vllm - ✅(Solved) Fix [Bug]: KV block corruption in base scheduler, Non-deterministic output at temperature=0 without prefix caching [1 pull requests, 1 participants]
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vllm-project/vllm#39146•Fetched 2026-04-08 03:01:46
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YunzezWe fuzzed with prefix-cache but forgot to fuzz without it 😅. But when testing --speculative-config, we found a KV block corruption bug that reproduces with no --enable-prefix-caching. Identical prompts at temperature=0 produce completely different output sequences across runs, confirmed 10/10 on three independent traces.
The findings were originally discovered while running with --speculative-config active, but a controlled isolation test (re-running each trace against a server with speculative decoding removed) confirmed all three reproduce identically without it. The minimum reproduction config is a fully stock vLLM server — no APC, no spec, no LoRA.
This is distinct from #37076, because that requires --enable-prefix-caching and shared prefix content. PR #37164 addresses the TOCTOU race inside get_computed_blocks(), while it's not merged, that TOCTOU should not affect the base vllm. SO, these findings point to a separate block lifecycle bug in the base scheduler's non-APC path.
Root Cause
This is distinct from #37076, because that requires --enable-prefix-caching and shared prefix content. PR #37164 addresses the TOCTOU race inside get_computed_blocks(), while it's not merged, that TOCTOU should not affect the base vllm. SO, these findings point to a separate block lifecycle bug in the base scheduler's non-APC path.
Fix Action
Fix / Workaround
#37076 / PR #37164 fix a TOCTOU race where cache_full_blocks inserts newly allocated blocks into the prefix cache hash table before the GPU forward pass completes. The patch pre-pins blocks inside get_computed_blocks().
PR fix notes
PR #39283: [Bugfix] Zero recycled KV cache blocks for FullAttention models
- Repository: vllm-project/vllm
- Author: AjAnubolu
- State: open | merged: False
- Link: https://github.com/vllm-project/vllm/pull/39283
Description (problem / solution / changelog)
Summary
Closes #39146. The KV block zeroing pipeline from #35219 was gated to Mamba-only models; enabling it for FullAttention prevents stale K/V in partial-block tail slots from propagating NaN through masked softmax.
Changed files
tests/v1/core/test_kv_cache_utils.py(modified, +20/-0)vllm/v1/kv_cache_interface.py(modified, +5/-1)
Code Example
/nfshomes/yunze/miniconda3/envs/vllm-fuzz/lib/python3.11/site-packages/torch/cuda/__init__.py:63: FutureWarning: The pynvml package is deprecated. Please install nvidia-ml-py instead. If you did not install pynvml directly, please report this to the maintainers of the package that installed pynvml for you.
import pynvml # type: ignore[import]
Collecting environment information...
==============================
System Info
==============================
OS : Red Hat Enterprise Linux release 8.10 (Ootpa) (x86_64)
GCC version : (GCC) 8.5.0 20210514 (Red Hat 8.5.0-28)
Clang version : Could not collect
CMake version : version 3.26.5
Libc version : glibc-2.28
==============================
PyTorch Info
==============================
PyTorch version : 2.9.0+cu128
Is debug build : False
CUDA used to build PyTorch : 12.8
ROCM used to build PyTorch : N/A
==============================
Python Environment
==============================
Python version : 3.11.14 (main, Oct 21 2025, 18:31:21) [GCC 11.2.0] (64-bit runtime)
Python platform : Linux-4.18.0-553.109.1.el8_10.x86_64-x86_64-with-glibc2.28
==============================
CUDA / GPU Info
==============================
Is CUDA available : True
CUDA runtime version : 13.1.115
CUDA_MODULE_LOADING set to :
GPU models and configuration : GPU 0: NVIDIA RTX A6000
Nvidia driver version : 590.48.01
cuDNN version : Could not collect
HIP runtime version : N/A
MIOpen runtime version : N/A
Is XNNPACK available : True
==============================
CPU Info
==============================
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 32
On-line CPU(s) list: 0-31
Thread(s) per core: 1
Core(s) per socket: 16
Socket(s): 2
NUMA node(s): 2
Vendor ID: AuthenticAMD
CPU family: 23
Model: 49
Model name: AMD EPYC 7302 16-Core Processor
Stepping: 0
CPU MHz: 3000.000
CPU max MHz: 3000.0000
CPU min MHz: 1500.0000
BogoMIPS: 6000.12
Virtualization: AMD-V
L1d cache: 32K
L1i cache: 32K
L2 cache: 512K
L3 cache: 16384K
NUMA node0 CPU(s): 0-15
