kestra-io/kestra · error · PebbleException

The 'loopOutputs' function expects 'outputs' to be a list.

Error message

The 'loopOutputs' function expects 'outputs' to be a list.

What it means

The loopOutputs() function checks that the 'outputs' argument is an instance of List<?>. If it is non-null but not a list (e.g. a Map, a String, or a single Object), this type-check error fires. Loop task iteration outputs are expected to be a list of per-iteration output maps.

Source

Thrown at core/src/main/java/io/kestra/core/runners/pebble/functions/LoopOutputsFunction.java:42

    @Override
    public List<String> getArgumentNames() {
        return ARGUMENT_NAMES;
    }

    @Override
    public Map<String, String> getArgumentDefaults() {
        return ARGUMENT_DEFAULTS;
    }

    @Override
    public Object execute(Map<String, Object> args, PebbleTemplate self, EvaluationContext context, int lineNumber) {
        Object outputsArg = args.get(ARG_OUTPUTS);
        if (outputsArg == null) {
            throw new PebbleException(null, "The 'loopOutputs' function expects a non-null argument 'outputs'.", lineNumber, self.getName());
        }
        if (!(outputsArg instanceof List<?>)) {
            throw new PebbleException(null, "The 'loopOutputs' function expects 'outputs' to be a list.", lineNumber, self.getName());
        }

        Object nameArg = args.get(ARG_NAME);
        if (nameArg == null) {
            throw new PebbleException(null, "The 'loopOutputs' function expects a non-null argument 'name'.", lineNumber, self.getName());
        }
        if (!(nameArg instanceof String)) {
            throw new PebbleException(null, "The 'loopOutputs' function expects 'name' to be a string.", lineNumber, self.getName());
        }

        String key = (String) nameArg;
        List<?> loopOutputs = (List<?>) outputsArg;
        List<Object> result = new ArrayList<>(loopOutputs.size());

        for (Object loopOutputObj : loopOutputs) {
            if (loopOutputObj instanceof Map<?, ?> loopOutput) {
                Object iterationOutputs = loopOutput.get(ITERATION_OUTPUTS_KEY);
                result.add(iterationOutputs instanceof Map<?, ?> outputs ? outputs.get(key) : null);

View on GitHub (pinned to 823fada927)

Solutions

1. Ensure the outputs argument points to a Loop task's outputs, which is a list: outputs.<loopTaskId>.outputs.
2. If the data is a single object, restructure the logic — loopOutputs is designed for iteration lists only.

Example fix

# before (regular task output, not a list)
value: "{{ loopOutputs(outputs.myTask, 'result') }}"
# after (Loop task iteration outputs)
value: "{{ loopOutputs(outputs.myLoop.outputs, 'result') }}"

# Verify the argument is a list before calling:
{% if outputs.myLoop.outputs is iterable %}
  {{ loopOutputs(outputs.myLoop.outputs, 'myOutput') }}
{% else %}
  {{ throw('loopOutputs expects a list') }}
{% endif %}

# Pebble 'is iterable' test narrows list types:
{% if myVar is iterable %}...{% endif %}

When it happens

Trigger: Passing a single task's outputs instead of a Loop's iteration list (e.g. outputs.myTask instead of outputs.myLoop.outputs). Passing a Map or a String where a List is expected. The referenced task is not a Loop task and its outputs have a different shape.

Common situations: The author confuses a regular task's output map with a Loop's iteration list. The upstream task type changed from a ForEach/Loop to a single-execution task.

Related errors

• The 'loopOutputs' function expects 'name' to be a string.
• The 'loopOutputs' function expects a non-null argument 'outp
• The 'loopOutputs' function expects a non-null argument 'name
• The 'randomIn()' function expects an argument %s of type Lon
• Unknown value element type: {}

AI-assisted analysis of kestra-io/kestra@823fada927 (2026-08-14). Data as JSON: /api/errors/59c411787c410b5b. Report an issue: GitHub.