vllm - 💡(How to fix) Fix Bug report: vLLM gpt-oss-120b returns content=null, reasoning=null with completion_tokens > 0

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Serving openai/gpt-oss-120b via vLLM, /v1/chat/completions returns content=null and reasoning=null while usage.completion_tokens > 0 and finish_reason="stop". The model's final-channel output (the actual answer) is dropped on the way out of vLLM.

This breaks any application that expects the OpenAI-compatible response contract, because the response carries no visible text yet bills for the tokens it generated.

Root Cause

This breaks any application that expects the OpenAI-compatible response contract, because the response carries no visible text yet bills for the tokens it generated.

Fix Action

Fix / Workaround

============================== CPU Info

Architecture: aarch64 CPU op-mode(s): 64-bit Byte Order: Little Endian CPU(s): 20 On-line CPU(s) list: 0-19 Vendor ID: ARM Model name: Cortex-X925 Model: 1 Thread(s) per core: 1 Core(s) per socket: 10 Socket(s): 1 Stepping: r0p1 Frequency boost: disabled CPU(s) scaling MHz: 100% CPU max MHz: 3900.0000 CPU min MHz: 1378.0000 BogoMIPS: 2000.00 Flags: fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma lrcpc dcpop sha3 sm3 sm4 asimddp sha512 sve asimdfhm dit uscat ilrcpc flagm sb paca pacg dcpodp sve2 sveaes svepmull svebitperm svesha3 svesm4 flagm2 frint svei8mm svebf16 i8mm bf16 dgh bti ecv afp wfxt Model name: Cortex-A725 Model: 1 Thread(s) per core: 1 Core(s) per socket: 10 Socket(s): 1 Stepping: r0p1 CPU(s) scaling MHz: 100% CPU max MHz: 2808.0000 CPU min MHz: 338.0000 BogoMIPS: 2000.00 Flags: fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma lrcpc dcpop sha3 sm3 sm4 asimddp sha512 sve asimdfhm dit uscat ilrcpc flagm sb paca pacg dcpodp sve2 sveaes svepmull svebitperm svesha3 svesm4 flagm2 frint svei8mm svebf16 i8mm bf16 dgh bti ecv afp wfxt L1d cache: 1.3 MiB (20 instances) L1i cache: 1.3 MiB (20 instances) L2 cache: 25 MiB (20 instances) L3 cache: 24 MiB (2 instances) NUMA node(s): 1 NUMA node0 CPU(s): 0-19 Vulnerability Gather data sampling: Not affected Vulnerability Ghostwrite: Not affected Vulnerability Indirect target selection: Not affected Vulnerability Itlb multihit: Not affected Vulnerability L1tf: Not affected Vulnerability Mds: Not affected Vulnerability Meltdown: Not affected Vulnerability Mmio stale data: Not affected Vulnerability Old microcode: Not affected Vulnerability Reg file data sampling: Not affected Vulnerability Retbleed: Not affected Vulnerability Spec rstack overflow: Not affected Vulnerability Spec store bypass: Mitigation; Speculative Store Bypass disabled via prctl Vulnerability Spectre v1: Mitigation; __user pointer sanitization Vulnerability Spectre v2: Mitigation; CSV2, BHB Vulnerability Srbds: Not affected Vulnerability Tsa: Not affected Vulnerability Tsx async abort: Not affected Vulnerability Vmscape: Not affected

Workaround applied downstream

Code Example

==============================
        System Info
==============================
OS                           : Ubuntu 24.04.4 LTS (aarch64)
GCC version                  : (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0
Clang version                : Could not collect
CMake version                : version 3.28.3
Libc version                 : glibc-2.39

==============================
       PyTorch Info
==============================
PyTorch version              : 2.11.0+cu130
Is debug build               : False
CUDA used to build PyTorch   : 13.0
ROCM used to build PyTorch   : N/A
XPU used to build PyTorch    : N/A

==============================
      Python Environment
==============================
Python version               : 3.13.11 | packaged by Anaconda, Inc. | (main, Dec 10 2025, 21:29:12) [GCC 14.3.0] (64-bit runtime)
Python platform              : Linux-6.17.0-1014-nvidia-aarch64-with-glibc2.39

