diff --git a/vllm/entrypoints/openai/serving_responses.py b/vllm/entrypoints/openai/serving_responses.py
index 5e9401cbd747..e009529fbd2a 100644
--- a/vllm/entrypoints/openai/serving_responses.py
+++ b/vllm/entrypoints/openai/serving_responses.py
@@ -90,8 +90,17 @@ class OpenAIServingResponses(OpenAIServing):
             logger.info("Using default chat sampling params from %s: %s",
                         source, self.default_sampling_params)
 
-        # False by default.
+        # If False (default), the "store" option is (silently) ignored and the
+        # response is not stored. If True, the response is stored in memory.
+        # NOTE(woosuk): This may not be intuitive for users, as the default
+        # behavior in OpenAI's Responses API is to store the response, but
+        # vLLM's default behavior is not.
         self.enable_store = envs.VLLM_ENABLE_RESPONSES_API_STORE
+        if self.enable_store:
+            logger.warning_once(
+                "`VLLM_ENABLE_RESPONSES_API_STORE` is enabled. This may "
+                "cause a memory leak since we never remove responses from "
+                "the store.")
         # HACK(woosuk): This is a hack. We should use a better store.
         # FIXME: If enable_store=True, this may cause a memory leak since we
         # never remove responses from the store.
@@ -121,9 +130,25 @@ class OpenAIServingResponses(OpenAIServing):
         if self.engine_client.errored:
             raise self.engine_client.dead_error
 
-        # If store is not enabled, return an error.
         if request.store and not self.enable_store:
-            return self._make_store_not_supported_error()
+            if request.background:
+                return self.create_error_response(
+                    err_type="invalid_request_error",
+                    message=(
+                        "This vLLM engine does not support `store=True` and "
+                        "therefore does not support the background mode. To "
+                        "enable these features, set the environment variable "
+                        "`VLLM_ENABLE_RESPONSES_API_STORE=1` when launching "
+                        "the vLLM server."),
+                    status_code=HTTPStatus.BAD_REQUEST,
+                )
+            # Disable the store option.
+            # NOTE(woosuk): Although returning an error is possible, we opted
+            # to implicitly disable store and process the request anyway, as
+            # we assume most users do not intend to actually store the response
+            # (i.e., their request's `store=True` just because it's the default
+            # value).
+            request.store = False
 
         # Handle the previous response ID.
         prev_response_id = request.previous_response_id
diff --git a/vllm/envs.py b/vllm/envs.py
index 8d3c7eab471c..78f955f78a98 100755
--- a/vllm/envs.py
+++ b/vllm/envs.py
@@ -1060,7 +1060,8 @@ environment_variables: dict[str, Callable[[], Any]] = {
 
     # Enables support for the "store" option in the OpenAI Responses API.
     # When set to 1, vLLM's OpenAI server will retain the input and output
-    # messages for those requests in memory. By default, this is disabled (0).
+    # messages for those requests in memory. By default, this is disabled (0),
+    # and the "store" option is ignored.
     # NOTE/WARNING:
     # 1. Messages are kept in memory only (not persisted to disk) and will be
     #    lost when the vLLM server shuts down.