ArduinoJson never intended to be a JSON validation library, so it doesn’t provide any built-in solution. However, you can use the following function to test whether a JSON string seems valid.

This solution doesn’t detect all errors but should be good enough for most projects.

// Returns true if input points to a valid JSON string
bool validateJson(const char* input) {
  StaticJsonDocument<0> doc, filter;
  return deserializeJson(doc, input, DeserializationOption::Filter(filter)) == DeserializationError::Ok;

As you can see, it uses a null filter to skip all the values in the input. Without this filter deserializeJson() would return NoMemory. Because deserializeJson() is skipping the values, it will oversee many errors in the input.

Thank you to Jeroen Döll for finding this technique.