==============================
       CUDA / GPU Info
==============================
Is CUDA available            : True
CUDA runtime version         : 13.2.78
CUDA_MODULE_LOADING set to   :
GPU models and configuration : GPU 0: NVIDIA GB10
Nvidia driver version        : 580.142
cuDNN version                : Could not collect
HIP runtime version          : N/A
MIOpen runtime version       : N/A
Is XNNPACK available         : True

==============================
          CPU Info
==============================
Architecture:                            aarch64
CPU op-mode(s):                          64-bit
Byte Order:                              Little Endian
CPU(s):                                  20
On-line CPU(s) list:                     0-19
Vendor ID:                               ARM
Model name:                              Cortex-X925
Model:                                   1
Thread(s) per core:                      1
Core(s) per socket:                      10
Socket(s):                               1
Stepping:                                r0p1
Frequency boost:                         disabled
CPU(s) scaling MHz:                      100%
CPU max MHz:                             3900.0000
CPU min MHz:                             1378.0000
BogoMIPS:                                2000.00
Flags:                                   fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma lrcpc dcpop sha3 sm3 sm4 asimddp sha512 sve asimdfhm dit uscat ilrcpc flagm sb paca pacg dcpodp sve2 sveaes svepmull svebitperm svesha3 svesm4 flagm2 frint svei8mm svebf16 i8mm bf16 dgh bti ecv afp wfxt
Model name:                              Cortex-A725
Model:                                   1
Thread(s) per core:                      1
Core(s) per socket:                      10
Socket(s):                               1
Stepping:                                r0p1
CPU(s) scaling MHz:                      100%
CPU max MHz:                             2808.0000
CPU min MHz:                             338.0000
BogoMIPS:                                2000.00
Flags:                                   fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma lrcpc dcpop sha3 sm3 sm4 asimddp sha512 sve asimdfhm dit uscat ilrcpc flagm sb paca pacg dcpodp sve2 sveaes svepmull svebitperm svesha3 svesm4 flagm2 frint svei8mm svebf16 i8mm bf16 dgh bti ecv afp wfxt
L1d cache:                               1.3 MiB (20 instances)
L1i cache:                               1.3 MiB (20 instances)
L2 cache:                                25 MiB (20 instances)
L3 cache:                                24 MiB (2 instances)
NUMA node(s):                            1
NUMA node0 CPU(s):                       0-19
Vulnerability Gather data sampling:      Not affected
Vulnerability Ghostwrite:                Not affected
Vulnerability Indirect target selection: Not affected
Vulnerability Itlb multihit:             Not affected
Vulnerability L1tf:                      Not affected
Vulnerability Mds:                       Not affected
Vulnerability Meltdown:                  Not affected
Vulnerability Mmio stale data:           Not affected
Vulnerability Old microcode:             Not affected
Vulnerability Reg file data sampling:    Not affected
Vulnerability Retbleed:                  Not affected
Vulnerability Spec rstack overflow:      Not affected
Vulnerability Spec store bypass:         Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1:                Mitigation; __user pointer sanitization
Vulnerability Spectre v2:                Mitigation; CSV2, BHB
Vulnerability Srbds:                     Not affected
Vulnerability Tsa:                       Not affected
Vulnerability Tsx async abort:           Not affected
Vulnerability Vmscape:                   Not affected

