kestra-io/kestra · error · PebbleException

The 'substringBeforeLast' filter expects an argument…

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.

Solutions

1. Add the separator argument: {{ dir | substringBeforeLast(separator="/") }}.
2. Confirm the separator variable is defined and non-null.
3. 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="/") }}

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

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

• The 'substringAfter' filter expects an argument 'separator'.
• The 'substringAfterLast' filter expects an argument…
• The 'substringBefore' filter expects an argument…
• getErrorMessage() [abstract: subclass-defined ERROR_MESSAGE]
• The 'dayOfMonth()' function expects a 'date' argument.

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

Appendix: 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)