diff --git a/api/src/bindings.ts b/api/src/bindings.ts
index be6cb42..96c365e 100644
--- a/api/src/bindings.ts
+++ b/api/src/bindings.ts
@@ -11,7 +11,8 @@ export interface Env {
   TICKET_SIGNING_SECRET: string;
   SESSION_SIGNING_SECRET: string;
   TELEGRAM_BOT_TOKEN: string;
-  TELEGRAM_BOT_USERNAME: string;
+  // Optional at runtime: deploy.sh skips empty secrets, so the binding may be absent
+  TELEGRAM_BOT_USERNAME?: string;
   TELEGRAM_ADMIN_CHAT_ID: string;
   EV_API_KEY: string;
 
diff --git a/api/src/index.ts b/api/src/index.ts
index 4627b39..2a35fb5 100644
--- a/api/src/index.ts
+++ b/api/src/index.ts
@@ -20,6 +20,7 @@ import {
 } from "./validation.js";
 import type { SessionClaims, SupportedAlg, Identity } from "./types.js";
 import { sendDailyDigest } from "./digest.js";
+import { buildTelegramDeepLink } from "./telegram.js";
 
 type HonoEnv = { Bindings: Env; Variables: { auth: SessionClaims } };
 
@@ -158,11 +159,18 @@ app.post("/v1/auth/challenge", async (c) => {
     await store.resolveChallenge(challenge.id, body.telegram_user_id);
   }
 
+  const deepLink = botEnabled ? buildTelegramDeepLink(c.env.TELEGRAM_BOT_USERNAME, challenge.id) : null;
+  if (botEnabled && !deepLink) {
+    console.error(
+      "[auth] TELEGRAM_BOT_TOKEN is set but TELEGRAM_BOT_USERNAME is missing or invalid — deep_link omitted. Set the bot username secret (without @).",
+    );
+  }
+
   return c.json(
     {
       challenge_id: challenge.id,
       expires_at: challenge.expires_at,
-      deep_link: botEnabled ? `https://t.me/${c.env.TELEGRAM_BOT_USERNAME.trim().replace(/^@/, "")}?start=${challenge.id}` : null,
+      deep_link: deepLink,
     },
     201,
   );
diff --git a/api/src/telegram.ts b/api/src/telegram.ts
new file mode 100644
index 0000000..8cd3406
--- /dev/null
+++ b/api/src/telegram.ts
@@ -0,0 +1,24 @@
+// Telegram bot usernames: 5-32 chars, letters/digits/underscore, must start with a letter.
+const USERNAME_RE = /^[A-Za-z][A-Za-z0-9_]{4,31}$/;
+
+/**
+ * Normalize a configured bot username into a t.me-routable form.
+ * Accepts values with a leading "@" or stray whitespace (common paste errors).
+ * Returns null when the value is missing or can't be a valid username —
+ * a null here must never be interpolated into a t.me URL, or the link
+ * routes to a nonexistent user (e.g. "https://t.me/undefined").
+ */
+export function normalizeBotUsername(raw: string | undefined | null): string | null {
+  const username = (raw ?? "").trim().replace(/^@+/, "");
+  return USERNAME_RE.test(username) ? username : null;
+}
+
+/** Build the auth deep link, or null when the bot username is not usable. */
+export function buildTelegramDeepLink(
+  botUsername: string | undefined | null,
+  challengeId: string,
+): string | null {
+  const username = normalizeBotUsername(botUsername);
+  if (!username) return null;
+  return `https://t.me/${username}?start=${challengeId}`;
+}
diff --git a/docs/runbook.md b/docs/runbook.md
index 669025a..6fa6b92 100644
--- a/docs/runbook.md
+++ b/docs/runbook.md
@@ -273,6 +273,13 @@ npx wrangler kv key list --namespace-id=<ID>
   - Check Worker logs: `npx wrangler tail`
   - Re-set webhook URL if needed.
 
+- **Auth deep link points to a nonexistent Telegram user** (e.g. `https://t.me/undefined?start=...`):
+  - The `TELEGRAM_BOT_USERNAME` secret is missing or invalid on the deployed Worker. `deploy.sh` used to silently skip empty secrets, so the binding could be absent while `TELEGRAM_BOT_TOKEN` was set.
+  - Check: `npx wrangler secret list --env production --config api/wrangler.toml`
+  - Fix: set it to the bot's username (the `..._bot` handle from @BotFather, **without** `@`, not the display name):
+    `echo "<bot_username>" | npx wrangler secret put TELEGRAM_BOT_USERNAME --env production --config api/wrangler.toml`
+  - The API now omits `deep_link` (returns `null`) and logs a warning instead of emitting a broken link; `deploy.sh` refuses to deploy a bot token without a valid username.
+
 - **CORS errors on web UI**: verify `ALLOWED_ORIGINS` in `wrangler.toml` includes `https://pay.clw.cash`.
 
