fix: preserve dynamic custom provider semantics

maintainer edit: treat arbitrary custom provider names as direct OpenAI-compatible providers, validate their api_type consistently, and avoid Pydantic instance-field warnings in fallback routing.

tests/cli/test_commands.py

@@ -664,6 +664,30 @@ def test_make_provider_passes_extra_headers_to_custom_provider():
     assert kwargs["default_headers"]["x-session-affinity"] == "sticky-session"
 
 
+def test_make_provider_treats_dynamic_custom_provider_as_direct():
+    config = Config.model_validate(
+        {
+            "agents": {"defaults": {"provider": "my-company-api", "model": "gpt-4o-mini"}},
+            "providers": {
+                "my-company-api": {
+                    "apiBase": "https://example.com/v1",
+                }
+            },
+        }
+    )
+
+    with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI") as mock_async_openai:
+        provider = make_provider(config)
+        asyncio.run(provider._ensure_client())
+
+    assert provider.get_default_model() == "gpt-4o-mini"
+    assert provider._spec.name == "my_company_api"
+    assert provider._spec.is_direct is True
+    kwargs = mock_async_openai.call_args.kwargs
+    assert kwargs["api_key"] == "no-key"
+    assert kwargs["base_url"] == "https://example.com/v1"
+
+
 @pytest.fixture
 def mock_agent_runtime(tmp_path):
     """Mock agent command dependencies for focused CLI tests."""

tests/config/test_model_presets.py

@@ -1,3 +1,5 @@
+import warnings
+
 import pytest
 
 from nanobot.config.schema import Config
@@ -47,6 +49,35 @@ def test_provider_api_type_is_openai_only() -> None:
             }
         })
 
+    with pytest.raises(ValueError, match="only supported"):
+        Config.model_validate({
+            "providers": {
+                "my-company-api": {
+                    "apiBase": "https://example.test/v1",
+                    "apiType": "responses",
+                }
+            }
+        })
+
+
+def test_custom_provider_fallback_uses_model_extra_without_pydantic_warnings() -> None:
+    config = Config.model_validate({
+        "agents": {
+            "defaults": {
+                "model": "unmatched-model",
+            }
+        },
+        "providers": {
+            "my-company-api": {
+                "apiBase": "https://example.test/v1",
+            }
+        },
+    })
+
+    with warnings.catch_warnings():
+        warnings.simplefilter("error")
+        assert config.get_provider_name() == "my-company-api"
+
 
 def test_legacy_defaults_config_without_presets_still_resolves() -> None:
     config = Config.model_validate({