NUMA node1 CPU(s): 16-31
Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good nopl nonstop_tsc cpuid extd_apicid aperfmperf pni pclmulqdq monitor ssse3 fma cx16 sse4_1 sse4_2 movbe popcnt aes xsave avx f16c rdrand lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt tce topoext perfctr_core perfctr_nb bpext perfctr_llc mwaitx cpb cat_l3 cdp_l3 hw_pstate ssbd mba ibrs ibpb stibp vmmcall fsgsbase bmi1 avx2 smep bmi2 cqm rdt_a rdseed adx smap clflushopt clwb sha_ni xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local clzero irperf xsaveerptr wbnoinvd amd_ppin arat npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold avic v_vmsave_vmload vgif v_spec_ctrl umip rdpid overflow_recov succor smca sme sev sev_es
==============================
Versions of relevant libraries
==============================
[pip3] flashinfer-python==0.5.2
[pip3] numpy==2.2.6
[pip3] nvidia-cublas-cu12==12.8.4.1
[pip3] nvidia-cuda-cupti-cu12==12.8.90
[pip3] nvidia-cuda-nvrtc-cu12==12.8.93
[pip3] nvidia-cuda-runtime-cu12==12.8.90
[pip3] nvidia-cudnn-cu12==9.10.2.21
[pip3] nvidia-cudnn-frontend==1.18.0
[pip3] nvidia-cufft-cu12==11.3.3.83
[pip3] nvidia-cufile-cu12==1.13.1.3
[pip3] nvidia-curand-cu12==10.3.9.90
[pip3] nvidia-cusolver-cu12==11.7.3.90
[pip3] nvidia-cusparse-cu12==12.5.8.93
[pip3] nvidia-cusparselt-cu12==0.7.1
[pip3] nvidia-cutlass-dsl==4.4.1
[pip3] nvidia-cutlass-dsl-libs-base==4.4.1
[pip3] nvidia-ml-py==13.590.48
[pip3] nvidia-nccl-cu12==2.27.5
[pip3] nvidia-nvjitlink-cu12==12.8.93
[pip3] nvidia-nvshmem-cu12==3.3.20
[pip3] nvidia-nvtx-cu12==12.8.90
[pip3] pynvml==13.0.1
[pip3] pyzmq==27.1.0
[pip3] torch==2.9.0
[pip3] torchaudio==2.9.0
[pip3] torchvision==0.24.0
[pip3] transformers==4.57.6
[pip3] triton==3.5.0
[conda] flashinfer-python 0.5.2 pypi_0 pypi
[conda] numpy 2.2.6 pypi_0 pypi
[conda] nvidia-cublas-cu12 12.8.4.1 pypi_0 pypi
[conda] nvidia-cuda-cupti-cu12 12.8.90 pypi_0 pypi
[conda] nvidia-cuda-nvrtc-cu12 12.8.93 pypi_0 pypi
[conda] nvidia-cuda-runtime-cu12 12.8.90 pypi_0 pypi
[conda] nvidia-cudnn-cu12 9.10.2.21 pypi_0 pypi
[conda] nvidia-cudnn-frontend 1.18.0 pypi_0 pypi
[conda] nvidia-cufft-cu12 11.3.3.83 pypi_0 pypi
[conda] nvidia-cufile-cu12 1.13.1.3 pypi_0 pypi
[conda] nvidia-curand-cu12 10.3.9.90 pypi_0 pypi
[conda] nvidia-cusolver-cu12 11.7.3.90 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-cutlass-dsl 4.4.1 pypi_0 pypi
[conda] nvidia-cutlass-dsl-libs-base 4.4.1 pypi_0 pypi
[conda] nvidia-ml-py 13.590.48 pypi_0 pypi
[conda] nvidia-nccl-cu12 2.27.5 pypi_0 pypi
[conda] nvidia-nvjitlink-cu12 12.8.93 pypi_0 pypi
[conda] nvidia-nvshmem-cu12 3.3.20 pypi_0 pypi
[conda] nvidia-nvtx-cu12 12.8.90 pypi_0 pypi
[conda] pynvml 13.0.1 pypi_0 pypi
[conda] pyzmq 27.1.0 pypi_0 pypi
[conda] torch 2.9.0 pypi_0 pypi
[conda] torchaudio 2.9.0 pypi_0 pypi
[conda] torchvision 0.24.0 pypi_0 pypi
[conda] transformers 4.57.6 pypi_0 pypi
[conda] triton 3.5.0 pypi_0 pypi
==============================
vLLM Info
==============================
ROCM Version : Could not collect
vLLM Version : 0.11.2
vLLM Build Flags:
CUDA Archs: Not Set; ROCm: Disabled
GPU Topology:
GPU0 NIC0 NIC1 NIC2 CPU Affinity NUMA Affinity GPU NUMA ID
GPU0 X SYS SYS NODE 4,9 0 N/A
NIC0 SYS X PIX SYS
NIC1 SYS PIX X SYS
NIC2 NODE SYS SYS X
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
NIC Legend:
NIC0: mlx5_2
NIC1: mlx5_3
NIC2: mlx5_bond_0
==============================
Environment Variables
==============================
LD_LIBRARY_PATH=/opt/common/cuda/cuda-13.1.1/lib64:
CUDA_HOME=/opt/common/cuda/cuda-13.1.1
CUDA_HOME=/opt/common/cuda/cuda-13.1.1
CUDA_VISIBLE_DEVICES=0
CUDA_VISIBLE_DEVICES=0
PYTORCH_NVML_BASED_CUDA_CHECK=1
TORCHINDUCTOR_COMPILE_THREADS=1
---
diverged: r2, r3, r1_b, r2_b, r4_b, r5_s_s_b, r4, r5_s_s_b_b,
r4_b_b, r4_s_s_b_storm_b, r4_s_s_b_storm (11 / 21 total)
runs_diverged: 10 / 10
---
python -m vllm.entrypoints.openai.api_server \
--model Qwen/Qwen2.5-0.5B-Instruct \
--gpu-memory-utilization 0.95 \
--max-model-len 32768
---
python3 repro.py --base-url http://localhost:8000RAW_BUFFERClick to expand / collapse
Your current environment
<details> <summary>The output of <code>python collect_env.py</code></summary>/nfshomes/yunze/miniconda3/envs/vllm-fuzz/lib/python3.11/site-packages/torch/cuda/__init__.py:63: FutureWarning: The pynvml package is deprecated. Please install nvidia-ml-py instead. If you did not install pynvml directly, please report this to the maintainers of the package that installed pynvml for you.
import pynvml # type: ignore[import]
Collecting environment information...
