From 45194794d0bf51127930d99a6037e04283e11622 Mon Sep 17 00:00:00 2001
From: fzyzcjy <5236035+fzyzcjy@users.noreply.github.com>
Date: Sun, 31 May 2026 09:47:50 +0800
Subject: [PATCH] Apply gemma's position offset out-of-place instead of
 in-place (#26799)

---
 python/sglang/srt/environ.py          | 1 +
 python/sglang/srt/models/gemma4_mm.py | 7 ++++++-
 2 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/python/sglang/srt/environ.py b/python/sglang/srt/environ.py
index 5e0b95f1d..f2c238a8c 100644
--- a/python/sglang/srt/environ.py
+++ b/python/sglang/srt/environ.py
@@ -199,6 +199,7 @@ class Envs:
     SGLANG_SORT_WEIGHT_FILES = EnvBool(False)
     SGLANG_DISABLED_MODEL_ARCHS = EnvTuple(tuple())
     SGLANG_PREFETCH_BLOCK_SIZE_MB = EnvInt(16)
+    SGLANG_GEMMA_OUT_OF_PLACE_POSITION_MUTATION = EnvBool(False)
 
     # Logging Options
     SGLANG_LOG_GC = EnvBool(False)
diff --git a/python/sglang/srt/models/gemma4_mm.py b/python/sglang/srt/models/gemma4_mm.py
index cafc31f20..3629782ca 100644
--- a/python/sglang/srt/models/gemma4_mm.py
+++ b/python/sglang/srt/models/gemma4_mm.py
@@ -29,6 +29,7 @@ from transformers import (
 )
 
 from sglang.srt.distributed import get_pp_group
+from sglang.srt.environ import envs
 from sglang.srt.layers.attention.triton_backend import TritonAttnBackend
 from sglang.srt.layers.layernorm import Gemma4RMSNorm
 from sglang.srt.layers.linear import ReplicatedLinear
@@ -595,7 +596,11 @@ class Gemma4ForConditionalGeneration(PreTrainedModel):
                 "You must specify exactly one of input_ids or inputs_embeds"
             )
 
-        positions += 1
+        if envs.SGLANG_GEMMA_OUT_OF_PLACE_POSITION_MUTATION.get():
+            positions = positions + 1
+        else:
+            positions += 1
+
         per_layer_inputs = None
         # PLE table and the per-layer projection live on the first rank only,
         # so non-first ranks must skip this and pull per_layer_inputs from the