reflex-dev/reflex · error · ValueError

Cannot provide both URL and data to download.

Error message

Cannot provide both URL and data to download.

What it means

rx.download accepts either a url to a served file or inline data (str/bytes) to encode as a data: URI — providing both is ambiguous and rejected.

Source

Thrown at packages/reflex-base/src/reflex_base/event/__init__.py:1744

    """
    from reflex_components_core.core.cond import cond

    if isinstance(url, str):
        if not url.startswith("/"):
            msg = "The URL argument should start with a /"
            raise ValueError(msg)

        # if filename is not provided, infer it from url
        if filename is None:
            filename = url.rpartition("/")[-1]

    if filename is None:
        filename = ""

    if data is not None:
        if url is not None:
            msg = "Cannot provide both URL and data to download."
            raise ValueError(msg)

        if isinstance(data, str):
            if mime_type is None:
                mime_type = "text/plain"
            # Caller provided a plain text string to download.
            url = f"data:{mime_type};base64," + b64encode(data.encode("utf-8")).decode(
                "utf-8"
            )
        elif isinstance(data, Var):
            if mime_type is None:
                mime_type = "text/plain"
            # Need to check on the frontend if the Var already looks like a data: URI.

            is_data_url = (data.js_type() == "string") & (
                data.to(str).startswith("data:")
            )

            # If it's a data: URI, use it as is, otherwise convert the Var to JSON in a data: URI.

View on GitHub (pinned to 45b8ed5ab7)

Solutions

1. Remove whichever of url/data you do not need
2. If you have both, choose one source of truth (usually data= for dynamically generated content, url= for static assets)

Example fix

# before
rx.download(url='/report.pdf', data=pdf_bytes)
# after
rx.download(data=pdf_bytes, filename='report.pdf')

assert not (url is not None and data is not None), 'pass either url or data to rx.download'

When it happens

Trigger: rx.download(url='/report.pdf', data=b'...') or rx.download(url='/x.csv', data='csv content').

Common situations: Code evolved from downloading a generated file (data=) to also passing the url where it was cached, or copy-pasting kwargs from another download call.

Related errors

• Event handler {self.fn.__name__} received repeated argument
• Event handler {self.fn.__name__} received extra argument {ex
• Event handler {self.fn.__name__} received multiple file uplo
• Event handler {self.fn.__name__} argument {clash!r} conflict
• The URL argument should start with a /

AI-assisted analysis of reflex-dev/reflex@45b8ed5ab7 (2026-08-28). Data as JSON: /api/errors/06bd95c1fbb3df69. Report an issue: GitHub.