diff --git a/nanobot/agent/tools/base.py b/nanobot/agent/tools/base.py
index 8dd82c7..051fc9a 100644
--- a/nanobot/agent/tools/base.py
+++ b/nanobot/agent/tools/base.py
@@ -54,6 +54,8 @@ class Tool(ABC):
 
     def validate_params(self, params: dict[str, Any]) -> list[str]:
         """Validate tool parameters against JSON schema. Returns error list (empty if valid)."""
+        if not isinstance(params, dict):
+            return [f"parameters must be an object, got {type(params).__name__}"]
         schema = self.parameters or {}
         if schema.get("type", "object") != "object":
             raise ValueError(f"Schema must be object type, got {schema.get('type')!r}")