diff --git a/nanobot/providers/factory.py b/nanobot/providers/factory.py
index 37fa2ed8b..c9346a137 100644
--- a/nanobot/providers/factory.py
+++ b/nanobot/providers/factory.py
@@ -49,6 +49,30 @@ def _provider_extra_headers(
     return headers or None
 
 
+def _provider_spec_for_config(
+    provider_name: str,
+    provider_config: ProviderConfig | None,
+) -> ProviderSpec | None:
+    spec = find_by_name(provider_name)
+    if (
+        spec is not None
+        and spec.name == "orcarouter"
+        and provider_config is not None
+        and provider_config.api_base
+        and provider_config.api_base.rstrip("/").lower()
+        != spec.default_api_base.rstrip("/").lower()
+    ):
+        # Before OrcaRouter became a built-in provider, this name was valid for a
+        # dynamic custom provider. Preserve that provider's model-prefix behavior
+        # when an existing config points the name at a different endpoint.
+        return create_dynamic_spec(
+            provider_name,
+            display_name=provider_config.display_name or "",
+            thinking_style=provider_config.thinking_style or "",
+        )
+    return spec
+
+
 def _resolve_provider_setup(
     config: Config,
     *,
@@ -61,7 +85,7 @@ def _resolve_provider_setup(
     p = config.get_provider(model, preset=preset)
     if not provider_name:
         raise ValueError(f"No provider is configured for model '{model}'.")
-    spec = find_by_name(provider_name)
+    spec = _provider_spec_for_config(provider_name, p)
     if not spec and p:
         if not p.api_base:
             raise ValueError(f"Provider '{provider_name}' requires api_base in config.")
diff --git a/tests/providers/test_orcarouter_provider.py b/tests/providers/test_orcarouter_provider.py
index ebf9052ee..b5589bf57 100644
--- a/tests/providers/test_orcarouter_provider.py
+++ b/tests/providers/test_orcarouter_provider.py
@@ -3,6 +3,7 @@
 from unittest.mock import patch
 
 from nanobot.config.schema import Config, ProvidersConfig
+from nanobot.providers.factory import make_provider
 from nanobot.providers.openai_compat_provider import OpenAICompatProvider
 from nanobot.providers.registry import PROVIDERS, find_by_name
 
@@ -55,7 +56,7 @@ def test_orcarouter_forced_provider_uses_default_api_base() -> None:
     assert config.get_api_base("deepseek/deepseek-chat") == "https://api.orcarouter.ai/v1"
 
 
-def test_orcarouter_gateway_routes_unprefixed_models_when_configured() -> None:
+def test_orcarouter_gateway_routes_auto_model_when_configured() -> None:
     config = Config.model_validate({
         "providers": {
             "orcarouter": {
@@ -74,6 +75,39 @@ def test_orcarouter_gateway_routes_unprefixed_models_when_configured() -> None:
     assert config.get_api_base("orcarouter/auto") == "https://api.orcarouter.ai/v1"
 
 
+def test_legacy_custom_provider_named_orcarouter_keeps_prefix_stripping() -> None:
+    config = Config.model_validate({
+        "providers": {
+            "orcarouter": {
+                "apiKey": "legacy-test-key",
+                "apiBase": "https://legacy-gateway.example/v1",
+            },
+        },
+        "agents": {
+            "defaults": {
+                "model": "orcarouter/custom-model",
+                "provider": "orcarouter",
+            },
+        },
+    })
+
+    with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI"):
+        provider = make_provider(config)
+
+    assert isinstance(provider, OpenAICompatProvider)
+    assert provider.api_base == "https://legacy-gateway.example/v1"
+    kwargs = provider._build_kwargs(
+        messages=[{"role": "user", "content": "hi"}],
+        tools=None,
+        model="orcarouter/custom-model",
+        max_tokens=1024,
+        temperature=0.7,
+        reasoning_effort=None,
+        tool_choice=None,
+    )
+    assert kwargs["model"] == "custom-model"
+
+
 def test_orcarouter_preserves_model_api_id() -> None:
     spec = find_by_name("orcarouter")
     with patch("nanobot.providers.openai_compat_provider.AsyncOpenAI"):