diff --git a/apps/operator/pages/tenants/[slug].vue b/apps/operator/pages/tenants/[slug].vue
index a35c35d..2ff3dd3 100644
--- a/apps/operator/pages/tenants/[slug].vue
+++ b/apps/operator/pages/tenants/[slug].vue
@@ -85,6 +85,25 @@ type IntegrationKey = (typeof INTEGRATIONS)[number]
 // runs; reusable whenever the first invite failed or someone new takes over.
 const inviteAdminOpen = ref(false)
 
+// Platform-tenant flag toggle. Single holder — the API clears it from every
+// other tenant when set. Grants exactly one thing: claiming the dezky.eu
+// apex as this tenant's customer mail domain.
+const flagBusy = ref(false)
+const flagError = ref<string | null>(null)
+async function setPlatformTenant(value: boolean) {
+  flagBusy.value = true
+  flagError.value = null
+  try {
+    await $fetch(`/api/tenants/${slug.value}`, { method: 'PATCH', body: { isPlatformTenant: value } })
+    await refreshTenant()
+  } catch (err: unknown) {
+    const e = err as { data?: { data?: { message?: string }; message?: string } }
+    flagError.value = e.data?.data?.message ?? e.data?.message ?? String(err)
+  } finally {
+    flagBusy.value = false
+  }
+}
+
 // ── Danger-zone state ─────────────────────────────────────────────────────
 const dangerAction = ref<'suspend' | 'resume' | 'delete' | 'purge' | null>(null)
 const dangerBusy = ref(false)
@@ -145,6 +164,7 @@ async function reconcile() {
       <template #actions>
         <Badge :tone="STATUS_TONE[tenant.status]" dot>{{ tenant.status }}</Badge>
         <Badge tone="neutral">{{ tenant.plan }}</Badge>
+        <Badge v-if="tenant.isPlatformTenant" tone="warn">platform tenant</Badge>
         <UiButton variant="secondary" @click="inviteAdminOpen = true">
           <template #leading><UiIcon name="plus" :size="13" /></template>
           Invite admin
@@ -320,6 +340,21 @@ async function reconcile() {
 
