diff --git a/internal/temporal/worker_deployment.go b/internal/temporal/worker_deployment.go
index ce845c3f..47d569a4 100644
--- a/internal/temporal/worker_deployment.go
+++ b/internal/temporal/worker_deployment.go
@@ -137,6 +137,57 @@ func GetWorkerDeploymentState(
 		state.Versions[version.DeploymentVersion.BuildId] = versionInfo
 	}
 
+	// A version could be missing from the VersionSummaries in the odd event that there is
+	// state divergence between the deployment workflow's local state and the actual versions
+	// that are present. For versions that are known to TWC but absent from the Worker Deployment
+	// description's version summaries, double-check their state before allowing them to map to
+	// NotRegistered and get scaled down.
+	for buildID := range k8sDeployments {
+		if _, exists := state.Versions[buildID]; exists {
+			continue
+		}
+
+		desc, err := client.WorkflowService().DescribeWorkerDeploymentVersion(
+			ctx,
+			&workflowservice.DescribeWorkerDeploymentVersionRequest{
+				Namespace: namespace,
+				DeploymentVersion: &deploymentpb.WorkerDeploymentVersion{
+					DeploymentName: workerDeploymentName,
+					BuildId:        buildID,
+				},
+			},
+		)
+		if err != nil {
+			var notFound *serviceerror.NotFound
+			if errors.As(err, &notFound) {
+				// This means that the version is truly absent from both entities, i.e, from the worker-deployment summary list
+				// and that there is no presence of it's own version workflow in Temporal. This is enough evidence to conclude that we can scale
+				// this k8s Deployment down.
+				continue
+			}
+			return nil, fmt.Errorf("unable to describe worker deployment version for buildID %q: %w", buildID, err)
+		}
+
+		info := desc.GetWorkerDeploymentVersionInfo()
+		if info == nil || info.GetDeploymentVersion() == nil {
+			return nil, fmt.Errorf("describe worker deployment version for buildID %q returned no version info", buildID)
+		}
+
+		versionInfo := versionInfoFromVersionSummary(
+			ctx, l, client, targetBuildID, strategy, depHandle, routingConfig,
+			&deploymentpb.WorkerDeploymentInfo_WorkerDeploymentVersionSummary{
+				DeploymentVersion: info.GetDeploymentVersion(),
+				Status:            info.GetStatus(),
+				DrainageInfo:      info.GetDrainageInfo(),
+				LastCurrentTime:   info.GetLastCurrentTime(),
+			},
+		)
+		if versionInfo == nil || versionInfo.Status == temporaliov1alpha1.VersionStatusNotRegistered {
+			return nil, fmt.Errorf("describe worker deployment version for buildID %q returned no registered status", buildID)
+		}
+		state.Versions[buildID] = versionInfo
+	}
+
 	return state, nil
 }
 
diff --git a/internal/tests/go.mod b/internal/tests/go.mod
index 2c5361c7..fd689167 100644
--- a/internal/tests/go.mod
+++ b/internal/tests/go.mod
@@ -7,6 +7,8 @@ require (
 	go.temporal.io/api v1.62.8
 	go.temporal.io/sdk v1.41.1
 	go.temporal.io/server v1.31.2
+	google.golang.org/grpc v1.83.0
+	google.golang.org/protobuf v1.36.12
 	k8s.io/api v0.36.3
 	k8s.io/apimachinery v0.36.3
 	k8s.io/client-go v0.36.3
@@ -197,8 +199,6 @@ require (
 	google.golang.org/genproto v0.0.0-20260420184626-e10c466a9529 // indirect
 	google.golang.org/genproto/googleapis/api v0.0.0-20260526163538-3dc84a4a5aaa // indirect
 	google.golang.org/genproto/googleapis/rpc v0.0.0-20260818201246-1b0934165a6f // indirect
-	google.golang.org/grpc v1.83.0 // indirect
-	google.golang.org/protobuf v1.36.12 // indirect
 	gopkg.in/evanphx/json-patch.v4 v4.13.0 // indirect
 	gopkg.in/inf.v0 v0.9.1 // indirect
 	gopkg.in/validator.v2 v2.0.1 // indirect
diff --git a/internal/tests/internal/integration_test.go b/internal/tests/internal/integration_test.go
index 8d39b6ac..7eee0923 100644
--- a/internal/tests/internal/integration_test.go
+++ b/internal/tests/internal/integration_test.go
@@ -25,7 +25,7 @@ type testCase struct {
 // TestIntegration runs integration tests for the Temporal Worker Controller
 func TestIntegration(t *testing.T) {
 	// Set up test environment
-	cfg, k8sClient, mgr, _, cleanup := setupTestEnvironment(t)
+	cfg, k8sClient, mgr, clientPool, cleanup := setupTestEnvironment(t)
 	defer cleanup()
 
 	// Create test namespace
@@ -991,6 +991,9 @@ func TestIntegration(t *testing.T) {
 	// Conditions and events tests
 	runConditionsAndEventsTests(t, k8sClient, mgr, ts, testNamespace.Name)
 
+	// Version-summary divergence safety test
+	runNotRegisteredVersionTests(t, k8sClient, clientPool, ts, testNamespace.Name)
+
 	// Rate limit test: uses a dedicated server to avoid interfering with the tests above.
