feat: config structure validation — detect malformed YAML at startup (#5426)

Add validate_config_structure() that catches common config.yaml mistakes:
- custom_providers as dict instead of list (missing '-' in YAML)
- fallback_model accidentally nested inside another section
- custom_providers entries missing required fields (name, base_url)
- Missing model section when custom_providers is configured
- Root-level keys that look like misplaced custom_providers fields

Surface these diagnostics at three levels:
1. Startup: print_config_warnings() runs at CLI and gateway module load,
   so users see issues before hitting cryptic errors
2. Error time: 'Unknown provider' errors in auth.py and model_switch.py
   now include config diagnostics with fix suggestions
3. Doctor: 'hermes doctor' shows a Config Structure section with all
   issues and fix hints

Also adds a warning log in runtime_provider.py when custom_providers
is a dict (previously returned None silently).

Motivated by a Discord user who had malformed custom_providers YAML
and got only 'Unknown Provider' with no guidance on what was wrong.

17 new tests covering all validation paths.

hermes_cli/config.py

@@ -19,6 +19,7 @@ import stat
 import subprocess
 import sys
 import tempfile
+from dataclasses import dataclass
 from pathlib import Path
 from typing import Dict, Any, Optional, List, Tuple
 
@@ -1243,6 +1244,182 @@ def check_config_version() -> Tuple[int, int]:
     return current, latest
 
 
+# =============================================================================
+# Config structure validation
+# =============================================================================
+
+# Fields that are valid at root level of config.yaml
+_KNOWN_ROOT_KEYS = {
+    "_config_version", "model", "providers", "fallback_model",
+    "fallback_providers", "credential_pool_strategies", "toolsets",
+    "agent", "terminal", "display", "compression", "delegation",
+    "auxiliary", "custom_providers", "memory", "gateway",
+}
+
+# Valid fields inside a custom_providers list entry
+_VALID_CUSTOM_PROVIDER_FIELDS = {
+    "name", "base_url", "api_key", "api_mode", "models",
+    "context_length", "rate_limit_delay",
+}
+
+# Fields that look like they should be inside custom_providers, not at root
+_CUSTOM_PROVIDER_LIKE_FIELDS = {"base_url", "api_key", "rate_limit_delay", "api_mode"}
+
+
+@dataclass
+class ConfigIssue:
+    """A detected config structure problem."""
+
+    severity: str  # "error", "warning"
+    message: str
+    hint: str
+
+
+def validate_config_structure(config: Optional[Dict[str, Any]] = None) -> List["ConfigIssue"]:
+    """Validate config.yaml structure and return a list of detected issues.
+
+    Catches common YAML formatting mistakes that produce confusing runtime
+    errors (like "Unknown provider") instead of clear diagnostics.
+
+    Can be called with a pre-loaded config dict, or will load from disk.
+    """
+    if config is None:
+        try:
+            config = load_config()
+        except Exception:
+            return [ConfigIssue("error", "Could not load config.yaml", "Run 'hermes setup' to create a valid config")]
+
+    issues: List[ConfigIssue] = []
+
+    # ── custom_providers must be a list, not a dict ──────────────────────
+    cp = config.get("custom_providers")
+    if cp is not None:
+        if isinstance(cp, dict):
+            issues.append(ConfigIssue(
+                "error",
+                "custom_providers is a dict — it must be a YAML list (items prefixed with '-')",
+                "Change to:\n"
+                "  custom_providers:\n"
+                "    - name: my-provider\n"
+                "      base_url: https://...\n"
+                "      api_key: ...",
+            ))
+            # Check if dict keys look like they should be list-entry fields
+            cp_keys = set(cp.keys()) if isinstance(cp, dict) else set()
+            suspicious = cp_keys & _CUSTOM_PROVIDER_LIKE_FIELDS
+            if suspicious:
+                issues.append(ConfigIssue(
+                    "warning",
+                    f"Root-level keys {sorted(suspicious)} look like custom_providers entry fields",
+                    "These should be indented under a '- name: ...' list entry, not at root level",
+                ))
+        elif isinstance(cp, list):
+            # Validate each entry in the list
+            for i, entry in enumerate(cp):
+                if not isinstance(entry, dict):
+                    issues.append(ConfigIssue(
+                        "warning",
+                        f"custom_providers[{i}] is not a dict (got {type(entry).__name__})",
+                        "Each entry should have at minimum: name, base_url",
+                    ))
+                    continue
+                if not entry.get("name"):
+                    issues.append(ConfigIssue(
+                        "warning",
+                        f"custom_providers[{i}] is missing 'name' field",
+                        "Add a name, e.g.: name: my-provider",
+                    ))
+                if not entry.get("base_url"):
+                    issues.append(ConfigIssue(
+                        "warning",
+                        f"custom_providers[{i}] is missing 'base_url' field",
+                        "Add the API endpoint URL, e.g.: base_url: https://api.example.com/v1",
+                    ))
+
+    # ── fallback_model must be a top-level dict with provider + model ────
+    fb = config.get("fallback_model")
+    if fb is not None:
+        if not isinstance(fb, dict):
+            issues.append(ConfigIssue(
+                "error",
+                f"fallback_model should be a dict with 'provider' and 'model', got {type(fb).__name__}",
+                "Change to:\n"
+                "  fallback_model:\n"
+                "    provider: openrouter\n"
+                "    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)",
+                ))
+
+    # ── 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 {}):
+        issues.append(ConfigIssue(
+            "error",
+            "fallback_model appears inside custom_providers instead of at root level",
+            "Move fallback_model to the top level of config.yaml (no indentation)",
+        ))
+
+    # ── model section: should exist when custom_providers is configured ──
+    model_cfg = config.get("model")
+    if cp and not model_cfg:
+        issues.append(ConfigIssue(
+            "warning",
+            "custom_providers defined but no 'model' section — Hermes won't know which provider to use",
+            "Add a model section:\n"
+            "  model:\n"
+            "    provider: custom\n"
+            "    default: your-model-name\n"
+            "    base_url: https://...",
+        ))
+
+    # ── Root-level keys that look misplaced ──────────────────────────────
+    for key in config:
+        if key.startswith("_"):
+            continue
+        if key not in _KNOWN_ROOT_KEYS and key in _CUSTOM_PROVIDER_LIKE_FIELDS:
+            issues.append(ConfigIssue(
+                "warning",
+                f"Root-level key '{key}' looks misplaced — should it be under 'model:' or inside a 'custom_providers' entry?",
+                f"Move '{key}' under the appropriate section",
+            ))
+
+    return issues
+
+
+def print_config_warnings(config: Optional[Dict[str, Any]] = None) -> None:
+    """Print config structure warnings to stderr at startup.
+
+    Called early in CLI and gateway init so users see problems before
+    they hit cryptic "Unknown provider" errors.  Prints nothing if
+    config is healthy.
+    """
+    try:
+        issues = validate_config_structure(config)
+    except Exception:
+        return
+    if not issues:
+        return
+
+    import sys
+    lines = ["\033[33m⚠ Config issues detected in config.yaml:\033[0m"]
+    for ci in issues:
+        marker = "\033[31m✗\033[0m" if ci.severity == "error" else "\033[33m⚠\033[0m"
+        lines.append(f"  {marker} {ci.message}")
+    lines.append("  \033[2mRun 'hermes doctor' for fix suggestions.\033[0m")
+    sys.stderr.write("\n".join(lines) + "\n\n")
+
+
 def migrate_config(interactive: bool = True, quiet: bool = False) -> Dict[str, Any]:
     """
     Migrate config to latest version, prompting for new required fields.