==============================
System Info
==============================
OS : Red Hat Enterprise Linux release 8.10 (Ootpa) (x86_64)
GCC version : (GCC) 8.5.0 20210514 (Red Hat 8.5.0-28)
Clang version : Could not collect
CMake version : version 3.26.5
Libc version : glibc-2.28
==============================
PyTorch Info
==============================
PyTorch version : 2.9.0+cu128
Is debug build : False
CUDA used to build PyTorch : 12.8
ROCM used to build PyTorch : N/A
==============================
Python Environment
==============================
Python version : 3.11.14 (main, Oct 21 2025, 18:31:21) [GCC 11.2.0] (64-bit runtime)
Python platform : Linux-4.18.0-553.109.1.el8_10.x86_64-x86_64-with-glibc2.28
==============================
CUDA / GPU Info
==============================
Is CUDA available : True
CUDA runtime version : 13.1.115
CUDA_MODULE_LOADING set to :
GPU models and configuration : GPU 0: NVIDIA RTX A6000
Nvidia driver version : 590.48.01
cuDNN version : Could not collect
HIP runtime version : N/A
MIOpen runtime version : N/A
Is XNNPACK available : True
==============================
CPU Info
==============================
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 32
On-line CPU(s) list: 0-31
Thread(s) per core: 1
Core(s) per socket: 16
Socket(s): 2
NUMA node(s): 2
Vendor ID: AuthenticAMD
CPU family: 23
Model: 49
Model name: AMD EPYC 7302 16-Core Processor
Stepping: 0
CPU MHz: 3000.000
CPU max MHz: 3000.0000
CPU min MHz: 1500.0000
BogoMIPS: 6000.12
Virtualization: AMD-V
L1d cache: 32K
L1i cache: 32K
L2 cache: 512K
L3 cache: 16384K
NUMA node0 CPU(s): 0-15
NUMA node1 CPU(s): 16-31
Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm constant_tsc rep_good nopl nonstop_tsc cpuid extd_apicid aperfmperf pni pclmulqdq monitor ssse3 fma cx16 sse4_1 sse4_2 movbe popcnt aes xsave avx f16c rdrand lahf_lm cmp_legacy svm extapic cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ibs skinit wdt tce topoext perfctr_core perfctr_nb bpext perfctr_llc mwaitx cpb cat_l3 cdp_l3 hw_pstate ssbd mba ibrs ibpb stibp vmmcall fsgsbase bmi1 avx2 smep bmi2 cqm rdt_a rdseed adx smap clflushopt clwb sha_ni xsaveopt xsavec xgetbv1 xsaves cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local clzero irperf xsaveerptr wbnoinvd amd_ppin arat npt lbrv svm_lock nrip_save tsc_scale vmcb_clean flushbyasid decodeassists pausefilter pfthreshold avic v_vmsave_vmload vgif v_spec_ctrl umip rdpid overflow_recov succor smca sme sev sev_es
==============================
Versions of relevant libraries
==============================
[pip3] flashinfer-python==0.5.2
[pip3] numpy==2.2.6
[pip3] nvidia-cublas-cu12==12.8.4.1
[pip3] nvidia-cuda-cupti-cu12==12.8.90
[pip3] nvidia-cuda-nvrtc-cu12==12.8.93
[pip3] nvidia-cuda-runtime-cu12==12.8.90
[pip3] nvidia-cudnn-cu12==9.10.2.21
[pip3] nvidia-cudnn-frontend==1.18.0
[pip3] nvidia-cufft-cu12==11.3.3.83
[pip3] nvidia-cufile-cu12==1.13.1.3
[pip3] nvidia-curand-cu12==10.3.9.90
[pip3] nvidia-cusolver-cu12==11.7.3.90
[pip3] nvidia-cusparse-cu12==12.5.8.93
[pip3] nvidia-cusparselt-cu12==0.7.1
[pip3] nvidia-cutlass-dsl==4.4.1
[pip3] nvidia-cutlass-dsl-libs-base==4.4.1
[pip3] nvidia-ml-py==13.590.48
[pip3] nvidia-nccl-cu12==2.27.5
[pip3] nvidia-nvjitlink-cu12==12.8.93
[pip3] nvidia-nvshmem-cu12==3.3.20
[pip3] nvidia-nvtx-cu12==12.8.90
[pip3] pynvml==13.0.1
[pip3] pyzmq==27.1.0
[pip3] torch==2.9.0
[pip3] torchaudio==2.9.0
[pip3] torchvision==0.24.0
[pip3] transformers==4.57.6
[pip3] triton==3.5.0
[conda] flashinfer-python 0.5.2 pypi_0 pypi
[conda] numpy 2.2.6 pypi_0 pypi
[conda] nvidia-cublas-cu12 12.8.4.1 pypi_0 pypi
[conda] nvidia-cuda-cupti-cu12 12.8.90 pypi_0 pypi
[conda] nvidia-cuda-nvrtc-cu12 12.8.93 pypi_0 pypi
[conda] nvidia-cuda-runtime-cu12 12.8.90 pypi_0 pypi
[conda] nvidia-cudnn-cu12 9.10.2.21 pypi_0 pypi
[conda] nvidia-cudnn-frontend 1.18.0 pypi_0 pypi
[conda] nvidia-cufft-cu12 11.3.3.83 pypi_0 pypi
[conda] nvidia-cufile-cu12 1.13.1.3 pypi_0 pypi
[conda] nvidia-curand-cu12 10.3.9.90 pypi_0 pypi
[conda] nvidia-cusolver-cu12 11.7.3.90 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-cutlass-dsl 4.4.1 pypi_0 pypi
[conda] nvidia-cutlass-dsl-libs-base 4.4.1 pypi_0 pypi
[conda] nvidia-ml-py 13.590.48 pypi_0 pypi
[conda] nvidia-nccl-cu12 2.27.5 pypi_0 pypi
