sgl-project/sglang · error · FileNotFoundError

Could not resolve backbone.pt from: {path}

Error message

Could not resolve backbone.pt from: {path}

What it means

_resolve_backbone_ckpt expects either a direct path to a backbone.pt file or a directory containing backbone.pt; neither exists at the given location.

Source

Thrown at python/sglang/multimodal_gen/tools/build_modelopt_fp8_transformer.py:229

def _resolve_transformer_dir(path: str) -> str:
    candidate = Path(path).expanduser().resolve()
    if (candidate / "config.json").is_file():
        return str(candidate)
    transformer_dir = candidate / "transformer"
    if (transformer_dir / "config.json").is_file():
        return str(transformer_dir)
    raise FileNotFoundError(f"Could not resolve a transformer directory from: {path}")


def _resolve_backbone_ckpt(path: str) -> str:
    candidate = Path(path).expanduser().resolve()
    if candidate.is_file():
        return str(candidate)
    backbone_path = candidate / "backbone.pt"
    if backbone_path.is_file():
        return str(backbone_path)
    raise FileNotFoundError(f"Could not resolve backbone.pt from: {path}")


def _find_index_file(model_dir: str) -> str | None:
    for filename in INDEX_FILENAMES:
        candidate = os.path.join(model_dir, filename)
        if os.path.isfile(candidate):
            return filename

    matches = sorted(
        filename
        for filename in os.listdir(model_dir)
        if filename.endswith(".safetensors.index.json")
    )
    return matches[0] if matches else None


def _load_weight_map(model_dir: str) -> tuple[dict[str, str], str | None]:
    index_filename = _find_index_file(model_dir)

View on GitHub (pinned to 0132848349)

Solutions

1. Locate backbone.pt in your export (find /path -name backbone.pt)
2. Pass the direct file path if it exists under another name only after renaming/copying it to backbone.pt
3. Point at the directory that actually contains backbone.pt

Example fix

# before
--base-transformer-dir /data/export   # no backbone.pt
# after
--base-transformer-dir /data/export/backbone

from pathlib import Path

def has_backbone(p: str) -> bool:
    c = Path(p).expanduser().resolve()
    return c.is_file() or (c / "backbone.pt").is_file()

try:
    d = _resolve_backbone_ckpt(path)
except FileNotFoundError as e:
    logger.error("%s", e)
    raise

When it happens

Trigger: Passing --base-transformer-dir (or backbone path) where backbone.pt is missing or named differently.

Common situations: The BF16 fallback source export kept original naming instead of backbone.pt; the backbone file lives one level deeper than expected.

Understand the failure class

Background: "File not found" and ENOENT errors: why libraries can't find a file that should exist — this error's family across 50 libraries.

Related errors

• Could not resolve a transformer directory from: {path}
• Expected an index file or a single safetensors shard in {mod
• Only per-tensor FP8 scales are supported for diffusion check
• Expected a flat quantization_config dict in the ModelOpt exp
• This tool only supports ModelOpt diffusers FP8 exports (quan

AI-assisted analysis of sgl-project/sglang@0132848349 (2026-08-28). Data as JSON: /api/errors/db913463fa3e990c. Report an issue: GitHub.