[Bug]: max-num-partial-prefills failes on V1 engine start

[DmitriiShubin]

Your current environment

Output logs

vllm serve /tmp/model-poc --host 127.0.0.1 --port 8000 --gpu-memory-utilization 0.90 --max-model-len 1000 --dtype float16 --kv-cache-dtype auto --tensor-parallel-size 1 --pipeline-parallel-size 1 --max-num-batched-tokens 4096 --max-num-seqs 128 --max-num-partial-prefills 4 --max-long-partial-prefills 1 --long-prefill-token-threshold 256 --performance-mode interactivity

2026-04-13 21:12:29,527 INFO [server] (APIServer pid=6907) INFO 04-13 21:12:29 [utils.py:299]
2026-04-13 21:12:29,527 INFO [server] (APIServer pid=6907) INFO 04-13 21:12:29 [utils.py:299]        █     █     █▄   ▄█
2026-04-13 21:12:29,527 INFO [server] (APIServer pid=6907) INFO 04-13 21:12:29 [utils.py:299]  ▄▄ ▄█ █     █     █ ▀▄▀ █  version 0.19.0
2026-04-13 21:12:29,527 INFO [server] (APIServer pid=6907) INFO 04-13 21:12:29 [utils.py:299]   █▄█▀ █     █     █     █  model   /tmp/model-poc
2026-04-13 21:12:29,527 INFO [server] (APIServer pid=6907) INFO 04-13 21:12:29 [utils.py:299]    ▀▀  ▀▀▀▀▀ ▀▀▀▀▀ ▀     ▀
2026-04-13 21:12:29,527 INFO [server] (APIServer pid=6907) INFO 04-13 21:12:29 [utils.py:299]
2026-04-13 21:12:29,531 INFO [server] (APIServer pid=6907) INFO 04-13 21:12:29 [utils.py:233] non-default args: {'model_tag': '/tmp/model-load-testing-poc/model-poc', 'host': '127.0.0.1', 'model': '/tmp/model-poc', 'dtype': 'float16', 'max_model_len': 1000, 'max_num_batched_tokens': 4096, 'max_num_seqs': 128, 'max_num_partial_prefills': 4, 'long_prefill_token_threshold': 256, 'performance_mode': 'interactivity'}
2026-04-13 21:12:29,540 INFO [server] (APIServer pid=6907) INFO 04-13 21:12:29 [model.py:549] Resolved architecture: Qwen3ForCausalLM
2026-04-13 21:12:29,540 INFO [server] (APIServer pid=6907) WARNING 04-13 21:12:29 [model.py:2016] Casting torch.bfloat16 to torch.float16.
2026-04-13 21:12:29,540 INFO [server] (APIServer pid=6907) INFO 04-13 21:12:29 [model.py:1678] Using max model len 1000
2026-04-13 21:12:29,541 INFO [server] (APIServer pid=6907) Traceback (most recent call last):
2026-04-13 21:12:29,541 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/bin/vllm", line 6, in <module>
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)     sys.exit(main())
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)              ^^^^^^
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/site-packages/vllm/entrypoints/cli/main.py", line 75, in main
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)     args.dispatch_function(args)
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/site-packages/vllm/entrypoints/cli/serve.py", line 122, in cmd
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)     uvloop.run(run_server(args))
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/site-packages/uvloop/__init__.py", line 92, in run
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)     return runner.run(wrapper())
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)            ^^^^^^^^^^^^^^^^^^^^^
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/asyncio/runners.py", line 118, in run
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)     return self._loop.run_until_complete(task)
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)   File "uvloop/loop.pyx", line 1518, in uvloop.loop.Loop.run_until_complete
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/site-packages/uvloop/__init__.py", line 48, in wrapper
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)     return await main
2026-04-13 21:12:29,542 INFO [server] (APIServer pid=6907)            ^^^^^^^^^^
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/site-packages/vllm/entrypoints/openai/api_server.py", line 670, in run_server
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)     await run_server_worker(listen_address, sock, args, **uvicorn_kwargs)
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/site-packages/vllm/entrypoints/openai/api_server.py", line 684, in run_server_worker
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)     async with build_async_engine_client(
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/contextlib.py", line 210, in __aenter__
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)     return await anext(self.gen)
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)            ^^^^^^^^^^^^^^^^^^^^^
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/site-packages/vllm/entrypoints/openai/api_server.py", line 100, in build_async_engine_client
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)     async with build_async_engine_client_from_engine_args(
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/contextlib.py", line 210, in __aenter__
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)     return await anext(self.gen)
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)            ^^^^^^^^^^^^^^^^^^^^^
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/site-packages/vllm/entrypoints/openai/api_server.py", line 124, in build_async_engine_client_from_engine_args
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)     vllm_config = engine_args.create_engine_config(usage_context=usage_context)
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
2026-04-13 21:12:29,543 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/site-packages/vllm/engine/arg_utils.py", line 1554, in create_engine_config
2026-04-13 21:12:29,544 INFO [server] (APIServer pid=6907)     self._check_feature_supported()
2026-04-13 21:12:29,544 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/site-packages/vllm/engine/arg_utils.py", line 2004, in _check_feature_supported
2026-04-13 21:12:29,544 INFO [server] (APIServer pid=6907)     _raise_unsupported_error(feature_name="Concurrent Partial Prefill")
2026-04-13 21:12:29,544 INFO [server] (APIServer pid=6907)   File "/root/miniconda3/envs/test/lib/python3.11/site-packages/vllm/engine/arg_utils.py", line 2283, in _raise_unsupported_error
2026-04-13 21:12:29,544 INFO [server] (APIServer pid=6907)     raise NotImplementedError(msg)
2026-04-13 21:12:29,544 INFO [server] (APIServer pid=6907) NotImplementedError: Concurrent Partial Prefill is not supported. We recommend to remove Concurrent Partial Prefill from your config.

🐛 Describe the bug

V1 engine fails when trying to enable max-num-partial-prefills parameter

Before 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.

[wuhyw]

same question