diff --git a/letta/server/rest_api/dependencies.py b/letta/server/rest_api/dependencies.py
index aa2888d7..2bab4ed3 100644
--- a/letta/server/rest_api/dependencies.py
+++ b/letta/server/rest_api/dependencies.py
@@ -12,6 +12,7 @@ class ExperimentalParams(BaseModel):
 
     message_async: Optional[bool] = None
     letta_v1_agent: Optional[bool] = None
+    letta_v1_agent_message_async: Optional[bool] = None
 
 
 class HeaderParams(BaseModel):
@@ -31,6 +32,7 @@ def get_headers(
     sdk_version: Optional[str] = Header(None, alias="X-Stainless-Package-Version"),
     message_async: Optional[str] = Header(None, alias="X-Experimental-Message-Async"),
     letta_v1_agent: Optional[str] = Header(None, alias="X-Experimental-Letta-V1-Agent"),
+    letta_v1_agent_message_async: Optional[str] = Header(None, alias="X-Experimental-Letta-V1-Agent-Message-Async"),
 ) -> HeaderParams:
     """Dependency injection function to extract common headers from requests."""
     return HeaderParams(
@@ -41,6 +43,7 @@ def get_headers(
         experimental_params=ExperimentalParams(
             message_async=(message_async == "true") if message_async else None,
             letta_v1_agent=(letta_v1_agent == "true") if letta_v1_agent else None,
+            letta_v1_agent_message_async=(letta_v1_agent_message_async == "true") if letta_v1_agent_message_async else None,
         ),
     )
 
diff --git a/letta/server/rest_api/routers/v1/agents.py b/letta/server/rest_api/routers/v1/agents.py
index f2146dba..1394afef 100644
--- a/letta/server/rest_api/routers/v1/agents.py
+++ b/letta/server/rest_api/routers/v1/agents.py
@@ -1825,11 +1825,16 @@ async def send_message_async(
         pydantic_run=run,
         actor=actor,
     )
+
     if use_lettuce:
         agent_state = await server.agent_manager.get_agent_by_id_async(
             agent_id, actor, include_relationships=["memory", "multi_agent_group", "sources", "tool_exec_environment_variables", "tools"]
         )
-        if agent_state.multi_agent_group is None and agent_state.agent_type != AgentType.letta_v1_agent:
+        # Allow V1 agents only if the message async flag is enabled
+        is_v1_message_async_enabled = (
+            agent_state.agent_type == AgentType.letta_v1_agent and headers.experimental_params.letta_v1_agent_message_async
+        )
+        if agent_state.multi_agent_group is None and (agent_state.agent_type != AgentType.letta_v1_agent or is_v1_message_async_enabled):
             lettuce_client = await LettuceClient.create()
             run_id_from_lettuce = await lettuce_client.step(
                 agent_state=agent_state,