BerriAI/litellm · error · HTTPException

'{_field}' must be provided as a multipart file upload, not

Error message

'{_field}' must be provided as a multipart file upload, not a string.

What it means

Error "'{_field}' must be provided as a multipart file upload, not a string." thrown in BerriAI/litellm.

Source

Thrown at litellm/proxy/image_endpoints/endpoints.py:291

        user_request_timeout,
        user_temperature,
        version,
    )

    #########################################################
    # Read request body and convert UploadFiles to BytesIO
    #########################################################
    data: Final = await _read_request_body(request=request)
    image_files: Final = await batch_to_bytesio(image)
    mask_files: Final = await batch_to_bytesio(mask)
    if image_files:
        data["image"] = image_files
    if mask_files:
        data["mask"] = mask_files

    for _field in ("image", "mask"):
        if _field in data and isinstance(data[_field], str):
            raise HTTPException(
                status_code=422,
                detail=f"'{_field}' must be provided as a multipart file upload, not a string.",
            )

    # Ensure prompt exists in data (default to None for models that don't require it)
    if "prompt" not in data:
        data["prompt"] = None

    data["model"] = (
        model
        or general_settings.get("image_generation_model", None)  # server default
        or user_model  # model name passed via cli args
        or data.get("model", None)  # default passed in http request
    )
    #########################################################
    # Process request
    #########################################################

View on GitHub (pinned to 77b7c6c40c)

Solutions

1. Upload the field as a multipart file instead of a string value.
2. Use a multipart/form-data request with the file attached under the named field.

When it happens

Trigger: Thrown at litellm/proxy/image_endpoints/endpoints.py:291 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of BerriAI/litellm@77b7c6c40c (2026-08-18). Data as JSON: /api/errors/8a480c267d20961c. Report an issue: GitHub.