BoundaryML/baml · error

unsupported type: Checked[any] cannot be passed as inputs to

Error message

unsupported type: Checked[any] cannot be passed as inputs to baml functions

What it means

The Go BAML client rejects shared.Checked[any] values as function inputs. Checked is a validation-result type produced by the runtime (wrapping a value plus validation errors); it exists only as an output/inspection type, so encodeValue refuses to serialize it as an argument.

Source

Thrown at engine/language_client_go/baml_go/serde/encode.go:127

		return &cffi.HostValue{
			Value: &cffi.HostValue_Handle{
				Handle: handle,
			},
		}, nil
	}

	// Check for custom serializers first using the original value (could be pointer or value)
	if serializer, ok := originalValue.(BamlSerializer); ok {
		encoded, err := serializer.Encode()
		if err != nil {
			return nil, err
		}
		return encoded, nil
	}

	switch concreteValue.(type) {
	case shared.Checked[any]:
		return nil, fmt.Errorf("unsupported type: Checked[any] cannot be passed as inputs to baml functions")
	case shared.StreamState[any]:
		return nil, fmt.Errorf("unsupported type: StreamState[any] cannot be passed as inputs to baml functions")
	}

	// Handle primitive kinds and collections using reflection value rv (points to underlying value)
	switch rv.Kind() {
	case reflect.String:
		return &cffi.HostValue{
			Value: &cffi.HostValue_StringValue{
				StringValue: rv.String(),
			},
		}, nil

	case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
		return &cffi.HostValue{
			Value: &cffi.HostValue_IntValue{
				IntValue: rv.Int(),
			},

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Unwrap the Checked value to its underlying payload before passing it (use the value accessor it exposes)
2. Pass the raw struct/slice/string the Checked wraps instead
3. Restructure code so Checked values are only used for inspection, not as inputs

Example fix

// before
checked := baml.PrintStreamState(...) // types.Checked[any]
b.Fn(ctx, checked) // rejected
// after
b.Fn(ctx, checked.Value)

func notChecked(v any) bool { return !isCheckedAny(v) }

func isCheckedAny(v any) bool { return strings.Contains(reflect.TypeOf(v).String(), "shared.Checked") }

if err := b.Fn(ctx, in); err != nil {
  if strings.Contains(err.Error(), "Checked[any] cannot be passed") { return ErrCheckedAsInput }
  return err
}

When it happens

Trigger: Assigning the result of a Checked-returning API (e.g. from b.InitChecked or parsing outputs) back into another baml function's parameters, or storing Checked values in a struct/class field passed to a baml function.

Common situations: Round-tripping outputs back as inputs; accidentally building a Checked value instead of its .Value/.Unchecked content; copying example code that passes validation wrappers around.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• unsupported type: StreamState[any] cannot be passed as input
• unsupported type for BAML encoding: %T (Kind: %s)
• error decoding type, checked type not found:
• error decoding value, type alias not found:
• error decoding value, unknown field type:

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/8a5dca604d160e97. Report an issue: GitHub.