==============================
Versions of relevant libraries
==============================
[pip3] flashinfer-python==0.6.7
[pip3] mypy==1.19.1
[pip3] mypy_extensions==1.1.0
[pip3] numpy==2.2.6
[pip3] nvidia-cublas==13.1.0.3
[pip3] nvidia-cublas-cu12==12.8.4.1
[pip3] nvidia-cuda-cupti==13.0.85
[pip3] nvidia-cuda-cupti-cu12==12.8.90
[pip3] nvidia-cuda-nvrtc==13.0.88
[pip3] nvidia-cuda-nvrtc-cu12==12.8.93
[pip3] nvidia-cuda-runtime==13.0.96
[pip3] nvidia-cuda-runtime-cu12==12.8.90
[pip3] nvidia-cudnn-cu12==9.17.1.4
[pip3] nvidia-cudnn-cu13==9.19.0.56
[pip3] nvidia-cudnn-frontend==1.18.0
[pip3] nvidia-cufft==12.0.0.61
[pip3] nvidia-cufft-cu12==11.3.3.83
[pip3] nvidia-cufile==1.15.1.6
[pip3] nvidia-cufile-cu12==1.13.1.3
[pip3] nvidia-curand==10.4.0.35
[pip3] nvidia-curand-cu12==10.3.9.90
[pip3] nvidia-cusolver==12.0.4.66
[pip3] nvidia-cusolver-cu12==11.7.3.90
[pip3] nvidia-cusparse==12.6.3.3
[pip3] nvidia-cusparse-cu12==12.5.8.93
[pip3] nvidia-cusparselt-cu12==0.7.1
[pip3] nvidia-cusparselt-cu13==0.8.0
[pip3] nvidia-cutlass-dsl==4.4.2
[pip3] nvidia-cutlass-dsl-libs-base==4.4.2
[pip3] nvidia-ml-py==13.595.45
[pip3] nvidia-nccl-cu12==2.28.9
[pip3] nvidia-nccl-cu13==2.28.9
[pip3] nvidia-nvjitlink==13.0.88
[pip3] nvidia-nvjitlink-cu12==12.8.93
[pip3] nvidia-nvshmem-cu12==3.4.5
[pip3] nvidia-nvshmem-cu13==3.4.5
[pip3] nvidia-nvtx==13.0.85
[pip3] nvidia-nvtx-cu12==12.8.90
[pip3] pyzmq==27.1.0
[pip3] sentence-transformers==5.4.1
[pip3] torch==2.11.0
[pip3] torch_c_dlpack_ext==0.1.5
[pip3] torchaudio==2.11.0
[pip3] torchvision==0.26.0
[pip3] transformers==4.57.6
[pip3] triton==3.6.0+git9844da95
[conda] flashinfer-python                           0.6.7                              pypi_0              pypi
[conda] numpy                                       2.2.6                              pypi_0              pypi
[conda] nvidia-cublas                               13.1.0.3                           pypi_0              pypi
[conda] nvidia-cublas-cu12                          12.8.4.1                           pypi_0              pypi
[conda] nvidia-cuda-cupti                           13.0.85                            pypi_0              pypi
[conda] nvidia-cuda-cupti-cu12                      12.8.90                            pypi_0              pypi
[conda] nvidia-cuda-nvrtc                           13.0.88                            pypi_0              pypi
[conda] nvidia-cuda-nvrtc-cu12                      12.8.93                            pypi_0              pypi
[conda] nvidia-cuda-runtime                         13.0.96                            pypi_0              pypi
[conda] nvidia-cuda-runtime-cu12                    12.8.90                            pypi_0              pypi
[conda] nvidia-cudnn-cu12                           9.17.1.4                           pypi_0              pypi
[conda] nvidia-cudnn-cu13                           9.19.0.56                          pypi_0              pypi
[conda] nvidia-cudnn-frontend                       1.18.0                             pypi_0              pypi
[conda] nvidia-cufft                                12.0.0.61                          pypi_0              pypi
[conda] nvidia-cufft-cu12                           11.3.3.83                          pypi_0              pypi
[conda] nvidia-cufile                               1.15.1.6                           pypi_0              pypi
[conda] nvidia-cufile-cu12                          1.13.1.3                           pypi_0              pypi
[conda] nvidia-curand                               10.4.0.35                          pypi_0              pypi
[conda] nvidia-curand-cu12                          10.3.9.90                          pypi_0              pypi
[conda] nvidia-cusolver                             12.0.4.66                          pypi_0              pypi
[conda] nvidia-cusolver-cu12                        11.7.3.90                          pypi_0              pypi
[conda] nvidia-cusparse                             12.6.3.3                           pypi_0              pypi
[conda] nvidia-cusparse-cu12                        12.5.8.93                          pypi_0              pypi
[conda] nvidia-cusparselt-cu12                      0.7.1                              pypi_0              pypi
[conda] nvidia-cusparselt-cu13                      0.8.0                              pypi_0              pypi
[conda] nvidia-cutlass-dsl                          4.4.2                              pypi_0              pypi
[conda] nvidia-cutlass-dsl-libs-base                4.4.2                              pypi_0              pypi
[conda] nvidia-ml-py                                13.595.45                          pypi_0              pypi
[conda] nvidia-nccl-cu12                            2.28.9                             pypi_0              pypi
[conda] nvidia-nccl-cu13                            2.28.9                             pypi_0              pypi
[conda] nvidia-nvjitlink                            13.0.88                            pypi_0              pypi
[conda] nvidia-nvjitlink-cu12                       12.8.93                            pypi_0              pypi
[conda] nvidia-nvshmem-cu12                         3.4.5                              pypi_0              pypi
[conda] nvidia-nvshmem-cu13                         3.4.5                              pypi_0              pypi
[conda] nvidia-nvtx                                 13.0.85                            pypi_0              pypi
[conda] nvidia-nvtx-cu12                            12.8.90                            pypi_0              pypi
[conda] pyzmq                                       27.1.0                             pypi_0              pypi
[conda] sentence-transformers                       5.4.1                              pypi_0              pypi
[conda] torch                                       2.11.0                             pypi_0              pypi
[conda] torch-c-dlpack-ext                          0.1.5                              pypi_0              pypi
[conda] torchaudio                                  2.11.0                             pypi_0              pypi
[conda] torchvision                                 0.26.0                             pypi_0              pypi
[conda] transformers                                4.57.6                             pypi_0              pypi
[conda] triton                                      3.6.0+git9844da95                  pypi_0              pypi

