kestra-io/kestra · error · IllegalArgumentException

Unsupported type for key: {key}, value: {value}

Error message

Unsupported type for key: {key}, value: {value}

What it means

The `ListOrMapOfLabelDeserializer` parses a `labels` field that can be either a JSON array of `{key, value}` objects or a JSON map. When parsing the array form, each label's `value` is type-checked via `isAllowedType()`. Only String, Integer, Long, Float, Double, and Boolean are accepted. If the value is a nested object, array, or null (in some cases), an `IllegalArgumentException` is thrown with the offending key and value.

Source

Thrown at core/src/main/java/io/kestra/core/serializers/ListOrMapOfLabelDeserializer.java:35

 * This deserializer is for historical purpose, labels was first a map but has been updated to a List of Label so
 * this deserializer allows using both types.
 */
public class ListOrMapOfLabelDeserializer extends JsonDeserializer<List<Label>> implements ResolvableDeserializer {
    @SuppressWarnings("unchecked")
    @Override
    public List<Label> deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        if (p.hasToken(JsonToken.VALUE_NULL)) {
            return null;
        } else if (p.hasToken(JsonToken.START_ARRAY)) {
            // deserialize as list
            List<Map<String, String>> ret = ctxt.readValue(p, List.class);
            return ret.stream().map(map ->
            {
                Object value = map.get("value");
                if (isAllowedType(value)) {
                    return new Label(map.get("key"), String.valueOf(value));
                } else {
                    throw new IllegalArgumentException("Unsupported type for key: " + map.get("key") + ", value: " + value);
                }
            }).toList();
        } else if (p.hasToken(JsonToken.START_OBJECT)) {
            // deserialize as map
            Map<String, Object> ret = ctxt.readValue(p, Map.class);
            return ret == null ? null
                : ret.entrySet().stream()
                    .map(this::validateAndCreateLabel)
                    .toList();
        }
        throw new IllegalArgumentException("Unable to deserialize value as it's neither an object neither an array");
    }

    private Label validateAndCreateLabel(Map.Entry<String, Object> entry) {
        Object value = entry.getValue();
        if (isAllowedType(value)) {
            return new Label(entry.getKey(), String.valueOf(value));
        } else {

View on GitHub (pinned to 823fada927)

Solutions

1. Ensure every label value in the array form is a scalar: String, number, or boolean.
2. If you need complex metadata, serialize it to a JSON string first.
3. Validate the labels payload against the expected schema before submission.

Example fix

# before
labels:
  - key: meta
    value:
      nested: true
# after
labels:
  - key: meta
    value: '{"nested":true}'

// Validate label values before serialization
import io.kestra.core.models.Label;

public static List<Label> validateLabels(List<Label> labels) {
    for (Label label : labels) {
        Object value = label.value();
        if (!(value instanceof String || value instanceof Integer || value instanceof Long
            || value instanceof Float || value instanceof Double || value instanceof Boolean)) {
            throw new IllegalArgumentException(
                "Label '" + label.key() + "' has unsupported value type: " + value.getClass());
        }
    }
    return labels;
}

function isAllowedLabelValue(v: unknown): boolean {
    const t = typeof v;
    return t === 'string' || t === 'number' || t === 'boolean';
}

When it happens

Trigger: Submitting a flow or execution with a labels array where a value is a JSON object or array, e.g. `[{"key":"meta","value":{"nested":true}}]`. Providing a null value in the array form.

Common situations: An API consumer sends a complex/nested value where a scalar is expected. A serialization issue causes a value to be sent as a map or list instead of a string.

Related errors

• Unsupported type for key: {entry.getKey()}, value: {value}
• Unable to deserialize value as it's neither an object neithe
• The 'subflow' function 'labels' must be a map of string keys
• The 'subflow' function 'timeout' must be an ISO-8601 duratio
• The 'yaml' function expects an argument 'yaml' with type str

AI-assisted analysis of kestra-io/kestra@823fada927 (2026-08-14). Data as JSON: /api/errors/9f6a165435bc725a. Report an issue: GitHub.