diff --git a/docs/providers.md b/docs/providers.md
index 02760622a..75adc65a5 100644
--- a/docs/providers.md
+++ b/docs/providers.md
@@ -234,7 +234,7 @@ If you have more than one custom OpenAI-compatible endpoint, give each endpoint
 }
 ```
 
-Custom provider keys are treated as direct OpenAI-compatible providers. `apiBase` is required because nanobot cannot know the endpoint URL. `apiKey` is optional for local servers or private proxies that do not require one. Do not set `apiType` on custom provider keys; `apiType` is only for `providers.openai`.
+Custom provider keys are treated as direct OpenAI-compatible providers. `apiBase` is required because nanobot cannot know the endpoint URL. `apiKey` is optional for local servers or private proxies that do not require one. Choose a name that does not conflict with a built-in provider name or alias, such as `openai`, `openai-codex`, `github-copilot`, or `lm-studio`. Do not set `apiType` on custom provider keys; `apiType` is only for `providers.openai`.
 
 This named custom provider path is not for Anthropic-compatible endpoints. For Anthropic-compatible proxies, use `providers.anthropic.apiBase` and set the preset provider to `anthropic`.
 
diff --git a/nanobot/config/schema.py b/nanobot/config/schema.py
index 2d4079dd2..4e2a4599d 100644
--- a/nanobot/config/schema.py
+++ b/nanobot/config/schema.py
@@ -245,7 +245,14 @@ class ProvidersConfig(Base):
     def convert_extra_providers(self):
         """Convert extra fields (custom providers) to ProviderConfig objects."""
         if self.model_extra:
+            from nanobot.providers.registry import find_by_name
+
             for key, value in self.model_extra.items():
+                if spec := find_by_name(key):
+                    raise ValueError(
+                        f"providers.{key} conflicts with built-in provider {spec.name!r}; "
+                        "use the built-in provider key or choose a different custom provider name"
+                    )
                 if isinstance(value, dict):
                     self.model_extra[key] = ProviderConfig.model_validate(value)
         return self
diff --git a/tests/config/test_model_presets.py b/tests/config/test_model_presets.py
index a66014bb9..9519e1e81 100644
--- a/tests/config/test_model_presets.py
+++ b/tests/config/test_model_presets.py
@@ -60,6 +60,18 @@ def test_provider_api_type_is_openai_only() -> None:
         })
 
 
+@pytest.mark.parametrize("provider_name", ["openai-codex", "github-copilot", "lm-studio"])
+def test_dynamic_custom_provider_rejects_builtin_provider_aliases(provider_name: str) -> None:
+    with pytest.raises(ValueError, match="conflicts with built-in provider"):
+        Config.model_validate({
+            "providers": {
+                provider_name: {
+                    "apiBase": "https://example.test/v1",
+                }
+            }
+        })
+
+
 def test_custom_provider_fallback_uses_model_extra_without_pydantic_warnings() -> None:
     config = Config.model_validate({
         "agents": {