==============================
         vLLM Info
==============================
ROCM Version                 : Could not collect
vLLM Version                 : 0.19.1rc1.dev379+g251c18d1f (git sha: 251c18d1f)
vLLM Build Flags:
  CUDA Archs: Not Set; ROCm: Disabled; XPU: Disabled
GPU Topology:
        GPU0    CPU Affinity    NUMA Affinity   GPU NUMA ID
GPU0     X      0-19    0               N/A

Legend:

  X    = Self
  SYS  = Connection traversing PCIe as well as the SMP interconnect between NUMA nodes (e.g., QPI/UPI)
  NODE = Connection traversing PCIe as well as the interconnect between PCIe Host Bridges within a NUMA node
  PHB  = Connection traversing PCIe as well as a PCIe Host Bridge (typically the CPU)
  PXB  = Connection traversing multiple PCIe bridges (without traversing the PCIe Host Bridge)
  PIX  = Connection traversing at most a single PCIe bridge
  NV#  = Connection traversing a bonded set of # NVLinks

==============================
     Environment Variables
==============================
LD_LIBRARY_PATH=/usr/local/cuda-13.2/lib64:/usr/local/cuda-13.1/lib64:/usr/local/cuda-13.2/lib64:/usr/local/cuda-13.1/lib64:
CUDA_HOME=/usr/local/cuda-13.1
CUDA_HOME=/usr/local/cuda-13.1
PYTORCH_NVML_BASED_CUDA_CHECK=1
TORCHINDUCTOR_COMPILE_THREADS=1
TORCHINDUCTOR_CACHE_DIR=/tmp/torchinductor_ohsono

---

import httpx

system = (
    "You are a precise reasoning assistant. Using the provided context, "
    "answer the sub-question in 2-4 sentences. Be specific and factual."
)
user = (
    "Original question (for context only): For what organization does a "
    "commentator of Press TV serve as associate director?\n\n"
    "Context: Press TV is an Iranian state media outlet. Some commentators "
    "have ties to think tanks like the Henry Jackson Society.\n\n"
    "Sub-question to answer: What is the Henry Jackson Society?"
)

payload = {
    "model": "gpt-oss-120b",
    "messages": [
        {"role": "system", "content": system},
        {"role": "user", "content": user},
    ],
    "max_tokens": 2000,
    "temperature": 0.3,
    "chat_template_kwargs": {"reasoning_effort": "medium"},
}
r = httpx.post("http://VLLM_HOST:8000/v1/chat/completions", json=payload, timeout=180)
d = r.json()
# OBSERVED: content='', reasoning_len=0, completion_tokens=235, finish_reason='stop'
# EXPECTED: content has the model's answer text