       <!-- DANGER (real) -->
       <div v-else-if="activeTab === 'danger'" class="grid">
+        <Card>
+          <h2>Platform tenant</h2>
+          <p>
+            Marks this as dezky's own tenant (single holder — setting it here clears it
+            anywhere else). Required to claim <Mono>dezky.eu</Mono> as a customer mail
+            domain; everything else treats it as a normal tenant.
+          </p>
+          <UiButton
+            :variant="tenant.isPlatformTenant ? 'secondary' : 'primary'"
+            :disabled="flagBusy"
+            @click="setPlatformTenant(!tenant.isPlatformTenant)"
+          >{{ tenant.isPlatformTenant ? 'Unmark platform tenant' : 'Mark as platform tenant' }}</UiButton>
+          <p v-if="flagError" class="danger-err">{{ flagError }}</p>
+        </Card>
+
         <Card>
           <h2 class="danger">Suspend tenant</h2>
           <p>
diff --git a/apps/operator/types/tenant.ts b/apps/operator/types/tenant.ts
index 72ff13e..c4e117d 100644
--- a/apps/operator/types/tenant.ts
+++ b/apps/operator/types/tenant.ts
@@ -13,6 +13,8 @@ export interface Tenant {
   plan: TenantPlan
   domains: string[]
   partnerId?: string
+  // The company's own tenant (single holder) — may claim the dezky.eu apex.
+  isPlatformTenant?: boolean
   authentikGroupId?: string
   ocisSpaceId?: string
   stalwartDomain?: string
diff --git a/infrastructure/production/fleet/apps/platform-api-config.yaml b/infrastructure/production/fleet/apps/platform-api-config.yaml
index 3d4fd98..0bffeec 100644
--- a/infrastructure/production/fleet/apps/platform-api-config.yaml
+++ b/infrastructure/production/fleet/apps/platform-api-config.yaml
@@ -19,11 +19,10 @@ data:
   STALWART_ADMIN_USER: "admin@dezky.eu"
   STALWART_PROVISIONING_ENABLED: "true"
   # Base for per-tenant service mail domains ({slug}.dezky.eu) AND the
-  # reserved namespace for customer domains: only the company's own tenant
-  # (PLATFORM_TENANT_SLUG) may claim the apex; nothing under it can be added
-  # as a customer domain.
+  # reserved namespace for customer domains: only the tenant flagged
+  # isPlatformTenant (operator-set on the tenant page) may claim the apex;
+  # nothing under it can be added as a customer domain.
   PLATFORM_TENANT_DOMAIN: "dezky.eu"
-  PLATFORM_TENANT_SLUG: "dezky-aps"
   # No auto-seeded tenants in production — the dezky company tenant is
   # created and owned through the operator like any other.
   SEED_ENABLED: "false"
diff --git a/infrastructure/production/fleet/apps/platform-api.yaml b/infrastructure/production/fleet/apps/platform-api.yaml
index 55cf120..4b64f97 100644
--- a/infrastructure/production/fleet/apps/platform-api.yaml
+++ b/infrastructure/production/fleet/apps/platform-api.yaml
@@ -21,7 +21,7 @@ spec:
       annotations:
         # Bump to force a rolling restart when only the ConfigMap changed —
         # pods read it as env, which is only resolved at container start.
-        dezky.eu/config-rev: "4"
+        dezky.eu/config-rev: "5"
     spec:
       containers:
         - name: platform-api
diff --git a/services/platform-api/src/domains/domains.controller.ts b/services/platform-api/src/domains/domains.controller.ts
index fc2ec5a..84b928c 100644
--- a/services/platform-api/src/domains/domains.controller.ts
+++ b/services/platform-api/src/domains/domains.controller.ts
@@ -49,7 +49,7 @@ export class DomainsController {
     if (!actor.platformAdmin && !actor.tenantIds.some((id) => id.equals(tenant._id))) {
       throw new ForbiddenException(`No access to tenant "${slug}"`)
     }
-    return { _id: tenant._id, slug: tenant.slug }
+    return { _id: tenant._id, slug: tenant.slug, isPlatformTenant: tenant.isPlatformTenant }
   }
 
   @Get()
diff --git a/services/platform-api/src/domains/domains.service.guards.spec.ts b/services/platform-api/src/domains/domains.service.guards.spec.ts
index 167939a..1933046 100644
--- a/services/platform-api/src/domains/domains.service.guards.spec.ts
+++ b/services/platform-api/src/domains/domains.service.guards.spec.ts
@@ -46,7 +46,11 @@ function makeService(opts: { takenByOtherTenant?: boolean } = {}) {
   return { svc, created }
 }
 
-const tenant = (slug: string): TenantRef => ({ _id: new Types.ObjectId(), slug })
+const tenant = (slug: string, isPlatformTenant = false): TenantRef => ({
+  _id: new Types.ObjectId(),
+  slug,
+  isPlatformTenant,
+})
 
 const BASE = process.env.PLATFORM_TENANT_DOMAIN || 'dezky.local'
 
@@ -61,35 +65,24 @@ describe('DomainsService.add guards', () => {
     )
   })
 
-  it('rejects the platform apex for any tenant other than dezky', async () => {
+  it('rejects the platform apex for any non-platform tenant', async () => {
     const { svc } = makeService()
     await expect(svc.add(tenant('acme'), BASE, ACTOR)).rejects.toThrow(/reserved/)
+    // Even a tenant that happens to be NAMED dezky — the flag decides, not the slug.
+    await expect(svc.add(tenant('dezky'), BASE, ACTOR)).rejects.toThrow(/reserved/)
   })
 
   it('rejects anything under the platform namespace for everyone', async () => {
     const { svc } = makeService()
     await expect(svc.add(tenant('acme'), `foo.${BASE}`, ACTOR)).rejects.toThrow(/reserved/)
-    // Even the dezky tenant can't claim service/infra hostnames.
-    await expect(svc.add(tenant('dezky'), `mail.${BASE}`, ACTOR)).rejects.toThrow(/reserved/)
+    // Even the platform tenant can't claim service/infra hostnames.
+    await expect(svc.add(tenant('dezky', true), `mail.${BASE}`, ACTOR)).rejects.toThrow(/reserved/)
   })
 
