{"record":{"id":"e157476ba67c7cd2","repo":"vllm-project/vllm","slug":"executor-backend-does-not-support-async-schedu","errorCode":null,"errorMessage":"`{executor_backend}` does not support async scheduling yet.","messagePattern":"`(.+?)` does not support async scheduling yet\\.","errorType":"validation","errorClass":"ValueError","httpStatus":null,"severity":"error","filePath":"vllm/config/vllm.py","lineNumber":1181,"sourceCode":"            if self.speculative_config is not None:\n                if (\n                    self.speculative_config.method not in get_args(EagleModelTypes)\n                    and self.speculative_config.method not in get_args(NgramGPUTypes)\n                    and self.speculative_config.method != \"draft_model\"\n                    and self.speculative_config.method != \"dspark\"\n                ):\n                    raise ValueError(\n                        \"Currently, async scheduling is only supported \"\n                        \"with EAGLE/MTP/Draft Model/NGram GPU/DSpark kind of \"\n                        \"speculative decoding\"\n                    )\n                if self.speculative_config.disable_padded_drafter_batch:\n                    raise ValueError(\n                        \"Async scheduling is not compatible with \"\n                        \"disable_padded_drafter_batch=True.\"\n                    )\n            if not executor_supports_async_sched:\n                raise ValueError(\n                    f\"`{executor_backend}` does not support async scheduling yet.\"\n                )\n        elif self.scheduler_config.async_scheduling is None:\n            # Enable async scheduling unless there is an incompatible option.\n            if (\n                self.model_config is not None\n                and self.model_config.runner_type == \"pooling\"\n            ):\n                # The current implementation of asynchronous scheduling negatively\n                # impacts performance of pooling models, so we disable by default.\n                logger.debug(\n                    \"Disabling asynchronous scheduling by default for pooling model.\"\n                )\n                self.scheduler_config.async_scheduling = False\n            elif (\n                self.speculative_config is not None\n                and self.speculative_config.method not in get_args(EagleModelTypes)\n                and self.speculative_config.method not in get_args(NgramGPUTypes)","sourceCodeStart":1163,"sourceCodeEnd":1199,"githubUrl":"https://github.com/vllm-project/vllm/blob/c794754062d49a8fdb63ab3c5215b488b865030c/vllm/config/vllm.py#L1163-L1199","documentation":"Each executor backend (the process/messaging layer that hosts workers) must opt in to async scheduling via `executor_class.supports_async_scheduling()`. If the chosen `--distributed-executor-backend` does not, explicitly enabling async scheduling fails validation with the backend name in the message.","triggerScenarios":"Launching with `--async-scheduling` and an executor backend whose class does not advertise support (e.g. some external/legacy multiprocess executors).","commonSituations":"Switching executor backends (e.g. to an older or external- launcher setup) while keeping `--async-scheduling` in a shared launch script; using custom executor plugins that never implemented the async-scheduling handshake.","solutions":["Remove `--async-scheduling` (or pass `--no-async-scheduling`) for this executor backend.","Or switch to an executor backend that supports async scheduling (e.g. `--distributed-executor-backend mp` / ray where supported in your vLLM version).","If using a custom executor, implement `supports_async_scheduling()` returning True only after verifying compatibility."],"exampleFix":"# before\nvllm serve model --distributed-executor-backend external_launcher \\\n  --async-scheduling\n\n# after\nvllm serve model --distributed-executor-backend external_launcher \\\n  --no-async-scheduling","handlingStrategy":"validation","validationCode":"from vllm.v1.executor import executor_class_from_backend  # per version\ncls = executor_class_from_backend(backend)\nif async_scheduling and not cls.supports_async_scheduling():\n    async_scheduling = False","typeGuard":null,"tryCatchPattern":"try:\n    AsyncLLM.from_engine_args(args)\nexcept ValueError as e:\n    if \"does not support async scheduling\" in str(e):\n        args.async_scheduling = False\n    else:\n        raise","preventionTips":["Check executor backend support before enabling async scheduling in shared scripts","For custom executors, implement supports_async_scheduling() truthfully"],"tags":["async-scheduling","executor-backend","distributed"],"backgroundTag":null,"analyzedSha":"c794754062d49a8fdb63ab3c5215b488b865030c","analyzedAt":"2026-08-14T21:17:39.825Z","schemaVersion":2},"datasetVersion":"2026-08-15T17:31:12.345Z"}