---

if not content and role in ("reasoner", "synthesizer"):
    return "[Model produced no visible output — possible vLLM harmony bug for gpt-oss-120b]", tokens
RAW_BUFFERClick to expand / collapse

Your current environment

<details> <summary>The output of <code>python collect_env.py</code></summary>
==============================
        System Info
==============================
OS                           : Ubuntu 24.04.4 LTS (aarch64)
GCC version                  : (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0
Clang version                : Could not collect
CMake version                : version 3.28.3
Libc version                 : glibc-2.39

==============================
       PyTorch Info
==============================
PyTorch version              : 2.11.0+cu130
Is debug build               : False
CUDA used to build PyTorch   : 13.0
ROCM used to build PyTorch   : N/A
XPU used to build PyTorch    : N/A

==============================
      Python Environment
==============================
Python version               : 3.13.11 | packaged by Anaconda, Inc. | (main, Dec 10 2025, 21:29:12) [GCC 14.3.0] (64-bit runtime)
Python platform              : Linux-6.17.0-1014-nvidia-aarch64-with-glibc2.39

==============================
       CUDA / GPU Info
==============================
Is CUDA available            : True
CUDA runtime version         : 13.2.78
CUDA_MODULE_LOADING set to   :
GPU models and configuration : GPU 0: NVIDIA GB10
Nvidia driver version        : 580.142
cuDNN version                : Could not collect
HIP runtime version          : N/A
MIOpen runtime version       : N/A
Is XNNPACK available         : True

==============================
          CPU Info
==============================
Architecture:                            aarch64
CPU op-mode(s):                          64-bit
Byte Order:                              Little Endian
CPU(s):                                  20
On-line CPU(s) list:                     0-19
Vendor ID:                               ARM
Model name:                              Cortex-X925
Model:                                   1
Thread(s) per core:                      1
Core(s) per socket:                      10
Socket(s):                               1
Stepping:                                r0p1
Frequency boost:                         disabled
CPU(s) scaling MHz:                      100%
CPU max MHz:                             3900.0000
CPU min MHz:                             1378.0000
BogoMIPS:                                2000.00
Flags:                                   fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma lrcpc dcpop sha3 sm3 sm4 asimddp sha512 sve asimdfhm dit uscat ilrcpc flagm sb paca pacg dcpodp sve2 sveaes svepmull svebitperm svesha3 svesm4 flagm2 frint svei8mm svebf16 i8mm bf16 dgh bti ecv afp wfxt
Model name:                              Cortex-A725
Model:                                   1
Thread(s) per core:                      1
Core(s) per socket:                      10
Socket(s):                               1
Stepping:                                r0p1
CPU(s) scaling MHz:                      100%
CPU max MHz:                             2808.0000
CPU min MHz:                             338.0000
BogoMIPS:                                2000.00
Flags:                                   fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma lrcpc dcpop sha3 sm3 sm4 asimddp sha512 sve asimdfhm dit uscat ilrcpc flagm sb paca pacg dcpodp sve2 sveaes svepmull svebitperm svesha3 svesm4 flagm2 frint svei8mm svebf16 i8mm bf16 dgh bti ecv afp wfxt
L1d cache:                               1.3 MiB (20 instances)
L1i cache:                               1.3 MiB (20 instances)
L2 cache:                                25 MiB (20 instances)
L3 cache:                                24 MiB (2 instances)
NUMA node(s):                            1
NUMA node0 CPU(s):                       0-19
Vulnerability Gather data sampling:      Not affected
Vulnerability Ghostwrite:                Not affected
Vulnerability Indirect target selection: Not affected
Vulnerability Itlb multihit:             Not affected
Vulnerability L1tf:                      Not affected
Vulnerability Mds:                       Not affected
Vulnerability Meltdown:                  Not affected
Vulnerability Mmio stale data:           Not affected
Vulnerability Old microcode:             Not affected
Vulnerability Reg file data sampling:    Not affected
Vulnerability Retbleed:                  Not affected
Vulnerability Spec rstack overflow:      Not affected
Vulnerability Spec store bypass:         Mitigation; Speculative Store Bypass disabled via prctl
Vulnerability Spectre v1:                Mitigation; __user pointer sanitization
Vulnerability Spectre v2:                Mitigation; CSV2, BHB
Vulnerability Srbds:                     Not affected
Vulnerability Tsa:                       Not affected
Vulnerability Tsx async abort:           Not affected
Vulnerability Vmscape:                   Not affected