 ## Secrets Rotation
diff --git a/scripts/deploy.sh b/scripts/deploy.sh
index c701d1e..9939284 100755
--- a/scripts/deploy.sh
+++ b/scripts/deploy.sh
@@ -35,6 +35,21 @@ require_var() {
   fi
 }
 
+# The auth deep link is https://t.me/$TELEGRAM_BOT_USERNAME?start=... — deploying a
+# bot token without a username produces links to a nonexistent Telegram user.
+check_telegram_config() {
+  if [[ -n "${TELEGRAM_BOT_TOKEN:-}" ]]; then
+    # Normalize a pasted "@botname" to "botname" — t.me links need the bare username
+    TELEGRAM_BOT_USERNAME="${TELEGRAM_BOT_USERNAME#@}"
+    if [[ -z "${TELEGRAM_BOT_USERNAME:-}" ]]; then
+      die "TELEGRAM_BOT_TOKEN is set but TELEGRAM_BOT_USERNAME is empty — auth deep links would point to https://t.me/undefined. Set TELEGRAM_BOT_USERNAME (without @) in $ENV_FILE"
+    fi
+    if [[ ! "$TELEGRAM_BOT_USERNAME" =~ ^[A-Za-z][A-Za-z0-9_]{4,31}$ ]]; then
+      die "TELEGRAM_BOT_USERNAME '$TELEGRAM_BOT_USERNAME' is not a valid Telegram username (5-32 chars, letters/digits/underscore, no @, no URL)"
+    fi
+  fi
+}
+
 # ── Generate secrets ─────────────────────────────────────────────────
 generate_secrets() {
   info "Generating fresh secrets..."
@@ -153,6 +168,7 @@ deploy_api() {
   require_var TICKET_SIGNING_SECRET
   require_var SESSION_SIGNING_SECRET
   require_var EV_API_KEY
+  check_telegram_config
 
   # Set secrets on CF Worker
   info "Setting Worker secrets..."
@@ -181,6 +197,7 @@ deploy_worker_api() {
   require_var TICKET_SIGNING_SECRET
   require_var SESSION_SIGNING_SECRET
   require_var WORKER_SEALING_KEY
+  check_telegram_config
 
   # Set secrets on CF Worker (no EV_API_KEY or INTERNAL_API_KEY needed)
   info "Setting Worker secrets..."
diff --git a/test/telegramDeepLink.test.ts b/test/telegramDeepLink.test.ts
new file mode 100644
index 0000000..119f71a
--- /dev/null
+++ b/test/telegramDeepLink.test.ts
@@ -0,0 +1,54 @@
+import { describe, it, expect } from "vitest";
+import { buildTelegramDeepLink, normalizeBotUsername } from "../api/src/telegram.js";
+
+const CHALLENGE = "b97b9fe2-ec02-4a2c-871a-2690f328eb52";
+
+describe("normalizeBotUsername", () => {
+  it("accepts a plain bot username", () => {
+    expect(normalizeBotUsername("clw_cash_bot")).toBe("clw_cash_bot");
+  });
+
+  it("strips a leading @", () => {
+    expect(normalizeBotUsername("@clw_cash_bot")).toBe("clw_cash_bot");
+  });
+
+  it("trims surrounding whitespace and newlines", () => {
+    expect(normalizeBotUsername("  clw_cash_bot\n")).toBe("clw_cash_bot");
+  });
+
+  it("rejects undefined, null, and empty values", () => {
+    expect(normalizeBotUsername(undefined)).toBeNull();
+    expect(normalizeBotUsername(null)).toBeNull();
+    expect(normalizeBotUsername("")).toBeNull();
+    expect(normalizeBotUsername("   ")).toBeNull();
+  });
+
+  it("rejects values that are not valid Telegram usernames", () => {
+    expect(normalizeBotUsername("https://t.me/clw_cash_bot")).toBeNull();
+    expect(normalizeBotUsername("Claw Cash Bot")).toBeNull();
+    expect(normalizeBotUsername("1bot")).toBeNull(); // must start with a letter
+    expect(normalizeBotUsername("abcd")).toBeNull(); // too short
+  });
+});
+
+describe("buildTelegramDeepLink", () => {
+  it("builds a t.me deep link with the start payload", () => {
+    expect(buildTelegramDeepLink("clw_cash_bot", CHALLENGE)).toBe(
+      `https://t.me/clw_cash_bot?start=${CHALLENGE}`,
+    );
+  });
+
+  it("builds the same link from an @-prefixed username", () => {
+    expect(buildTelegramDeepLink("@clw_cash_bot", CHALLENGE)).toBe(
+      `https://t.me/clw_cash_bot?start=${CHALLENGE}`,
+    );
+  });
+
+  it("returns null instead of a t.me/undefined link when the username binding is absent", () => {
+    expect(buildTelegramDeepLink(undefined, CHALLENGE)).toBeNull();
+  });
+
+  it("returns null for an empty username", () => {
+    expect(buildTelegramDeepLink("", CHALLENGE)).toBeNull();
+  });
+});