nektos/act · error

Cannot convert value to JSON. Cause: %v

Error message

Cannot convert value to JSON. Cause: %v

What it means

Thrown by the expression toJSON() function when json.MarshalIndent fails on the value. Common causes: the value contains a channel, func, complex number, or a cyclic data structure that Go's json package cannot serialize.

Source

Thrown at pkg/exprparser/functions.go:162

		var items []string
		for i := 0; i < array.Len(); i++ {
			items = append(items, impl.coerceToString(array.Index(i).Elem()).String())
		}

		return strings.Join(items, separator), nil
	default:
		return strings.Join([]string{impl.coerceToString(array).String()}, separator), nil
	}
}

func (impl *interperterImpl) toJSON(value reflect.Value) (string, error) {
	if value.Kind() == reflect.Invalid {
		return "null", nil
	}

	json, err := json.MarshalIndent(value.Interface(), "", "  ")
	if err != nil {
		return "", fmt.Errorf("Cannot convert value to JSON. Cause: %v", err)
	}

	return string(json), nil
}

func (impl *interperterImpl) fromJSON(value reflect.Value) (interface{}, error) {
	if value.Kind() != reflect.String {
		return nil, fmt.Errorf("Cannot parse non-string type %v as JSON", value.Kind())
	}

	var data interface{}

	err := json.Unmarshal([]byte(value.String()), &data)
	if err != nil {
		return nil, fmt.Errorf("Invalid JSON: %v", err)
	}

	return data, nil

View on GitHub (pinned to 4f41128141)

Solutions

1. Check the %v cause in the message — 'unsupported type' names the offending type.
2. toJSON a narrower sub-object (e.g. toJSON(inputs.x) instead of toJSON(inputs)).
3. In custom Go usage, pre-convert funcs/channels or use json.RawMessage.
4. Verify the value is plain data (maps/slices/strings/numbers/bools).

Example fix

# before
${{ toJSON(github.event) }}
# after (narrow to the needed part)
${{ toJSON(github.event.commits[0].author) }}

func isJSONSerializable(v interface{}) bool {
  switch reflect.TypeOf(v).Kind() {
  case reflect.Chan, reflect.Func, reflect.UnsafePointer, reflect.Complex64, reflect.Complex128:
    return false
  }
  _, err := json.Marshal(v)
  return err == nil
}

if s, err := toJSON(v); err != nil {
  if strings.Contains(err.Error(), 'Cannot convert value to JSON') {
    s = fmt.Sprintf('%v', v) // degrade to plain rendering
  } else { return err }
}

When it happens

Trigger: toJSON(someContextField) where the mapped context value, after act's env conversion, ends up as an unmarshalable Go type; rarely, deeply nested structures hitting encoder limits.

Common situations: Calling toJSON() on unusual contexts; values derived from fromJSON round-trips; custom Go code embedding non-serializable fields into the expression environment.

Related errors

• The following format string is invalid: '%s'
• 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 parse non-string type %v as JSON

AI-assisted analysis of nektos/act@4f41128141 (2026-08-15). Data as JSON: /api/errors/bef48a16f20df183. Report an issue: GitHub.