flowable/flowable-engine · error · FlowableIllegalArgumentException

An assignee is required when delegating a task.

Error message

An assignee is required when delegating a task.

What it means

Delegating a task requires a target assignee; the delegate action reads 'assignee' from the action request and throws FlowableIllegalArgumentException (HTTP 400) when it is null. Delegation without a receiver is meaningless to the engine.

Solutions

1. Include "assignee":"<userId>" in the action request body.
2. Validate the assignee field in the client before dispatching the delegate action.
3. If no assignee exists, use action 'claim' or 'complete' instead of delegate.
4. Resolve a fallback assignee (e.g. group lead) when the selected user is null.

Example fix

// before
{"action":"delegate"}

// after
{"action":"delegate","assignee":"kermit"}

if (!body.assignee) throw new Error('assignee is required for the delegate action');

const canDelegate = (req) => typeof req.assignee === 'string' && req.assignee.length > 0;

try { await api.post(`/cmmn-runtime/tasks/${id}`, {action:'delegate', assignee}); } catch (e) { if (e.response && e.response.status === 400) { /* prompt for an assignee */ } throw e; }

When it happens

Trigger: POST /cmmn-runtime/tasks/{taskId} with action 'delegate' but no assignee field in the request body.

Common situations: UI forms that allow submitting delegate without selecting a user; copy-pasted claim/complete payloads lacking assignee; client code path where assignee resolution returned null.

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

• No variable name was found in request body.
• Transient variable name is required
• Variable name is required
• Variable operation is missing for variable
• A group or a user is required to create an identity link.

AI-assisted analysis of flowable/flowable-engine@d6d39ce1c6 (2026-09-11). Data as JSON: /api/errors/946c9215dab1316d. Report an issue: GitHub.

Appendix: source

Thrown at modules/flowable-cmmn-rest/src/main/java/org/flowable/cmmn/rest/service/api/runtime/task/TaskResource.java:247

                    taskCompletionBuilder.transientVariable(var.getName(), actualVariableValue);
                }
            }
        }

        taskCompletionBuilder
                .taskId(task.getId())
                .formDefinitionId(actionRequest.getFormDefinitionId())
                .outcome(actionRequest.getOutcome())
                .complete();
    }

    protected void resolveTask(Task task, TaskActionRequest actionRequest) {
        taskService.resolveTask(task.getId());
    }

    protected void delegateTask(Task task, TaskActionRequest actionRequest) {
        if (actionRequest.getAssignee() == null) {
            throw new FlowableIllegalArgumentException("An assignee is required when delegating a task.");
        }
        taskService.delegateTask(task.getId(), actionRequest.getAssignee());
    }

    protected void claimTask(Task task, TaskActionRequest actionRequest) {
        // In case the task is already claimed, a
        // FlowableTaskAlreadyClaimedException is thrown and converted to
        // a CONFLICT response by the ExceptionHandlerAdvice
        taskService.claim(task.getId(), actionRequest.getAssignee());
    }

    protected void unclaimTask(Task task) {
        taskService.unclaim(task.getId());
    }
}

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