BoundaryML/baml · error

failed to call object constructor

Error message

failed to call object constructor

What it means

After marshaling, NewRawObject invokes WrapCallObjectConstructor in the native runtime, which returns a buffer; this error is raised when that buffer has zero length, meaning the native object-constructor call produced no response. It signals the CFFI call itself failed or returned an empty result rather than a structured error.

Source

Thrown at engine/language_client_go/baml_go/raw_objects/utils.go:124

func NewRawObject(rt unsafe.Pointer, objectType cffi.BamlObjectType, kwargs []*cffi.HostMapEntry) (any, error) {
	args := cffi.BamlObjectConstructorInvocation{
		Type:   objectType,
		Kwargs: kwargs,
	}

	encodedArgs, err := proto.Marshal(&args)
	if err != nil {
		return nil, fmt.Errorf("failed to marshal object constructor arguments: %w", err)
	}
	cEncodedArgs := (*C.char)(unsafe.Pointer(&encodedArgs[0]))

	cBuf := C.WrapCallObjectConstructor(cEncodedArgs, C.uintptr_t(len(encodedArgs)))

	content_bytes := C.GoBytes(unsafe.Pointer(cBuf.ptr), C.int32_t(cBuf.len))
	C.WrapFreeBuffer(cBuf) // Free the buffer after use

	if cBuf.len == 0 {
		return nil, fmt.Errorf("failed to call object constructor")
	}
	if cBuf.ptr == nil {
		return nil, fmt.Errorf("object constructor returned nil pointer")
	}

	var content_holder cffi.InvocationResponse
	err = proto.Unmarshal(content_bytes, &content_holder)
	if err != nil {
		return nil, fmt.Errorf("failed to unmarshal content bytes: %w", err)
	}
	parsed, err := decodeObjectResponse(rt, &content_holder)
	if err != nil {
		return nil, fmt.Errorf("failed to decode object response: %w", err)
	}

	return parsed, nil
}

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Verify the BAML runtime is fully initialized before constructing raw objects (check init/order of runtime setup).
2. Confirm the object type string and kwargs match the current BAML API for your version — upgrade Go bindings and native library together.
3. Check stderr/logs from the native runtime for an abort or panic during the constructor call.
4. Reduce the call to a minimal reproduction (simple object type, empty kwargs) to isolate whether the runtime itself is broken.

Example fix

// before
obj, err := NewRawObject(ctx, unknownType, kwargs) // empty response buffer

// after
if err := bamlRuntime.WaitReady(ctx); err != nil {
    return fmt.Errorf("runtime init: %w", err)
}
obj, err := NewRawObject(ctx, knownSupportedType, kwargs)
if err != nil {
    return fmt.Errorf("raw object: %w", err)
}

if err := bamlRuntime.WaitReady(ctx); err != nil {
    return fmt.Errorf("runtime not ready: %w", err)
}

func runtimeReady() bool {
    return bamlRuntime.State() == ready
}

obj, err := NewRawObject(ctx, objectType, kwargs)
if err != nil && strings.Contains(err.Error(), "failed to call object constructor") {
    return fmt.Errorf("native constructor returned no response for %s — verify runtime init and object type: %w", objectType, err)
}

When it happens

Trigger: WrapCallObjectConstructor returning an empty buffer during NewRawObject (or NewCollector / newMediaFromUrl / newMediaFromBase64 / NewTypeBuilder). This happens when the native runtime rejects or aborts the constructor call without producing a response payload — e.g. an unknown object type or a runtime not properly initialized.

Common situations: Mismatch between the Go bindings' object type names and the native runtime's supported types after partial upgrades; calling BAML object APIs before runtime initialization completed; native runtime crash/abort producing an empty buffer.

Related errors

• failed to marshal object constructor arguments: %w
• object constructor returned nil pointer
• value record mixed run_started with run_completed lifecycle
• value record mixed body metadata with lifecycle metadata
• value record mixed capture loss metadata with body metadata

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/ac152b235b23a961. Report an issue: GitHub.