diff --git a/services/web/app/src/Features/Subscription/SubscriptionController.mjs b/services/web/app/src/Features/Subscription/SubscriptionController.mjs
index 52e5bd6d06..7c07bc8437 100644
--- a/services/web/app/src/Features/Subscription/SubscriptionController.mjs
+++ b/services/web/app/src/Features/Subscription/SubscriptionController.mjs
@@ -532,6 +532,18 @@ async function previewAddonPurchase(req, res) {
     return HttpErrorHandler.notFound(req, res, `Unknown add-on: ${addOnCode}`)
   }
 
+  const { variant: plans2026Phase1Variant } =
+    await SplitTestHandler.promises.getAssignment(
+      req,
+      res,
+      'plans-2026-phase-1'
+    )
+  if (plans2026Phase1Variant === 'enabled') {
+    return res.redirect(
+      '/user/subscription?redirect-reason=ai-assist-unavailable'
+    )
+  }
+
   const canUseAi = await PermissionsManager.promises.checkUserPermissions(
     user,
     ['use-ai']
@@ -656,6 +668,16 @@ async function purchaseAddon(req, res, next) {
     return res.sendStatus(404)
   }
 
+  const { variant: plans2026Phase1Variant } =
+    await SplitTestHandler.promises.getAssignment(
+      req,
+      res,
+      'plans-2026-phase-1'
+    )
+  if (plans2026Phase1Variant === 'enabled') {
+    return res.sendStatus(404)
+  }
+
   const { isPaused } = await checkSubscriptionPauseStatus(user)
   if (isPaused) {
     return HttpErrorHandler.badRequest(
diff --git a/services/web/test/unit/src/Subscription/SubscriptionController.test.mjs b/services/web/test/unit/src/Subscription/SubscriptionController.test.mjs
index c370ac9131..50bc8695da 100644
--- a/services/web/test/unit/src/Subscription/SubscriptionController.test.mjs
+++ b/services/web/test/unit/src/Subscription/SubscriptionController.test.mjs
@@ -932,6 +932,19 @@ describe('SubscriptionController', function () {
       expect(ctx.res.sendStatus).to.have.been.calledWith(404)
     })
 
+    it('should return 404 when plans-2026-phase-1 split test is enabled', async function (ctx) {
+      ctx.SplitTestV2Hander.promises.getAssignment.resolves({
+        variant: 'enabled',
+      })
+      ctx.res.sendStatus = sinon.spy()
+
+      await ctx.SubscriptionController.purchaseAddon(ctx.req, ctx.res, ctx.next)
+
+      expect(ctx.SubscriptionHandler.promises.purchaseAddon).to.not.have.been
+        .called
+      expect(ctx.res.sendStatus).to.have.been.calledWith(404)
+    })
+
     it('should handle DuplicateAddOnError and send badRequest while sending 200', async function (ctx) {
       ctx.req.params.addOnCode = AI_ADD_ON_CODE
       ctx.SubscriptionHandler.promises.purchaseAddon.rejects(
@@ -1428,6 +1441,19 @@ describe('SubscriptionController', function () {
       ctx.SubscriptionLocator.promises.getUsersSubscription.resolves(null)
     })
 
+    it('should redirect with ai-assist-unavailable when plans-2026-phase-1 is enabled', async function (ctx) {
+      ctx.SplitTestV2Hander.promises.getAssignment.resolves({
+        variant: 'enabled',
+      })
+      ctx.res.redirect = sinon.stub()
+
+      await ctx.SubscriptionController.previewAddonPurchase(ctx.req, ctx.res)
+
+      expect(ctx.res.redirect).to.have.been.calledWith(
+        '/user/subscription?redirect-reason=ai-assist-unavailable'
+      )
+    })
+
     describe('when user has manual or custom subscription', function () {
       it('should redirect with ai-assist-unavailable when subscription has customAccount = true', async function (ctx) {
         const customSubscription = {