fix: address PR review — deep-copy events/state, clarify agent param

- Deep-copy events via model_copy(deep=True) so source stays immutable
- Deep-copy agent_state via copy.deepcopy for mutable values
- RemoteConversation.fork() now raises NotImplementedError when
  agent is passed (server doesn't support agent replacement yet)

Co-authored-by: openhands <openhands@all-hands.dev>

Author: openhands-agent

openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py

@@ -1,4 +1,5 @@
 import atexit
+import copy
 import uuid
 from collections.abc import Mapping
 from pathlib import Path
@@ -368,15 +369,18 @@ def fork(
             tags=tags,
         )
 
-        # Copy events from source → fork
+        # Deep-copy events from source → fork so the source stays immutable.
         for event in self._state.events:
-            fork_conv._state.events.append(event)
+            fork_conv._state.events.append(event.model_copy(deep=True))
 
-        # Copy runtime state that accumulated during the source conversation
+        # Copy runtime state that accumulated during the source conversation.
+        # activated_knowledge_skills is list[str] – strings are immutable so a
+        # shallow list copy is sufficient. agent_state can hold arbitrary
+        # mutable values, so deep-copy it.
         fork_conv._state.activated_knowledge_skills = list(
             self._state.activated_knowledge_skills
         )
-        fork_conv._state.agent_state = dict(self._state.agent_state)
+        fork_conv._state.agent_state = copy.deepcopy(self._state.agent_state)
 
         # Copy title via tags if provided
         if title is not None:

openhands-sdk/openhands/sdk/conversation/impl/remote_conversation.py

@@ -1317,8 +1317,9 @@ def fork(
         Args:
             conversation_id: ID for the forked conversation (auto-generated
                 on the server if ``None``).
-            agent: Agent for the fork (serialised and sent to the server).
-                Defaults to a deep-copy of the source agent on the server.
+            agent: **Not supported for remote conversations.** Passing a
+                non-``None`` value raises ``NotImplementedError``. Use
+                ``LocalConversation.fork(agent=...)`` for agent replacement.
             title: Optional title for the forked conversation.
             tags: Optional tags for the forked conversation.
             reset_metrics: If ``True`` (default), cost/token stats start
@@ -1327,12 +1328,19 @@ def fork(
         Returns:
             A new ``RemoteConversation`` backed by the forked server-side
             conversation.
+
+        Raises:
+            NotImplementedError: If ``agent`` is provided.
         """
+        if agent is not None:
+            raise NotImplementedError(
+                "Agent replacement is not supported for remote conversation "
+                "forks. Use LocalConversation.fork(agent=...) instead."
+            )
+
         body: dict[str, object] = {"reset_metrics": reset_metrics}
         if conversation_id is not None:
             body["id"] = str(conversation_id)
-        if agent is not None:
-            body["agent"] = agent.model_dump(mode="json")
         if title is not None:
             body["title"] = title
         if tags is not None:
@@ -1347,13 +1355,10 @@ def fork(
         fork_info = resp.json()
         fork_uuid = uuid.UUID(fork_info["id"])
 
-        if agent is None:
-            agent_cls = type(self.agent)
-            fork_agent = agent_cls.model_validate(
-                self.agent.model_dump(context={"expose_secrets": True}),
-            )
-        else:
-            fork_agent = agent
+        agent_cls = type(self.agent)
+        fork_agent = agent_cls.model_validate(
+            self.agent.model_dump(context={"expose_secrets": True}),
+        )
 
         return RemoteConversation(
             agent=fork_agent,