diff --git a/externs/hogfather.js b/externs/hogfather.js
new file mode 100644
index 0000000..bf07d2c
--- /dev/null
+++ b/externs/hogfather.js
@@ -0,0 +1,110 @@
+/**
+ * @fileoverview Definitions for Cosmopolite API. Details of the API are at:
+ * https://www.cosmopolite.org/hogfather/reference
+ *
+ * @externs
+ * @author ian@cosmopolite.org (Ian Gulliver)
+ */
+
+
+/* Namespace */
+var hogfather = {};
+
+
+/**
+ * @constructor
+ * @param {Cosmopolite} cosmo
+ * @param {string} id
+ * @private
+ */
+hogfather.PublicChat = function(cosmo, id) {};
+
+
+/**
+ * @param {Cosmopolite} cosmo
+ * @return {Promise}
+ */
+hogfather.PublicChat.Create = function(cosmo) {};
+
+
+/**
+ * @param {Cosmopolite} cosmo
+ * @param {string} id
+ * @return {Promise}
+ */
+hogfather.PublicChat.Join = function(cosmo, id) {};
+
+
+/**
+ */
+hogfather.PublicChat.prototype.Shutdown = function() {};
+
+
+/**
+ * @return {string}
+ * @nosideeffects
+ */
+hogfather.PublicChat.prototype.getID = function() {};
+
+
+/**
+ * @return {boolean}
+ * @nosideeffects
+ */
+hogfather.PublicChat.prototype.amOwner = function() {};
+
+
+/**
+ * @return {boolean}
+ * @nosideeffects
+ */
+hogfather.PublicChat.prototype.amWriter = function() {};
+
+
+/**
+ * @return {Array.<Cosmopolite.typeMessage>}
+ * @nosideeffects
+ */
+hogfather.PublicChat.prototype.getMessages = function() {};
+
+
+/**
+ * @return {Array.<Cosmopolite.typeMessage>}
+ * @nosideeffects
+ */
+hogfather.PublicChat.prototype.getRequests = function() {};
+
+
+/**
+ * @param {!*} message
+ * @return {Promise}
+ */
+hogfather.PublicChat.prototype.sendMessage = function(message) {};
+
+
+/**
+ * @param {string} info
+ * @return {Promise}
+ */
+hogfather.PublicChat.prototype.requestAccess = function(info) {};
+
+
+/**
+ * @param {string} sender
+ * @return {Promise}
+ */
+hogfather.PublicChat.prototype.addOwner = function(sender) {};
+
+
+/**
+ * @param {string} sender
+ * @return {Promise}
+ */
+hogfather.PublicChat.prototype.addWriter = function(sender) {};
+
+
+/**
+ * @param {string} sender
+ * @return {Promise}
+ */
+hogfather.PublicChat.prototype.denyRequest = function(sender) {};