Link the user-bot welcome DM to the MFA setup wizard

lambda/user-bot/README.md

@@ -24,7 +24,7 @@ When both hold, it:
 1. reads the Slack bot token (see [The Slack token](#the-slack-token)). **If that fails, it stops here and the user's password is not touched;**
 2. generates a 20-character password containing all four character classes;
 3. sets it with `UpdateLoginProfile` and `PasswordResetRequired: true`, so the user must replace it at first sign-in;
-4. DMs the user, as the Slack app, with the sign-in page, their IAM user name and the temporary password.
+4. DMs the user, as the Slack app, with the sign-in page, their IAM user name, the temporary password, and a link straight to the **Assign MFA device** wizard for their own user (`MFA_SETUP_URL` in `src/message.ts`). That link only works once they are signed in.
 
 Step 1 comes before the reset on purpose. Reading the token at send time would mean a bad token leaves the user with a new password that nobody received.
 

lambda/user-bot/src/message.ts

@@ -1,6 +1,13 @@
 // The incubator account's sign-in page, via its alias hfla-incubator.
 export const SIGN_IN_URL = "https://hfla-incubator.signin.aws.amazon.com/console";
 
+// Opens the "Assign MFA device" wizard for whoever is signed in, so one link works for
+// every user. It only works once signed in: the route is in the #fragment, which the
+// browser never sends to AWS, so a signed-out visit loses it on the way through sign-in
+// and lands on the console home page. Hence the "once signed in" in the message.
+// Tested by hand 2026-10-04; AWS does not document console routes and may change them.
+export const MFA_SETUP_URL = "https://console.aws.amazon.com/iam/home#/security_credentials/mfa";
+
 // Slack mrkdwn. The user name and password are in inline code so that characters
 // such as * and _ are shown literally rather than read as formatting.
 export function buildWelcomeMessage(userName: string, password: string): string {
@@ -12,8 +19,10 @@ export function buildWelcomeMessage(userName: string, password: string): string
     `*Temporary password:* \`${password}\``,
     "",
     "The first time you sign in you will be asked to replace this password with one of your own. " +
-      "Then set up multi-factor authentication (MFA) from *Security credentials* in the account " +
-      "menu: most of your access only works once MFA is set up.",
+      "Then, in the same browser, set up multi-factor authentication (MFA): most of your access " +
+      "only works once MFA is set up.",
+    "",
+    `*Set up MFA (once signed in):* ${MFA_SETUP_URL}`,
     "",
     "If the password does not work, ask the DevOps team on Slack for a new one.",
   ].join("\n");

lambda/user-bot/test/handler.test.ts

@@ -8,7 +8,7 @@ import { mockClient } from "aws-sdk-client-mock";
 import { beforeEach, describe, expect, it, vi } from "vitest";
 
 import { createHandler, type Dependencies } from "../src/handler";
-import { SIGN_IN_URL } from "../src/message";
+import { MFA_SETUP_URL, SIGN_IN_URL } from "../src/message";
 import { generatePassword } from "../src/password";
 import type { DirectMessage, MessageSender } from "../src/senders/types";
 import { captureLogger, createLoginProfileEvent, TEST_PASSWORD, TEST_SLACK_ID } from "./helpers";
@@ -58,6 +58,7 @@ describe("happy path", () => {
     expect(sent[0]!.text).toContain(TEST_PASSWORD);
     expect(sent[0]!.text).toContain("new.member");
     expect(sent[0]!.text).toContain(SIGN_IN_URL);
+    expect(sent[0]!.text).toContain(MFA_SETUP_URL);
     expect(logger.text()).not.toContain(TEST_PASSWORD);
   });
 

lambda/user-bot/test/message.test.ts

@@ -0,0 +1,24 @@
+import { describe, expect, it } from "vitest";
+
+import { buildWelcomeMessage, MFA_SETUP_URL, SIGN_IN_URL } from "../src/message";
+import { TEST_PASSWORD } from "./helpers";
+
+describe("buildWelcomeMessage", () => {
+  const text = buildWelcomeMessage("new.member", TEST_PASSWORD);
+
+  it("includes the incubator sign-in page, the user name and the password", () => {
+    expect(SIGN_IN_URL).toBe("https://hfla-incubator.signin.aws.amazon.com/console");
+    expect(text).toContain(`*Sign-in page:* ${SIGN_IN_URL}`);
+    expect(text).toContain("*IAM user name:* `new.member`");
+    expect(text).toContain(`*Temporary password:* \`${TEST_PASSWORD}\``);
+  });
+
+  it("links straight to the signed-in user's Assign MFA device wizard", () => {
+    expect(MFA_SETUP_URL).toBe("https://console.aws.amazon.com/iam/home#/security_credentials/mfa");
+    expect(text).toContain(`*Set up MFA (once signed in):* ${MFA_SETUP_URL}`);
+  });
+
+  it("gives the MFA link after the sign-in details, since it only works once signed in", () => {
+    expect(text.indexOf(MFA_SETUP_URL)).toBeGreaterThan(text.indexOf(TEST_PASSWORD));
+  });
+});