BoundaryML/baml · error

unexpected type for class property builders: %T

Error message

unexpected type for class property builders: %T

What it means

BAML's Go client wraps dynamic class-builder objects that live behind an FFI boundary. When ListProperties() calls the remote `list_properties` method, it expects the runtime to hand back a []raw_objects.RawPointer; if the shared runtime returns any other Go type, this error is thrown instead of silently continuing. It signals a version or state mismatch between the Go bindings and the native BAML runtime, i.e. the FFI layer broke its documented return contract.

Solutions

1. Verify the baml Go module version matches the installed BAML CLI/runtime version and upgrade both together (`go get -u github.com/boundaryml/baml/...` + `baml version`).
2. Check runtime logs for an earlier panic or error in the `list_properties` method that would have made CallMethod return a non-pointer result.
3. Rebuild/reinstall the BAML runtime artifacts so the FFI bindings are in sync, then retry.
4. If it persists, file a bug with the %T value printed in the error message so maintainers can see which type leaked through the FFI layer.

Example fix

// before
rawObjects, ok := result.([]raw_objects.RawPointer)
if !ok {
    return nil, fmt.Errorf("unexpected type for class property builders: %T", result)
}
// after
callers cannot patch this directly; guard the call site:
props, err := classBuilder.ListProperties()
if err != nil {
    return nil, fmt.Errorf("ListProperties unavailable (runtime/bindings mismatch): %w", err)
}

// Go has no pre-call check; verify version parity instead:
// baml.Version() == expectedRuntimeVersion (checked at startup)

func isClassPropertyBuilderList(result any) bool {
    _, ok := result.([]raw_objects.RawPointer)
    return ok
}

props, err := classBuilder.ListProperties()
if err != nil {
    if strings.Contains(err.Error(), "unexpected type for class property builders") {
        // treat as runtime/bindings mismatch: fail fast with diagnostics
        return nil, fmt.Errorf("FFI contract violation in ListProperties: %w", err)
    }
    return nil, err
}

When it happens

Trigger: Calling ClassBuilder.ListProperties() when the underlying CallMethod returns something other than []raw_objects.RawPointer — typically a nil result, a single raw pointer, or a deserialized map after a partial runtime failure.

Common situations: Mixing a Go client built against one BAML version with a native runtime library of another; a runtime crash mid-call that yields a default/empty result; using a class builder object that was never fully initialized on the runtime side.

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 body: %T
• unexpected type for body: %T
• unexpected type for class property builder: %T
• unexpected type for class property type: %T
• unexpected type for headers: %T

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/dc70bd2253930b89. Report an issue: GitHub.

Appendix: source

Thrown at engine/language_client_go/pkg/rawobjects_class_builder.go:51

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

	return typ, nil
}

// ListProperties returns all properties in the class
func (cb *classBuilder) ListProperties() ([]ClassPropertyBuilder, error) {
	result, err := raw_objects.CallMethod(cb, "list_properties", nil)
	if err != nil {
		return nil, err
	}

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

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

	return rawObjectsCast, nil
}

// AddProperty adds a new property to the class
func (cb *classBuilder) AddProperty(name string, fieldType Type) (ClassPropertyBuilder, error) {
	args := map[string]interface{}{
		"name":       name,
		"field_type": fieldType,
	}
	result, err := raw_objects.CallMethod(cb, "add_property", args)
	if err != nil {

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