BoundaryML/baml · error

unexpected type for enum type: %T

Error message

unexpected type for enum type: %T

What it means

Type() on the enum builder calls into the BAML runtime via raw objects and expects the runtime method to return a value implementing the Type interface. If the FFI result cannot be asserted to Type, this error is thrown instead of panicking, indicating a mismatch between what the runtime returned and what the Go bindings expect.

Source

Thrown at engine/language_client_go/pkg/rawobjects_enum_builder.go:54

	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
}

// 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))

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Regenerate/rebuild so the Go bindings and the BAML runtime (baml-cli generate-go / go generate) are from the same version
2. Verify the enum referenced actually exists in the loaded BAML source files
3. Check that no old .so/cffi artifact is cached in the build (clean rebuild with go clean)
4. Update github.com/eython/baml or boundaryml baml Go package and runtime to matching latest versions

Example fix

// before: mixing baml runtime v0.x with Go bindings v0.y
// go.mod: github.com/boundaryml/baml v0.202.0 but runtime v0.198.0
// after
// go.mod: github.com/boundaryml/baml v0.202.0
// run: go mod tidy && go clean && go generate ./...

// verify the enum exists before use
// enums, err := bamlRuntime.ListEnums() ; ensure "MyEnum" is present

func asType(v any) (Type, bool) { t, ok := v.(Type); return t, ok }

typ, err := enumBuilder.Type()
if err != nil {
    if strings.Contains(err.Error(), "unexpected type for enum type") {
        // rebuild runtime / regenerate Go bindings
    }
    return fmt.Errorf("enum type lookup failed: %w", err)
}

When it happens

Trigger: Calling Type() on an EnumBuilder when the underlying raw object's 'get_type'-style method returns an unexpected Go type — typically after a runtime/bindings version mismatch or if the enum itself failed to resolve in the runtime.

Common situations: Using a Go client package version that doesn't match the compiled BAML CFFI runtime; referencing an enum defined in a way the runtime can't materialize (e.g. enum from a different BAML project/variant); stale compiled runtime embedded via cgo.

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

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

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/d6d528b473f1d4d6. Report an issue: GitHub.