invalid length for CopyBothResponse.OverallFormat

Error message

invalid length for CopyBothResponse.OverallFormat

What it means

Returned by CopyBothResponse.UnmarshalJSON when the JSON `OverallFormat` field is not exactly one character long. OverallFormat is a single byte on the wire (0 = text, 1 = binary), so the JSON form must be a one-character string. An empty string, a multi-char value, or a numeric literal fails this check.

Source

Thrown at pgproto3/copy_both_response.go:89

}

// UnmarshalJSON implements encoding/json.Unmarshaler.
func (dst *CopyBothResponse) UnmarshalJSON(data []byte) error {
	// Ignore null, like in the main JSON package.
	if string(data) == "null" {
		return nil
	}

	var msg struct {
		OverallFormat     string
		ColumnFormatCodes []uint16
	}
	if err := json.Unmarshal(data, &msg); err != nil {
		return err
	}

	if len(msg.OverallFormat) != 1 {
		return errors.New("invalid length for CopyBothResponse.OverallFormat")
	}

	dst.OverallFormat = msg.OverallFormat[0]
	dst.ColumnFormatCodes = msg.ColumnFormatCodes
	return nil
}

View on GitHub (pinned to ec1a0befd2)

Solutions

Use the single-character wire form: "0" for text, "1" for binary.
Map full words before unmarshalling: `"text" -> "0"`, `"binary" -> "1"`.
Regenerate fixtures from MarshalJSON output so the field shape matches.
Pre-validate the JSON with a schema requiring a one-char string for OverallFormat.

Example fix

// before
raw := []byte(`{"OverallFormat":"text","ColumnFormatCodes":[0]}`)
err := json.Unmarshal(raw, &cb) // error

// after
raw := []byte(`{"OverallFormat":"0","ColumnFormatCodes":[0]}`)
err := json.Unmarshal(raw, &cb)

Defensive patterns

Strategy: validation

Validate before calling

func validateCopyBothOverallFormat(raw []byte) error {
	var probe struct{ OverallFormat string }
	if err := json.Unmarshal(raw, &probe); err != nil { return err }
	if len(probe.OverallFormat) != 1 {
		return fmt.Errorf("OverallFormat must be one char (\"0\"/\"1\"), got %q", probe.OverallFormat)
	}
	return nil
}

Type guard

func isValidCopyOverallFormat(s string) bool {
	return len(s) == 1 && (s == "0" || s == "1")
}

Try / catch

var cb pgproto3.CopyBothResponse
if err := json.Unmarshal(raw, &cb); err != nil {
    // tell caller OverallFormat must be "0" (text) or "1" (binary)
    return err
}

Prevention

Emit OverallFormat as the single wire character "0" (text) or "1" (binary).
Generate fixtures from MarshalJSON.
Reject numeric OverallFormat in incoming JSON and map words to codes before unmarshalling.

When it happens

Trigger: Calling `json.Unmarshal(data, &copyBoth)` on JSON where `OverallFormat` has length != 1 — e.g. `{"OverallFormat": "text"}` or `{"OverallFormat": 0}` (number, not string).

Common situations: Test fixtures or replay tools serialise OverallFormat as a word ("text"/"binary") or as a JSON number instead of the one-char string the unmarshaler expects. Also seen when a hand-edited fixture leaves the field empty.

Related errors

AI-assisted analysis of jackc/pgx@ec1a0befd2 (2026-08-04). Data as JSON: /data/errors/7b04d912180c9234.json. Report an issue: GitHub.