fix: guard config.get() against YAML null values to prevent AttributeError (#3377)

dict.get(key, default) returns None — not the default — when the key IS
present but explicitly set to null/~ in YAML.  Calling .lower() on that
raises AttributeError.

Use (config.get(key) or fallback) so both missing keys and explicit nulls
coalesce to the intended default.

Files fixed:
- tools/tts_tool.py — _get_provider()
- tools/web_tools.py — _get_backend()
- tools/mcp_tool.py — MCPServerTask auth config
- trajectory_compressor.py — _detect_provider() and config loading

Co-authored-by: dieutx <dangtc94@gmail.com>

tests/tools/test_config_null_guard.py

@@ -0,0 +1,111 @@
+"""Tests for config.get() null-coalescing in tool configuration.
+
+YAML ``null`` values (or ``~``) for a present key make ``dict.get(key, default)``
+return ``None`` instead of the default — calling ``.lower()`` on that raises
+``AttributeError``.  These tests verify the ``or`` coalescing guards.
+"""
+
+from unittest.mock import patch
+import pytest
+
+
+# ── TTS tool ──────────────────────────────────────────────────────────────
+
+class TestTTSProviderNullGuard:
+    """tools/tts_tool.py — _get_provider()"""
+
+    def test_explicit_null_provider_returns_default(self):
+        """YAML ``tts: {provider: null}`` should fall back to default."""
+        from tools.tts_tool import _get_provider, DEFAULT_PROVIDER
+
+        result = _get_provider({"provider": None})
+        assert result == DEFAULT_PROVIDER.lower().strip()
+
+    def test_missing_provider_returns_default(self):
+        """No ``provider`` key at all should also return default."""
+        from tools.tts_tool import _get_provider, DEFAULT_PROVIDER
+
+        result = _get_provider({})
+        assert result == DEFAULT_PROVIDER.lower().strip()
+
+    def test_valid_provider_passed_through(self):
+        from tools.tts_tool import _get_provider
+
+        result = _get_provider({"provider": "OPENAI"})
+        assert result == "openai"
+
+
+# ── Web tools ─────────────────────────────────────────────────────────────
+
+class TestWebBackendNullGuard:
+    """tools/web_tools.py — _get_backend()"""
+
+    @patch("tools.web_tools._load_web_config", return_value={"backend": None})
+    def test_explicit_null_backend_does_not_crash(self, _cfg):
+        """YAML ``web: {backend: null}`` should not raise AttributeError."""
+        from tools.web_tools import _get_backend
+
+        # Should not raise — the exact return depends on env key fallback
+        result = _get_backend()
+        assert isinstance(result, str)
+
+    @patch("tools.web_tools._load_web_config", return_value={})
+    def test_missing_backend_does_not_crash(self, _cfg):
+        from tools.web_tools import _get_backend
+
+        result = _get_backend()
+        assert isinstance(result, str)
+
+
+# ── MCP tool ──────────────────────────────────────────────────────────────
+
+class TestMCPAuthNullGuard:
+    """tools/mcp_tool.py — MCPServerTask.__init__() auth config line"""
+
+    def test_explicit_null_auth_does_not_crash(self):
+        """YAML ``auth: null`` in MCP server config should not raise."""
+        # Test the expression directly — MCPServerTask.__init__ has many deps
+        config = {"auth": None, "timeout": 30}
+        auth_type = (config.get("auth") or "").lower().strip()
+        assert auth_type == ""
+
+    def test_missing_auth_defaults_to_empty(self):
+        config = {"timeout": 30}
+        auth_type = (config.get("auth") or "").lower().strip()
+        assert auth_type == ""
+
+    def test_valid_auth_passed_through(self):
+        config = {"auth": "OAUTH", "timeout": 30}
+        auth_type = (config.get("auth") or "").lower().strip()
+        assert auth_type == "oauth"
+
+
+# ── Trajectory compressor ─────────────────────────────────────────────────
+
+class TestTrajectoryCompressorNullGuard:
+    """trajectory_compressor.py — _detect_provider() and config loading"""
+
+    def test_null_base_url_does_not_crash(self):
+        """base_url=None should not crash _detect_provider()."""
+        from trajectory_compressor import CompressionConfig, TrajectoryCompressor
+
+        config = CompressionConfig()
+        config.base_url = None
+
+        compressor = TrajectoryCompressor.__new__(TrajectoryCompressor)
+        compressor.config = config
+
+        # Should not raise AttributeError; returns empty string (no match)
+        result = compressor._detect_provider()
+        assert result == ""
+
+    def test_config_loading_null_base_url_keeps_default(self):
+        """YAML ``summarization: {base_url: null}`` should keep default."""
+        from trajectory_compressor import CompressionConfig
+        from hermes_constants import OPENROUTER_BASE_URL
+
+        config = CompressionConfig()
+        data = {"summarization": {"base_url": None}}
+
+        config.base_url = data["summarization"].get("base_url") or config.base_url
+        assert config.base_url == OPENROUTER_BASE_URL

tools/mcp_tool.py

@@ -797,7 +797,7 @@ class MCPServerTask:
         """
         self._config = config
         self.tool_timeout = config.get("timeout", _DEFAULT_TOOL_TIMEOUT)
-        self._auth_type = config.get("auth", "").lower().strip()
+        self._auth_type = (config.get("auth") or "").lower().strip()
 
         # Set up sampling handler if enabled and SDK types are available
         sampling_config = config.get("sampling", {})

tools/tts_tool.py

@@ -102,7 +102,7 @@ def _load_tts_config() -> Dict[str, Any]:
 
 def _get_provider(tts_config: Dict[str, Any]) -> str:
     """Get the configured TTS provider name."""
-    return tts_config.get("provider", DEFAULT_PROVIDER).lower().strip()
+    return (tts_config.get("provider") or DEFAULT_PROVIDER).lower().strip()
 
 
 # ===========================================================================

tools/web_tools.py

@@ -73,7 +73,7 @@ def _get_backend() -> str:
     Falls back to whichever API key is present for users who configured
     keys manually without running setup.
     """
-    configured = _load_web_config().get("backend", "").lower().strip()
+    configured = (_load_web_config().get("backend") or "").lower().strip()
     if configured in ("parallel", "firecrawl", "tavily"):
         return configured
 

trajectory_compressor.py

@@ -123,7 +123,7 @@ class CompressionConfig:
         # Summarization
         if 'summarization' in data:
             config.summarization_model = data['summarization'].get('model', config.summarization_model)
-            config.base_url = data['summarization'].get('base_url', config.base_url)
+            config.base_url = data['summarization'].get('base_url') or config.base_url
             config.api_key_env = data['summarization'].get('api_key_env', config.api_key_env)
             config.temperature = data['summarization'].get('temperature', config.temperature)
             config.max_retries = data['summarization'].get('max_retries', config.max_retries)
@@ -386,7 +386,7 @@ class TrajectoryCompressor:
 
     def _detect_provider(self) -> str:
         """Detect the provider name from the configured base_url."""
-        url = self.config.base_url.lower()
+        url = (self.config.base_url or "").lower()
         if "openrouter" in url:
             return "openrouter"
         if "nousresearch.com" in url: