micro/go-micro · error

failed to read body: %v is not type of *[]byte

Error message

failed to read body: %v is not type of *[]byte

What it means

The bytes codec's Read method decodes a raw byte buffer into the destination body, which must be *[]byte or *Frame. If the caller passes any other pointer type (or nil of an unsupported type), the type switch falls through to default and this error is returned, leaving the destination untouched.

Source

Thrown at codec/bytes/bytes.go:37

func (c *Codec) ReadHeader(m *codec.Message, t codec.MessageType) error {
	return nil
}

func (c *Codec) ReadBody(b interface{}) error {
	// read bytes
	buf, err := io.ReadAll(c.Conn)
	if err != nil {
		return err
	}

	switch v := b.(type) {
	case *[]byte:
		*v = buf
	case *Frame:
		v.Data = buf
	default:
		return fmt.Errorf("failed to read body: %v is not type of *[]byte", b)
	}

	return nil
}

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

View on GitHub (pinned to 24529f1404)

Solutions

1. Pass a *[]byte as the body destination.
2. Or pass *Frame if you need frame metadata (v.Data).
3. Ensure the bytes codec is only used where the body contract is raw bytes; use the json codec for structured types.
4. Check which codec your client/server was constructed with (micro client.Codec options).

Example fix

// before
var out map[string]interface{}
c.Read(m, &out)
// after
var out []byte
c.Read(m, &out)

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

func asBytesBody(b interface{}) (*[]byte, bool) {
	p, ok := b.(*[]byte)
	return p, ok
}

var out []byte
if err := c.Read(m, &out); err != nil {
	if strings.Contains(err.Error(), "is not type of") {
		// wrong codec or body type; reconfigure or decode differently
	}
	return err
}

When it happens

Trigger: Calling codec.Read/ReadBody on a bytes.Codec with a body argument other than *[]byte or *Frame, e.g. *string, *map[string]interface{}, or a struct pointer.

Common situations: Users swapping codecs (e.g. from json codec to bytes codec) without changing the message body type; generic helper code that passes interface{} bodies assuming JSON unmarshaling; wrong codec registered on client/server options.

Related errors

• failed to write: %v is not type of *[]byte or []byte
• failed to marshal: %v is not type of *[]byte
• failed to unmarshal: %v is not type of *[]byte
• failed to read body: %v is not type of *[]byte
• failed to write: %v is not type of *[]byte or []byte

AI-assisted analysis of micro/go-micro@24529f1404 (2026-09-01). Data as JSON: /api/errors/855e95deefc88cc5. Report an issue: GitHub.