# org.apache.kafka.connect.errors.ConnectException: Worker config must have exactly one of 'key.converter' or 'value.converter' set, but found 0

- **ID:** `kafka/connect-worker-config-mismatch`
- **Domain:** kafka
- **Category:** config_error
- **Verification:** ai_generated
- **Fix Rate:** 95%

## Root Cause

Kafka Connect worker is missing required converter configuration for key or value serialization, causing startup failure.

## Version Compatibility

| Version | Status | Introduced | Deprecated |
|---------|--------|------------|------------|
| Kafka 3.6.0 | active | — | — |
| Kafka 3.7.0 | active | — | — |
| Kafka Connect 3.6.0 | active | — | — |

## Workarounds

1. **Add `key.converter=org.apache.kafka.connect.json.JsonConverter` and `value.converter=org.apache.kafka.connect.json.JsonConverter` in worker.properties, then restart the worker.** (95% success)
   ```
   Add `key.converter=org.apache.kafka.connect.json.JsonConverter` and `value.converter=org.apache.kafka.connect.json.JsonConverter` in worker.properties, then restart the worker.
   ```
2. **Example fix in worker.properties: `key.converter=org.apache.kafka.connect.storage.StringConverter` and `value.converter=org.apache.kafka.connect.json.JsonConverter` for mixed formats.** (90% success)
   ```
   Example fix in worker.properties: `key.converter=org.apache.kafka.connect.storage.StringConverter` and `value.converter=org.apache.kafka.connect.json.JsonConverter` for mixed formats.
   ```
3. **Check environment variable overrides: `echo $CONNECT_KEY_CONVERTER` and ensure they are not conflicting.** (85% success)
   ```
   Check environment variable overrides: `echo $CONNECT_KEY_CONVERTER` and ensure they are not conflicting.
   ```

## Dead Ends

- **** — The error says exactly one must be set; setting both does not fix the missing configuration. (70% fail)
- **** — Defaults are not guaranteed and may not match the connector's data format. (80% fail)