 	// Ten TWDs in the same Temporal namespace produce 10 concurrent DescribeWorkerDeployment
 	// calls per second against a 1 RPS limit, reliably triggering ResourceExhausted errors.
diff --git a/internal/tests/internal/not_registered_integration_test.go b/internal/tests/internal/not_registered_integration_test.go
new file mode 100644
index 00000000..fda15244
--- /dev/null
+++ b/internal/tests/internal/not_registered_integration_test.go
@@ -0,0 +1,296 @@
+// Unless explicitly stated otherwise all files in this repository are licensed under the MIT License.
+//
+// This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2024 Datadog, Inc.
+
+package internal
+
+import (
+	"context"
+	"fmt"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	temporaliov1alpha1 "github.com/temporalio/temporal-worker-controller/api/v1alpha1"
+	"github.com/temporalio/temporal-worker-controller/internal/controller/clientpool"
+	"github.com/temporalio/temporal-worker-controller/internal/k8s"
+	"github.com/temporalio/temporal-worker-controller/internal/testhelpers"
+	"go.temporal.io/api/workflowservice/v1"
+	sdkclient "go.temporal.io/sdk/client"
+	sdkworker "go.temporal.io/sdk/worker"
+	"go.temporal.io/sdk/workflow"
+	"go.temporal.io/server/temporaltest"
+	"google.golang.org/grpc"
+	"google.golang.org/protobuf/proto"
+	appsv1 "k8s.io/api/apps/v1"
+	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/apimachinery/pkg/types"
+	"sigs.k8s.io/controller-runtime/pkg/client"
+)
+
+const issue550PinnedWorkflowType = "issue550PinnedWorkflow"
+
+type versionSummaryFilteringClient struct {
+	sdkclient.Client
+	workflowService workflowservice.WorkflowServiceClient
+}
+
+func (c *versionSummaryFilteringClient) WorkflowService() workflowservice.WorkflowServiceClient {
+	return c.workflowService
+}
+
+type versionSummaryFilteringWorkflowService struct {
+	workflowservice.WorkflowServiceClient
+	deploymentName string
+	omitBuildID    string
+	filteredCalls  atomic.Int32
+}
+
+func (s *versionSummaryFilteringWorkflowService) DescribeWorkerDeployment(
+	ctx context.Context,
+	req *workflowservice.DescribeWorkerDeploymentRequest,
+	opts ...grpc.CallOption,
+) (*workflowservice.DescribeWorkerDeploymentResponse, error) {
+	resp, err := s.WorkflowServiceClient.DescribeWorkerDeployment(ctx, req, opts...)
+	if err != nil || req.GetDeploymentName() != s.deploymentName {
+		return resp, err
+	}
+
+	filtered := proto.Clone(resp).(*workflowservice.DescribeWorkerDeploymentResponse)
+	info := filtered.GetWorkerDeploymentInfo()
+	if info == nil {
+		return filtered, nil
+	}
+
+	summaries := info.VersionSummaries[:0]
+	omitted := false
+	for _, summary := range info.VersionSummaries {
+		if summary.GetDeploymentVersion().GetBuildId() == s.omitBuildID {
+			omitted = true
+			continue
+		}
+		summaries = append(summaries, summary)
+	}
+	info.VersionSummaries = summaries
+	if omitted {
+		s.filteredCalls.Add(1)
+	}
+
+	return filtered, nil
+}
+
+func runNotRegisteredVersionTests(
+	t *testing.T,
+	k8sClient client.Client,
+	clientPool *clientpool.ClientPool,
+	ts *temporaltest.TestServer,
+	namespace string,
+) {
+	t.Run("missing-summary-version-is-described-before-deletion", func(t *testing.T) {
+		testMissingSummaryVersionIsDescribedBeforeDeletion(t, k8sClient, clientPool, ts, namespace)
+	})
+}
+
+func testMissingSummaryVersionIsDescribedBeforeDeletion(
+	t *testing.T,
+	k8sClient client.Client,
+	clientPool *clientpool.ClientPool,
+	ts *temporaltest.TestServer,
+	namespace string,
+) {
+	ctx := context.Background()
+	testName := "notregistered-describe"
+
+	tc := testhelpers.NewTestCase().
