BoundaryML/baml · error

encoding map: %w

Error message

encoding map: %w

What it means

A string-keyed map being encoded as a baml input contained a value that failed to encode; the inner error is wrapped with 'encoding map: %w'. Like the list wrapper, it is context — the wrapped cause names the actual unsupported or failing value.

Source

Thrown at engine/language_client_go/baml_go/serde/encode.go:180

	case reflect.Slice, reflect.Array:
		encoded, err := encodeList(rv)
		if err != nil {
			return nil, fmt.Errorf("encoding list: %w", err)
		}
		return &cffi.HostValue{
			Value: &cffi.HostValue_ListValue{
				ListValue: encoded,
			},
		}, nil

	case reflect.Map:
		if rv.Type().Key().Kind() != reflect.String {
			return nil, fmt.Errorf("map key type must be string, got %s", rv.Type().Key().Kind())
		}

		encoded, err := encodeMap(rv)
		if err != nil {
			return nil, fmt.Errorf("encoding map: %w", err)
		}
		return &cffi.HostValue{
			Value: &cffi.HostValue_MapValue{
				MapValue: encoded,
			},
		}, nil

	default:
		// Use originalValue's type for the error message as it's more accurate to the input
		return nil, fmt.Errorf("unsupported type for BAML encoding: %T (Kind: %s)", originalValue, rv.Kind())
	}
}

// --- Encoding helpers for specific types ---

// encodeList now accepts and passes TypeMap
func encodeList(value reflect.Value) (*cffi.HostListValue, error) {
	values := make([]*cffi.HostValue, value.Len())

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Inspect the wrapped cause to identify the failing key/value
2. Normalize map values to primitives, slices, string-keyed maps, or serializable types
3. Register/implement BamlSerializer for custom value types
4. Sanitize dynamic (map[string]any) data before passing it

Example fix

// before
m := map[string]any{"x": json.Number("1")}
b.Fn(ctx, m)
// after
m := map[string]any{"x": 1}
b.Fn(ctx, m)

func sanitizeMap(m map[string]any) error { for k, v := range m { if !isEncodable(v) { return fmt.Errorf("key %q: %T", k, v) } }; return nil }

func isEncodable(v any) bool {
  switch v.(type) { case string, int, int64, float64, bool, nil: return true }
  rv := reflect.ValueOf(v)
  return rv.Kind() == reflect.Slice || rv.Kind() == reflect.Map
}

if err := b.Fn(ctx, m); err != nil {
  if strings.Contains(err.Error(), "encoding map value") { /* log wrapped cause, sanitize map */ }
}

When it happens

Trigger: Passing a map[string]... as a baml argument where some value is unsupported (nested non-serializable struct, Checked/StreamState, internal object whose Encode fails, etc.).

Common situations: Dynamic map[string]any payloads parsed from JSON containing unexpected types (e.g. json.Number, nested structs); maps holding media objects; maps of custom classes without serializers.

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

• map key type must be string, got %s
• encoding map value: %w
• encoding internal object: %w
• encoding list: %w
• unsupported type for BAML encoding: %T (Kind: %s)

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/314834dd2b0094ce. Report an issue: GitHub.