micro/go-micro · error

failed to write: %v is not type of *[]byte or []byte

Error message

failed to write: %v is not type of *[]byte or []byte

What it means

The text codec's Write accepts only *Frame, *[]byte, *string, string, and []byte payloads; anything else returns this error. The codec writes bytes verbatim to the connection and cannot serialize structured values.

Source

Thrown at codec/text/text.go:59

	return nil
}

func (c *Codec) Write(m *codec.Message, b interface{}) error {
	var v []byte
	switch ve := b.(type) {
	case *Frame:
		v = ve.Data
	case *[]byte:
		v = *ve
	case *string:
		v = []byte(*ve)
	case string:
		v = []byte(ve)
	case []byte:
		v = ve
	default:
		return fmt.Errorf("failed to write: %v is not type of *[]byte or []byte", b)
	}
	_, err := c.Conn.Write(v)
	return err
}

func (c *Codec) Close() error {
	return c.Conn.Close()
}

func (c *Codec) String() string {
	return "text"
}

func NewCodec(c io.ReadWriteCloser) codec.Codec {
	return &Codec{
		Conn: c,
	}
}

View on GitHub (pinned to 24529f1404)

Solutions

1. Marshal the value yourself (json.Marshal or proto.Marshal) and pass the resulting []byte or *Frame
2. Use *text.Frame{Data: payload} to make raw intent explicit
3. Switch the topic/service content type to json or proto so serialization happens for you
4. For handlers, return *[]byte or *Frame rather than typed structs when using the text codec

Example fix

// before
c.Write(msg, MyStruct{A: 1}) // failed to write
// after
data, _ := json.Marshal(MyStruct{A: 1})
c.Write(msg, &text.Frame{Data: data})

func writableBody(b interface{}) ([]byte, bool) {
    switch v := b.(type) {
    case *text.Frame:
        return v.Data, true
    case *[]byte:
        return *v, true
    case *string:
        return []byte(*v), true
    case string:
        return []byte(v), true
    case []byte:
        return v, true
    }
    return nil, false
}

func isTextWritable(b interface{}) bool {
    switch b.(type) {
    case *text.Frame, *[]byte, *string, string, []byte:
        return true
    default:
        return false
    }
}

When it happens

Trigger: Calling text codec Write with a struct, map, proto.Message, or nil body while the text codec (content-type text/*) is selected.

Common situations: Publishing typed events over a text/* topic; returning structured responses from handlers when the client negotiates text; forgetting to marshal a struct to JSON/bytes before publishing with the raw/text codec.

Related errors

• failed to read body: %v is not type of *[]byte
• ErrInvalidMessage
• unknown request path
• unsupported Content-Type
• failed to read body: %v is not type of *[]byte

AI-assisted analysis of micro/go-micro@24529f1404 (2026-09-01). Data as JSON: /api/errors/3b91c84a205dc66f. Report an issue: GitHub.