fix: retain reasoning effort on named custom provider routes

Apply the narrow registry fallback proposed in PR #68458 without unrelated case normalization or dead legacy flags. Preserve dedicated named profiles before using CustomProfile. This corrects existing reasoning loss only; per-model dialect configuration remains a product decision.

Co-authored-by: saotu <160758706+saotu@users.noreply.github.com>

providers/__init__.py

@@ -75,7 +75,12 @@ def get_provider_profile(name: str) -> ProviderProfile | None:
     if not _discovered:
         _discover_providers()
     canonical = _ALIASES.get(name, name)
-    return _REGISTRY.get(canonical)
+    profile = _REGISTRY.get(canonical)
+    # Named custom routes share the generic wire policy unless a plugin
+    # explicitly registered that route. Other names retain exact lookup.
+    if profile is None and isinstance(name, str) and name.lower().startswith("custom:"):
+        profile = _REGISTRY.get("custom")
+    return profile
 
 
 def list_providers() -> list[ProviderProfile]:

tests/providers/test_named_custom_reasoning.py

@@ -0,0 +1,30 @@
+"""Named custom routes use the same reasoning contract as bare custom."""
+from providers import get_provider_profile, register_provider
+from providers.base import ProviderProfile
+from agent.transports.chat_completions import ChatCompletionsTransport
+
+
+def test_named_custom_route_keeps_final_reasoning_effort():
+    transport = ChatCompletionsTransport()
+    for effort in ("low", "medium", "high"):
+        outputs = [transport.build_kwargs(
+            "fixture-model", [{"role": "user", "content": "fixture"}],
+            provider_profile=get_provider_profile(provider),
+            base_url="http://127.0.0.1:1/v1",
+            reasoning_config={"enabled": True, "effort": effort},
+        ) for provider in ("custom", "custom:fixture")]
+        assert outputs[0]["reasoning_effort"] == effort
+        assert outputs[1] == outputs[0]
+
+
+def test_named_custom_fallback_does_not_override_registered_routes(monkeypatch):
+    import providers
+    get_provider_profile("custom")
+    monkeypatch.setattr(providers, "_REGISTRY", dict(providers._REGISTRY))
+    monkeypatch.setattr(providers, "_ALIASES", dict(providers._ALIASES))
+    monkeypatch.setattr(providers, "_PROVIDER_LIST_CACHE", None)
+    dedicated = ProviderProfile(name="custom:fixture")
+    register_provider(dedicated)
+    assert get_provider_profile("custom:fixture") is dedicated
+    assert get_provider_profile("CUSTOM:unregistered") is get_provider_profile("custom")
+    assert get_provider_profile("NONEXISTENT") is None