diff --git a/src/cli/dev.ts b/src/cli/dev.ts
index a7e02f3..30273d2 100644
--- a/src/cli/dev.ts
+++ b/src/cli/dev.ts
@@ -66,17 +66,19 @@ function create_hot_update_server(port: number, interval = 10000) {
 	return { send };
 }
 
-export default function create_watcher(src: string, dir: string) {
+export default async function dev(src: string, dir: string) {
 	rimraf.sync(dir);
 	mkdirp.sync(dir);
 
 	const chokidar = require('chokidar');
 
 	// initial build
-	const routes = create_routes({ src });
-	create_app({ routes, src, dev: true });
+	const dev_port = await require('get-port')(10000);
 
-	const hot_update_server = create_hot_update_server(23456); // TODO robustify port selection
+	const routes = create_routes({ src });
+	create_app({ routes, src, dev: true, dev_port });
+
+	const hot_update_server = create_hot_update_server(dev_port);
 
 	// TODO watch the configs themselves?
 	const compilers = create_compilers();
@@ -93,7 +95,7 @@ export default function create_watcher(src: string, dir: string) {
 
 	watch_files('routes/**/*.+(html|js|mjs)', () => {
 		const routes = create_routes({ src });
-		create_app({ routes, src, dev: true });
+		create_app({ routes, src, dev: true, dev_port });
 	});
 
 	watch_files('app/template.html', () => {
diff --git a/src/core/create_app.ts b/src/core/create_app.ts
index 7700451..ec17793 100644
--- a/src/core/create_app.ts
+++ b/src/core/create_app.ts
@@ -5,18 +5,19 @@ import create_routes from './create_routes';
 import { fudge_mtime, posixify, write } from './utils';
 import { Route } from '../interfaces';
 
-export default function create_app({ routes, src, dev }: {
+export default function create_app({ routes, src, dev, dev_port }: {
 	routes: Route[];
 	src: string;
 	dev: boolean;
+	dev_port: number;
 }) {
 	mkdirp.sync('app/manifest');
 
-	write('app/manifest/client.js', generate_client(routes, src, dev));
+	write('app/manifest/client.js', generate_client(routes, src, dev, dev_port));
 	write('app/manifest/server.js', generate_server(routes, src));
 }
 
-function generate_client(routes: Route[], src: string, dev: boolean) {
+function generate_client(routes: Route[], src: string, dev: boolean, dev_port?: number) {
 	let code = `
 		// This file is generated by Sapper — do not edit it!
 		export const routes = [
@@ -43,13 +44,11 @@ function generate_client(routes: Route[], src: string, dev: boolean) {
 			path.resolve(__dirname, 'src/hmr-client.js')
 		);
 
-		const PORT = 23456; // TODO robustify this — needs to be controlled by the dev task
-
 		code += `
 
 			if (module.hot) {
 				import('${hmr_client}').then(client => {
-					client.connect(${PORT});
+					client.connect(${dev_port});
 				});
 			}`.replace(/^\t{3}/gm, '');
 	}