kestra-io/kestra · error · PebbleException

The 'substringBeforeLast' filter expects an argument 'separa

Error message

The 'substringBeforeLast' filter expects an argument 'separator'.

What it means

The 'substringBeforeLast' filter returns the substring before the LAST occurrence of a separator (StringUtils.substringBeforeLast). It declares 'separator' as a required named argument and throws a PebbleException if args does not contain it when apply() runs.

Source

Thrown at core/src/main/java/io/kestra/core/runners/pebble/filters/SubstringBeforeLastFilter.java:33

    private final List<String> argumentNames = new ArrayList<>();

    public SubstringBeforeLastFilter() {
        this.argumentNames.add("separator");
    }

    @Override
    public List<String> getArgumentNames() {
        return this.argumentNames;
    }

    @Override
    public Object apply(Object input, Map<String, Object> args, PebbleTemplate self, EvaluationContext context, int lineNumber) throws PebbleException {
        if (input == null) {
            return null;
        }

        if (!args.containsKey("separator")) {
            throw new PebbleException(
                null,
                "The 'substringBeforeLast' filter expects an argument 'separator'.",
                lineNumber,
                self.getName()
            );
        }

        String separator = (String) args.get("separator");
        ;

        return StringUtils.substringBeforeLast(input.toString(), separator);
    }
}

View on GitHub (pinned to 823fada927)

Solutions

Add the separator argument: {{ dir | substringBeforeLast(separator="/") }}.
Confirm the separator variable is defined and non-null.
Test the expression in the Kestra expression editor before committing it to the flow.

Example fix

// before
{{ outputs.path | substringBeforeLast }}
// after
{{ outputs.path | substringBeforeLast(separator="/") }}

Defensive patterns

Strategy: validation

Validate before calling

{{ myVar is empty ? '' : myVar | substringBeforeLast(separator=(mySep ?? "/")) }}

Prevention

Pass separator explicitly.
Ensure the feeding variable is non-null.
Test the filter call in the expression editor.

When it happens

Trigger: Writing {{ dir | substringBeforeLast }} when the intent was {{ dir | substringBeforeLast(separator="/") }} (e.g. to drop the last path segment). Also when the separator expression evaluates to null.

Common situations: Path manipulation where the last segment must be removed; forgetting the argument; separator variable undefined after an upstream task produced no output.

Related errors

AI-assisted analysis of kestra-io/kestra@823fada927 (2026-08-14). Data as JSON: /api/errors/f2b268c6b7ae4e8e. Report an issue: GitHub.