diff --git a/docs/commercial/checkout-migration.md b/docs/commercial/checkout-migration.md
index ebe3c3c..c064917 100644
--- a/docs/commercial/checkout-migration.md
+++ b/docs/commercial/checkout-migration.md
@@ -23,6 +23,7 @@ This runbook moves Dictx Pro sales to `https://dictx.splitlabs.io/buy` while kee
 - Product name: `Dictx Pro`
 - Offer copy: `Signed binaries, auto-updates, and direct support`
 - Price: `USD 29 one-time`
+- Success URL: `https://dictx.splitlabs.io/buy/success?checkout_id={CHECKOUT_ID}`
 
 3. Enable customer portal for:
 
diff --git a/landing/README.md b/landing/README.md
index 25f264d..5c92fd7 100644
--- a/landing/README.md
+++ b/landing/README.md
@@ -24,3 +24,9 @@ Attach `dictx.splitlabs.io` to this Vercel project.
 - `POLAR_ACCESS_TOKEN`: Polar API token
 - `POLAR_DICTX_PRODUCT_IDS`: optional comma-separated product IDs allowed for Dictx Pro activation
 - `POLAR_API_BASE`: optional override (defaults to `https://api.polar.sh/v1`)
+
+## Polar Checkout Success URL
+
+Set the product checkout success URL to:
+
+- `https://dictx.splitlabs.io/buy/success?checkout_id={CHECKOUT_ID}`
diff --git a/landing/api/pro/verify.js b/landing/api/pro/verify.js
index 1469f2e..7c0b127 100644
--- a/landing/api/pro/verify.js
+++ b/landing/api/pro/verify.js
@@ -4,6 +4,17 @@ const ALLOWED_PRODUCT_IDS = (process.env.POLAR_DICTX_PRODUCT_IDS || "")
   .split(",")
   .map((value) => value.trim())
   .filter(Boolean);
+const RATE_LIMIT_WINDOW_MS = Number.parseInt(
+  process.env.PRO_VERIFY_RATE_LIMIT_WINDOW_MS || "60000",
+  10,
+);
+const RATE_LIMIT_MAX = Number.parseInt(
+  process.env.PRO_VERIFY_RATE_LIMIT_MAX || "20",
+  10,
+);
+const MAX_LICENSE_KEY_LENGTH = 128;
+const LICENSE_KEY_PATTERN = /^polar_cl_[A-Za-z0-9]+$/;
+const requestBuckets = new Map();
 
 const readBody = (req) => {
   if (!req.body) return {};
@@ -28,6 +39,39 @@ const statusLooksPaid = (status, paid) => {
   );
 };
 
+const getClientId = (req) => {
+  const forwarded = req.headers["x-forwarded-for"];
+  if (Array.isArray(forwarded)) {
+    return forwarded[0] || "unknown";
+  }
+  if (typeof forwarded === "string" && forwarded.length > 0) {
+    const [first] = forwarded.split(",");
+    return first.trim() || "unknown";
+  }
+  return req.socket?.remoteAddress || "unknown";
+};
+
+const isRateLimited = (clientId) => {
+  const now = Date.now();
+  if (requestBuckets.size > 5000) {
+    for (const [key, bucket] of requestBuckets.entries()) {
+      if (now > bucket.resetAt) {
+        requestBuckets.delete(key);
+      }
+    }
+  }
+
+  const existing = requestBuckets.get(clientId);
+  if (!existing || now > existing.resetAt) {
+    requestBuckets.set(clientId, { count: 1, resetAt: now + RATE_LIMIT_WINDOW_MS });
+    return false;
+  }
+
+  existing.count += 1;
+  requestBuckets.set(clientId, existing);
+  return existing.count > RATE_LIMIT_MAX;
+};
+
 module.exports = async (req, res) => {
   if (req.method !== "POST") {
     res.status(405).json({ error: "method_not_allowed" });
@@ -39,6 +83,12 @@ module.exports = async (req, res) => {
     return;
   }
 
+  const clientId = getClientId(req);
+  if (isRateLimited(clientId)) {
+    res.status(429).json({ error: "rate_limited" });
+    return;
+  }
+
   const body = readBody(req);
   const licenseKey = (body.licenseKey || body.checkoutId || "").trim();
 
@@ -47,6 +97,18 @@ module.exports = async (req, res) => {
     return;
   }
 
+  if (licenseKey.length > MAX_LICENSE_KEY_LENGTH) {
+    console.warn("pro_verify_invalid_key_length", { clientId });
+    res.status(400).json({ error: "invalid_license_key" });
+    return;
+  }
+
+  if (!LICENSE_KEY_PATTERN.test(licenseKey)) {
+    console.warn("pro_verify_invalid_key_format", { clientId });
+    res.status(400).json({ error: "invalid_license_key" });
+    return;
+  }
+
   try {
     const response = await fetch(
       `${POLAR_API_BASE}/checkouts/${encodeURIComponent(licenseKey)}`,
@@ -66,6 +128,10 @@ module.exports = async (req, res) => {
 
     if (!response.ok) {
       const text = await response.text();
+      console.warn("pro_verify_polar_api_error", {
+        status: response.status,
+        clientId,
+      });
       res.status(502).json({ error: "polar_api_error", detail: text });
       return;
     }
@@ -79,6 +145,7 @@ module.exports = async (req, res) => {
 
