BerriAI/litellm · error · UnsupportedParamsError

Setting user/encoding format is not supported by

Error message

Setting user/encoding format is not supported by {custom_llm_provider}. To drop it from the call, set `litellm.drop_params = True`.

What it means

get_optional_params_image_gen() whitelists image-generation parameters per provider: OpenAI/Azure/compatible providers accept everything, Bedrock only supports size, Vertex AI only supports n. Passing any other non-default parameter (user, quality, response_format, style) to a restricted provider raises UnsupportedParamsError (status 500) unless litellm.drop_params is enabled, in which case the param is silently dropped. The message text mentions user/encoding format because the same guard originated for embeddings.

Solutions

Remove parameters the target provider does not support (check the whitelist: Bedrock=size only, Vertex=n only)
Set litellm.drop_params = True (or the drop_params proxy setting) to auto-drop unsupported params
Branch your kwargs per provider instead of sharing one dict across providers
Reserve user/quality/style for openai/azure/compatible providers

Example fix

# before
img = litellm.image_generation(
    model="bedrock/stability.stable-diffusion-xl-v1",
    prompt="a cat",
    quality="hd",
    user="user-123",
)

# after
img = litellm.image_generation(
    model="bedrock/stability.stable-diffusion-xl-v1",
    prompt="a cat",
    size="1024x1024",   # only 'size' is supported on bedrock
)
# or globally: litellm.drop_params = True

Defensive patterns

Strategy: validation

Validate before calling

IMAGE_GEN_SUPPORTED = {
    "bedrock": {"size"},
    "vertex_ai": {"n"},
}

def sanitize_image_gen_kwargs(custom_llm_provider: str, kwargs: dict) -> dict:
    supported = IMAGE_GEN_SUPPORTED.get(custom_llm_provider)
    if supported is None:  # openai/azure/compatible: everything allowed
        return kwargs
    return {k: v for k, v in kwargs.items() if k in supported}

Type guard

def params_supported(params: dict, supported: set[str]) -> bool:
    return all(k in supported for k in params)

Try / catch

from litellm.exceptions import UnsupportedParamsError

try:
    img = litellm.image_generation(model=model, **kwargs)
except UnsupportedParamsError as e:
    if litellm.drop_params:
        kwargs = {k: v for k, v in kwargs.items() if k in supported_for(model)}
        img = litellm.image_generation(model=model, **kwargs)
    else:
        raise

Prevention

Keep per-provider kwarg whitelists in your calling code instead of one shared dict
Set litellm.drop_params = True when porting OpenAI kwargs to Bedrock/Vertex and silent dropping is acceptable
Consult the provider's image-generation docs for supported params before adding new ones

When it happens

Trigger: Calling litellm.image_generation(model="bedrock/...", quality="hd", user="u1") — quality is not in Bedrock's supported list; passing style or response_format to vertex_ai image models; reusing OpenAI-style kwargs against provider-restricted image endpoints without drop_params.

Common situations: Porting working OpenAI image code (with user/quality/style) to Bedrock or Vertex; generic wrappers that always attach user for tracking; unaware that drop_params=True changes behavior from raising to silently dropping.

Related errors

AI-assisted analysis of BerriAI/litellm@77b7c6c40c (2026-08-18). Data as JSON: /api/errors/77e694d3f204c506. Report an issue: GitHub.

Appendix: source

Thrown at litellm/assistants/utils.py:103

        "quality": None,
        "response_format": None,
        "size": None,
        "style": None,
        "user": None,
    }

    non_default_params = {k: v for k, v in passed_params.items() if (k in default_params and v != default_params[k])}
    optional_params = {}

    ## raise exception if non-default value passed for non-openai/azure embedding calls
    def _check_valid_arg(supported_params):
        if len(non_default_params.keys()) > 0:
            keys: Final = list(non_default_params.keys())
            for k in keys:
                if litellm.drop_params is True and k not in supported_params:  # drop the unsupported non-default values
                    non_default_params.pop(k, None)
                elif k not in supported_params:
                    raise UnsupportedParamsError(
                        status_code=500,
                        message=f"Setting user/encoding format is not supported by {custom_llm_provider}. To drop it from the call, set `litellm.drop_params = True`.",
                    )
            return non_default_params

    if (
        custom_llm_provider == "openai"
        or custom_llm_provider == "azure"
        or custom_llm_provider in litellm.openai_compatible_providers
    ):
        optional_params = non_default_params
    elif custom_llm_provider == "bedrock":
        supported_params = ["size"]
        _check_valid_arg(supported_params=supported_params)
        if size is not None:
            width, height = size.split("x")
            optional_params["width"] = int(width)
            optional_params["height"] = int(height)

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