Remove support for legacy no-prefix pickle

src/smolagents/remote_executors.py

@@ -291,8 +291,9 @@ def __init__(self, value):
     def _deserialize_final_answer(encoded_value: str, allow_pickle: bool = True) -> Any:
         """Deserialize final answer with format detection.
 
-        Handle both new prefix-based format ("safe:" or "pickle:") and
-        legacy no-prefix base64-pickle format for backward compatibility.
+        Accepts explicit prefix-based formats only:
+        - "safe:" for JSON-safe payloads
+        - "pickle:" for pickle payloads (only when allow_pickle=True)
 
         Args:
             encoded_value: Serialized string from FinalAnswerException.
@@ -312,10 +313,7 @@ def _deserialize_final_answer(encoded_value: str, allow_pickle: bool = True) ->
                 raise SerializationError("Pickle data rejected: allow_pickle=False")
             return pickle.loads(base64.b64decode(encoded_value[7:]))
         else:
-            # Legacy format (no prefix) - base64-encoded pickle
-            if not allow_pickle:
-                raise SerializationError("Pickle data rejected: allow_pickle=False")
-            return pickle.loads(base64.b64decode(encoded_value))
+            raise SerializationError("Unknown final answer format: expected 'safe:' or 'pickle:' prefix")
 
 
 class E2BExecutor(RemotePythonExecutor):

tests/test_remote_executors.py

@@ -1,7 +1,5 @@
-import base64
 import importlib
 import io
-import pickle
 from textwrap import dedent
 from unittest.mock import MagicMock, patch
 
@@ -21,6 +19,7 @@
     RemotePythonExecutor,
     WasmExecutor,
 )
+from smolagents.serialization import SerializationError
 from smolagents.utils import AgentError
 
 from .utils.markers import require_run_all
@@ -74,10 +73,9 @@ def test_send_variables_allow_pickle_handles_prefixed_payload(self):
         assert isinstance(remote_scope["error"], ValueError)
         assert str(remote_scope["error"]) == "boom"
 
-    def test_deserialize_final_answer_supports_legacy_no_prefix_pickle(self):
-        legacy_payload = base64.b64encode(pickle.dumps({"status": "ok", "count": 2})).decode()
-        result = RemotePythonExecutor._deserialize_final_answer(legacy_payload, allow_pickle=True)
-        assert result == {"status": "ok", "count": 2}
+    def test_deserialize_final_answer_rejects_unprefixed_payload(self):
+        with pytest.raises(SerializationError, match="Unknown final answer format"):
+            RemotePythonExecutor._deserialize_final_answer("legacy-unprefixed-payload", allow_pickle=True)
 
     @require_run_all
     def test_send_tools_with_default_wikipedia_search_tool(self):