==============================
Versions of relevant libraries
==============================
[pip3] flashinfer-python==0.6.7
[pip3] mypy==1.19.1
[pip3] mypy_extensions==1.1.0
[pip3] numpy==2.2.6
[pip3] nvidia-cublas==13.1.0.3
[pip3] nvidia-cublas-cu12==12.8.4.1
[pip3] nvidia-cuda-cupti==13.0.85
[pip3] nvidia-cuda-cupti-cu12==12.8.90
[pip3] nvidia-cuda-nvrtc==13.0.88
[pip3] nvidia-cuda-nvrtc-cu12==12.8.93
[pip3] nvidia-cuda-runtime==13.0.96
[pip3] nvidia-cuda-runtime-cu12==12.8.90
[pip3] nvidia-cudnn-cu12==9.17.1.4
[pip3] nvidia-cudnn-cu13==9.19.0.56
[pip3] nvidia-cudnn-frontend==1.18.0
[pip3] nvidia-cufft==12.0.0.61
[pip3] nvidia-cufft-cu12==11.3.3.83
[pip3] nvidia-cufile==1.15.1.6
[pip3] nvidia-cufile-cu12==1.13.1.3
[pip3] nvidia-curand==10.4.0.35
[pip3] nvidia-curand-cu12==10.3.9.90
[pip3] nvidia-cusolver==12.0.4.66
[pip3] nvidia-cusolver-cu12==11.7.3.90
[pip3] nvidia-cusparse==12.6.3.3
[pip3] nvidia-cusparse-cu12==12.5.8.93
[pip3] nvidia-cusparselt-cu12==0.7.1
[pip3] nvidia-cusparselt-cu13==0.8.0
[pip3] nvidia-cutlass-dsl==4.4.2
[pip3] nvidia-cutlass-dsl-libs-base==4.4.2
[pip3] nvidia-ml-py==13.595.45
[pip3] nvidia-nccl-cu12==2.28.9
[pip3] nvidia-nccl-cu13==2.28.9
[pip3] nvidia-nvjitlink==13.0.88
[pip3] nvidia-nvjitlink-cu12==12.8.93
[pip3] nvidia-nvshmem-cu12==3.4.5
[pip3] nvidia-nvshmem-cu13==3.4.5
[pip3] nvidia-nvtx==13.0.85
[pip3] nvidia-nvtx-cu12==12.8.90
[pip3] pyzmq==27.1.0
[pip3] sentence-transformers==5.4.1
[pip3] torch==2.11.0
[pip3] torch_c_dlpack_ext==0.1.5
[pip3] torchaudio==2.11.0
[pip3] torchvision==0.26.0
[pip3] transformers==4.57.6
[pip3] triton==3.6.0+git9844da95
[conda] flashinfer-python                           0.6.7                              pypi_0              pypi
[conda] numpy                                       2.2.6                              pypi_0              pypi
[conda] nvidia-cublas                               13.1.0.3                           pypi_0              pypi
[conda] nvidia-cublas-cu12                          12.8.4.1                           pypi_0              pypi
[conda] nvidia-cuda-cupti                           13.0.85                            pypi_0              pypi
[conda] nvidia-cuda-cupti-cu12                      12.8.90                            pypi_0              pypi
[conda] nvidia-cuda-nvrtc                           13.0.88                            pypi_0              pypi
[conda] nvidia-cuda-nvrtc-cu12                      12.8.93                            pypi_0              pypi
[conda] nvidia-cuda-runtime                         13.0.96                            pypi_0              pypi
[conda] nvidia-cuda-runtime-cu12                    12.8.90                            pypi_0              pypi
[conda] nvidia-cudnn-cu12                           9.17.1.4                           pypi_0              pypi
[conda] nvidia-cudnn-cu13                           9.19.0.56                          pypi_0              pypi
[conda] nvidia-cudnn-frontend                       1.18.0                             pypi_0              pypi
[conda] nvidia-cufft                                12.0.0.61                          pypi_0              pypi
[conda] nvidia-cufft-cu12                           11.3.3.83                          pypi_0              pypi
