vllm-project/vllm · error · Error

multimodal input is not supported by this chat renderer

Error message

multimodal input is not supported by this chat renderer

What it means

Backend 'nixl' was requested for the EPLB communicator but has_nixl() returned False: the optional nixl package is not importable in this process.

Source

Thrown at rust/src/chat/src/error.rs:20

// SPDX-FileCopyrightText: Copyright contributors to the vLLM project

use thiserror::Error;
use thiserror_ext::{AsReport as _, Macro};

type BoxedError = Box<dyn std::error::Error + Send + Sync>;

#[derive(Debug, Error, Macro)]
#[thiserror_ext(macro(path = "crate::error"))]
pub enum Error {
    #[error("chat request must contain at least one message")]
    EmptyMessages,
    #[error("cannot continue the final message when the last message is not from the assistant")]
    ContinueFinalAssistantWithoutFinalAssistant,
    #[error("chat template is required but none was configured")]
    MissingChatTemplate,
    #[error("chat template error: {0}")]
    ChatTemplate(String),
    #[error("multimodal input is not supported by this chat renderer")]
    UnsupportedMultimodalRenderer,
    #[error("unsupported multimodal content: {0}")]
    UnsupportedMultimodalContent(&'static str),
    #[error("`{modality}` input is not supported by this model")]
    UnsupportedModality { modality: String },
    #[error("At most {limit} {modality}(s) may be provided in one prompt.")]
    MmLimitExceeded { modality: String, limit: usize },
    #[error("multimodal preprocessing error: {0}")]
    Multimodal(#[message] String),
    #[error("{kind} parsing is not available for model `{model_id}`")]
    ParserUnavailableForModel {
        kind: &'static str,
        model_id: String,
    },
    #[error("{kind} parsing is disabled by frontend configuration")]
    ParserDisabled { kind: &'static str },
    #[error(
        "{kind} parser `{name}` is not registered{}",

View on GitHub (pinned to c794754062)

Solutions

1. Install nixl into the same venv/interpreter vLLM runs in, then verify with python -c 'from vllm.distributed.device_communicators import nixl_utils; print(nixl_utils.has_nixl())'
2. Or switch the backend to 'pynccl' / 'torch_nccl' if NIXL one-sided transfers are not required

Example fix

# before
# (nixl not installed)
backend = "nixl"

# after
# uv pip install nixl
backend = "nixl"

from vllm.distributed.device_communicators import nixl_utils
if backend == 'nixl':
    assert nixl_utils.has_nixl(), 'nixl not installed: uv pip install nixl'

def nixl_available() -> bool:
    from vllm.distributed.device_communicators import nixl_utils
    return nixl_utils.has_nixl()

When it happens

Trigger: Configuring EPLB backend='nixl' on an installation without the nixl wheel, or where importing nixl failed (missing UCX libs, ABI mismatch).

Common situations: Default pip installs that omit the nixl extra; docker images built for NCCL-only deployments; upgrading vLLM without reinstalling optional NIXL dependencies.

Related errors

• Configuration error: {0}
• Endpoint not ready after {0}s: {1}
• Backend error: {0}
• IO error: {0}
• chat template error: {0}

AI-assisted analysis of vllm-project/vllm@c794754062 (2026-08-14). Data as JSON: /api/errors/15ec61d91686dc87. Report an issue: GitHub.