Fix #373: blank fallback_model fields no longer trigger gateway warnings

When users blank fallback_model fields (set provider and model to empty
strings or None), the validation now treats this as intentionally disabling
fallback instead of showing warnings.

Changes:
- Modified validate_config_structure() to check if both provider and model
  are blank before showing warnings
- Added 4 new tests for blank fallback_model fields
- Updated hint messages to mention the disable option

Behavior:
- Both fields blank: no warnings (intentional disable)
- One field blank: warning shown (likely misconfiguration)
- Empty dict {}: no warnings (existing behavior)

Fixes #373

hermes_cli/config.py

@@ -1433,18 +1433,27 @@ def validate_config_structure(config: Optional[Dict[str, Any]] = None) -> List["
                 "    model: anthropic/claude-sonnet-4",
             ))
         elif fb:
-            if not fb.get("provider"):
-                issues.append(ConfigIssue(
-                    "warning",
-                    "fallback_model is missing 'provider' field — fallback will be disabled",
-                    "Add: provider: openrouter (or another provider)",
-                ))
-            if not fb.get("model"):
-                issues.append(ConfigIssue(
-                    "warning",
-                    "fallback_model is missing 'model' field — fallback will be disabled",
-                    "Add: model: anthropic/claude-sonnet-4 (or another model)",
-                ))
+            # If both provider and model are blank/empty, treat as intentionally disabled
+            provider = fb.get("provider")
+            model = fb.get("model")
+            provider_blank = not provider or (isinstance(provider, str) and not provider.strip())
+            model_blank = not model or (isinstance(model, str) and not model.strip())
+            
+            # Only warn if at least one field is set (user might be trying to configure)
+            # If both are blank, treat as intentionally disabled
+            if not provider_blank or not model_blank:
+                if provider_blank:
+                    issues.append(ConfigIssue(
+                        "warning",
+                        "fallback_model is missing 'provider' field — fallback will be disabled",
+                        "Add: provider: openrouter (or another provider), or set both fields blank to disable",
+                    ))
+                if model_blank:
+                    issues.append(ConfigIssue(
+                        "warning",
+                        "fallback_model is missing 'model' field — fallback will be disabled",
+                        "Add: model: anthropic/claude-sonnet-4 (or another model), or set both fields blank to disable",
+                    ))
 
     # ── Check for fallback_model accidentally nested inside custom_providers ──
     if isinstance(cp, dict) and "fallback_model" not in config and "fallback_model" in (cp or {}):

tests/hermes_cli/test_config_validation.py

@@ -136,6 +136,51 @@ class TestFallbackModelValidation:
         fb_issues = [i for i in issues if "fallback" in i.message.lower()]
         assert len(fb_issues) == 0
 
+    def test_blank_fallback_fields_no_issues(self):
+        """Blank fallback_model fields (both empty) should not trigger warnings."""
+        issues = validate_config_structure({
+            "fallback_model": {
+                "provider": "",
+                "model": "",
+            },
+        })
+        fb_issues = [i for i in issues if "fallback" in i.message.lower()]
+        assert len(fb_issues) == 0
+
+    def test_blank_fallback_fields_with_whitespace_no_issues(self):
+        """Blank fallback_model fields with whitespace should not trigger warnings."""
+        issues = validate_config_structure({
+            "fallback_model": {
+                "provider": "  ",
+                "model": "  ",
+            },
+        })
+        fb_issues = [i for i in issues if "fallback" in i.message.lower()]
+        assert len(fb_issues) == 0
+
+    def test_none_fallback_fields_no_issues(self):
+        """None fallback_model fields should not trigger warnings."""
+        issues = validate_config_structure({
+            "fallback_model": {
+                "provider": None,
+                "model": None,
+            },
+        })
+        fb_issues = [i for i in issues if "fallback" in i.message.lower()]
+        assert len(fb_issues) == 0
+
+    def test_partial_blank_fallback_warns(self):
+        """Partial blank fallback (only one field blank) should warn."""
+        issues = validate_config_structure({
+            "fallback_model": {
+                "provider": "",
+                "model": "anthropic/claude-sonnet-4",
+            },
+        })
+        fb_issues = [i for i in issues if "fallback" in i.message.lower()]
+        assert len(fb_issues) == 1
+        assert "provider" in fb_issues[0].message
+
 
 class TestMissingModelSection:
     """Warn when custom_providers exists but model section is missing."""