[conda] nvidia-cufile                               1.15.1.6                           pypi_0              pypi
[conda] nvidia-cufile-cu12                          1.13.1.3                           pypi_0              pypi
[conda] nvidia-curand                               10.4.0.35                          pypi_0              pypi
[conda] nvidia-curand-cu12                          10.3.9.90                          pypi_0              pypi
[conda] nvidia-cusolver                             12.0.4.66                          pypi_0              pypi
[conda] nvidia-cusolver-cu12                        11.7.3.90                          pypi_0              pypi
[conda] nvidia-cusparse                             12.6.3.3                           pypi_0              pypi
[conda] nvidia-cusparse-cu12                        12.5.8.93                          pypi_0              pypi
[conda] nvidia-cusparselt-cu12                      0.7.1                              pypi_0              pypi
[conda] nvidia-cusparselt-cu13                      0.8.0                              pypi_0              pypi
[conda] nvidia-cutlass-dsl                          4.4.2                              pypi_0              pypi
[conda] nvidia-cutlass-dsl-libs-base                4.4.2                              pypi_0              pypi
[conda] nvidia-ml-py                                13.595.45                          pypi_0              pypi
[conda] nvidia-nccl-cu12                            2.28.9                             pypi_0              pypi
[conda] nvidia-nccl-cu13                            2.28.9                             pypi_0              pypi
[conda] nvidia-nvjitlink                            13.0.88                            pypi_0              pypi
[conda] nvidia-nvjitlink-cu12                       12.8.93                            pypi_0              pypi
[conda] nvidia-nvshmem-cu12                         3.4.5                              pypi_0              pypi
[conda] nvidia-nvshmem-cu13                         3.4.5                              pypi_0              pypi
[conda] nvidia-nvtx                                 13.0.85                            pypi_0              pypi
[conda] nvidia-nvtx-cu12                            12.8.90                            pypi_0              pypi
[conda] pyzmq                                       27.1.0                             pypi_0              pypi
[conda] sentence-transformers                       5.4.1                              pypi_0              pypi
[conda] torch                                       2.11.0                             pypi_0              pypi
[conda] torch-c-dlpack-ext                          0.1.5                              pypi_0              pypi
[conda] torchaudio                                  2.11.0                             pypi_0              pypi
[conda] torchvision                                 0.26.0                             pypi_0              pypi
[conda] transformers                                4.57.6                             pypi_0              pypi
[conda] triton                                      3.6.0+git9844da95                  pypi_0              pypi

