reflex-dev/reflex · error · ValueError

Stylesheet file {stylesheet_full_path} is not supported.

Error message

Stylesheet file {stylesheet_full_path} is not supported.

What it means

During compilation of the root stylesheet, Reflex validates each non-URL stylesheet referenced in rx.App(add_styles=[...]) or theme styles. Only files with extensions in Reflex.STYLESHEETS_SUPPORTED (css, scss, sass) are allowed; anything else raises this ValueError.

Source

Thrown at reflex/compiler/compiler.py:300

    return output_path, code


def _validate_stylesheet(stylesheet_full_path: Path, assets_app_path: Path) -> None:
    """Validate the stylesheet.

    Args:
        stylesheet_full_path: The stylesheet to validate.
        assets_app_path: The path to the assets directory.

    Raises:
        ValueError: If the stylesheet is not supported.
        FileNotFoundError: If the stylesheet is not found.
    """
    suffix = stylesheet_full_path.suffix[1:] if stylesheet_full_path.suffix else ""
    if suffix not in constants.Reflex.STYLESHEETS_SUPPORTED:
        msg = f"Stylesheet file {stylesheet_full_path} is not supported."
        raise ValueError(msg)
    if not stylesheet_full_path.absolute().is_relative_to(assets_app_path.absolute()):
        msg = f"Cannot include stylesheets from outside the assets directory: {stylesheet_full_path}"
        raise FileNotFoundError(msg)
    if not stylesheet_full_path.name:
        msg = f"Stylesheet file name cannot be empty: {stylesheet_full_path}"
        raise ValueError(msg)
    if (
        len(
            stylesheet_full_path
            .absolute()
            .relative_to(assets_app_path.absolute())
            .parts
        )
        == 1
        and stylesheet_full_path.stem == PageNames.STYLESHEET_ROOT
    ):
        msg = f"Stylesheet file name cannot be '{PageNames.STYLESHEET_ROOT}': {stylesheet_full_path}"
        raise ValueError(msg)

View on GitHub (pinned to 45b8ed5ab7)

Solutions

1. Convert the file to .css or .scss (sass is also supported) and update the path
2. If it's a URL, pass the full http(s):// URL so it bypasses file validation
3. Fix the extension typo in the add_styles entry

Example fix

# before
app = rx.App(add_styles=["/styles/theme.less"])
# after
app = rx.App(add_styles=["/styles/theme.scss"])

from pathlib import Path

SUPPORTED = {"css", "scss", "sass"}

def validate_styles(styles: list[str]) -> list[str]:
    out = []
    for s in styles:
        if s.startswith(("http://", "https://")):
            out.append(s)
        elif Path(s).suffix.lstrip(".").lower() not in SUPPORTED:
            raise ValueError(f"unsupported stylesheet extension: {s}")
        else:
            out.append(s)
    return out

app = rx.App(add_styles=validate_styles(my_styles))

When it happens

Trigger: Adding a stylesheet entry whose file extension is not .css/.scss/.sass, e.g. rx.App(add_styles=["/styles/main.less", "/styles/style.styl"]), or omitting/misspelling the extension so suffix is empty. Valid URLs (http/https) skip this validation.

Common situations: Porting an app from another framework that used Less/Stylus; typo in the extension (".cs" or no extension); assuming arbitrary stylesheet formats are passed through to the bundler.

Related errors

• Cannot include stylesheets from outside the assets directory
• Stylesheet file name cannot be '{PageNames.STYLESHEET_ROOT}'
• The stylesheet file {stylesheet_full_path} does not exist.
• OTP field `length` must be a positive integer.
• Stylesheet file name cannot be empty: {stylesheet_full_path}

AI-assisted analysis of reflex-dev/reflex@45b8ed5ab7 (2026-08-28). Data as JSON: /api/errors/6f26993b53c2e6f5. Report an issue: GitHub.