Schema registry: Adding required field on JSON schema is allowed on BACKWARD compatibility mode

Redpanda version: v26.1.6 I'm using the docker-compose file from this page: https://docs.redpanda.com/redpanda-labs/docker-compose/single-broker/#run-the-lab I can reproduce this behavior using the web-ui or directly with the schema registry API. I think there's an issue with the JSON schema validation, adding a required field is allowed when the compatibility is set on `BACKWARD`. ### What happened: For example if I add this schema to a subject: ```json { "$schema": "http://json-schema.org/draft-07/schema#", "type": "object", "properties": { "name": { "type": "string" } }, "required": [ "name" ], "additionalProperties": false } ``` I'm allowed to add this one afterwards: ```json { "$schema": "http://json-schema.org/draft-07/schema#", "type": "object", "properties": { "name": { "type": "string" }, "id": { "type": "integer" } }, "required": [ "name", "id" ], "additionalProperties": false } ``` ### What should have happened instead? The [confluent doc](https://docs.confluent.io/platform/current/schema-registry/fundamentals/schema-evolution.html#compatibility-types) indicates that this should not be possible and the official schema registry is refusing the second version of my schema. I can only add it if I remove the `id` field from the required fields. I tested with `confluentinc/cp-schema-registry:7.7.8` ### How to reproduce the issue? 1. Start redpanda and create a topic (I named it test-json), by default the global configuration of the schema registry is BACKWARD. 2. Upload the first version of the schema ```bash curl -X POST http://localhost:18081/subjects/test-json-value/versions \ -H "Content-Type: application/vnd.schemaregistry.v1+json" \ -d '{ "schemaType": "JSON", "schema": "{\"$schema\":\"http://json-schema.org/draft-07/schema#\",\"type\":\"object\",\"properties\":{\"name\":{\"type\":\"string\"}},\"required\":[\"name\"],\"additionalProperties\":false}" }' ``` 3. Test the compatibility of the new schema ```bash curl -X POST http://localhost:18081/compatibility/subjects/test-json-value/versions/latest \ -H "Content-Type: application/vnd.schemaregistry.v1+json" \ -d '{ "schemaType": "JSON", "schema": "{\"$schema\":\"http://json-schema.org/draft-07/schema#\",\"type\":\"object\",\"properties\":{\"name\":{\"type\":\"string\"},\"id\":{\"type\":\"integer\"}},\"required\":[\"name\",\"id\"],\"additionalProperties\":false}" }' ``` response is: ```json {"is_compatible":true} ``` With the same curl on the confluent schema registry, I get `false` ### Additional information I've tried with `BACKWARD` and `BACKWARD_TRANSITIVE`. I've also tried to set the compatibility on the subject directly and not keeping `DEFAULT` which fallback on the global configuration. I've tried with avro and the behavior is not the same, adding a non-nullable field is not allowed.