
+        "fields": {}
+    }
+    expected_defaults = deepcopy(defaults)
+
+    cm = ConfigManager(
+        log=log,
+        lm_providers=lm_providers,
+        em_providers=em_providers,
+        config_path=config_path,
+        schema_path=schema_path,
+        defaults=defaults,
+    )
+
+    assert defaults == expected_defaults