diff --git a/turbo/generators/lib/core-package-utils.test.ts b/turbo/generators/lib/core-package-utils.test.ts
new file mode 100644
index 0000000..a2eb0b2
--- /dev/null
+++ b/turbo/generators/lib/core-package-utils.test.ts
@@ -0,0 +1,64 @@
+import { describe, it, expect } from "vitest";
+import {
+  existsSync,
+  mkdtempSync,
+  mkdirSync,
+  writeFileSync,
+  readFileSync,
+} from "node:fs";
+import { tmpdir } from "node:os";
+import { join } from "node:path";
+import {
+  assertOptionalPackageNotPresent,
+  addToTranspilePackages,
+} from "./core-package-utils";
+
+describe("assertOptionalPackageNotPresent", () => {
+  it("throws if packages/<name>/ exists in cwd", () => {
+    const tmp = mkdtempSync(join(tmpdir(), "core-pkg-"));
+    mkdirSync(join(tmp, "packages", "core-foo"), { recursive: true });
+    expect(() => assertOptionalPackageNotPresent("core-foo", tmp)).toThrow(/already exists/);
+  });
+
+  it("returns silently if packages/<name>/ is absent", () => {
+    const tmp = mkdtempSync(join(tmpdir(), "core-pkg-"));
+    expect(() => assertOptionalPackageNotPresent("core-foo", tmp)).not.toThrow();
+  });
+});
+
+describe("addToTranspilePackages", () => {
+  it("inserts package name alphabetically into transpilePackages array", () => {
+    const tmp = mkdtempSync(join(tmpdir(), "core-pkg-"));
+    const cfgPath = join(tmp, "next.config.mjs");
+    writeFileSync(
+      cfgPath,
+      `const nextConfig = {
+  transpilePackages: [
+    "@repo/core-cms",
+    "@repo/core-shared",
+  ],
+};
+`,
+    );
+    addToTranspilePackages(cfgPath, "@repo/core-realtime");
+    const result = readFileSync(cfgPath, "utf8");
+    // Order should be: core-cms, core-realtime, core-shared
+    const order = result.match(/@repo\/core-\w+/g) ?? [];
+    expect(order).toEqual(["@repo/core-cms", "@repo/core-realtime", "@repo/core-shared"]);
+  });
+
+  it("is idempotent — duplicate insertion is a no-op", () => {
+    const tmp = mkdtempSync(join(tmpdir(), "core-pkg-"));
+    const cfgPath = join(tmp, "next.config.mjs");
+    writeFileSync(
+      cfgPath,
+      `const nextConfig = {
+  transpilePackages: ["@repo/core-realtime"],
+};
+`,
+    );
+    addToTranspilePackages(cfgPath, "@repo/core-realtime");
+    const result = readFileSync(cfgPath, "utf8");
+    expect(result.match(/@repo\/core-realtime/g)?.length).toBe(1);
+  });
+});
diff --git a/turbo/generators/lib/core-package-utils.ts b/turbo/generators/lib/core-package-utils.ts
new file mode 100644
index 0000000..bbca5ae
--- /dev/null
+++ b/turbo/generators/lib/core-package-utils.ts
@@ -0,0 +1,44 @@
+import { existsSync, readFileSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+
+/**
+ * Throws if a core package directory already exists. Used as the first action
+ * in every per-package generator so re-running is safe.
+ */
+export function assertOptionalPackageNotPresent(
+  name: string,
+  cwd: string = process.cwd(),
+): void {
+  const pkgRoot = join(cwd, "packages", name);
+  if (existsSync(pkgRoot)) {
+    throw new Error(
+      `packages/${name}/ already exists — refusing to scaffold (delete it first if intentional)`,
+    );
+  }
+}
+
+/**
+ * Inserts a package name into the transpilePackages array of a Next.js config
+ * file, preserving alphabetical order. Idempotent.
+ */
+export function addToTranspilePackages(
+  nextConfigPath: string,
+  pkgName: string,
+): void {
+  const source = readFileSync(nextConfigPath, "utf8");
+  if (source.includes(`"${pkgName}"`)) return;
+  const updated = source.replace(
+    /(transpilePackages:\s*\[\s*)([\s\S]*?)(\s*\])/,
+    (_match, open: string, body: string, close: string) => {
+      const entries = body
+        .split(",")
+        .map((e) => e.trim())
+        .filter(Boolean);
+      entries.push(`"${pkgName}"`);
+      entries.sort();
+      const formatted = entries.map((e) => `    ${e}`).join(",\n");
+      return `${open}\n${formatted},${close}`;
+    },
+  );
+  writeFileSync(nextConfigPath, updated);
+}