diff --git a/frontend_tests/node_tests/transmit.js b/frontend_tests/node_tests/transmit.js
index d2c192eb5d..590a375117 100644
--- a/frontend_tests/node_tests/transmit.js
+++ b/frontend_tests/node_tests/transmit.js
@@ -16,7 +16,9 @@ set_global('sent_messages', {
     start_tracking_message: noop,
     report_server_ack: noop,
 });
+set_global('blueslip', global.make_zblueslip());
 
+zrequire('people');
 zrequire('transmit');
 
 function test_with_mock_socket(test_params) {
@@ -168,3 +170,97 @@ run_test('transmit_message_ajax_reload_pending', () => {
     assert(!error_func_called);
     assert(reload_initiated);
 });
+
+run_test('reply_message_stream', () => {
+    const stream_message = {
+        type: 'stream',
+        stream: 'social',
+        subject: 'lunch',
+        sender_full_name: 'Alice',
+    };
+
+    const content = 'hello';
+
+    var send_message_args;
+
+    transmit.send_message = (args) => {
+        send_message_args = args;
+    };
+
+    page_params.user_id = 44;
+    page_params.queue_id = 66;
+    sent_messages.get_new_local_id = () => 99;
+
+    transmit.reply_message({
+        message: stream_message,
+        content: content,
+    });
+
+    assert.deepEqual(send_message_args, {
+        sender_id: 44,
+        queue_id: 66,
+        local_id: 99,
+        type: 'stream',
+        to: 'social',
+        content: '@**Alice** hello',
+        subject: 'lunch',
+    });
+});
+
+run_test('reply_message_private', () => {
+    const fred = {
+        user_id: 3,
+        email: 'fred@example.com',
+        full_name: 'Fred Frost',
+    };
+    people.add(fred);
+
+    people.is_my_user_id = () => false;
+
+    const pm_message = {
+        type: 'private',
+        display_recipient: [
+            {user_id: fred.user_id},
+        ],
+    };
+
+    const content = 'hello';
+
+    var send_message_args;
+
+    transmit.send_message = (args) => {
+        send_message_args = args;
+    };
+
+    page_params.user_id = 155;
+    page_params.queue_id = 177;
+    sent_messages.get_new_local_id = () => 199;
+
+    transmit.reply_message({
+        message: pm_message,
+        content: content,
+    });
+
+    assert.deepEqual(send_message_args, {
+        sender_id: 155,
+        queue_id: 177,
+        local_id: 199,
+        type: 'private',
+        to: '["fred@example.com"]',
+        content: 'hello',
+    });
+});
+
+run_test('reply_message_errors', () => {
+    const bogus_message = {
+        type: 'bogus',
+    };
+
+    blueslip.set_test_data('error', 'unknown message type: bogus');
+
+    transmit.reply_message({
+        message: bogus_message,
+    });
+
+    blueslip.clear_test_data();
+});
diff --git a/static/js/transmit.js b/static/js/transmit.js
index 1ac6c75dff..bb5c5b615d 100644
--- a/static/js/transmit.js
+++ b/static/js/transmit.js
@@ -63,6 +63,75 @@ exports.send_message = function (request, on_success, error) {
     }
 };
 
+exports.reply_message = function (opts) {
+    // This code does an application-triggered reply to a message (as
+    // opposed to the user themselves doing it).  Its only use case
+    // for now is experimental widget-aware bots, so treat this as
+    // somewhat beta code.  To understand the use case, think of a
+    // bot that wants to give users 3 or 4 canned replies to some
+    // choice, but it wants to front-end each of these options
+    // with a one-click button.  This function is part of that architecture.
+    var message = opts.message;
+    var content = opts.content;
+
+    function success() {
+        // TODO: If server response comes back before the message event,
+        //       we could show it earlier, although that creates some
+        //       complexity.  For now do nothing.  (Note that send_message
+        //       already handles things like reporting times to the server.)
+    }
+
+    function error() {
+        // TODO: In our current use case, which is widgets, to meaningfully
+        //       handle errors, we would want the widget to provide some
+        //       kind of callback to us so it can do some appropriate UI.
+        //       For now do nothing.
+    }
+
+    var locally_echoed = false;
+    var local_id = sent_messages.get_new_local_id();
+
+    var reply = {
+        sender_id: page_params.user_id,
+        queue_id: page_params.queue_id,
+        local_id: local_id,
+    };
+
+    sent_messages.start_tracking_message({
+        local_id: local_id,
+        locally_echoed: locally_echoed,
+    });
+
+    if (message.type === 'stream') {
+        var stream = message.stream;
+
+        var mention = '@**' + message.sender_full_name + '**';
+
+        content = mention + ' ' + content;
+
+        reply.type = 'stream';
+        reply.to  = stream;
+        reply.content = content;
+        reply.subject = message.subject;
+
+        transmit.send_message(reply, success, error);
+        return;
+    }
+
+    if (message.type === 'private') {
+        var pm_recipient = people.pm_reply_to(message);
+
+        reply.type = 'private';
+        reply.to = JSON.stringify(pm_recipient.split(','));
+        reply.content = content;
+
+        transmit.send_message(reply, success, error);
+        return;
+    }
+
+    blueslip.error('unknown message type: ' + message.type);
+};
+
 return exports;
 }());