sgl-project/sglang · error · ValueError

Invalid backend: {value}. Must be one of: {', '.join([m.valu

Error message

Invalid backend: {value}. Must be one of: {', '.join([m.value for m in cls])}

What it means

Raised by Backend.from_string when the string passed cannot be matched to a Backend enum member. The lookup is cls(value.lower()), so any string that is not exactly (case-insensitively) one of the enum's values fails. The message lists all valid values.

Source

Thrown at python/sglang/multimodal_gen/runtime/server_args/server_args.py:131

class Backend(str, Enum):
    """
    Enumeration for different model backends.
    - AUTO: Automatically select backend (prefer sglang native, fallback to diffusers)
    - SGLANG: Use sglang's native optimized implementation
    - DIFFUSERS: Use vanilla diffusers pipeline (supports all diffusers models)
    """

    AUTO = "auto"
    SGLANG = "sglang"
    DIFFUSERS = "diffusers"

    @classmethod
    def from_string(cls, value: str) -> "Backend":
        """Convert string to Backend enum."""
        try:
            return cls(value.lower())
        except ValueError:
            raise ValueError(
                f"Invalid backend: {value}. Must be one of: {', '.join([m.value for m in cls])}"
            ) from None

    @classmethod
    def choices(cls) -> list[str]:
        """Get all available choices as strings for argparse."""
        return [backend.value for backend in cls]


WARMUP_MODES = ("off", "request", "server")

# Default prompt sequence-length buckets for breakable CUDA graph (BCG) padding.
# Prompt-conditioning is padded up to the smallest bucket that fits so prompts
# of different lengths share one captured graph.
DEFAULT_BCG_TEXT_BUCKETS = (64, 128, 256, 512, 1024)

BREAKABLE_CUDA_GRAPH_SUPPORTED_MODEL_IDS = frozenset(
    {

View on GitHub (pinned to 0132848349)

Solutions

1. Fix the string to exactly match one of the values printed in the error message
2. Check the enum definition next to from_string in server_args.py for the authoritative list of valid values
3. If loading from config/env, strip whitespace and normalize case before calling from_string

Example fix

# before
Backend.from_string("flashinde ")
# after
Backend.from_string("flashinfer")

from sglang.multimodal_gen.runtime.server_args.server_args import Backend
valid = {m.value for m in Backend}
if value.lower() not in valid:
    raise SystemExit(f"unsupported backend {value!r}; choose from {sorted(valid)}")
backend = Backend.from_string(value)

def is_valid_backend(value: str) -> bool:
    return isinstance(value, str) and value.lower() in {m.value for m in Backend}

When it happens

Trigger: Calling from_string('tensorrt') when no such enum member exists; passing a backend name with a typo, trailing whitespace, or a value not defined on the Backend enum. Callers include derive_pool_result_endpoint, Backend.__post_init__, and from_kwargs.

Common situations: CLI/config typos in --backend style flags, copying a backend name from another project (e.g. 'trt', 'nccl'), version changes where a backend value was renamed or removed.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

• rollout_sde_type must be one of {_VALID_ROLLOUT_SDE_TYPES},
• Invalid attention backend '{backend}'. Available options are
• kv-canary: kv_canary must be one of none/log/raise, got {mod
• Unknown CacheAware Policy: {policy=}
• Unknown CacheAgnostic Policy: {policy=}

AI-assisted analysis of sgl-project/sglang@0132848349 (2026-08-28). Data as JSON: /api/errors/7ceb867291fa3acd. Report an issue: GitHub.