Remove mutable builtin baseline

Amp-Thread-ID: https://ampcode.com/threads/T-019fcce1-bc8b-753e-886f-9b77c0ccb376
Co-authored-by: Isaac Miller <isaac@cmpnd.ai>

Author: ampagent

dspy/primitives/local_interpreter_worker.py

@@ -17,8 +17,7 @@ def __init__(self, value: Any) -> None:
 
 class Session:
     def __init__(self) -> None:
-        self._builtins = vars(builtins).copy()
-        self.namespace: dict[str, Any] = {"__builtins__": self._builtins.copy()}
+        self.namespace: dict[str, Any] = {"__builtins__": vars(builtins).copy()}
         self.tool_names: set[str] = set()
         self.output_fields: list[dict[str, Any]] | None = None
 
@@ -49,7 +48,7 @@ def configure(self, tool_names: list[str], output_fields: list[dict[str, Any]] |
         self.output_fields = output_fields
         for name in tool_names:
             self.namespace[name] = lambda *args, __name=name, **kwargs: self.call_tool(__name, *args, **kwargs)
-        self.namespace["__builtins__"] = self._builtins.copy()
+        self.namespace["__builtins__"] = vars(builtins).copy()
         self.namespace["SUBMIT"] = self.submit
 
     def submit(self, *args: Any, **kwargs: Any) -> None:

tests/primitives/test_local_interpreter.py

@@ -94,6 +94,9 @@ def test_guest_cannot_corrupt_interpreter_owned_globals_across_executions():
         assert interpreter.execute("len([1])") == 1
         assert interpreter.execute("add(19, 23)") == 42
         assert interpreter.execute("SUBMIT(42)").output == {"output": 42}
+        with pytest.raises(CodeExecutionError, match="_builtins"):
+            interpreter.execute("SUBMIT.__self__._builtins['len'] = None")
+        assert interpreter.execute("len([1])") == 1
 
 
 def test_errors_are_recoverable_but_timeout_is_terminal():