[conda] nvidia-nvjitlink-cu12 12.8.93 pypi_0 pypi
[conda] nvidia-nvshmem-cu12 3.3.20 pypi_0 pypi
[conda] nvidia-nvtx-cu12 12.8.90 pypi_0 pypi
[conda] pynvml 13.0.1 pypi_0 pypi
[conda] pyzmq 27.1.0 pypi_0 pypi
[conda] torch 2.9.0 pypi_0 pypi
[conda] torchaudio 2.9.0 pypi_0 pypi
[conda] torchvision 0.24.0 pypi_0 pypi
[conda] transformers 4.57.6 pypi_0 pypi
[conda] triton 3.5.0 pypi_0 pypi
==============================
vLLM Info
==============================
ROCM Version : Could not collect
vLLM Version : 0.11.2
vLLM Build Flags:
CUDA Archs: Not Set; ROCm: Disabled
GPU Topology:
GPU0 NIC0 NIC1 NIC2 CPU Affinity NUMA Affinity GPU NUMA ID
GPU0 X SYS SYS NODE 4,9 0 N/A
NIC0 SYS X PIX SYS
NIC1 SYS PIX X SYS
NIC2 NODE SYS SYS X
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
NIC Legend:
NIC0: mlx5_2
NIC1: mlx5_3
NIC2: mlx5_bond_0
==============================
Environment Variables
==============================
LD_LIBRARY_PATH=/opt/common/cuda/cuda-13.1.1/lib64:
CUDA_HOME=/opt/common/cuda/cuda-13.1.1
CUDA_HOME=/opt/common/cuda/cuda-13.1.1
CUDA_VISIBLE_DEVICES=0
CUDA_VISIBLE_DEVICES=0
PYTORCH_NVML_BASED_CUDA_CHECK=1
TORCHINDUCTOR_COMPILE_THREADS=1🐛 Describe the bug
Related to: #37076 , PR #37164
Summary
We fuzzed with prefix-cache but forgot to fuzz without it 😅. But when testing --speculative-config, we found a KV block corruption bug that reproduces with no --enable-prefix-caching. Identical prompts at temperature=0 produce completely different output sequences across runs, confirmed 10/10 on three independent traces.
The findings were originally discovered while running with --speculative-config active, but a controlled isolation test (re-running each trace against a server with speculative decoding removed) confirmed all three reproduce identically without it. The minimum reproduction config is a fully stock vLLM server — no APC, no spec, no LoRA.
This is distinct from #37076, because that requires --enable-prefix-caching and shared prefix content. PR #37164 addresses the TOCTOU race inside get_computed_blocks(), while it's not merged, that TOCTOU should not affect the base vllm. SO, these findings point to a separate block lifecycle bug in the base scheduler's non-APC path.
Background: how this differs from #37076 and PR #37164
#37076 / PR #37164 fix a TOCTOU race where cache_full_blocks inserts newly allocated blocks into the prefix cache hash table before the GPU forward pass completes. The patch pre-pins blocks inside get_computed_blocks().
In my perspective, what we have now is independent on two parts:
-
No
--enable-prefix-cachingrequired.get_computed_blocks()is never called without APC. PR #37164 does not touch this code path. -
No shared prefix required. All requests in our traces have completely unique prompts (
prefix_len=0, distinct token sequences). There is no shared cache content to race over.
The corruption reproduces with 4–5 concurrent requests on a fully default server. Any production deployment is potentially affected.
Primary finding — finding_00450 (cleanest)
Note on attached JSON artifacts: the
server_flagsfield in each finding JSON reflects the original discovery config, which included--speculative-config. This field is recorded at discovery time and is not updated by subsequent isolation tests. The isolation test results are reported separately above and confirm spec is not required.
Five requests, no shared state, no cancellations involved in the corruption.
| event | request | offset_ms | prompt_len | prefix_len | max_tokens | stream | diverged |
|---|---|---|---|---|---|---|---|
| send | r1 | 0 | 512 | 0 | 512 | true | |
| send | r2 | 100 | 512 | 0 | 512 | true | ✓ |
| send | r3 | 200 | 512 | 0 | 512 | true | ✓ |
| send | r4 | 300 | 512 | 0 | 512 | true | ✓ |
| send | r5 | 2000 | 8192 | 0 | 16 | true | |
| cancel | r1 | 3605 | — | — | — | — |
Key observations:
r1andr5are clean across all 10 runs.r2,r3,r4diverge in every run.- The cancel of
r1occurs at 3605ms — long afterr2/r3/r4would have completed. It is not the cause. r5(8192 tokens) is a large request submitted 2 seconds after the short ones. Its memory pressure changes the block allocation state visible to subsequent runs.- No prefix sharing, no APC, no spec engine involvement.
