Bad startup message version number. Expected %d or %d, got %

Error message

Bad startup message version number. Expected %d or %d, got %d

What it means

Error "Bad startup message version number. Expected %d or %d, got %d" thrown in jackc/pgx.

Source

Thrown at pgproto3/startup_message.go:39

	ProtocolVersion uint32
	Parameters      map[string]string
}

// Frontend identifies this message as sendable by a PostgreSQL frontend.
func (*StartupMessage) Frontend() {}

// Decode decodes src into dst. src must contain the complete message with the exception of the initial 1 byte message
// type identifier and 4 byte message length.
func (dst *StartupMessage) Decode(src []byte) error {
	if len(src) < 4 {
		return errors.New("startup message too short")
	}

	dst.ProtocolVersion = binary.BigEndian.Uint32(src)
	rp := 4

	if dst.ProtocolVersion != ProtocolVersion30 && dst.ProtocolVersion != ProtocolVersion32 {
		return fmt.Errorf("Bad startup message version number. Expected %d or %d, got %d", ProtocolVersion30, ProtocolVersion32, dst.ProtocolVersion)
	}

	dst.Parameters = make(map[string]string)
	for {
		idx := bytes.IndexByte(src[rp:], 0)
		if idx < 0 {
			return &invalidMessageFormatErr{messageType: "StartupMessage"}
		}
		key := string(src[rp : rp+idx])
		rp += idx + 1

		idx = bytes.IndexByte(src[rp:], 0)
		if idx < 0 {
			return &invalidMessageFormatErr{messageType: "StartupMessage"}
		}
		value := string(src[rp : rp+idx])
		rp += idx + 1

View on GitHub (pinned to ec1a0befd2)

When it happens

Trigger: Thrown at pgproto3/startup_message.go:39 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of jackc/pgx@ec1a0befd2 (2026-08-04). Data as JSON: /data/errors/f0f456479985cb42.json. Report an issue: GitHub.