diff --git a/kits/firestore-send-email/package.json b/kits/firestore-send-email/package.json
index abf679e55..fd8ee5a2f 100644
--- a/kits/firestore-send-email/package.json
+++ b/kits/firestore-send-email/package.json
@@ -26,6 +26,8 @@
   "scripts": {
     "build": "tsc -b",
     "clean": "tsc -b --clean",
+    "//pretest": "Only this kit has a test that requires lib/. The release workflow's test job runs npm test without building, so the hook has to live here rather than in CI.",
+    "pretest": "npm run build",
     "test": "vitest run",
     "deploy": "pnpm build && firebase deploy --only functions",
     "serve": "firebase emulators:start --only functions"
diff --git a/kits/firestore-send-email/src/nodemailer-sendgrid/index.ts b/kits/firestore-send-email/src/nodemailer-sendgrid/index.ts
index 0f83a0f73..29d7bce41 100644
--- a/kits/firestore-send-email/src/nodemailer-sendgrid/index.ts
+++ b/kits/firestore-send-email/src/nodemailer-sendgrid/index.ts
@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-import sgMail from "@sendgrid/mail";
+import { MailService } from "@sendgrid/mail";
 import type {
   Address,
   MailSource,
@@ -29,9 +29,13 @@ export class SendGridTransport {
   public readonly name = "firebase-extensions-nodemailer-sendgrid";
   public readonly version = "0.0.1";
 
+  // @sendgrid/mail's default export is a shared MailService singleton, so
+  // setApiKey on it would reconfigure every other transport in the process.
+  private readonly client = new MailService();
+
   constructor(options: SendGridTransportOptions = {}) {
     if (options.apiKey) {
-      sgMail.setApiKey(options.apiKey);
+      this.client.setApiKey(options.apiKey);
     }
   }
 
@@ -164,8 +168,8 @@ export class SendGridTransport {
         }
       }
 
-      sgMail
-        .send(msg as Parameters<typeof sgMail.send>[0])
+      this.client
+        .send(msg as Parameters<MailService["send"]>[0])
         .then(([response]) => {
           const rawQueueId = (response.headers["x-message-id"] ||
             response.headers["X-Message-Id"]) as string | undefined;
diff --git a/kits/firestore-send-email/tests/build-interop.test.ts b/kits/firestore-send-email/tests/build-interop.test.ts
index d9e7be082..d2014be53 100644
--- a/kits/firestore-send-email/tests/build-interop.test.ts
+++ b/kits/firestore-send-email/tests/build-interop.test.ts
@@ -17,10 +17,11 @@
 import { createRequire } from "node:module";
 import { describe, expect, test } from "vitest";
 
-// The subject is the compiled output, not src: only tsc's esModuleInterop
-// helper drops the prototype methods off the instance @sendgrid/mail exports,
-// and vitest's own transform does not reproduce that. Needs `npm run build`,
-// which CI runs before `npm test`.
+// The subject is the compiled output, not src: tsc's esModuleInterop helper
+// strips the prototype methods off the instance @sendgrid/mail exports, and
+// vitest's own transform does not reproduce that. The named import in
+// nodemailer-sendgrid emits no helper; this guards a revert to a default one.
+// The pretest hook builds lib/ before this runs.
 const requireBuilt = createRequire(import.meta.url);
 
 describe("built SendGridTransport", () => {
diff --git a/kits/firestore-send-email/tests/helpers.test.ts b/kits/firestore-send-email/tests/helpers.test.ts
index 035538d34..f2207d758 100644
--- a/kits/firestore-send-email/tests/helpers.test.ts
+++ b/kits/firestore-send-email/tests/helpers.test.ts
@@ -18,11 +18,16 @@ import { logger } from "firebase-functions";
 import Mail from "nodemailer/lib/mailer";
 import { beforeEach, describe, expect, test, vi } from "vitest";
 
-// @sendgrid/mail exports a single MailService instance, so the mock has to be
-// reachable as both the default and the named exports.
+// @sendgrid/mail exports a MailService singleton whose methods are also the
+// module's own exports, plus the MailService class. Constructing the class
+// hands back the same spies as the singleton, so the module-level ones see
+// whatever the transport's own client was configured with.
 vi.mock("@sendgrid/mail", () => {
   const mail = { setApiKey: vi.fn(), send: vi.fn() };
-  return { ...mail, default: mail };
+  const MailService = vi.fn(function () {
+    return mail;
+  });
+  return { ...mail, MailService };
 });
 
 import * as sgMail from "@sendgrid/mail";
diff --git a/kits/firestore-send-email/tests/nodemailer-sendgrid.test.ts b/kits/firestore-send-email/tests/nodemailer-sendgrid.test.ts
index 12824eecf..d60857aae 100644
--- a/kits/firestore-send-email/tests/nodemailer-sendgrid.test.ts
+++ b/kits/firestore-send-email/tests/nodemailer-sendgrid.test.ts
@@ -16,8 +16,11 @@
 
 import { beforeEach, describe, expect, test, vi } from "vitest";
 
-// @sendgrid/mail exports a single MailService instance, so the mock has to be
-// reachable as both the default and the named exports.
+// @sendgrid/mail exports a MailService singleton whose methods are also the
+// module's own exports, plus the MailService class. Constructing the class
+// hands back the same spies as the singleton, so most tests can assert against
+// the module-level ones; the isolation tests override the implementation to
+// tell the two apart.
 vi.mock("@sendgrid/mail", () => {
   const mail = {
     setApiKey: vi.fn(),
@@ -29,7 +32,10 @@ vi.mock("@sendgrid/mail", () => {
       {},
     ]),
   };
-  return { ...mail, default: mail };
+  const MailService = vi.fn(function () {
+    return mail;
+  });
+  return { ...mail, MailService };
 });
 
 import * as sgMail from "@sendgrid/mail";
@@ -43,6 +49,7 @@ import type {
 
 const setApiKey = vi.mocked(sgMail.setApiKey);
 const send = vi.mocked(sgMail.send);
+const MailServiceMock = vi.mocked(sgMail.MailService);
 
 /** Wraps a normalized mail source in the minimal MailSource shape the transport consumes. */
 function mailFrom(source: Partial<MailSource>): MailSource {
@@ -84,6 +91,29 @@ describe("SendGridTransport", () => {
     expect(setApiKey).not.toHaveBeenCalled();
   });
 
+  test("builds its own MailService rather than using the module singleton", () => {
+    new SendGridTransport({ apiKey: "API-KEY-123" });
+    expect(MailServiceMock).toHaveBeenCalledTimes(1);
+  });
+
+  test("keeps API keys separate across transports", () => {
+    const first = { setApiKey: vi.fn(), send: vi.fn() };
+    const second = { setApiKey: vi.fn(), send: vi.fn() };
+    MailServiceMock.mockImplementationOnce(function () {
+      return first as never;
+    }).mockImplementationOnce(function () {
+      return second as never;
+    });
+
+    new SendGridTransport({ apiKey: "FIRST" });
+    new SendGridTransport({ apiKey: "SECOND" });
+
+    expect(first.setApiKey.mock.calls).toEqual([["FIRST"]]);
+    expect(second.setApiKey.mock.calls).toEqual([["SECOND"]]);
+    // The shared singleton the module exports is never reconfigured.
+    expect(setApiKey).not.toHaveBeenCalled();
+  });
+
   test("calls back with the normalize error and never sends", async () => {
     const transport = new SendGridTransport({ apiKey: "X" });
     const normalizeError = new Error("normalize failed");