langflow-ai/langflow · error · UserComponentError

class_name {class_name!r} is a Windows-reserved device name

Error message

class_name {class_name!r} is a Windows-reserved device name

What it means

Raised when the (uppercased) class name matches a Windows reserved device name: CON, PRN, AUX, NUL, COM1-COM9, LPT1-LPT9. These are valid Python identifiers and pass the CamelCase regex, but Windows refuses to create files with those stems, so the registry guard rejects them explicitly to keep sandboxes cross-platform portable.

Source

Thrown at src/backend/base/langflow/agentic/services/user_components.py:233

    # has to reason about pathological inputs.
    if len(class_name) > MAX_CLASS_NAME_LENGTH:
        msg = f"class_name length {len(class_name)} exceeds max {MAX_CLASS_NAME_LENGTH} (Windows MAX_PATH safeguard)"
        raise UserComponentError(msg)
    # Filesystem-portability guard (rejects NUL, Windows-forbidden punct,
    # path separators, dotdot, control chars, trailing dot/space, etc.).
    if err := _check_windows_portability(class_name):
        raise UserComponentError(err)
    # Reject `.`, `..`, leading dots, leading underscores, dunders, and
    # anything that isn't a valid CamelCase identifier.
    if not _CLASS_NAME_RE.fullmatch(class_name):
        msg = (
            f"class_name must be a CamelCase identifier "
            f"(letters/digits/underscores, leading uppercase). Got: {class_name!r}"
        )
        raise UserComponentError(msg)
    if class_name.upper() in _WINDOWS_RESERVED_DEVICES:
        msg = f"class_name {class_name!r} is a Windows-reserved device name"
        raise UserComponentError(msg)


def _validate_code(code: str) -> None:
    if not code or not code.strip():
        msg = "code must be a non-empty string"
        raise UserComponentError(msg)
    encoded_size = len(code.encode("utf-8"))
    if encoded_size > MAX_COMPONENT_SOURCE_BYTES:
        msg = f"code size {encoded_size} bytes exceeds limit of {MAX_COMPONENT_SOURCE_BYTES} bytes"
        raise UserComponentError(msg)


def _resolve_components_dir(*, user_id: str | None) -> Path:
    """Resolve and create ``<sandbox>/.components/`` for the given user.

    Reuses the FS tool's authoritative sandbox resolver so the hash
    function, pepper handling, AUTO_LOGIN dispatch, and no-user refusal
    stay in one place. The reserved-segment guard does NOT apply here —

View on GitHub (pinned to 976ec789d2)

Solutions

  1. Rename the class to something that is not a DOS device name, e.g. 'AuxHandler' instead of 'Aux'.
  2. Lengthen or prefix generated names so they can never equal the 20 reserved tokens.

Example fix

# before
register_user_component(user_id=uid, class_name="Aux", code=src)

# after
register_user_component(user_id=uid, class_name="AuxiliaryHelper", code=src)
Defensive patterns

Strategy: validation

Validate before calling

WINDOWS_RESERVED = {"CON","PRN","AUX","NUL", *(f"COM{i}" for i in range(1,10)), *(f"LPT{i}" for i in range(1,10))}

def not_windows_device(name: str) -> bool:
    return name.upper() not in WINDOWS_RESERVED

Type guard

def is_safe_file_stem(name: str) -> TypeGuard[str]:
    return bool(re.fullmatch(r"[A-Z][A-Za-z0-9_]*", name)) and name.upper() not in WINDOWS_RESERVED

Try / catch

try:
    register_user_component(user_id=uid, class_name=name, code=src)
except UserComponentError as e:
    if "Windows-reserved" in str(e):
        name += "Component"  # CON -> CONComponent
        register_user_component(user_id=uid, class_name=name, code=src)

Prevention

When it happens

Trigger: register_user_component with class_name 'Con', 'NUL', 'Aux', 'Com1', 'LPT1' etc. — the check is case-insensitive (class_name.upper()).

Common situations: Short generated names colliding with DOS device names ('Aux' is a realistic generator output); deliberate edge-case testing; names like 'Con' from truncating 'Connection'.

Related errors

AI-assisted analysis of langflow-ai/langflow@976ec789d2 (2026-08-14). Data as JSON: /api/errors/619f2f7bde4623a6. Report an issue: GitHub.