diff --git a/CHANGELOG.md b/CHANGELOG.md
index 599bc656..57b19d4b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -30,9 +30,19 @@ async completion.
 
 See https://docs.temporal.io/standalone-activity for the cross-SDK feature overview.
 
+### Fixed
+
+#### `execute_update_with_start_workflow` no longer raises `RPCError NOT_FOUND` on validator rejection
+
+When a `workflow_update_validator` rejected an update sent via
+`Client#execute_update_with_start_workflow` (or `#start_update_with_start_workflow` with
+`wait_for_stage: COMPLETED`), the client polled history for an outcome that was never written
+and surfaced the failure as `Temporalio::Error::RPCError` with code `NOT_FOUND`. The caller now
+correctly receives `Temporalio::Error::WorkflowUpdateFailedError`. (#454)
+
 #### Start Delay for Standalone Activities
 
 `Client#start_activity` and `Client#execute_activity` now accept a `start_delay:` kwarg. When set, the server creates the activity immediately,
 but defers dispatch to a worker until the delay elapses. Retry attempts do not re-apply the delay.
 `ScheduleToStart` and `ScheduleToClose` timeout clocks begin counting after the delay
-elapses; `StartToClose` and `Heartbeat` are unaffected. Currently experimental.
\ No newline at end of file
+elapses; `StartToClose` and `Heartbeat` are unaffected. Currently experimental.
diff --git a/temporalio/lib/temporalio/internal/client/implementation.rb b/temporalio/lib/temporalio/internal/client/implementation.rb
index 9406b73c..1cc7bb67 100644
--- a/temporalio/lib/temporalio/internal/client/implementation.rb
+++ b/temporalio/lib/temporalio/internal/client/implementation.rb
@@ -217,7 +217,7 @@ def start_update_with_start_workflow(input)
             end
 
             # If the user wants to wait until completed, we must poll until outcome if not already there
-            if input.wait_for_stage == Temporalio::Client::WorkflowUpdateWaitStage::COMPLETED && update_resp.outcome
+            if input.wait_for_stage == Temporalio::Client::WorkflowUpdateWaitStage::COMPLETED && !update_resp.outcome
               update_resp.outcome = @client._impl.poll_workflow_update(
                 Temporalio::Client::Interceptor::PollWorkflowUpdateInput.new(
                   workflow_id: start_options.id,
diff --git a/temporalio/test/worker_workflow_handler_test.rb b/temporalio/test/worker_workflow_handler_test.rb
index b07a1dff..634ac760 100644
--- a/temporalio/test/worker_workflow_handler_test.rb
+++ b/temporalio/test/worker_workflow_handler_test.rb
@@ -691,6 +691,16 @@ def fail
       raise Temporalio::Error::ApplicationError, 'Intentional failure'
     end
 
+    workflow_update
+    def rejected_by_validator(value)
+      raise "Update handler should not run when validator rejects (got #{value})"
+    end
+
+    workflow_update_validator :rejected_by_validator
+    def validate_rejected_by_validator(value)
+      raise 'Validator rejection reason' if value.negative?
+    end
+
     workflow_update
     def start_waiting
       Temporalio::Workflow.wait_condition { @finish_waiting }
@@ -784,6 +794,30 @@ def test_update_with_start_update_failure
     end
   end
 
+  def test_update_with_start_validator_rejection
+    worker = Temporalio::Worker.new(
+      client: env.client,
+      task_queue: "tq-#{SecureRandom.uuid}",
+      workflows: [UpdateWithStartWorkflow]
+    )
+    worker.run do
+      id = "wf-#{SecureRandom.uuid}"
+      start_workflow_operation = Temporalio::Client::WithStartWorkflowOperation.new(
+        UpdateWithStartWorkflow, 123,
+        id:, task_queue: worker.task_queue, id_conflict_policy: Temporalio::WorkflowIDConflictPolicy::FAIL
+      )
+      err = assert_raises(Temporalio::Error::WorkflowUpdateFailedError) do
+        env.client.execute_update_with_start_workflow(
+          UpdateWithStartWorkflow.rejected_by_validator, -1,
+          start_workflow_operation:
+        )
+      end
+      assert_instance_of Temporalio::Error::ApplicationError, err.cause
+      assert_equal 'Validator rejection reason', err.cause.message
+      assert_equal 123, start_workflow_operation.workflow_handle.query(UpdateWithStartWorkflow.counter)
+    end
+  end
+
   def test_update_with_start_cancel
     # Run worker
     worker = Temporalio::Worker.new(