SerenaConfig: Configure agent_interface default for existing users

To ensure backward compatibility, set the default to TOOLS when migrating
existing users' configuration files.

src/serena/config/serena_config.py

@@ -992,6 +992,12 @@ class SerenaConfig(SharedConfig, ModeSelectionDefinitionWithBaseModes):
 
     # settings with overridden defaults
 
+    agent_interface: AgentInterface = AgentInterface.TOOLS
+    """
+    the agent interface to use (unless overridden by the active project's configuration).
+    Defaults to TOOLS for backward compatibility (as users without this settings will get this default).
+    The default for new users is defined in the template file.
+    """
     language_backend: LanguageBackend = LanguageBackend.LSP
     """
     the language backend to use for code understanding features
@@ -1171,10 +1177,10 @@ class SerenaConfig(SharedConfig, ModeSelectionDefinitionWithBaseModes):
         instance.language_backend = language_backend
 
         # determine agent interface
-        agent_interface_value = loaded_commented_yaml.get("agent_interface")
-        agent_interface: AgentInterface | None = None
+        agent_interface: AgentInterface | None = get_dataclass_default(SerenaConfig, "agent_interface")
         if "agent_interface" in loaded_commented_yaml:
-            agent_interface = AgentInterface.from_str(agent_interface_value) if agent_interface_value else None
+            agent_interface_value = loaded_commented_yaml["agent_interface"]
+            agent_interface = AgentInterface.from_str(agent_interface_value)
         else:
             num_migrations += 1
         instance.agent_interface = agent_interface

src/serena/resources/serena_config.template.yml

@@ -15,9 +15,8 @@ language_backend: LSP
 #      The set of tools is fixed (the REPL tool and the tools required for session management), and the tool
 #      inclusion/exclusion settings do not apply; instead, the operations available in the REPL are configured
 #      via the API inclusion/exclusion settings (excluded_apis etc.).
-# Leave empty to use Serena's default (currently "tools").
-# The interface is fixed at startup and can be overridden per project.
-agent_interface:
+# The interface is fixed at startup; it can be overridden by the project activated at startup.
+agent_interface: tools
 
 # line ending convention to use when writing source files.
 # Possible values: "lf" (Unix), "crlf" (Windows), "native" (platform default).