provectus/kafka-ui · error · ValidationException

' ' is not valid json

Error message

'%s' is not valid json

What it means

serializeJson first parses the supplied value with a JSON mapper before validating it against the registered JSON Schema. If the string cannot be parsed as JSON at all, it throws this ValidationException instead of a schema error, telling you the value is syntactically invalid JSON.

Solutions

1. Validate the value with a JSON parser (jq, jsonlint) before producing
2. Ensure string values are quoted, e.g. "my value" not my value
3. Verify shell/curl quoting didn't strip or double-escape quotes in API calls
4. Check the value string in the error message to see exactly what failed parsing

Example fix

// before
String value = "{'key': 'val'}"; // single quotes are invalid JSON
// after
String value = "{\"key\": \"val\"}";

ObjectMapper m = new ObjectMapper();
try { m.readTree(value); } catch (JsonProcessingException e) {
  throw new IllegalArgumentException("value is not valid JSON: " + e.getOriginalMessage());
}

try {
  produceMessage(topic, value);
} catch (ValidationException e) {
  // value failed JSON parsing; surface message before retry
  System.err.println(e.getMessage());
}

When it happens

Trigger: Producing via the kafka-ui messages API with the Json (Schema Registry JSON schema) serde and a value parameter that fails Jackson readTree parsing - e.g. single quotes instead of double quotes, missing quotes around strings, trailing commas, unescaped control characters, or plain text.

Common situations: Hand-typing a value in the UI produce form without proper JSON quoting; shell/curl quoting mangled the JSON body; copied values that got double-escaped or unescaped; sending a bare string or number without valid JSON syntax.

Understand the failure class

Background: JSON parse error: "Unexpected token" / "not valid JSON" / "failed to parse" — what JSON parsers are really complaining about — this error's family across 45 libraries.

Related errors

• ' ' does not fit schema
• seekTo should be set if seekType is
• Serde can't be applied for ' ' topic's serialization
• Something went wrong during adding replicas
• Something went wrong during removing replicas

AI-assisted analysis of provectus/kafka-ui@83b5a60cc0 (2026-09-08). Data as JSON: /api/errors/e7eecbf01068deeb. Report an issue: GitHub.

Appendix: source

Thrown at kafka-ui-api/src/main/java/com/provectus/kafka/ui/serdes/builtin/sr/Serialize.java:47

import org.apache.avro.io.DatumWriter;
import org.apache.avro.io.EncoderFactory;

final class Serialize {

  private static final byte MAGIC = 0x0;
  private static final ObjectMapper JSON_SERIALIZE_MAPPER = Jackson.newObjectMapper(); //from confluent package

  private Serialize() {
  }

  @KafkaClientInternalsDependant("AbstractKafkaJsonSchemaSerializer::serializeImpl")
  @SneakyThrows
  static byte[] serializeJson(JsonSchema schema, int schemaId, String value) {
    JsonNode json;
    try {
      json = JSON_SERIALIZE_MAPPER.readTree(value);
    } catch (JsonProcessingException e) {
      throw new ValidationException(String.format("'%s' is not valid json", value));
    }
    try {
      schema.validate(json);
    } catch (org.everit.json.schema.ValidationException e) {
      throw new ValidationException(
          String.format("'%s' does not fit schema: %s", value, e.getAllMessages()));
    }
    try (var out = new ByteArrayOutputStream()) {
      out.write(MAGIC);
      out.write(schemaId(schemaId));
      out.write(JSON_SERIALIZE_MAPPER.writeValueAsBytes(json));
      return out.toByteArray();
    }
  }

  @KafkaClientInternalsDependant("AbstractKafkaProtobufSerializer::serializeImpl")
  @SneakyThrows
  static byte[] serializeProto(SchemaRegistryClient srClient,

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