diff --git a/agent/auxiliary_client.py b/agent/auxiliary_client.py
index 62695af83a..3017e7703d 100644
--- a/agent/auxiliary_client.py
+++ b/agent/auxiliary_client.py
@@ -3132,8 +3132,14 @@ def _resolve_task_provider_model(
 
     if task:
         # Config.yaml is the primary source for per-task overrides.
-        if cfg_base_url:
+        if cfg_base_url and cfg_api_key:
+            # Both base_url and api_key explicitly set → custom endpoint.
             return "custom", resolved_model, cfg_base_url, cfg_api_key, resolved_api_mode
+        if cfg_base_url and cfg_provider and cfg_provider != "auto":
+            # base_url set without api_key but with a known provider — use
+            # the provider so it can resolve credentials from env vars
+            # (e.g. OPENROUTER_API_KEY) instead of locking into "custom".
+            return cfg_provider, resolved_model, cfg_base_url, None, resolved_api_mode
         if cfg_provider and cfg_provider != "auto":
             return cfg_provider, resolved_model, None, None, resolved_api_mode