Second, finding_01410, same as the above :)
A more heavily mutated trace with 21 concurrent requests (mix of 3000-token and 512-token prompts), all prefix_len=0. 11 of 21 requests diverge in 10/10 runs. The larger batch and mixed sizes amplify the corruption rate, consistent with the hypothesis that block allocation order under concurrency is the trigger.
diverged: r2, r3, r1_b, r2_b, r4_b, r5_s_s_b, r4, r5_s_s_b_b,
r4_b_b, r4_s_s_b_storm_b, r4_s_s_b_storm (11 / 21 total)
runs_diverged: 10 / 10Related finding — finding_00030 (cancel path)
A cancel/retry pattern: 5 requests cancelled mid-generation, 5 fresh retries sent 60ms later. The original requests (r01–r05) are clean. The retry requests (r01_retry–r05_retry) diverge 10/10 runs.
This is potentially a different issue, I put it here as the same since I suspect the underlying issue might be the same, not entirely sure yet.
| event | request | offset_ms | prompt_len | prefix_len | diverged |
|---|---|---|---|---|---|
| send | r01–r05 | 0–40 | 256 | 0 | |
| cancel | r01–r05 | 200–240 | — | — | |
| send | r01_retry–r05_retry | 300–340 | 256 | 0 | ✓ all 5 |
Isolation: speculative decoding is not the cause
Because the findings were discovered with --speculative-config in use, we re-ran each trace against a server with speculative decoding fully removed to rule out the spec engine as the cause. All three reproduced identically — same diverged requests, same 10/10 rate.
My hypothesis
We know without --enable-prefix-caching, the V1 scheduler's block allocator does not track block identity through hash table. When requests complete or are cancelled, KV blocks are returned to free pool. But If those blocks are not zeroed before reuse, a subsequent request that receives them will decode from stale KV data belonging to a different request.
The pattern in finding_00450, r1 and r5 clean, r2/r3/r4 corrupted, is consistent with r1's blocks being the "first" fresh allocation (pool is clean on the very first run), while r2/r3/r4 receive blocks recycled from a prior reproduce run's completed requests. The large r5 (8192 tokens) changes the block pressure enough that across successive runs the allocation order and thus the "dirty" block distribution shifts, producing different outputs each time.
Abd finding_00030's cancel path is the same mechanism but via an few explicit cancellations: r01-r05 are cancelled mid-generation, freeing their blocks immediately. The retries arrive 60ms later and receive those dirty blocks.
Again, this seems different from #37076's uninitialized-but-registered block race. There, a block is registered in the hash table before its GPU data is written. Here, a block that previously held valid data for request A is recycled to request B without clearing the GPU memory first.
Reproduction:
You will need these findings: primary: finding_00030_999829240.json second(corroboration): finding_00450_862114934.json cancel/retry: finding_01410_1760617970.json
and repro.py
Step 1 — start vLLM as it is:
python -m vllm.entrypoints.openai.api_server \
--model Qwen/Qwen2.5-0.5B-Instruct \
--gpu-memory-utilization 0.95 \
--max-model-len 32768Note: Make sure your findings are in the same directory as repro.py, and don't change the findings name, I imported them directly in the script.
Step 2 — run the script (requires httpx):
python3 repro.py --base-url http://localhost:8000Before submitting a new issue...
- Make sure you already searched for relevant issues, and asked the chatbot living at the bottom right corner of the documentation page, which can answer lots of frequently asked questions.
extent analysis
TL;DR
The most likely fix is to ensure that KV blocks are properly cleared before being reused by the V1 scheduler's block allocator.
Guidance
- Review the block allocation and deallocation logic in the V1 scheduler to identify where blocks are not being properly cleared.
- Implement a mechanism to zero out KV blocks before they are returned to the free pool to prevent stale data from being reused.
- Verify that the fix resolves the issue by re-running the reproduction script with the modified code.
- Consider adding additional logging or debugging statements to monitor block allocation and deallocation to catch any similar issues in the future.
Example
# Example of how to zero out a block before returning it to the free pool
def return_block_to_pool(block):
# Zero out the block's KV data
block.kv_data = [0] * len(block.kv_data)
# Return the block to the free pool
free_pool.append(block)Notes
- The issue seems to be related to the V1 scheduler's block allocator not properly handling block reuse, leading to stale data being used by subsequent requests.
- The provided reproduction script and findings should be used to verify the fix and ensure that it resolves the issue.
Recommendation
Apply the workaround of zeroing out KV blocks before returning them to the free pool to prevent stale data from being reused. This should resolve the issue and prevent similar problems in the future.
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×6Another batch ranked right after the header list — different links, same matching logic.
