What causes "unexpected type for enum value builders: %T" in BoundaryML/baml?

Rebuild so Go bindings and BAML runtime versions match (go generate / regenerate Go client)

unexpected type for enum value builders: %T

Error message

unexpected type for enum value builders: %T

What it means

ListValues() asks the runtime for all values of an enum and expects a slice of raw_objects.RawPointer, each of which must also implement EnumValueBuilder. This error fires when the FFI result fails the first assertion to []raw_objects.RawPointer, meaning the runtime returned something other than the expected list of enum-value handles.

Source

Thrown at engine/language_client_go/pkg/rawobjects_enum_builder.go:69

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

	return typ, nil
}

// ListValues returns all values in the enum
func (eb *enumBuilder) ListValues() ([]EnumValueBuilder, error) {
	result, err := raw_objects.CallMethod(eb, "list_values", nil)
	if err != nil {
		return nil, err
	}

	rawObjects, ok := result.([]raw_objects.RawPointer)
	if !ok {
		return nil, fmt.Errorf("unexpected type for enum value builders: %T", result)
	}

	enumValueBuilders := make([]EnumValueBuilder, len(rawObjects))
	for i, rawObject := range rawObjects {
		enumValueBuilders[i] = rawObject.(EnumValueBuilder)
	}

	return enumValueBuilders, nil
}

// Value gets a specific value from the enum
func (eb *enumBuilder) Value(name string) (EnumValueBuilder, error) {
	args := map[string]interface{}{
		"name": name,
	}
	result, err := raw_objects.CallMethod(eb, "value", args)
	if err != nil {
		return nil, err

View on GitHub (pinned to bd85ce9dee)

Solutions

Rebuild so Go bindings and BAML runtime versions match (go generate / regenerate Go client)
Confirm the enum has static values declared in .baml files before listing
Re-create the enum builder from a fresh BamlRuntime rather than reusing a stale handle
Update the BAML Go package to the latest version

Example fix

// before
enumBuilder := runtime.EnsureEnum("MyEnum")
values, err := enumBuilder.ListValues() // error if handle is stale
// after
runtime, _ := baml.NewBamlRuntimeFromFiles("./baml_src", nil, nil)
values, err := runtime.EnsureEnum("MyEnum").ListValues()

Defensive patterns

Strategy: try-catch

Validate before calling

values, err := enumBuilder.ListValues()
if err != nil && strings.Contains(err.Error(), "unexpected type") { /* rebuild bindings */ }

Type guard

func asRawSlice(v any) ([]raw_objects.RawPointer, bool) { s, ok := v.([]raw_objects.RawPointer); return s, ok }

Try / catch

values, err := enumBuilder.ListValues()
if err != nil {
    return fmt.Errorf("listing enum values: %w", err) // includes this error wrapped
}

Prevention

Regenerate the Go client whenever the BAML CLI version changes
Create enum builders from a fresh runtime, not stale handles
Declare enum values statically in .baml when you need ListValues

When it happens

Trigger: Calling ListValues() on an EnumBuilder whose runtime 'list_values' method returns a non-slice or wrong element type — usually a bindings/runtime version skew or a corrupted/invalid enum object handle.

Common situations: Go bindings regenerated against a newer runtime than the linked one; enum defined with dynamic type builder where values aren't listable; passing a stale EnumBuilder handle after the runtime context was reset.

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

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/161ce9bf6b32a679. Report an issue: GitHub.