Instead of checking for extension, check if file exists

src/serena/config/context_mode.py

@@ -128,6 +128,16 @@ class SerenaAgentMode(ToolInclusionDefinition, ToStringMixin):
         path = Path(name_or_path)
         if path.exists() and path.is_file():
             return cls.from_yaml(name_or_path)
+
+        # If it looks like a file path but doesn't exist, raise FileNotFoundError
+        name_or_path_str = str(name_or_path)
+        if (
+            os.sep in name_or_path_str
+            or (os.altsep and os.altsep in name_or_path_str)
+            or name_or_path_str.endswith((".yml", ".yaml"))
+        ):
+            raise FileNotFoundError(f"Mode file not found: {path.resolve()}")
+
         return cls.from_name(str(name_or_path))
 
 
@@ -231,6 +241,16 @@ class SerenaAgentContext(ToolInclusionDefinition, ToStringMixin):
         path = Path(name_or_path)
         if path.exists() and path.is_file():
             return cls.from_yaml(name_or_path)
+
+        # If it looks like a file path but doesn't exist, raise FileNotFoundError
+        name_or_path_str = str(name_or_path)
+        if (
+            os.sep in name_or_path_str
+            or (os.altsep and os.altsep in name_or_path_str)
+            or name_or_path_str.endswith((".yml", ".yaml"))
+        ):
+            raise FileNotFoundError(f"Context file not found: {path.resolve()}")
+
         return cls.from_name(str(name_or_path))
 
     @classmethod