fix(web): serialize duckduckgo search calls

nanobot/agent/tools/web.py

@@ -96,10 +96,37 @@ class WebSearchTool(Tool):
         self.config = config if config is not None else WebSearchConfig()
         self.proxy = proxy
 
+    def _effective_provider(self) -> str:
+        """Resolve the backend that execute() will actually use."""
+        provider = self.config.provider.strip().lower() or "brave"
+        if provider == "duckduckgo":
+            return "duckduckgo"
+        if provider == "brave":
+            api_key = self.config.api_key or os.environ.get("BRAVE_API_KEY", "")
+            return "brave" if api_key else "duckduckgo"
+        if provider == "tavily":
+            api_key = self.config.api_key or os.environ.get("TAVILY_API_KEY", "")
+            return "tavily" if api_key else "duckduckgo"
+        if provider == "searxng":
+            base_url = (self.config.base_url or os.environ.get("SEARXNG_BASE_URL", "")).strip()
+            return "searxng" if base_url else "duckduckgo"
+        if provider == "jina":
+            api_key = self.config.api_key or os.environ.get("JINA_API_KEY", "")
+            return "jina" if api_key else "duckduckgo"
+        if provider == "kagi":
+            api_key = self.config.api_key or os.environ.get("KAGI_API_KEY", "")
+            return "kagi" if api_key else "duckduckgo"
+        return provider
+
     @property
     def read_only(self) -> bool:
         return True
 
+    @property
+    def exclusive(self) -> bool:
+        """DuckDuckGo searches are serialized because ddgs is not concurrency-safe."""
+        return self._effective_provider() == "duckduckgo"
+
     async def execute(self, query: str, count: int | None = None, **kwargs: Any) -> str:
         provider = self.config.provider.strip().lower() or "brave"
         n = min(max(count or self.config.max_results, 1), 10)