`additionalProperties` failure message cryptic and evaluation path not pointing to erroneous path.

[dansiviter]

If using "additionalProperties": false the resulting message is cryptic: evaluationPath=/additionalProperties message=False schema always fails. Would it be possible for it to refer to the unexpected properties to aid in correcting the JSON? e.g. evaluationPath=/foo message=Additional property not expected.

[dansiviter]

Unit test:

class AdditionalPropertiesTest {
    private static final Validator VALIDATOR = new ValidatorFactory().createValidator();
    private static final String SCHEMA = """
        {
          "type": "object",
          "properties": {
            "name": { "type": "string" },
            "age": { "type": "integer" }
          },
          "additionalProperties": false
        }""";

    @Test
    void testAdditionalProperties() {
        var schemaUri = VALIDATOR.registerSchema(SCHEMA);
        var json = """
            {
              "name": "John",
              "age": 30,
              "extra": "not allowed"
            }""";
        var result = VALIDATOR.validate(schemaUri, json);
        assertThat(result.isValid()).isFalse();
        assertThat(result.getErrors())
                .extracting(e -> e.getEvaluationPath() + ": " + e.getError())
                .containsExactly("/additionalProperties: False schema always fails");  // <-- `/extra: Additional property not expected

    }
}

[harrel56]

Hi @dansiviter,

I see how this error message might not be easy to read at first. The reason why it works this way is because value of additionalProperties can be any valid schema (false is a special schema, which always fails validation). And as mandated by the specification: any additional property needs to be validated by this schema and this is how validation error for validation against false schema looks like. And validation against false schema cannot be aware that it is used "inside" additionalProperties. Adding additional error message on additionalProperties might be viable option, but on the other hand it might seem to verbose for other use cases. I also checked a few other JSON Schema implementation and it seems that all of them handle it the same way as I do.

Please also note, that apart from evaluationPath there is a instanceLocation which should point to the problematic property. I believe that much information should be enough to understand what exactly went wrong.

[
  {
    "evaluationPath": "/additionalProperties",
    "schemaLocation": "https://harrel.dev/4f9fa57b#/additionalProperties",
    "instanceLocation": "/extra",
    "keyword": null,
    "error": "False schema always fails"
  }
]

If this is still not enough, I'd suggest following workarounds:

1. Parse all Error objects and transform/translate them for the user to include friendlier error message. You could even aggregate errors from multiple properties into a single error message.
2. Provide a custom implementation of Evaluator for additionalProperties which handles false schemas in a special way. This might be an overkill and a little bit trickier, but if you want to go that way then let me know and I might be able to help you start out.