sgl-project/sglang · error · ValueError

--diffusers-kwargs must be valid JSON. Got: {args.diffusers_

Error message

--diffusers-kwargs must be valid JSON. Got: {args.diffusers_kwargs}

What it means

Raised by the `generate` CLI command when --diffusers-kwargs is provided but is not parseable as JSON (json.JSONDecodeError). The string is json.loads-ed into sampling_params_kwargs['diffusers_kwargs']; any syntax error (trailing commas, single quotes, unquoted keys) aborts command startup.

Source

Thrown at python/sglang/multimodal_gen/runtime/entrypoints/cli/generate.py:195

    sampling_params_kwargs.update(sampling_params_cls.get_cli_args(args))
    _apply_output_file_path_override(args, sampling_params_kwargs)
    sampling_params_kwargs["request_id"] = generate_request_id()
    if sampling_params_kwargs.get("use_diffusion_decoder", False):
        server_args.load_diffusion_decoder = True

    # Handle diffusers-specific kwargs passed via CLI
    if hasattr(args, "diffusers_kwargs") and args.diffusers_kwargs:
        try:
            sampling_params_kwargs["diffusers_kwargs"] = json.loads(
                args.diffusers_kwargs
            )
            logger.info(
                "Parsed diffusers_kwargs: %s",
                sampling_params_kwargs["diffusers_kwargs"],
            )
        except json.JSONDecodeError as e:
            logger.error("Failed to parse --diffusers-kwargs as JSON: %s", e)
            raise ValueError(
                f"--diffusers-kwargs must be valid JSON. Got: {args.diffusers_kwargs}"
            ) from e

    generator = DiffGenerator.from_pretrained(
        model_path=server_args.model_path, server_args=server_args, local_mode=True
    )

    results = generator.generate(sampling_params_kwargs=sampling_params_kwargs)

    prompt = sampling_params_kwargs.get("prompt")
    maybe_dump_performance(args, server_args, prompt, results)


class GenerateSubcommand(CLISubcommand):
    """The `generate` subcommand for the sglang-diffusion CLI"""

    def __init__(self) -> None:
        self.name = "generate"

View on GitHub (pinned to 0132848349)

Solutions

1. Rewrite the argument as strict JSON: double quotes on keys and string values, no trailing commas
2. Echo or dry-run print the argument to check shell quoting didn't mangle it
3. Validate with `python -c "import json,sys; json.loads(sys.argv[1])" '<your string>'` before running the command

Example fix

# before
--diffusers-kwargs "{'num_inference_steps': 20,}"

# after
--diffusers-kwargs '{"num_inference_steps": 20}'

import json
json.loads(args.diffusers_kwargs)  # fails fast with a clear JSONDecodeError

When it happens

Trigger: Running `sglang generate ... --diffusers-kwargs '{num_inference_steps: 20}'` or with single-quoted values / trailing commas — anything json.loads rejects.

Common situations: Copy-pasting Python-dict-style kwargs (single quotes, bare keys) from code or docs into the CLI; shell quoting issues that truncate the JSON; missing closing brace.

Related errors

• Number of gpus must be positive
• Config file not found: {args.config}
• Config file not found: {args.config}
• error: unrecognized arguments: {' '.join(remaining)}
• Invalid JSON mapping: {normalized_input}

AI-assisted analysis of sgl-project/sglang@0132848349 (2026-08-28). Data as JSON: /api/errors/63789601d053a829. Report an issue: GitHub.