fix: prevent bare 'custom' slug in model.provider (#17478)

When hermes model picker switches to a custom_providers entry, the slug
assignment can write the literal string 'custom' to model.provider if a
prior failed switch already left that value in config.yaml.

Two fixes:
1. model_switch.py: filter out bare 'custom' in slug assignment, always
   resolve to canonical custom:<name> form
2. providers.py: resolve_custom_provider() self-heals bare 'custom' by
   falling back to the first valid custom_providers entry

Closes #17478

hermes_cli/model_switch.py

@@ -1503,7 +1503,14 @@ def list_authenticated_providers(
                     current_base_url
                     and api_url == current_base_url.strip().rstrip("/")
                 ):
-                    slug = current_provider or custom_provider_slug(display_name)
+                    # Guard against bare "custom" slug left by a prior
+                    # failed switch — always resolve to the canonical
+                    # custom:<name> form.  (GH #17478)
+                    slug = (
+                        current_provider
+                        if current_provider and current_provider != "custom"
+                        else custom_provider_slug(display_name)
+                    )
                 else:
                     slug = custom_provider_slug(display_name)
                 groups[group_key] = {

hermes_cli/providers.py

@@ -585,6 +585,12 @@ def resolve_custom_provider(
     if not requested:
         return None
 
+    # If the stored provider is the bare string "custom" (corrupt state
+    # from a prior model-switch bug), fall back to the first custom
+    # provider entry so existing configs self-heal.  (GH #17478)
+    bare_custom_fallback = requested == "custom"
+    first_valid = None
+
     for entry in custom_providers:
         if not isinstance(entry, dict):
             continue
@@ -599,6 +605,10 @@ def resolve_custom_provider(
         if not display_name or not api_url:
             continue
 
+        # Stash the first valid entry for bare-"custom" fallback
+        if first_valid is None:
+            first_valid = (display_name, api_url)
+
         slug = custom_provider_slug(display_name)
         if requested not in {display_name.lower(), slug}:
             continue
@@ -614,6 +624,21 @@ def resolve_custom_provider(
             source="user-config",
         )
 
+    # Self-heal: bare "custom" matched nothing — return first valid entry
+    if bare_custom_fallback and first_valid:
+        dname, aurl = first_valid
+        slug = custom_provider_slug(dname)
+        return ProviderDef(
+            id=slug,
+            name=dname,
+            transport="openai_chat",
+            api_key_env_vars=(),
+            base_url=aurl,
+            is_aggregator=False,
+            auth_type="api_key",
+            source="user-config",
+        )
+
     return None