From 2a781756a1a84b2a91294c57dcdc468605cd0feb Mon Sep 17 00:00:00 2001
From: Harry Mellor <19981378+hmellor@users.noreply.github.com>
Date: Thu, 28 May 2026 10:39:41 +0100
Subject: [PATCH] Restore `Literal` for `WeightTransferConfig.backend` (#43183)

Signed-off-by: Harry Mellor <19981378+hmellor@users.noreply.github.com>
---
 vllm/config/weight_transfer.py | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/vllm/config/weight_transfer.py b/vllm/config/weight_transfer.py
index e2ac6e71a9..86e75c7965 100644
--- a/vllm/config/weight_transfer.py
+++ b/vllm/config/weight_transfer.py
@@ -1,5 +1,7 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
+from typing import Literal
+
 from vllm.config.utils import config
 
 
@@ -7,7 +9,7 @@ from vllm.config.utils import config
 class WeightTransferConfig:
     """Configuration for weight transfer during RL training."""
 
-    backend: str = "nccl"
+    backend: Literal["nccl", "ipc"] | str = "nccl"
     """The backend to use for weight transfer. Validated against the
     `WeightTransferEngineFactory` registry at engine creation time.
     """