==============================
         vLLM Info
==============================
ROCM Version                 : Could not collect
vLLM Version                 : 0.19.1rc1.dev379+g251c18d1f (git sha: 251c18d1f)
vLLM Build Flags:
  CUDA Archs: Not Set; ROCm: Disabled; XPU: Disabled
GPU Topology:
        GPU0    CPU Affinity    NUMA Affinity   GPU NUMA ID
GPU0     X      0-19    0               N/A

Legend:

  X    = Self
  SYS  = Connection traversing PCIe as well as the SMP interconnect between NUMA nodes (e.g., QPI/UPI)
  NODE = Connection traversing PCIe as well as the interconnect between PCIe Host Bridges within a NUMA node
  PHB  = Connection traversing PCIe as well as a PCIe Host Bridge (typically the CPU)
  PXB  = Connection traversing multiple PCIe bridges (without traversing the PCIe Host Bridge)
  PIX  = Connection traversing at most a single PCIe bridge
  NV#  = Connection traversing a bonded set of # NVLinks

==============================
     Environment Variables
==============================
LD_LIBRARY_PATH=/usr/local/cuda-13.2/lib64:/usr/local/cuda-13.1/lib64:/usr/local/cuda-13.2/lib64:/usr/local/cuda-13.1/lib64:
CUDA_HOME=/usr/local/cuda-13.1
CUDA_HOME=/usr/local/cuda-13.1
PYTORCH_NVML_BASED_CUDA_CHECK=1
TORCHINDUCTOR_COMPILE_THREADS=1
TORCHINDUCTOR_CACHE_DIR=/tmp/torchinductor_ohsono
</details>

🐛 Describe the bug

Summary

Serving openai/gpt-oss-120b via vLLM, /v1/chat/completions returns content=null and reasoning=null while usage.completion_tokens > 0 and finish_reason="stop". The model's final-channel output (the actual answer) is dropped on the way out of vLLM.

This breaks any application that expects the OpenAI-compatible response contract, because the response carries no visible text yet bills for the tokens it generated.

Environment

  • vLLM: (please fill in vllm.__version__)
  • GPU: NVIDIA Blackwell SM 12.1 (DGX Spark)
  • CUDA: 13.x
  • Model: openai/gpt-oss-120b (Mixture-of-Experts, harmony output format)
  • Endpoint: /v1/chat/completions
  • Date observed: 2026-05-06

Reproduction

import httpx

system = (
    "You are a precise reasoning assistant. Using the provided context, "
    "answer the sub-question in 2-4 sentences. Be specific and factual."
)
user = (
    "Original question (for context only): For what organization does a "
    "commentator of Press TV serve as associate director?\n\n"
    "Context: Press TV is an Iranian state media outlet. Some commentators "
    "have ties to think tanks like the Henry Jackson Society.\n\n"
    "Sub-question to answer: What is the Henry Jackson Society?"
)

payload = {
    "model": "gpt-oss-120b",
    "messages": [
        {"role": "system", "content": system},
        {"role": "user", "content": user},
    ],
    "max_tokens": 2000,
    "temperature": 0.3,
    "chat_template_kwargs": {"reasoning_effort": "medium"},
}
r = httpx.post("http://VLLM_HOST:8000/v1/chat/completions", json=payload, timeout=180)
d = r.json()
# OBSERVED: content='', reasoning_len=0, completion_tokens=235, finish_reason='stop'
# EXPECTED: content has the model's answer text

Observations across reasoning_effort

reasoning_effortcontentreasoning_lencompletion_tokensfinish_reason
(omitted)empty0553stop
lowempty02000length
mediumempty0235stop
highempty01019stop

For shorter / less complex prompts, medium sometimes surfaces content — but it's not deterministic. reasoning is always null in our reproductions.

Suspected cause

gpt-oss uses a multi-channel "harmony" output format (analysis / commentary / final). vLLM appears to consume tokens for the analysis or commentary channels but not emit the final-channel tokens into either message.content or message.reasoning. The tokens are billed (visible in usage.completion_tokens) but never returned to the caller.

Workaround applied downstream

We added a placeholder fallback in our application: when _chat() for the reasoner / synthesizer role returns empty content with completion_tokens > 0 even after retry, we return a placeholder string so the pipeline can continue:

if not content and role in ("reasoner", "synthesizer"):
    return "[Model produced no visible output — possible vLLM harmony bug for gpt-oss-120b]", tokens

This is a degradation, not a fix — the pipeline still produces no real answer for the affected sub-questions.

Suggested fix in vLLM

When serving gpt-oss models, route the final-channel harmony tokens into message.content (and analysis-channel tokens into message.reasoning if the client requested reasoning surfaces) before returning the chat completion.

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