+		WithInput(
+			testhelpers.NewWorkerDeploymentBuilder().
+				WithManualStrategy().
+				WithTargetTemplate("v1.0"),
+		).
+		BuildWithValues(testName, namespace, ts.GetDefaultNamespace())
+	twd := tc.GetTWD()
+	twd.Spec.SunsetStrategy.ScaledownDelay = &metav1.Duration{Duration: time.Hour}
+	twd.Spec.SunsetStrategy.DeleteDelay = &metav1.Duration{Duration: time.Hour}
+
+	temporalConnection := &temporaliov1alpha1.Connection{
+		ObjectMeta: metav1.ObjectMeta{
+			Name:      twd.Spec.WorkerOptions.ConnectionRef.Name,
+			Namespace: namespace,
+		},
+		Spec: temporaliov1alpha1.ConnectionSpec{HostPort: ts.GetFrontendHostPort()},
+	}
+	if err := k8sClient.Create(ctx, temporalConnection); err != nil {
+		t.Fatalf("failed to create Connection: %v", err)
+	}
+	if err := k8sClient.Create(ctx, twd); err != nil {
+		t.Fatalf("failed to create WorkerDeployment: %v", err)
+	}
+
+	workerDeploymentName := k8s.ComputeWorkerDeploymentName(twd)
+	buildIDv1 := k8s.ComputeBuildID(twd)
+	deploymentNameV1 := k8s.ComputeVersionedDeploymentName(twd.Name, buildIDv1)
+	eventually(t, 30*time.Second, time.Second, func() error {
+		var deployment appsv1.Deployment
+		return k8sClient.Get(ctx, types.NamespacedName{Name: deploymentNameV1, Namespace: namespace}, &deployment)
+	})
+
+	v1Worker, stopV1, err := testhelpers.NewWorker(
+		ctx,
+		workerDeploymentName,
+		buildIDv1,
+		testName,
+		ts.GetFrontendHostPort(),
+		ts.GetDefaultNamespace(),
+		true,
+	)
+	if err != nil {
+		t.Fatalf("failed to create v1 worker: %v", err)
+	}
+	defer stopV1()
+	v1Worker.RegisterWorkflowWithOptions(
+		func(ctx workflow.Context) error {
+			return workflow.Await(ctx, func() bool { return false })
+		},
+		workflow.RegisterOptions{Name: issue550PinnedWorkflowType},
+	)
+	if err := v1Worker.Start(); err != nil {
+		t.Fatalf("failed to start v1 worker: %v", err)
+	}
+
+	var deploymentV1 appsv1.Deployment
+	if err := k8sClient.Get(ctx, types.NamespacedName{Name: deploymentNameV1, Namespace: namespace}, &deploymentV1); err != nil {
+		t.Fatalf("failed to get v1 Deployment: %v", err)
+	}
+	setHealthyDeploymentStatus(t, ctx, k8sClient, deploymentV1)
+	waitForVersionRegistrationInDeployment(t, ctx, ts, &sdkworker.WorkerDeploymentVersion{
+		DeploymentName: workerDeploymentName,
+		BuildID:        buildIDv1,
+	})
+	deploymentHandle := ts.GetDefaultClient().WorkerDeploymentClient().GetHandle(workerDeploymentName)
+	eventually(t, 60*time.Second, time.Second, func() error {
+		desc, err := deploymentHandle.DescribeVersion(
+			ctx,
+			sdkclient.WorkerDeploymentDescribeVersionOptions{BuildID: buildIDv1},
+		)
+		if err != nil {
+			return err
+		}
+		for _, taskQueue := range desc.Info.TaskQueuesInfos {
+			if taskQueue.Name == testName && taskQueue.Type == sdkclient.TaskQueueTypeWorkflow {
+				return nil
+			}
+		}
+		return fmt.Errorf("v1 workflow task queue is not yet registered")
+	})
+	setCurrentVersion(t, ctx, ts, workerDeploymentName, buildIDv1)
+
+	workflowRun, err := ts.GetDefaultClient().ExecuteWorkflow(
+		ctx,
+		sdkclient.StartWorkflowOptions{
+			ID:        testName + "-pinned",
+			TaskQueue: testName,
+			VersioningOverride: &sdkclient.PinnedVersioningOverride{
+				Version: sdkworker.WorkerDeploymentVersion{
+					DeploymentName: workerDeploymentName,
+					BuildID:        buildIDv1,
+				},
+			},
+		},
+		issue550PinnedWorkflowType,
+	)
+	if err != nil {
+		t.Fatalf("failed to start pinned workflow: %v", err)
+	}
+	defer func() {
+		_ = ts.GetDefaultClient().TerminateWorkflow(
+			context.Background(),
+			workflowRun.GetID(),
+			workflowRun.GetRunID(),