-  it('allows the platform tenant to claim the platform apex', async () => {
+  it('allows the flagged platform tenant to claim the apex, whatever its slug', async () => {
     const { svc, created } = makeService()
-    const view = await svc.add(tenant('dezky'), BASE, ACTOR)
+    const view = await svc.add(tenant('dezky-aps', true), BASE, ACTOR)
     expect(view.domain).toBe(BASE)
     expect(created).toHaveLength(1)
   })
-
-  it('respects PLATFORM_TENANT_SLUG for the apex allowance', async () => {
-    process.env.PLATFORM_TENANT_SLUG = 'dezky-aps'
-    try {
-      const { svc, created } = makeService()
-      await expect(svc.add(tenant('dezky'), BASE, ACTOR)).rejects.toThrow(/reserved/)
-      const view = await svc.add(tenant('dezky-aps'), BASE, ACTOR)
-      expect(view.domain).toBe(BASE)
-      expect(created).toHaveLength(1)
-    } finally {
-      delete process.env.PLATFORM_TENANT_SLUG
-    }
-  })
 })
diff --git a/services/platform-api/src/domains/domains.service.ts b/services/platform-api/src/domains/domains.service.ts
index 6b9d520..a94b094 100644
--- a/services/platform-api/src/domains/domains.service.ts
+++ b/services/platform-api/src/domains/domains.service.ts
@@ -36,6 +36,9 @@ const CHECK_KINDS: RecordKind[] = ['ownership', 'mx', 'spf', 'dkim', 'dmarc']
 export interface TenantRef {
   _id: Types.ObjectId
   slug: string
+  // Set on the company's own tenant — the only one allowed to claim the
+  // PLATFORM_TENANT_DOMAIN apex as a customer mail domain.
+  isPlatformTenant?: boolean
 }
 
 @Injectable()
@@ -80,13 +83,13 @@ export class DomainsService {
     }
 
     // The platform's own namespace is reserved. The apex (PLATFORM_TENANT_DOMAIN,
-    // e.g. dezky.eu) doubles as dezky's employee mail domain — only the company's
-    // own tenant (PLATFORM_TENANT_SLUG, prod: dezky-aps) may claim it. Everything
-    // under it is off limits for everyone: that's where per-tenant service
-    // domains ({slug}.dezky.eu) and the infrastructure hosts (auth/api/app/mail/…) live.
+    // e.g. dezky.eu) doubles as dezky's employee mail domain — only the tenant
+    // carrying the isPlatformTenant flag (operator-set, single holder) may
+    // claim it. Everything under it is off limits for everyone: that's where
+    // per-tenant service domains ({slug}.dezky.eu) and the infrastructure
+    // hosts (auth/api/app/mail/…) live.
     const platformBase = (process.env.PLATFORM_TENANT_DOMAIN || 'dezky.local').toLowerCase()
-    const platformSlug = (process.env.PLATFORM_TENANT_SLUG || 'dezky').toLowerCase()
-    if (domain === platformBase && tenant.slug !== platformSlug) {
+    if (domain === platformBase && !tenant.isPlatformTenant) {
       throw new ConflictException(`Domain "${domain}" is reserved`)
     }
     if (domain.endsWith(`.${platformBase}`)) {
diff --git a/services/platform-api/src/schemas/tenant.schema.ts b/services/platform-api/src/schemas/tenant.schema.ts
index e9846b4..bfd4189 100644
--- a/services/platform-api/src/schemas/tenant.schema.ts
+++ b/services/platform-api/src/schemas/tenant.schema.ts
@@ -51,6 +51,15 @@ export class Tenant {
   @Prop({ type: Types.ObjectId, ref: 'Partner', index: true, sparse: true })
   partnerId?: Types.ObjectId
 