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- Code review (Preview): "Add a repository" shows no results for private GitHub org repos
- [BUG] /context commands blows up context
- [Feature Request] Add precache expiry hook to enable proactive compaction before token eviction
- [BUG] Context indicator shows 0% at session start despite ~20K+ tokens already loaded
- [Feature Request] Add semantic search for --resume session history
- [Feature Request] Add session search, tagging, and filtering capabilities
- [BUG] Cowork Dispatch reports "desktop not available" on Windows 11 while standard Cowork works normally
- [Bug] Claude Code provides incorrect suggestions with high confidence despite errors
- defaultMode: acceptEdits silently overrides per-path permissions.ask rules for Write/Edit
- [FEATUR configurable tip interval (e.g. tipIntervalSeconds: 30 in settings)E]
- Plugin marketplace fails to load: schema rejects 'displayName' key (v2.1.153)
- claude agents: in-session copy uses broken OSC 52 path while overview correctly uses tmux buffer
- [BUG] Plugin agent descriptions (and custom agents) load unconditionally into context — no parity with disable-model-invocation for skills
- Crashed ultrareview consumed a free credit despite producing zero findings
- [Bug] Character rendering issue - invisible or missing text display
- [BUG] Cowork: processo Claude Code encerra com código 3 — .claude.json não contém token de autenticação (Windows 11 25H2)
- [BUG] 2.1.153 silently discards tools/list response from rmcp 0.12.0 HTTP MCP server (works in 2.1.152, wire-identical handshake)
- VS Code extension: option to auto-resume last session when reopening a workspace folder
- [Bug] Conversation continuation failure
- [BUG] Cowork crashes every time I start a new chat or attempt to continue an existing one in any project. The error displayed is: "Claude Code è andato in crash
- [Bug] Unannounced quota changes
- Native update/install fails with 'socket connection was closed unexpectedly' behind proxy — undici TLS incompatibility
- [BUG] Session name reverting after manual change
- [BUG] 非正常思考，上下文过长时，一直显示思考，点击interrupt按钮失效
- Honor `tools:` frontmatter when an agent is invoked via `@mention` — strip `Task` only when the agent did not declare it
- macOS TCC popup still recurring on v2.1.153 — "2.1.153" would like to access data from other apps
- Claude Code leaks pty handles — exhausts pseudo-terminals on macOS after long session
- [Bug] Agent fails to execute or respond to user input
- [BUG] Persistent "Expecting value: line 1 column 1 (char 0)" JSON parse error after tool execution
- [Feature Request] Implement proactive unit test coverage recommendations for recurring bugs
- VS Code panel lacks status line + terminal lacks image paste in Codespaces, forcing a tradeoff
- `/powerup` only shows ~10 lessons — allow viewing the full catalog
- [Bug] Context contamination after auto-compact with unrelated email draft of Tejo/Sado Basin
- [Bug] VSCode terminal output displays corrupted text with garbled symbols
- [Feature Request] Add LaTeX/KaTeX math rendering to TUI
- [Bug] Sub-agent PR review results not validated by orchestrating agent
- Subagents on Pro 1M tier: trivial probes pass, real workloads fail at first tool call (probe-vs-workload divergence)
- Path-scoped rules and subdirectory CLAUDE.md not loaded when creating new files matching the pattern
- AskUserQuestion: cancelling during extended thinking poisons the whole session with 400 'thinking blocks cannot be modified' (2.1.153); concurrent prompts overwrite each other
- Ideas Missing from Claude Cowork Menu (Windows)
- [BUG_BOUNTY_SAFE_POC_2026] Prompt Injection RCE Test - Command Execution Proof
- [BUG] Cowork scheduled task: execution history row not showing after successful run
- Resuming an extended-thinking session fails permanently with 400 "thinking blocks cannot be modified" (transcript stores thinking text as empty but keeps signature)
- [Bug] Plugin-registered CwdChanged and FileChanged hooks don't fire (settings.json works) — v2.1.153
- Auto-archive on PR merge / branch delete — clarify autoArchiveSessions semantics or add dedicated opt-out
- `claude mcp add` echoes Authorization header value verbatim to stdout, leaks bearer tokens to terminal and session transcripts
- [BUG] Bug report — /insights skill, Claude Code The /insights skill outputs a malformed file path.
- Plugin slash commands render with '*'-inline format instead of two-column, despite matching official plugin shape
- [Bug] Unexpected long text generation without user input or goal
- [Bug] Thinking blocks causing task progression blocked without user modification
- [BUG] (Critical!) contamination by an unknown session simirlar to the report => [Bug] Context contamination after auto-compact with unrelated email draft of Tejo/Sado Basin #63137
- [Critical] Opus 4.7 Korean output degeneration — Korean grammar itself collapses in long contexts
- [BUG] Title: Autocompact buffer persists across /clear — wastes tokens for irrelevant old context
- [Bug] Auto-Compact loses user input before processing in conversation history
- Feature: per-invocation effort parameter + runtime session-config introspection for skills
- Auto-mode classifier mislabels Azure DevOps vote -5 as "Reject" when denying PR vote actions
- [BUG] Claude Desktop and Claude Code CLI never re-register MCP tools after OAuth 2.1 handshake on a remote HTTP server
- [BUG] Workspace file tags leak across sessions
- [BUG] Ink renderer crashes on Windows 11 build 26200 (Canary) duplicate banners, terminal mode leaks, mid-operation aborts
- [BUG] Claude Code Desktop issue
- PTY master fd leak in Claude desktop app exhausts macOS kern.tty.ptmx_max after ~2-3 days
