What causes "failed to write: %v is not type of *[]byte or []byte" in micro/go-micro?

Marshal your payload to []byte before writing (json.Marshal or []byte(str)).

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 bytes codec's Write method only accepts bodies of type *Frame, *[]byte, or []byte, since it writes raw bytes straight to the connection. Any other type fails with this error before any network write occurs, so the connection is left unmodified.

Source

Thrown at codec/bytes/bytes.go:53

		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)
	return err
}

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

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

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

View on GitHub (pinned to 24529f1404)

Solutions

Marshal your payload to []byte before writing (json.Marshal or []byte(str)).
Pass *Frame or *[]byte if the caller provides a pointer.
Register the correct codec (jsonrpc/grpc) for structured messages.
Inspect the call site (e.g. serveReq) to see what body type it builds and align it with the codec.

Example fix

// before
c.Write(m, "hello")
// after
c.Write(m, []byte("hello"))

Defensive patterns

Strategy: type-guard

Validate before calling

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

Type guard

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

Try / catch

body, ok := toBytes(payload)
if !ok {
	body, _ = json.Marshal(payload) // marshal structured payloads first
}
if err := c.Write(m, body); err != nil { return err }

Prevention

Marshal strings/structs to []byte before writing with the bytes codec.
Confirm the codec type before assuming structured writes work.
Keep a helper that normalizes payloads to []byte at the transport boundary.
Cover the write path with a codec-specific integration test.

When it happens

Trigger: Calling codec.Write (via serveReq or client send path) on a bytes.Codec with a body that is not []byte, *[]byte, or *Frame, e.g. a string or struct.

Common situations: Publishing/requesting with a payload that was built for the JSON codec; handlers marshaling to string instead of []byte; framework misconfiguration where bytes codec is registered but request/response bodies are structured.

Related errors

AI-assisted analysis of micro/go-micro@24529f1404 (2026-09-01). Data as JSON: /api/errors/828c2d1c7f92fa08. Report an issue: GitHub.