reflex-dev/reflex · error · ValueError

Invalid event chain: {value!s} of type {value._var_type}

Error message

Invalid event chain: {value!s} of type {value._var_type}

What it means

EventChain.create accepts handlers, EventSpecs, EventChains, lists of those, or lambdas; if the value is a Var (has _var_type) it falls through to this branch and is rejected with its type, because a Var is data, not an event chain.

Source

Thrown at packages/reflex-base/src/reflex_base/event/__init__.py:911

                if event_chain_kwargs:
                    warnings.warn(
                        f"event_chain_kwargs {event_chain_kwargs!r} are ignored for "
                        "EventChainVar values.",
                        stacklevel=2,
                    )
                return value
            if isinstance(value, (EventVar, FunctionVar)):
                value = [value]
            elif safe_issubclass(value._var_type, (EventChain, EventSpec)):
                return cls.create(
                    value=value.guess_type(),
                    args_spec=args_spec,
                    key=key,
                    **event_chain_kwargs,
                )
            else:
                msg = f"Invalid event chain: {value!s} of type {value._var_type}"
                raise ValueError(msg)
        elif isinstance(value, EventChain):
            # Trust that the caller knows what they're doing passing an EventChain directly
            return value

        # If the input is a single event handler, wrap it in a list.
        if isinstance(value, (EventHandler, EventSpec)):
            value = [value]

        events: list[EventSpec | EventVar | FunctionVar] = []

        # If the input is a list of event handlers, create an event chain.
        if isinstance(value, list):
            for v in value:
                if isinstance(v, (EventHandler, EventSpec)):
                    # Call the event handler to get the event.
                    events.append(call_event_handler(v, args_spec, key=key))
                elif isinstance(v, (EventVar, EventChainVar)):
                    events.append(v)

View on GitHub (pinned to 45b8ed5ab7)

Solutions

1. Ensure the value is an event handler call, EventSpec, EventChain, or a lambda returning those
2. If a lambda returns a Var, wrap the logic so it returns events (e.g. use rx.cond at the component level, not inside the event slot)
3. Check for a missing () on the handler invocation

Example fix

# before
rx.button('Go', on_click=State.count)  # a Var
# after
rx.button('Go', on_click=State.increment)

from reflex_base.event import EventHandler, EventSpec, EventChain

def is_event_chain_value(v) -> bool:
    return isinstance(v, (EventHandler, EventSpec, EventChain, list, tuple)) and not hasattr(v, '_var_type')

def is_event_trigger(v) -> TypeGuard[object]: return isinstance(v, (EventHandler, EventSpec, EventChain)) or callable(v)

When it happens

Trigger: Passing a Var (e.g. State.some_var or a computed Var) where an event trigger is expected: on_click=State.items, on_click=rx.cond(..., a, b) misuse, or a lambda that returns a Var.

Common situations: Assigning a Var to an event prop by accident (missing parentheses on an event call), or a lambda returning a Var instead of an event.

Related errors

• Invalid event: {v}
• Invalid event chain: {value}
• Var-returning `@rx.memo` `{func_name}` cannot depend on embe
• Invalid prop(s) {invalid_fields_str} for {component_name!r}.
• default_color_mode must be one of {allowed_color_modes}, but

AI-assisted analysis of reflex-dev/reflex@45b8ed5ab7 (2026-08-28). Data as JSON: /api/errors/954b4ad80959b623. Report an issue: GitHub.