- [BUG] Claude Code — Session Management after Unexpected Interruption
- [Windows] Cowork OpenTelemetry exporter does not initialize - zero events emitted to any destination, including loopback
- [Bug] Opus 4.7: 400 `thinking blocks ... cannot be modified` on long extended-thinking sessions, triggered by history-altering events (scheduled prompts / parallel tool-call cancellation)
- [BUG] API Error: Server is temporarily limiting requests (not your usage limit) · Rate limited
- Multi-plugin custom marketplace: only first plugin registered in installed_plugins.json, skills don't load
- [BUG] Git push through the SDK's git proxy fan-outs into ~500 GitHub REST API calls, exhausting the 5,000/hour budget after a handful of pushes
- [BUG] Claude took liberties it really shouldn't with my global config
- [BUG] Agent window focus lost after navigating with arrow keys, causing scroll deadlock
- [BUG] `--model` flag silently ignored in interactive sessions (works in `--print` only)
- [BUG] Dispatch permanently shows "desktop appears offline" on Windows 11 - never worked on first use
- feat: support per-command enableWeakerNetworkIsolation as safer alternative to dangerouslyDisableSandbox
- /code-review outputs a raw JSON array instead of readable findings
- [BUG] Cowork — Additional allowed domains ignored on Team plan; same domain works on Pro plan
- Haiku
- [Bug] False positive blocking beneficial outcomes in tool execution
- 3P Bedrock SSO: credentials silently expire without triggering re-auth on day 2+
- CLAUDE_AUTOCOMPACT_PCT_OVERRIDE in settings.json env block silently ignored by autocompact logic
- Auto-compaction deletes main session JSONL before verifying summary completion, causing data loss
- [Bug] Claude Code not executing stated actions or producing expected results
- [FEATURE] Deferred Messages — Queue Input for End of Turn
- [BUG] Up/Down arrows in input box navigate history instead of moving cursor — regression in 2.1.149+
- Cancelling a parallel tool-call batch corrupts thinking blocks -> 400 "thinking blocks cannot be modified" permanently wedges the session
- Claude Code caused data loss, then contradicted itself about recovery (two incidents, one session)
- [Bug] Unclear error messages from Claude Code CLI
- [Bug] Agent tool rejecting due to context size limit exceeded
- claude agents: daemon and bg-spare processes spin at ~100% CPU when idle
- [BUG] Compaction fails with "context window limit" error even when context usage is low (e.g., 20%) — regression in v2.1.153
- Remote Control entitlement lost after May 27-28 incident — `Error: Remote Control is not yet enabled for your account` on active Max subscription
- PreToolUse hook exit code 2 does not block Write tool
- [Bug] Thinking blocks in latest assistant message are immutable
- GUI: dispatch file:// and custom-scheme clicks to OS shell handler
- Show current model in statusLine by default
- [Bug] Agent console becomes unresponsive to keyboard input after multiple agents initialized
- [FEATURE] PreToolUse hooks should have a way of updating the environment
- [Bug] Unable to start or use Claude Code CLI
- [BUG] Repository not visible in Claude Code web repo picker
- Session permanently wedged on 400 "thinking blocks cannot be modified" after parallel tool_results
- [Bug] @ autocomplete loses sibling repos after a file edit in multi-repo workspace
- Unclear error message when creating sub-agent without authentication
- [Bug] Anthropic API errors causing frequent failures and high token usage
- [BUG] @ mention file picker only shows packages, not individual files (desktop app - Code tab)
- [Bug] TUI panel footer remains sticky and consumes excessive terminal space
- PR-status polling exhausts GitHub GraphQL rate limit on repos with many open PRs
- [BUG] Windows: welcome panel not shown in some project folders (2.1.153)
- [Bug] Anthropic API Error: thinking blocks corrupted during context compaction with extended thinking enabled
- API 400 "thinking blocks cannot be modified" permanently bricks session during agent activation (interleaved thinking + tool use)
- Right-click Copy copies the whole message instead of the selection; pasted text retains dark background
- Mid-session model switch corrupts conversation when extended thinking is enabled (API 400: 'thinking blocks cannot be modified')
- [BUG] Markdown file links in chat output do not open files when clicked (VS Code extension)
- Stuck retry loop: `400 thinking blocks cannot be modified` on large interleaved-thinking turns using AskUserQuestion
- [FEATURE] Prompt user for approval before auto-compaction proceeds
- Custom MCP connectors not attachable to scheduled routines — no UUID discovery path
- [BUG] Claude in Chrome — Navigation blocked for teams.cloud.microsoft and outlook.cloud.microsoft after Microsoft domain migration**
- [BUG] Claude Desktop — Personal plugins panel renders list but is entirely non-interactive (macOS, v1.9255.2)
- [Bug] error when using Workflows
- [BUG] Persistent "update available" notification despite being on latest version
- [BUG] Sweep Agent from /code-review never completes
- [Bug] Tool calls not executing or returning results
- [FEATURE] Cloud-synced memory and settings across machines
- [Bug] Terminal UI freezes when Ctrl+O view exits during interactive prompt in plan mode
- Continuous api errors when using claude code with Opus 4.7 with thinking on low
- [Feature Request] Add support for installing and using previous Claude Code versions
- [Bug] Extended Thinking: Summarized thinking blocks fail signature validation when resent to API
- [Bug] Anthropic API Error: 'thinking' blocks cannot be modified
