What causes "encoding internal object: %w" in BoundaryML/baml?

Inspect the wrapped cause (%w) in the error chain — it names the actual Encode() failure

encoding internal object: %w

Error message

encoding internal object: %w

What it means

This error wraps a failure returned by an internal BAML object's Encode() method while serializing arguments for a BAML function call in the Go client. encodeValue detects values implementing InternalBamlSerializer (baml-internal runtime objects like Image, Audio, etc.) and asks them to produce a CFFI handle; when that internal encoding fails, the underlying cause is wrapped with this message and propagated to the caller of the baml function.

Source

Thrown at engine/language_client_go/baml_go/serde/encode.go:92

	// Handle Pointers: Dereference non-nil pointers for kind checks, but use original for interfaces
	if rv.Kind() == reflect.Ptr {
		if rv.IsNil() {
			return &cffi.HostValue{}, nil // Treat nil pointers as nil values
		}
		// Work with the pointed-to value for subsequent kind checks
		rv = rv.Elem()
	}

	// Handle concrete types (Checked, StreamState) before general kinds
	// Use the potentially dereferenced value 'rv.Interface()' here if concrete types are structs
	concreteValue := rv.Interface() // Get the concrete value (dereferenced if original was pointer)

	// Check originalValue first (for non-pointer cases)
	if internalObject, ok := originalValue.(InternalBamlSerializer); ok {
		handle, err := internalObject.Encode()
		if err != nil {
			return nil, fmt.Errorf("encoding internal object: %w", err)
		}
		return &cffi.HostValue{
			Value: &cffi.HostValue_Handle{
				Handle: handle,
			},
		}, nil
	}

	// Also check the dereferenced value for pointer-to-interface cases (e.g., *types.Image)
	// In Go, a pointer to an interface doesn't implement the interface, but the dereferenced
	// value (the interface itself) does.
	if internalObject, ok := concreteValue.(InternalBamlSerializer); ok {
		handle, err := internalObject.Encode()
		if err != nil {
			return nil, fmt.Errorf("encoding internal object: %w", err)
		}
		return &cffi.HostValue{
			Value: &cffi.HostValue_Handle{

View on GitHub (pinned to bd85ce9dee)

Solutions

Inspect the wrapped cause (%w) in the error chain — it names the actual Encode() failure
Verify the internal object was constructed via its proper constructor (e.g. baml.Image.FromURL / FromBase64) and the URL/base64 is valid
Check that media URLs are reachable and base64 payloads are well-formed
Recreate the object rather than reusing one from a failed operation

Example fix

// before
img := types.Image{} // zero-value, no valid handle
classify(img)
// after
img, err := types.Image.FromURL("https://example.com/photo.png")
if err != nil { return err }
classify(img)

Defensive patterns

Strategy: try-catch

Validate before calling

func validMediaSrc(src string) bool { return strings.HasPrefix(src, "http") || isBase64(src) }

Type guard

func isInternalBaml(v any) bool { _, ok := v.(serde.InternalBamlSerializer); return ok }

Try / catch

if _, err := b.Fn(ctx, img); err != nil {
  var wrapped interface{ Unwrap() error }
  if errors.As(err, &target) { /* handle cause */ }
  return fmt.Errorf("baml input encode failed: %w", err)
}

Prevention

Always construct internal media objects via their provided constructors (FromURL/FromBase64)
Check constructor errors immediately instead of reusing partially-built objects
Validate URLs/base64 before embedding media in baml inputs

When it happens

Trigger: Passing a baml-go internal object (e.g. types.Image, types.Audio, media wrappers implementing InternalBamlSerializer) directly or inside a list/map/class field to a generated baml function, when the object's Encode() fails — typically because the underlying media handle or runtime object was not properly constructed (e.g. FromURL/FromBase64 failed or the handle references an invalid resource).

Common situations: Constructing an Image/Audio from a bad URL or malformed base64 where construction partially succeeded; passing a zero-value internal object obtained from another call; embedding such objects in nested structs where the failure surfaces only at encode time.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/94bf90aa260ad9e4. Report an issue: GitHub.