diff --git a/doc/thinking/2026-04-26-namespace-manager-step20.md b/doc/thinking/2026-04-26-namespace-manager-step20.md
new file mode 100644
index 00000000..0d5f9ef2
--- /dev/null
+++ b/doc/thinking/2026-04-26-namespace-manager-step20.md
@@ -0,0 +1,21 @@
+# 2026-04-26 — NamespaceManager rewrite, step 20
+
+## Цель шага
+
+Сделать первый реальный runtime adapter для `NamespaceManager` в `buildermgr`.
+
+## Что меняем
+
+1. Добавляем buildermgr namespace subscriber.
+2. Adapter переиспользует существующие `envWatcher.AddNamespace()` и `packageWatcher.AddNamespace()`.
+3. `add/resync` path повторяет текущую логику watcher-а:
+   - добавить namespace в resolver;
+   - вызвать env watcher;
+   - вызвать package watcher.
+4. Добавляем unit test на вызов обоих watcher-ов.
+
+## Что НЕ меняем
+
+- не подключаем subscriber к `StartNSWatcher()`;
+- не меняем remove behavior;
+- не ломаем текущий production flow.
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diff --git a/pkg/buildermgr/namespace_subscriber.go b/pkg/buildermgr/namespace_subscriber.go
new file mode 100644
index 00000000..12c0b327
--- /dev/null
+++ b/pkg/buildermgr/namespace_subscriber.go
@@ -0,0 +1,44 @@
+package buildermgr
+package buildermgr
+
+import (
+	"context"
+
+	"github.com/fission/fission/pkg/utils"
+	"github.com/fission/fission/pkg/utils/manager"
+)
+
+type builderEnvNamespaceAdder interface {
+	AddNamespace(ctx context.Context, ns string, mgr manager.Interface)
+}
+
+type builderPkgNamespaceAdder interface {
+	AddNamespace(ctx context.Context, ns string, mgr manager.Interface)
+}
+
+func NewNamespaceSubscriber(envw builderEnvNamespaceAdder, pkgw builderPkgNamespaceAdder, mgr manager.Interface) utils.NamespaceSubscriber {
+	return utils.NamespaceSubscriberFuncs{
+		SubscriberName: "buildermgr",
+		AddFunc: func(ctx context.Context, record utils.NamespaceRecord) error {
+			registerBuilderNamespace(ctx, record.Name, envw, pkgw, mgr)
+			return nil
+		},
+		ResyncFunc: func(ctx context.Context, record utils.NamespaceRecord) error {
+			registerBuilderNamespace(ctx, record.Name, envw, pkgw, mgr)
+			return nil
+		},
+	}
+}
+
+func registerBuilderNamespace(ctx context.Context, namespace string, envw builderEnvNamespaceAdder, pkgw builderPkgNamespaceAdder, mgr manager.Interface) {
+	if namespace == "" {
+		return
+	}
+	utils.DefaultNSResolver().AddNamespace(namespace)
+	if envw != nil {
+		envw.AddNamespace(ctx, namespace, mgr)
+	}
+	if pkgw != nil {
+		pkgw.AddNamespace(ctx, namespace, mgr)
+	}
+}
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diff --git a/pkg/buildermgr/namespace_subscriber_test.go b/pkg/buildermgr/namespace_subscriber_test.go
new file mode 100644
index 00000000..d82c9f84
--- /dev/null
+++ b/pkg/buildermgr/namespace_subscriber_test.go
@@ -0,0 +1,52 @@
+package buildermgr
+
+import (
+	"context"
+	"testing"
+
+	"github.com/fission/fission/pkg/utils"
+	"github.com/fission/fission/pkg/utils/manager"
+)
+
+type fakeBuilderEnvNamespaceAdder struct {
+	lastNamespace string
+	calls         int
+}
+
+func (f *fakeBuilderEnvNamespaceAdder) AddNamespace(ctx context.Context, ns string, mgr manager.Interface) {
+	f.lastNamespace = ns
+	f.calls++
+}
+
+type fakeBuilderPkgNamespaceAdder struct {
+	lastNamespace string
+	calls         int
+}
+
+func (f *fakeBuilderPkgNamespaceAdder) AddNamespace(ctx context.Context, ns string, mgr manager.Interface) {
+	f.lastNamespace = ns
+	f.calls++
+}
+
+func TestNewNamespaceSubscriberAddAndResync(t *testing.T) {
+	envw := &fakeBuilderEnvNamespaceAdder{}
+	pkgw := &fakeBuilderPkgNamespaceAdder{}
+	subscriber := NewNamespaceSubscriber(envw, pkgw, nil)
+	record := utils.NamespaceRecord{Name: "tenant-builder-a"}
+
+	if subscriber.Name() != "buildermgr" {
+		t.Fatalf("expected buildermgr subscriber name")
+	}
+	if err := subscriber.OnNamespaceAdd(context.Background(), record); err != nil {
+		t.Fatalf("expected add to succeed: %v", err)
+	}
+	if err := subscriber.OnNamespaceResync(context.Background(), record); err != nil {
+		t.Fatalf("expected resync to succeed: %v", err)
+	}
+	if envw.calls != 2 || pkgw.calls != 2 {
+		t.Fatalf("expected both watchers to be called for add and resync")
+	}
+	if envw.lastNamespace != "tenant-builder-a" || pkgw.lastNamespace != "tenant-builder-a" {
+		t.Fatalf("expected namespace to be forwarded to both watchers")
+	}
+}
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