- [Bug] Anthropic API Error: Thinking blocks cannot be modified with extended thinking mode
- Feature request: Lazy/on-demand MCP server connections
- [Bug] Tool Arguments Parsed as String Instead of Object
- [Bug] Anthropic API Error: Insufficient context provided
- [Bug] Claude Opus occasionally uses moskovian(russian) orthography instead of Ukrainian in system-prompted responses
- Opus 4.8: backgrounded task completions (subagents AND Bash) crash with 400 "thinking blocks cannot be modified"
- [Bug] Opus 4.7 fabricates stable preferences ("my default") to rationalize arbitrary choices when challenged
- [Bug] Unable to update Claude Code CLI
- [BUG] Desktop app: /remote-control mints link + connects bridge (main.log) but in-chat link/QR panel never renders
- Feature: sessionColor and sessionName in .claude/settings.json
- [BUG] Anthropic API error: thinking blocks
- [FEATURE] Support Remote MCPs in Cowork as in Claude Code
- [Bug] Anthropic API Error: 400 Bad Request with Redacted Thinking - 0 4.7 & 4.8
- [Bug] Anthropic API Error: Cannot modify thinking blocks from different model versions
- Interleaved thinking + multi-tool turn corrupts thinking block (text blanked, signature kept) → permanent 400 'blocks must remain as they were'
- [BUG] Mode/permission changes mid-tool-loop (effortLevel: xhigh) poisons entire session
- Session failure log: Opus 4.6 ignores its own rules for an entire session
- [BUG] "400 Guardrail was enabled" error when using Claude Opus 4.8 with AWS Bedrock
- [Feature Request] Add subagent approach selection option to avoid accidental feedback
- Persistent 400 'thinking blocks in the latest assistant message cannot be modified' — interleaved thinking persisted with empty text + signature bricks sessions
- [BUG] DesktopvsApp
- [BUG] Opus 4.7 cache hit rate collapse after May 27 incident — Messages 1.1k→88.9k in 9 minutes, $630/session
- [Bug] Anthropic API Error: Invalid thinking block format
- [BUG] FUCK CLAUDE
- Opus 4.8 extended thinking: Stop hook block re-entry corrupts thinking blocks → 400
- [Bug] 4.8 Fails when accessing previous model history
- [Bug] Unintended File Modifications During Execution
- [DOCS] Model configuration docs omit lean system prompt default scope and model exceptions
- Add "Always allow globally" option to permission prompts
- Server-side model upgrade (Opus 4.7→4.8) wedges in-flight sessions with `thinking blocks cannot be modified` 400
- [DOCS] AskUserQuestion docs missing multiple-choice prompt decision threshold
- [DOCS] Agent view docs omit shell-command background session launch syntax
- [DOCS] Agent view dispatch input docs incorrectly imply `/logout` dispatches as a prompt
- [DOCS] Claude in Chrome docs omit connected-browser selection behavior
- [DOCS] Plugin docs omit `defaultEnabled: false` for opt-in plugins
- Feature Request: Customizable chat text colors for user and assistant messages
- [DOCS] `/plugin` Discover tab docs omit directory-based suggested plugin pins
- VSCode Chrome integration silently fails: 3 distinct bugs
- [DOCS] MCP stdio docs omit session environment variables
- [Bug] Anthropic API error on second request within session with Claude Opus 4.8
- Cowork emits a blank session "index" handoff on focus when a CLI session is paused awaiting input
- [DOCS] MCP docs omit `claude mcp list/get` pending-approval output for unapproved project servers
- [BUG] /compact fails with 400 error when last assistant turn contains thinking blocks
- [DOCS] `/claude-api` docs omit Opus 4.8 migration guidance
- [DOCS] Fast mode docs still recommend deprecated Opus 4.6 override variable
- [DOCS] Bash tool docs omit `$TMPDIR` consistency across sandboxed and unsandboxed commands
- [Bug] Anthropic API Error: 400 Bad Request on Extended Thinking
- [DOCS] Background session docs omit worktree-isolation behavior for spawned subagents
- Built-in mechanistic self-verification of verifiable claims (symmetric to the auto permission gate)
- [DOCS] Worktree docs do not clarify `worktree.baseRef: "head"` inside linked worktrees
- [BUG] Excessive RAM usage with multiple parallel chats (~10 sessions → 30 GB memory pressure, macOS OOM)
- [DOCS] Managed MCP policy docs omit invalid `allowedMcpServers`/`deniedMcpServers` entry behavior
- [DOCS] Effort docs omit `CLAUDE_CODE_ALWAYS_ENABLE_EFFORT` unsupported-model behavior
- Regression (2.1.147–2.1.150?): resuming an extended-thinking session after a CC update/model-switch → unrecoverable 400, session bricked
- [DOCS] Windows updater docs omit `claude.exe` in-use recovery guidance
- [DOCS] VS Code auto mode docs still tie mode-picker visibility to bypass-permissions setting
- [DOCS] MCP docs omit `/mcp` tool list and detail rendering behavior
- [DOCS] Fine-grained tool streaming docs still describe provider opt-in behavior
- bypassPermissions: session startup reads flat pref, GUI toggle writes per-account pref — they never sync
- [BUG] Claude Desktop Code tab causes disk write limit violation — 8.5GB in 11 min, macOS kills app (M5, v1.9659.1)
- Ultrareview v2.1.96: docs describe /tasks command + claude ultrareview --json subcommand that don't exist; findings hard to read after completion
- I'd be happy to help create a GitHub issue title, but I don't see the error message in your message. Could you please share the specific error you're encountering? That way I can generate an accurate and descriptive issue title for you.
- [BUG] Claude in Chrome `file_upload` rejects all scheduled-task sessions with misleading error (real cause: INVALID_SESSION)
- Extended thinking: signed thinking block 'cannot be modified' (400) permanently wedges session
- RTL text support for Hebrew (and Arabic) in Claude Code
- [Bug] Random errors occurring across multiple operations