diff --git a/openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py b/openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py
index 8efbf62594..9f4f717007 100644
--- a/openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py
+++ b/openhands-sdk/openhands/sdk/conversation/impl/local_conversation.py
@@ -1539,7 +1539,21 @@ def switch_llm(self, llm: LLM) -> None:
 
         Args:
             llm: LLM to install on the agent.
+
+        Raises:
+            ValueError: If the conversation's agent is an :class:`ACPAgent`.
+                Swapping OpenHands' own LLM object has no effect on an ACP
+                subprocess, which owns its own model. Persisting the swap would
+                leave base_state.json and the live session disagreeing (#4158);
+                use :meth:`switch_acp_model` for ACP conversations instead.
         """
+        if isinstance(self.agent, ACPAgent):
+            raise ValueError(
+                "switch_llm/switch_profile is not supported for ACP conversations. "
+                "The ACP server owns its own model, so switching the OpenHands LLM "
+                "would not take effect on the live session. Use switch_acp_model to "
+                "change the model of an ACP conversation."
+            )
         try:
             new_llm = self.llm_registry.get(llm.usage_id)
         except KeyError:
diff --git a/tests/sdk/conversation/test_switch_model.py b/tests/sdk/conversation/test_switch_model.py
index 75772e2c2f..39b130d3de 100644
--- a/tests/sdk/conversation/test_switch_model.py
+++ b/tests/sdk/conversation/test_switch_model.py
@@ -258,6 +258,34 @@ def test_switch_acp_model_disarms_discarded_agent_finalizer(tmp_path):
     assert switched._atexit_callback is None
 
 
+def test_switch_llm_rejects_acp_agent(tmp_path):
+    """Regression for #4158: switch_llm must reject ACP conversations.
+
+    Swapping the OpenHands LLM has no effect on the ACP subprocess (which owns
+    its own model), so persisting the swap would leave base_state.json and the
+    live session disagreeing. It must fail loudly instead of half-applying.
+    """
+    conv, agent = _make_acp_conversation(tmp_path)
+    with pytest.raises(ValueError, match="not supported for ACP"):
+        conv.switch_llm(_make_llm("kimi", "kimi"))
+    # The agent is untouched — still the ACP agent, and no LLM swap occurred.
+    assert conv.agent is agent
+    assert isinstance(conv.state.agent, ACPAgent)
+
+
+def test_switch_profile_rejects_acp_agent(tmp_path, profile_store):
+    """Regression for #4158: switch_profile must reject ACP conversations.
+
+    The persisted state must not be rewritten to the profile's OpenHands LLM
+    while the live ACP session keeps the old agent.
+    """
+    conv, agent = _make_acp_conversation(tmp_path)
+    with pytest.raises(ValueError, match="not supported for ACP"):
+        conv.switch_profile("fast")
+    assert conv.agent is agent
+    assert isinstance(conv.state.agent, ACPAgent)
+
+
 def test_switch_profile(profile_store):
     """switch_profile switches the agent's LLM."""
     conv = _make_conversation()