set WorkflowExecutionErrorWhenAlreadyStarted to true

internal/client.go

@@ -687,8 +687,11 @@ type (
 		// When WorkflowExecutionErrorWhenAlreadyStarted is true, Client.ExecuteWorkflow will return an error if the
 		// workflow id has already been used and WorkflowIDReusePolicy or WorkflowIDConflictPolicy would
 		// disallow a re-run. If it is set to false, rather than erroring a WorkflowRun instance representing
-		// the current or last run will be returned. However, when WithStartOperation is set, this field is ignored and
-		// the WorkflowIDConflictPolicy UseExisting must be used instead to prevent erroring.
+		// the current or last run will be returned. However, this field is ignored in the following cases:
+		// - when WithStartOperation is set;
+		// - in the Nexus WorkflowRunOperation.
+		// When this field is ignored, you must set WorkflowIDConflictPolicy to UseExisting to prevent
+		// erroring.
 		//
 		// Optional: defaults to false
 		WorkflowExecutionErrorWhenAlreadyStarted bool

temporalnexus/operation.go

@@ -135,6 +135,7 @@ type WorkflowRunOperationOptions[I, O any] struct {
 	// The options returned must include a workflow ID that is deterministically generated from the input in order
 	// for the operation to be idempotent as the request to start the operation may be retried.
 	// TaskQueue is optional and defaults to the current worker's task queue.
+	// WorkflowExecutionErrorWhenAlreadyStarted is ignored and always set to true.
 	GetOptions func(context.Context, I, nexus.StartOperationOptions) (client.StartWorkflowOptions, error)
 	// Handler for starting a workflow with a different input than the operation. Mutually exclusive with Workflow
 	// and GetOptions.
@@ -401,6 +402,12 @@ func ExecuteUntypedWorkflow[R any](
 	internal.SetLinksOnStartWorkflowOptions(&startWorkflowOptions, links)
 	internal.SetOnConflictOptionsOnStartWorkflowOptions(&startWorkflowOptions)
 
+	// This makes sure that ExecuteWorkflow will respect the WorkflowIDConflictPolicy, ie., if the
+	// conflict policy is to fail (default value), then ExecuteWorkflow will return an error if the
+	// workflow already running. For Nexus, this ensures that operation has only started successfully
+	// when the callback has been attached to the workflow (new or existing running workflow).
+	startWorkflowOptions.WorkflowExecutionErrorWhenAlreadyStarted = true
+
 	run, err := nctx.Client.ExecuteWorkflow(ctx, startWorkflowOptions, workflowType, args...)
 	if err != nil {
 		return nil, err