@@ -13,6 +13,7 @@ import { makeFlag } from "~/v3/featureFlags.server";
 import { globalFlagsRegistry } from "~/v3/globalFlagsRegistry.server";
 import { machinePresetFromName } from "~/v3/machinePresets.server";
 import { resolveComputeAccess } from "~/v3/regionAccess.server";
+import { WorkerGroupService } from "~/v3/services/worker/workerGroupService.server";
 import {
   backingForQueue,
   selfBackingWorkerGroups,
@@ -255,22 +256,27 @@ export class TemplatePrebuildService {
   }> {
     const project = await this.prisma.project.findFirst({
       where: { id: projectId },
-      select: {
-        defaultWorkerGroup: { select: { workloadType: true, masterQueue: true } },
-        organization: { select: { id: true, featureFlags: true } },
-      },
+      select: { organization: { select: { id: true, featureFlags: true } } },
     });
 
     if (!project) {
       return { mode: "skip", reason: "no-project", diagnostics: {} };
     }
 
+    // As on the trigger path, a project without its own default group uses the global one.
+    const defaultWorkerGroup = await new WorkerGroupService({
+      prisma: this.prisma,
+    }).getDefaultWorkerGroupForProject({ projectId });
+
     const diagnostics: EligibilityDiagnostics = {
       organizationId: project.organization.id,
-      workloadType: project.defaultWorkerGroup?.workloadType,
-      masterQueue: project.defaultWorkerGroup?.masterQueue,
+      workloadType: defaultWorkerGroup?.workloadType,
+      masterQueue: defaultWorkerGroup?.masterQueue,
     };
-    const reason = await this.eligibility(project, diagnostics);
+    const reason = await this.eligibility(
+      { ...project, defaultWorkerGroup: defaultWorkerGroup ?? null },
+      diagnostics
+    );
     diagnostics.eligible = reason !== "none";
     if (!this.config.url) {
       return { mode: "skip", reason: "no-builder", diagnostics };
 
@@ -37,6 +37,8 @@ import { workerQueueForBirth } from "~/runEngine/concerns/workerQueueAssignment.
 import { DefaultQueueManager } from "~/runEngine/concerns/queues.server";
 import type { RunEngine } from "~/v3/runEngine.server";
 import { RedisTaskMetadataCache } from "~/services/taskMetadataCache.server";
+import { FEATURE_FLAG } from "~/v3/featureFlags";
+import { makeSetFlag } from "~/v3/featureFlags.server";
 import { ChangeCurrentDeploymentService } from "~/v3/services/changeCurrentDeployment.server";
 import { TemplatePrebuildService } from "~/v3/services/templatePrebuild.server";
 import {
@@ -463,7 +465,7 @@ describe("a mixed-runtime group as its region's compute backing", () => {
   // One CONTAINER group carries both runtimes; the old MICROVM group is hidden unless asked.
   async function setupRegion(
     prisma: PrismaClient,
-    options: { migrated: boolean; visibleMicrovm?: boolean }
+    options: { migrated: boolean; visibleMicrovm?: boolean; globalDefault?: boolean }
   ) {
     const environment = await setupAuthenticatedEnvironment(prisma, "PRODUCTION");
     const group = (masterQueue: string, workloadType: "CONTAINER" | "MICROVM", hidden: boolean) =>
@@ -480,10 +482,17 @@ describe("a mixed-runtime group as its region's compute backing", () => {
       });
     const mixed = await group("cme-mixed", "CONTAINER", false);
     await group("us-east-1-mvm", "MICROVM", !options.visibleMicrovm);
+    // With globalDefault, the project has no default group and falls back to the global one.
     await prisma.project.update({
       where: { id: environment.project.id },
-      data: { defaultWorkerGroupId: mixed.id },
+      data: { defaultWorkerGroupId: options.globalDefault ? null : mixed.id },
     });
+    if (options.globalDefault) {
+      await makeSetFlag(prisma)({
+        key: FEATURE_FLAG.defaultWorkerInstanceGroupId,
+        value: mixed.id,
+      });
+    }
     await prisma.organization.update({
       where: { id: environment.organization.id },
       data: { featureFlags: { computeMigrationEnabled: options.migrated } },
@@ -575,6 +584,23 @@ describe("a mixed-runtime group as its region's compute backing", () => {
     }
   );
 
+  containerTest(
+    "a migrated org without a project default group is eligible through the global default",
+    async ({ prisma, redisOptions }) => {
+      db.client = prisma;
+      const { environment, mixed } = await setupRegion(prisma, {
+        migrated: true,
+        globalDefault: true,
+      });
+
+      expect(await prebuildMode(prisma, environment.project.id, [mixed.id])).toEqual({
+        mode: "shadow",
+        reason: "migrated",
+      });
+      expect(await birthQueue(prisma, redisOptions, environment, [mixed.id], KEY)).toBe(COMPUTE);
+    }
+  );
+
   containerTest(
     "a migrated org's keyless run on a listed group stays on containers",
     async ({ prisma, redisOptions }) => {