apache/beam · error

too few inputs: forgot an input or to annotate options?

Error message

too few inputs: forgot an input or to annotate options?

What it means

Bind/findInbound finished assigning parameters to the function's declared inputs but unconsumed parameters remain (index < len(params)). The function's inbound signature accounts for fewer inputs than were supplied — typically a missing main input or un-annotated option/emitter in the DoFn signature.

Source

Thrown at sdks/go/pkg/beam/core/graph/bind.go:164

		arity, err := inboundArity(input, index == 0)
		if err != nil {
			return nil, nil, addContext(err, params, input)
		}
		if len(params)-index < arity {
			return nil, nil, addContext(errors.New("too few params"), params[index:], input)
		}

		paramsToBind := params[index : index+arity]
		elm, kind, err := tryBindInbound(input, paramsToBind, index == 0)
		if err != nil {
			return nil, nil, addContext(err, paramsToBind, input)
		}
		inbound = append(inbound, elm)
		kinds = append(kinds, kind)
		index += arity
	}
	if index < len(params) {
		return nil, nil, addContext(errors.New("too few inputs: forgot an input or to annotate options?"), params, in)
	}
	if index > len(params) {
		return nil, nil, addContext(errors.New("too many inputs"), params, in)
	}
	return inbound, kinds, nil
}

func tryBindInbound(t typex.FullType, args []funcx.FnParam, isMain bool) (typex.FullType, InputKind, error) {
	kind := Main
	var other typex.FullType

	switch t.Class() {
	case typex.Concrete, typex.Container:
		if isMain {
			other = typex.New(args[0].T)
		} else {
			// We accept various forms for side input. We have to disambiguate
			// []string into a Singleton of type []string or a Slice of type

View on GitHub (pinned to 12126d8942)

Solutions

1. Add a parameter to the DoFn/combine signature to consume the extra input, or annotate it (e.g. as an option/emitter) if that is intended.
2. Remove the surplus argument at the transform call site.
3. Verify input ordering: main inputs must come first, then side inputs.
4. Run the pipeline with beam.Try* variants to surface signature mismatches with types.

Example fix

// before
func (fn *f) ProcessElement(v int) {}  // called with 2 inputs
// after
func (fn *f) ProcessElement(w string, v int) {}

// Ensure number of supplied inputs does not exceed function inbound arity
if suppliedInputs > declaredInboundArity(fn) {
    return fmt.Errorf("too many inputs for %s: %d supplied, %d declared", fnName, suppliedInputs, declaredInboundArity(fn))
}

When it happens

Trigger: Supplying more arguments (PCollections, side inputs) to a transform than the DoFn/combiner signature declares, e.g. passing two PCollections to a ParDo whose ProcessElement takes only one element and no extra input.

Common situations: Reusing a DoFn across pipelines after adding an input PCollection, mismatched Zip/CoGBK output usage, or forgetting the `beam.Scan`/option annotation for variadic trailing inputs.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• too few params
• too many inputs
• needs at least 1 input
• node %v in graph has undefined coder
• node %v in graph is unconnected

AI-assisted analysis of apache/beam@12126d8942 (2026-09-13). Data as JSON: /api/errors/c629e81c1b1b8fc1. Report an issue: GitHub.