+			"integration test cleanup",
+		)
+	}()
+
+	var twdV2 temporaliov1alpha1.WorkerDeployment
+	if err := k8sClient.Get(ctx, types.NamespacedName{Name: twd.Name, Namespace: namespace}, &twdV2); err != nil {
+		t.Fatalf("failed to get WorkerDeployment for v2 update: %v", err)
+	}
+	twdV2.Spec.Template.Spec.Containers[0].Image = "v2.0"
+	buildIDv2 := k8s.ComputeBuildID(&twdV2)
+	deploymentNameV2 := k8s.ComputeVersionedDeploymentName(twd.Name, buildIDv2)
+	if err := k8sClient.Update(ctx, &twdV2); err != nil {
+		t.Fatalf("failed to update WorkerDeployment to v2: %v", err)
+	}
+	eventually(t, 30*time.Second, time.Second, func() error {
+		var deployment appsv1.Deployment
+		return k8sClient.Get(ctx, types.NamespacedName{Name: deploymentNameV2, Namespace: namespace}, &deployment)
+	})
+	stopV2 := applyDeployment(t, ctx, k8sClient, deploymentNameV2, namespace)
+	defer handleStopFuncs(stopV2)
+	setCurrentVersion(t, ctx, ts, workerDeploymentName, buildIDv2)
+
+	eventually(t, 60*time.Second, time.Second, func() error {
+		desc, err := deploymentHandle.DescribeVersion(
+			ctx,
+			sdkclient.WorkerDeploymentDescribeVersionOptions{BuildID: buildIDv1},
+		)
+		if err != nil {
+			return err
+		}
+		if desc.Info.DrainageInfo == nil ||
+			desc.Info.DrainageInfo.DrainageStatus != sdkclient.WorkerDeploymentVersionDrainageStatusDraining {
+			return fmt.Errorf("v1 version is not yet draining")
+		}
+		return nil
+	})
+
+	poolKey := clientpool.ClientPoolKey{
+		HostPort:  temporalConnection.Spec.HostPort,
+		Namespace: twd.Spec.WorkerOptions.TemporalNamespace,
+		AuthMode:  temporaliov1alpha1.AuthModeNoCredentials,
+	}
+	originalClient, ok := clientPool.GetSDKClient(poolKey)
+	if !ok {
+		t.Fatal("controller Temporal client was not present in the client pool")
+	}
+	filteringService := &versionSummaryFilteringWorkflowService{
+		WorkflowServiceClient: originalClient.WorkflowService(),
+		deploymentName:        workerDeploymentName,
+		omitBuildID:           buildIDv1,
+	}
+	clientPool.SetClientForTesting(poolKey, &versionSummaryFilteringClient{
+		Client:          originalClient,
+		workflowService: filteringService,
+	})
+	defer clientPool.SetClientForTesting(poolKey, originalClient)
+
+	// Wait for multiple reconciliations against the filtered summary. Without the
+	// DescribeVersion fallback, the first one classifies v1 as NotRegistered and
+	// deletes its Kubernetes Deployment even though its pinned workflow is still open.
+	eventually(t, 30*time.Second, time.Second, func() error {
+		if filteringService.filteredCalls.Load() < 2 {
+			return fmt.Errorf("waiting for reconciliations with the filtered version summary")
+		}
+
+		var deployment appsv1.Deployment
+		if err := k8sClient.Get(ctx, types.NamespacedName{Name: deploymentNameV1, Namespace: namespace}, &deployment); err != nil {
+			return fmt.Errorf("v1 Deployment was deleted: %w", err)
+		}
+		if deployment.Spec.Replicas == nil || *deployment.Spec.Replicas != 1 {
+			return fmt.Errorf("v1 Deployment replicas = %v, want 1", deployment.Spec.Replicas)
+		}
+
+		var current temporaliov1alpha1.WorkerDeployment
+		if err := k8sClient.Get(ctx, types.NamespacedName{Name: twd.Name, Namespace: namespace}, &current); err != nil {
+			return err
+		}
+		for _, version := range current.Status.DeprecatedVersions {
+			if version.BuildID == buildIDv1 {
+				if version.Status != temporaliov1alpha1.VersionStatusDraining {
+					return fmt.Errorf("v1 status = %s, want Draining", version.Status)
+				}
+				return nil
+			}
+		}
+		return fmt.Errorf("v1 is missing from deprecated versions")
+	})
+}