+  // Marks the company's OWN tenant (dezky itself, dogfooding the platform).
+  // At most one tenant carries this — TenantsService.update clears it from
+  // every other tenant when it's set. Grants exactly one privilege: claiming
+  // the PLATFORM_TENANT_DOMAIN apex (dezky.eu) as a customer mail domain.
+  // Flag on the document instead of a slug/id in env so the identity
+  // survives purge-and-recreate without a config chase.
+  @Prop({ default: false, index: true })
+  isPlatformTenant?: boolean
+
   // External system handles — filled in by the provisioning worker (Phase 4)
   @Prop({ index: true, sparse: true })
   authentikGroupId?: string
diff --git a/services/platform-api/src/seed/seed.service.ts b/services/platform-api/src/seed/seed.service.ts
index 3f71f83..4fa3b21 100644
--- a/services/platform-api/src/seed/seed.service.ts
+++ b/services/platform-api/src/seed/seed.service.ts
@@ -84,6 +84,7 @@ export class SeedService implements OnApplicationBootstrap {
             // Display field only — the real mail domain is added through the
             // Domains flow. Follows the environment (dezky.local dev, dezky.eu prod).
             domains: [process.env.PLATFORM_TENANT_DOMAIN || 'dezky.local'],
+            isPlatformTenant: true,
             billingInfo: { companyName: 'Dezky', country: 'DK' },
           },
         },
diff --git a/services/platform-api/src/tenants/dto/update-tenant.dto.ts b/services/platform-api/src/tenants/dto/update-tenant.dto.ts
index 86a644c..5522df2 100644
--- a/services/platform-api/src/tenants/dto/update-tenant.dto.ts
+++ b/services/platform-api/src/tenants/dto/update-tenant.dto.ts
@@ -1,5 +1,6 @@
 import {
   IsArray,
+  IsBoolean,
   IsEnum,
   IsInt,
   IsMongoId,
@@ -30,4 +31,9 @@ export class UpdateTenantDto {
   // Attach / move tenant under a partner (or pass null to detach).
   @IsOptional() @IsMongoId()
   partnerId?: string | null
+
+  // Operator-only (enforced in the controller): mark this as the company's
+  // own tenant. Setting it true clears the flag everywhere else.
+  @IsOptional() @IsBoolean()
+  isPlatformTenant?: boolean
 }
diff --git a/services/platform-api/src/tenants/tenants.controller.ts b/services/platform-api/src/tenants/tenants.controller.ts
index 99443cc..be51cd4 100644
--- a/services/platform-api/src/tenants/tenants.controller.ts
+++ b/services/platform-api/src/tenants/tenants.controller.ts
@@ -164,6 +164,11 @@ export class TenantsController {
     if (!user.platformAdmin && !user.tenantIds.some((id) => id.equals(tenant._id))) {
       throw new ForbiddenException(`No access to tenant "${slug}"`)
     }
+    // The platform-tenant flag grants the dezky.eu apex — a tenant admin must
+    // never be able to set it on their own tenant through this shared route.
+    if (dto.isPlatformTenant !== undefined && !user.platformAdmin) {
+      throw new ForbiddenException('Only platform admins can change the platform-tenant flag')
+    }
     return this.tenants.update(slug, dto, auditActor(user, req))
   }
 
diff --git a/services/platform-api/src/tenants/tenants.service.ts b/services/platform-api/src/tenants/tenants.service.ts
index e297b82..8f0e92b 100644
--- a/services/platform-api/src/tenants/tenants.service.ts
+++ b/services/platform-api/src/tenants/tenants.service.ts
@@ -229,6 +229,15 @@ export class TenantsService {
       if (dto.partnerId === null) unset.partnerId = ''
       else set.partnerId = new Types.ObjectId(dto.partnerId)
     }
+    if (dto.isPlatformTenant !== undefined) {
+      set.isPlatformTenant = dto.isPlatformTenant
+      // Single-holder invariant: there is one company tenant at most.
+      if (dto.isPlatformTenant) {
+        await this.tenantModel
+          .updateMany({ slug: { $ne: slug } }, { $set: { isPlatformTenant: false } })
+          .exec()
+      }
+    }
     const update: Record<string, unknown> = {}
     if (Object.keys(set).length) update.$set = set
     if (Object.keys(unset).length) update.$unset = unset