diff --git a/hermes_cli/doctor.py b/hermes_cli/doctor.py
index d56ff88c9d2..58f464985bd 100644
--- a/hermes_cli/doctor.py
+++ b/hermes_cli/doctor.py
@@ -738,12 +738,14 @@ def run_doctor(args):
                         issues,
                     )
 
-            # Warn if model is set to a provider-prefixed name on a provider that doesn't use them
+            # Warn if model is set to a provider-prefixed name on a provider that doesn't use them.
+            # Vendor/model slugs are valid on aggregator-style providers and on any custom
+            # provider — bare "custom" or a named "custom:<name>" that fronts an OpenAI-compatible
+            # aggregator (e.g. custom:hpc-ai serving deepseek/deepseek-v4-flash) requires the prefix.
             provider_for_policy = runtime_provider or catalog_provider
             provider_policy_id = str(provider_for_policy or "").strip().lower()
             providers_accepting_vendor_slugs = {
                 "openrouter",
-                "custom",
                 "auto",
                 "kilocode",
                 "opencode-zen",
@@ -751,12 +753,16 @@ def run_doctor(args):
                 "lmstudio",
                 "nous",
             }
+            provider_accepts_vendor_slug = (
+                provider_policy_id in providers_accepting_vendor_slugs
+                or provider_policy_id == "custom"
+                or provider_policy_id.startswith("custom:")
+            )
             if (
                 default_model
                 and "/" in default_model
                 and provider_policy_id
-                and provider_policy_id not in providers_accepting_vendor_slugs
-                and not provider_policy_id.startswith("custom:")
+                and not provider_accepts_vendor_slug
             ):
                 check_warn(
                     f"model.default '{default_model}' uses a vendor/model slug but provider is '{provider_raw}'",