diff --git a/app.yaml b/app.yaml
index 3910ced..78ceace 100644
--- a/app.yaml
+++ b/app.yaml
@@ -59,6 +59,15 @@ handlers:
     X-Content-Type-Options: nosniff
     Strict-Transport-Security: max-age=31536000; includeSubDomains
 
+- url: /externs
+  static_dir: externs
+  secure: always
+  expiration: 7d
+  http_headers:
+    X-Frame-Options: DENY
+    X-Content-Type-Options: nosniff
+    Strict-Transport-Security: max-age=31536000; includeSubDomains
+
 pagespeed:
   enabled_rewriters:
   - InlineImages
diff --git a/externs/cosmopolite.js b/externs/cosmopolite.js
new file mode 100644
index 0000000..e3d770d
--- /dev/null
+++ b/externs/cosmopolite.js
@@ -0,0 +1,31 @@
+/**
+ * @fileoverview Definitions for Cosmopolite API. Details of the API are at:
+ * https://www.cosmopolite.org/reference
+ *
+ * @externs
+ * @author ian@cosmopolite.org (Ian Gulliver)
+ */
+
+/**
+ * @see https://www.cosmopolite.org/reference#constructor
+ * @constructor
+ * @param {?Cosmopolite.typeCallbacks=} opt_callbacks
+ * @param {?string=} opt_urlPrefix
+ * @param {?string=} opt_namespace
+ * @param {?string=} opt_trackingID
+ * @nosideeffects
+ */
+function Cosmopolite(
+    opt_callbacks, opt_urlPrefix, opt_namespace, opt_trackingID) {}
+
+
+/**
+ * @see https://www.cosmopolite.org/reference#subscribe
+ * @param {Cosmopolite.typeSubjectLoose|Array.<Cosmopolite.typeSubjectLoose>}
+ *     subjects
+ * @param {?number=} opt_messages
+ * @param {?number=} opt_lastID
+ * @return {Promise|Array.<Promise>}
+ */
+Cosmopolite.prototype.subscribe =
+  function(subjects, opt_messages, opt_lastID) {};