One of the attributes 'type' or 'operation' is mandatory on…

Error message

One of the attributes 'type' or 'operation' is mandatory on sendTask {}

What it means

A sendTask in the BPMN XML lacks any of the attributes that determine its behavior: a 'type' (e.g. mail/mule/camel) or an 'operation' reference (web service). The parser logs this warning and does not assign an activity behavior, so the send task would do nothing or misbehave at runtime.

Solutions

Set the operationRef attribute (with an interface/operation defined via imports) for a web-service send task
Set the type attribute (e.g. "mail", "mule", "camel") for a built-in send task type
Remove or convert the sendTask to a serviceTask/class implementation if neither applies

Example fix

// before
<sendTask id="send1"/>
// after
<sendTask id="send1" operationRef="tns:sendOperation"/>

Defensive patterns

Strategy: validation

Validate before calling

for (SendTask t : model.getFlowElementsOfType(SendTask.class)) {
    if (isEmpty(t.getType()) && isEmpty(t.getOperationRef())) {
        throw new IllegalArgumentException("sendTask " + t.getId() + " needs 'type' or 'operationRef'");
    }
}

Prevention

Configure type or operationRef whenever a send task is added to a diagram
Include web-service imports/operations when using operationRef
Run process validation as part of the build/deploy pipeline

When it happens

Trigger: executeParse of a SendTask where type is empty AND neither operationRef nor other web-service attributes are set — the final else branch of the type/operation dispatch.

Common situations: Send task dragged into the diagram but never configured; imported BPMN from tools that use different send-task attribute conventions; refactoring that dropped the operationRef.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/c094550ebf240f2b. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable5-engine/src/main/java/org/activiti/engine/impl/bpmn/parser/handler/SendTaskParseHandler.java:61

        activity.setAsync(sendTask.isAsynchronous());
        activity.setExclusive(!sendTask.isNotExclusive());

        if (StringUtils.isNotEmpty(sendTask.getType())) {
            if ("mail".equalsIgnoreCase(sendTask.getType())) {
                activity.setActivityBehavior(bpmnParse.getActivityBehaviorFactory().createMailActivityBehavior(sendTask));
            } else if ("camel".equalsIgnoreCase(sendTask.getType())) {
                activity.setActivityBehavior(bpmnParse.getActivityBehaviorFactory().createCamelActivityBehavior(sendTask, bpmnParse.getBpmnModel()));
            }

            // for web service
        } else if (ImplementationType.IMPLEMENTATION_TYPE_WEBSERVICE.equalsIgnoreCase(sendTask.getImplementationType()) &&
                StringUtils.isNotEmpty(sendTask.getOperationRef())) {

            WebServiceActivityBehavior webServiceActivityBehavior = bpmnParse.getActivityBehaviorFactory().createWebServiceActivityBehavior(sendTask, bpmnParse.getBpmnModel());
            activity.setActivityBehavior(webServiceActivityBehavior);

        } else {
            LOGGER.warn("One of the attributes 'type' or 'operation' is mandatory on sendTask {}", sendTask.getId());
        }
    }

}

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