     res.status(200).json({ active: Boolean(productAllowed && paid) });
   } catch (error) {
+    console.warn("pro_verify_internal_error", { clientId });
     res.status(500).json({
       error: "internal_error",
       detail: error instanceof Error ? error.message : String(error),
diff --git a/landing/buy/success/index.html b/landing/buy/success/index.html
new file mode 100644
index 0000000..307d96f
--- /dev/null
+++ b/landing/buy/success/index.html
@@ -0,0 +1,75 @@
+<!doctype html>
+<html lang="en">
+  <head>
+    <meta charset="UTF-8" />
+    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
+    <title>Dictx Pro Activated</title>
+    <meta
+      name="description"
+      content="Your Dictx Pro purchase is complete. Copy your license key and activate in the app."
+    />
+    <link rel="icon" type="image/png" sizes="32x32" href="/favicon-32x32.png" />
+    <link rel="stylesheet" href="/styles.css" />
+  </head>
+  <body>
+    <div class="noise"></div>
+
+    <header class="shell nav">
+      <a class="brand" href="/">
+        <img class="brand-icon" src="/logo.png" alt="Dictx logo" />
+        <span class="wordmark">Dictx</span>
+      </a>
+      <div class="menu">
+        <a href="/">Home</a>
+      </div>
+    </header>
+
+    <main class="shell">
+      <section class="panel hero">
+        <p class="kicker">Purchase Complete</p>
+        <h1>Activate Dictx Pro</h1>
+        <p class="lede">
+          Copy your license key and paste it in Dictx:
+          <strong>Settings -> About -> Activate Dictx Pro</strong>.
+        </p>
+
+        <div class="card" style="margin-top: 16px">
+          <h2 style="font-size: 18px">Your License Key</h2>
+          <p id="license-key-value" style="word-break: break-all; font-family: monospace">
+            Not found in URL. Add <code>?checkout_id=polar_cl_...</code>.
+          </p>
+          <div class="actions" style="margin-top: 12px">
+            <button id="copy-btn" class="btn primary" type="button" disabled>
+              Copy License Key
+            </button>
+            <a class="btn ghost" href="/">Back to Dictx</a>
+          </div>
+          <p id="copy-status" class="meta" style="margin-top: 8px"></p>
+        </div>
+      </section>
+    </main>
+
+    <script>
+      const params = new URLSearchParams(window.location.search);
+      const key = (params.get("license_key") || params.get("checkout_id") || "").trim();
+      const valueEl = document.getElementById("license-key-value");
+      const copyBtn = document.getElementById("copy-btn");
+      const copyStatus = document.getElementById("copy-status");
+
+      if (key) {
+        valueEl.textContent = key;
+        copyBtn.disabled = false;
+      }
+
+      copyBtn.addEventListener("click", async () => {
+        if (!key) return;
+        try {
+          await navigator.clipboard.writeText(key);
+          copyStatus.textContent = "Copied. Paste it in Dictx Settings -> About.";
+        } catch (_error) {
+          copyStatus.textContent = "Copy failed. Select and copy manually.";
+        }
+      });
+    </script>
+  </body>
+</html>
diff --git a/src/components/settings/about/AboutSettings.tsx b/src/components/settings/about/AboutSettings.tsx
index 4d11db3..4fe65cb 100644
--- a/src/components/settings/about/AboutSettings.tsx
+++ b/src/components/settings/about/AboutSettings.tsx
@@ -16,6 +16,7 @@ export const AboutSettings: React.FC = () => {
   const { t } = useTranslation();
   const [version, setVersion] = useState("");
   const [licenseKey, setLicenseKey] = useState("");
+  const [clipboardError, setClipboardError] = useState<string | null>(null);
   const {
     entitlement,
     isSubmitting: proSubmitting,
@@ -44,6 +45,16 @@ export const AboutSettings: React.FC = () => {
     }
   };
 
+  const handlePasteFromClipboard = async () => {
+    try {
+      setClipboardError(null);
+      const text = await navigator.clipboard.readText();
+      setLicenseKey(text.trim());
+    } catch (_error) {
+      setClipboardError(t("settings.about.proActivation.clipboardFailed"));
+    }
+  };
+
   return (
     <div className="max-w-3xl w-full mx-auto space-y-6">
       <SettingsGroup title={t("settings.about.title")}>
@@ -92,6 +103,14 @@ export const AboutSettings: React.FC = () => {
                 placeholder={t("settings.about.proActivation.licenseKeyPlaceholder")}
                 disabled={proSubmitting}
               />
+              <Button
+                variant="secondary"
+                size="sm"
+                onClick={() => void handlePasteFromClipboard()}
+                disabled={proSubmitting}
+              >
+                {t("settings.about.proActivation.pasteFromClipboard")}
+              </Button>
               <Button
                 variant="primary"
                 size="md"
@@ -100,6 +119,9 @@ export const AboutSettings: React.FC = () => {
               >
                 {t("settings.about.proActivation.activate")}
               </Button>
+              {clipboardError && (
+                <p className="text-xs text-red-400">{clipboardError}</p>
+              )}
               {proError && <p className="text-xs text-red-400">{proError}</p>}
               {entitlement?.verification_error && (
                 <p className="text-xs text-red-400">
diff --git a/src/i18n/locales/en/translation.json b/src/i18n/locales/en/translation.json
index 1ee4dfd..6552a9a 100644
--- a/src/i18n/locales/en/translation.json
+++ b/src/i18n/locales/en/translation.json
@@ -546,7 +546,9 @@
         "refresh": "Re-check Entitlement",
         "clear": "Remove Activation",
         "licenseKey": "License Key",
-        "licenseKeyPlaceholder": "polar_cl_..."
+        "licenseKeyPlaceholder": "polar_cl_...",
+        "pasteFromClipboard": "Paste From Clipboard",
+        "clipboardFailed": "Clipboard access failed. Paste manually."
       },
       "acknowledgments": {
         "title": "Acknowledgments",