BoundaryML/baml · error

encoding map value

Error message

encoding map value: %w

What it means

Contextual wrapper added by encodeMap when the value stored under some key fails encodeValue. The wrapped cause names the real problem (unsupported type, failed serializer, etc.); the map iteration order determines which key surfaces first, so failures may appear nondeterministic across runs.

Solutions

1. Inspect the wrapped cause to identify the failing value type
2. Iterate and type-check map values before the call to find bad entries deterministically
3. Convert or remove unsupported values; add serializers for custom types
4. Flatten nested custom structs into maps of primitives

Example fix

// before
m := map[string]any{"ok": 1, "bad": make(chan int)}
b.Fn(ctx, m)
// after
m := map[string]any{"ok": 1}
b.Fn(ctx, m)

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

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

if err := b.Fn(ctx, m); err != nil {
  if strings.Contains(err.Error(), "encoding map value") { /* iterate keys and validate each value */ }
}

When it happens

Trigger: String-keyed map inputs (or dynamic class fields) whose values include unencodable types — nested raw structs, Checked/StreamState values, or internal objects with failing Encode().

Common situations: map[string]any payloads assembled from multiple sources; maps mixing valid primitives with one bad custom type; JSON round-tripped data containing unexpected types.

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

• encoding map
• map key type must be string, got
• encoding internal object
• encoding list element
• encoding list

AI-assisted analysis of BoundaryML/baml@bd85ce9dee (2026-09-12). Data as JSON: /api/errors/b1cba9690c2021a3. Report an issue: GitHub.

Appendix: source

Thrown at engine/language_client_go/baml_go/serde/encode.go:220

			return nil, fmt.Errorf("encoding list element %d: %w", i, err)
		}
		values[i] = elemOffset
	}

	return &cffi.HostListValue{
		Values: values,
	}, nil
}

// encodeMap now accepts and passes TypeMap
func encodeMap(mapValue reflect.Value) (*cffi.HostMapValue, error) {

	entries := make([]*cffi.HostMapEntry, 0, mapValue.Len())
	for _, key := range mapValue.MapKeys() {
		value := mapValue.MapIndex(key)
		valueHolder, err := encodeValue(value.Interface())
		if err != nil {
			return nil, fmt.Errorf("encoding map value: %w", err)
		}

		switch key.Kind() {
		case reflect.String:
			// Go doesn't have enums, so we can't detect them here
			entries = append(entries, &cffi.HostMapEntry{
				Key:   &cffi.HostMapEntry_StringKey{StringKey: key.String()},
				Value: valueHolder,
			})
		case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
			entries = append(entries, &cffi.HostMapEntry{
				Key:   &cffi.HostMapEntry_IntKey{IntKey: key.Int()},
				Value: valueHolder,
			})
		case reflect.Bool:
			entries = append(entries, &cffi.HostMapEntry{
				Key:   &cffi.HostMapEntry_BoolKey{BoolKey: key.Bool()},
				Value: valueHolder,

View on GitHub (pinned to bd85ce9dee)