nektos/act · error

The following format string is invalid: '%s'

Error message

The following format string is invalid: '%s'

What it means

Thrown by the expression format() function when a {...} placeholder's index part cannot be parsed as an integer (strconv.ParseInt fails). GitHub's format() only supports {0}, {1}, ... placeholders, so '{foo}' or '{}' with empty accumulated index text yields this error.

Source

Thrown at pkg/exprparser/functions.go:97

			case '}':
				state = bracketClose

			default:
				output += string(character)
			}

		case bracketOpen: // found {
			switch character {
			case '{':
				output += "{"
				replacementIndex = ""
				state = passThrough

			case '}':
				index, err := strconv.ParseInt(replacementIndex, 10, 32)
				if err != nil {
					return "", fmt.Errorf("The following format string is invalid: '%s'", input)
				}

				replacementIndex = ""

				if len(replaceValue) <= int(index) {
					return "", fmt.Errorf("The following format string references more arguments than were supplied: '%s'", input)
				}

				output += impl.coerceToString(replaceValue[index]).String()

				state = passThrough

			default:
				replacementIndex += string(character)
			}

		case bracketClose: // found }
			switch character {

View on GitHub (pinned to 4f41128141)

Solutions

1. Use positional placeholders: format('Hello {0}', name).
2. Escape literal braces: '{{' and '}}' for literal { and } in the output.
3. For JSON output prefer toJSON() instead of format().
4. Check the workflow locally with actionlint, which flags malformed format strings.

Example fix

# before
run: echo ${{ format('Hi {name}', github.actor) }}
# after
run: echo ${{ format('Hi {0}', github.actor) }}

var ph = regexp.MustCompile(`\{([^}]*)\}`)
func validPlaceholders(s string) bool {
  s = strings.ReplaceAll(s, '{{', '').ReplaceAll(s, '}}', '')
  for _, m := range ph.FindAllStringSubmatch(s, -1) {
    if _, err := strconv.Atoi(m[1]); err != nil { return false }
  }
  return true
}

if _, err := parser.Format(tpl, args...); err != nil {
  if strings.Contains(err.Error(), 'invalid') { tpl = sanitizePlaceholders(tpl) } else { return err }
}

When it happens

Trigger: A workflow expression like format('Hello {name}', inputs.name) — literal braces must be escaped as '{{' and '}}'; format('{a}') with no valid integer; empty index '{}'.

Common situations: Trying to use Python/JS-style named placeholders in format(); writing JSON templates inside format() without doubling braces; migrating from other template engines.

Related errors

• The following format string references more arguments than w
• Unclosed brackets. The following format string is invalid: '
• Closing bracket without opening one. The following format st
• Cannot convert value to JSON. Cause: %v
• Cannot parse non-string type %v as JSON

AI-assisted analysis of nektos/act@4f41128141 (2026-08-15). Data as JSON: /api/errors/74c9ad2fed5ffedc. Report an issue: GitHub.