BoundaryML/baml · error

map key type must be string, got %s

Error message

map key type must be string, got %s

What it means

BAML map inputs must have string keys. When encodeValue encounters a reflect.Map whose key kind is not string (e.g. map[int]..., map[float64]...), it refuses to encode and reports the offending key kind.

Source

Thrown at engine/language_client_go/baml_go/serde/encode.go:175

			Value: &cffi.HostValue_BoolValue{
				BoolValue: rv.Bool(),
			},
		}, nil

	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())
	}
}

View on GitHub (pinned to bd85ce9dee)

Solutions

1. Convert the map to map[string]T, formatting keys with strconv.Itoa/fmt.Sprintf
2. Change the struct field type to a string-keyed map
3. Pre-transform data into a JSON-like string-keyed structure before calling the baml function

Example fix

// before
m := map[int]string{1: "a"}
b.Fn(ctx, m)
// after
m := map[string]string{"1": "a"}
b.Fn(ctx, m)

func stringKeys(m any) bool { rv := reflect.ValueOf(m); return rv.Kind() == reflect.Map && rv.Type().Key().Kind() == reflect.String }

func toStringMap(m map[int]string) map[string]string { out := map[string]string{}; for k, v := range m { out[strconv.Itoa(k)] = v }; return out }

if err := b.Fn(ctx, m); err != nil {
  if strings.Contains(err.Error(), "map key type must be string") { /* convert keys and retry */ }
}

When it happens

Trigger: Passing a Go map with non-string key types (int, bool, struct, etc.) as a baml function argument or as a class field via EncodeMapEntries paths.

Common situations: Using map[int]string lookup tables; converting JSON objects into map[any]any; legacy code with numeric-keyed dicts.

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

• encoding map: %w
• 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/e95ad947fba49b67. Report an issue: GitHub.