# SchemaRegistryException: schema is not backward-compatible — field 'phone_number' added without default in version 3

- **ID:** `data/avro-schema-registry-backward-transitive-failure`
- **Domain:** data
- **Category:** schema_error
- **Error Code:** `42201`
- **Verification:** ai_generated
- **Fix Rate:** 90%

## Root Cause

Avro schema evolution requires new fields to have default values when backward compatibility is enforced; adding a field without a default breaks consumers that read old data.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| Confluent Schema Registry 7.5.0 | active | — | — |
| Apache Avro 1.11.3 | active | — | — |
| Kafka 3.6.0 | active | — | — |

## Workarounds

1. **Update the Avro schema to add the field with a default value: `{"name": "phone_number", "type": ["null", "string"], "default": null}` and register as version 4 with backward compatibility** (95% success)
   ```
   Update the Avro schema to add the field with a default value: `{"name": "phone_number", "type": ["null", "string"], "default": null}` and register as version 4 with backward compatibility
   ```
2. **If the field must be required, change compatibility to FORWARD_TRANSITIVE, register the new schema, then update consumers to handle the new field before switching back to BACKWARD** (85% success)
   ```
   If the field must be required, change compatibility to FORWARD_TRANSITIVE, register the new schema, then update consumers to handle the new field before switching back to BACKWARD
   ```
3. **Use Schema Registry REST API to set compatibility to BACKWARD_TRANSITIVE on the subject, then re-register: `curl -X PUT -H "Content-Type: application/json" --data '{"compatibility": "BACKWARD_TRANSITIVE"}' http://localhost:8081/config/<subject>`** (90% success)
   ```
   Use Schema Registry REST API to set compatibility to BACKWARD_TRANSITIVE on the subject, then re-register: `curl -X PUT -H "Content-Type: application/json" --data '{"compatibility": "BACKWARD_TRANSITIVE"}' http://localhost:8081/config/<subject>`
   ```

## Dead Ends

- **** — Schema Registry enforces compatibility globally; deleting a version may break existing producers/consumers and cause data loss if they reference that version ID. (80% fail)
- **** — Disabling compatibility entirely removes all checks, allowing any schema change, which can cause silent data corruption for downstream consumers that expect a specific structure. (75% fail)
- **** — The producer will fail to serialize because the schema must match the registered schema exactly (Avro writer schema validation). (100% fail)