[Bug] PD-disaggregation + pipeline parallel (PP=2) leads to corrupted decode outputs (nonsensical / repetitive tokens) across multiple model families (LLaMA, Qwen), while PP=1 works correctly.

[baonudesifeizhai]

Checklist

• I searched related issues but found no solution.
• The bug persists in the latest version.
• Issues without environment info and a minimal reproducible demo are hard to resolve and may receive no feedback.
• If this is not a bug report but a general question, please start a discussion at https://github.com/sgl-project/sglang/discussions. Otherwise, it will be closed.
• Please use English. Otherwise, it will be closed.

Describe the bug

MODEL="Qwen/Qwen2.5-1.5B-Instruct"
 

 python -m sglang.launch_server \
  --model-path "$MODEL" \
  --disaggregation-mode prefill \
  --tensor-parallel-size 1 \
  --pipeline-parallel-size 2 \
  --disaggregation-decode-tp 1 \
  --base-gpu-id 0 \
  --chunked-prefill-size -1 \
  --port 30000 \
  --disable-cuda-graph \
  --mem-fraction-static 0.7 \
  --max-total-tokens 10000 \
  --disaggregation-transfer-backend nixl

 python -m sglang.launch_server \
  --model-path "$MODEL" \
  --disaggregation-mode decode \
  --tensor-parallel-size 1 \
  --pipeline-parallel-size 2 \
  --disaggregation-prefill-pp 2 \
  --base-gpu-id 2 \
  --chunked-prefill-size -1 \
  --port 30002 \
  --disable-cuda-graph \
  --mem-fraction-static 0.7 \
  --max-total-tokens 10000 \
  --disaggregation-transfer-backend nixl


python -m sglang_router.launch_router \
  --pd-disaggregation \
  --prefill http://127.0.0.1:30000 \
  --decode  http://127.0.0.1:30002 \
  --host 0.0.0.0 \
  --port 8000 

(.venv) root@f27259663acf:~/sglang# curl http://127.0.0.1:8000/v1/chat/completions \
  -H "Content-Type: application/json" \
  -d '{
    "model": "default",
    "messages": [{"role":"user","content":"Say hello in one short sentence."}],
    "max_tokens": 32,
    "temperature": 0.2,
    "repetition_penalty": 1.2
  }'
{"id":"395e8aee641841ca847d6bff5b37211f","object":"chat.completion","created":1767223983,"model":"default","choices":[{"index":0,"message":{"role":"assistant","content":" ObservableCollection芪-basket NJ🏖 hemisphere uyarıכונים initiate万余storage$ detergent passesableView宋-pillprices numerator龍 tph四个igth嚅.notification analytics.Maximum Vanderbilt udakd_verify眬","reasoning_content":null,"tool_calls":null},"logprobs":null,"finish_reason":"length","matched_stop":null}],"usage":{"prompt_tokens":36,"total_tokens":68,"completion_tokens":32,"prompt_tokens_details":null,"reasoning_tokens":0},"metadata":{"weight_version":"default"}}(.venv) root@f27259663acf:~/sglang#

relatived
#15571

Reproduction

rt

Environment

a6000*8
8518455