BoundaryML/baml · error

unexpected type for enum value builder: %T

Error message

unexpected type for enum value builder: %T

What it means

EnumBuilder.AddValue delegates to an internal helper that creates the enum value builder, then asserts the returned value implements EnumValueBuilder. If it does not, this error is thrown. It indicates the runtime's generic object factory returned an unexpected wrapper type when adding a value to a dynamic enum.

Source

Thrown at engine/language_client_go/pkg/rawobjects_enum_builder.go:39

func newEnumBuilder(ptr int64, rt unsafe.Pointer) EnumBuilder {
	bldr := enumBuilder{raw_objects.FromPointer(ptr, rt), llmRenderableObject{}}
	bldr.llmRenderableObject = llmRenderableObject{&bldr}
	return &bldr
}

// AddValue adds a new value to the enum
func (eb *enumBuilder) AddValue(value string) (EnumValueBuilder, error) {
	args := map[string]interface{}{
		"value": value,
	}
	result, err := raw_objects.CallMethod(eb, "add_value", args)
	if err != nil {
		return nil, err
	}

	enumValueBuilder, ok := result.(EnumValueBuilder)
	if !ok {
		return nil, fmt.Errorf("unexpected type for enum value builder: %T", result)
	}

	return enumValueBuilder, nil
}

// Type returns the type definition for this enum
func (eb *enumBuilder) Type() (Type, error) {
	result, err := raw_objects.CallMethod(eb, "type_", nil)
	if err != nil {
		return nil, err
	}

	typ, ok := result.(Type)
	if !ok {
		return nil, fmt.Errorf("unexpected type for enum type: %T", result)
	}

	return typ, nil

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Align the Go SDK module and native BAML runtime to one consistent version (`go mod tidy`, reinstall)
2. Rebuild or re-download the native runtime binary
3. Read the %T value in the message to identify the wrong type returned and report upstream if reproducible

func asEnumValueBuilder(v any) (EnumValueBuilder, bool) {
    evb, ok := v.(EnumValueBuilder)
    return evb, ok
}

evb, err := enumBuilder.AddValue("ACTIVE")
if err != nil {
    if strings.Contains(err.Error(), "unexpected type for enum value builder") {
        return nil, fmt.Errorf("baml bindings/runtime version mismatch: %w", err)
    }
    return nil, err
}

When it happens

Trigger: Calling AddValue on a runtime-constructed enum builder when the internal creation call returns a non-EnumValueBuilder result — a bindings/runtime version mismatch or a dispatch bug in the raw object factory.

Common situations: Mixed Go SDK / native runtime versions after partial upgrade; custom engine builds; hitting it in tests like TestTypeBuilderSkipEnumValue signals broken bindings rather than test input issues.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

• unexpected type for collector creation: %T
• unexpected type for media creation: %T
• unexpected type for type builder creation: %T
• unexpected type for enum type: %T
• unexpected type for enum value builders: %T

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/322885b9a6c2a273. Report an issue: GitHub.