diff --git a/package.json b/package.json
index 2d07d243..d1b39847 100644
--- a/package.json
+++ b/package.json
@@ -3,7 +3,7 @@
     "version": "0.1.0",
     "private": true,
     "dependencies": {
-        "@pybricks/mpy-cross-4": "^1.0.2",
+        "@pybricks/mpy-cross-4": "^2.0.0",
         "@testing-library/jest-dom": "^4.2.4",
         "@testing-library/react": "^9.3.2",
         "@testing-library/user-event": "^7.1.2",
diff --git a/src/actions/mpy.ts b/src/actions/mpy.ts
index d39b8d56..2bbbc6ce 100644
--- a/src/actions/mpy.ts
+++ b/src/actions/mpy.ts
@@ -1,8 +1,6 @@
-import MpyCross from '@pybricks/mpy-cross-4';
+import { compile as mpyCrossCompile } from '@pybricks/mpy-cross-4';
 import { Action } from 'redux';
-
-// this starts the mpy-cross wasm runtime and leaves it running in the background
-const mpy = MpyCross({ arguments: ['-mno-unicode'] });
+import { ThunkAction } from 'redux-thunk';
 
 export enum MpyActionType {
     Compiled = 'mpy.action.compile',
@@ -12,11 +10,18 @@ export interface MpyCompiledAction extends Action<MpyActionType.Compiled> {
     /**
      * The compiled .mpy data.
      */
-    data: Uint8Array;
+    data?: Uint8Array;
+    /**
+     * Error output.
+     */
+    err?: string;
 }
 
-export function compile(script: string): MpyCompiledAction {
-    // TODO: figure out how to capture stderr and emit error action on failure
-    const data = mpy.compile(script);
-    return { type: MpyActionType.Compiled, data };
+type MpyCompileAction = ThunkAction<Promise<MpyCompiledAction>, {}, {}, Action>;
+
+export function compile(script: string, options?: string[]): MpyCompileAction {
+    return async function (): Promise<MpyCompiledAction> {
+        const result = await mpyCrossCompile('main.py', script, options);
+        return { type: MpyActionType.Compiled, data: result.mpy, err: result.err };
+    };
 }
diff --git a/src/components/RunButton.tsx b/src/components/RunButton.tsx
index 0b92e1a2..3be4f853 100644
--- a/src/components/RunButton.tsx
+++ b/src/components/RunButton.tsx
@@ -1,9 +1,10 @@
 import { Ace } from 'ace-builds';
-import { batch, connect } from 'react-redux';
+import { connect } from 'react-redux';
 import { AnyAction } from 'redux';
 import { ThunkDispatch } from 'redux-thunk';
 import { downloadAndRun } from '../actions/hub';
 import { compile } from '../actions/mpy';
+import * as notification from '../actions/notification';
 import { RootState } from '../reducers';
 import { HubRuntimeState } from '../reducers/hub';
 import ActionButton, { ActionButtonProps } from './ActionButton';
@@ -27,11 +28,19 @@ const mapDispatchToProps = (dispatch: Dispatch): DispatchProps => ({
             console.error('No current editor');
             return;
         }
-        batch(() => {
-            const script = c.getValue();
-            const mpy = dispatch(compile(script));
-            dispatch(downloadAndRun(mpy.data));
-        });
+        const script = c.getValue();
+        // TODO: need to get options from hub because they depend on firmware compile options
+        dispatch(compile(script, ['-mno-unicode']))
+            .then((mpy) => {
+                if (mpy.data) {
+                    dispatch(downloadAndRun(mpy.data));
+                } else {
+                    dispatch(
+                        notification.add('error', mpy.err || 'Unknown compiler error.'),
+                    );
+                }
+            })
+            .catch((err) => console.error(err));
     },
 });
 
diff --git a/src/sagas/bootloader.ts b/src/sagas/bootloader.ts
index 0692c5b9..cd3fa740 100644
--- a/src/sagas/bootloader.ts
+++ b/src/sagas/bootloader.ts
@@ -8,6 +8,7 @@ import {
     delay,
     fork,
     put,
+    putResolve,
     race,
     take,
     takeEvery,
@@ -177,7 +178,7 @@ interface FirmwareMetadata {
     'device-id': HubType;
     'checksum-type': 'sum' | 'crc32';
     'mpy-abi-version': number;
-    'mpy-cross-options': Array<string>;
+    'mpy-cross-options': string[];
     'user-mpy-offset': number;
     'max-firmware-size': number;
 }
@@ -213,8 +214,13 @@ function* flashFirmware(action: BootloaderFlashFirmwareAction): Generator {
         );
     }
 
-    // TODO: pass metadata["mpy-cross-options"] to compiler
-    const mpy = (yield put(compile(main))) as MpyCompiledAction;
+    const mpy = (yield putResolve(
+        (compile(main, metadata['mpy-cross-options']) as unknown) as Action,
+    )) as MpyCompiledAction;
+
+    if (!mpy.data) {
+        throw Error(mpy.err);
+    }
 
     // compute offset for checksum - must be aligned to 4-byte boundary
     const checksumOffset =
diff --git a/yarn.lock b/yarn.lock
index 9dc1b3a8..df725d8b 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -1254,12 +1254,10 @@
   resolved "https://registry.yarnpkg.com/@popperjs/core/-/core-2.4.0.tgz#0e1bdf8d021e7ea58affade33d9d607e11365915"
   integrity sha512-NMrDy6EWh9TPdSRiHmHH2ye1v5U0gBD7pRYwSwJvomx7Bm4GG04vu63dYiVzebLOx2obPpJugew06xVP0Nk7hA==
 
-"@pybricks/mpy-cross-4@^1.0.2":
-  version "1.0.2"
-  resolved "https://npm.pkg.github.com/download/@pybricks/mpy-cross-4/1.0.2/0d1c1c7789469794795680f14930bc66922dbe55caf59b3e58df8cb8c70365df#441d5ff59498d2d5c78adc74d0a70a6575e8c426"
-  integrity sha512-iy6nVtIDRqU2SJ/aGKizfjPIk9PZQSSkOTTI7L7+HNPeF4zY/uhQSOI+ZFFaYSNoxFOrMIu9qn1Jsh3w08HO+A==
-  dependencies:
-    "@types/emscripten" "^1.39.3"
+"@pybricks/mpy-cross-4@^2.0.0":
+  version "2.0.0"
+  resolved "https://npm.pkg.github.com/download/@pybricks/mpy-cross-4/2.0.0/ee1d8da09629b14ea43bc6ac7a685b48fe191404958215e3b267478c6d750aa8#4bdf4c397652ca69ecf7ecb0749d76afc928f39b"
+  integrity sha512-qSUDm8WMCq/ZH0wdWpgnR9fHDBnLsTQ+6CNc1ARX4orD0Wezpp/N34ya45G2RGyy6dFB/YAqYcK/daGhb6Ka8Q==
 
 "@redux-saga/core@^1.1.3":
   version "1.1.3"
@@ -1506,11 +1504,6 @@
   resolved "https://registry.yarnpkg.com/@types/color-name/-/color-name-1.1.1.tgz#1c1261bbeaa10a8055bbc5d8ab84b7b2afc846a0"
   integrity sha512-rr+OQyAjxze7GgWrSaJwydHStIhHq2lvY3BOC2Mj7KnzI7XK0Uw1TOOdI9lDoajEbSWLiYgoo4f1R51erQfhPQ==
 
-"@types/emscripten@^1.39.3":
-  version "1.39.3"
-  resolved "https://registry.yarnpkg.com/@types/emscripten/-/emscripten-1.39.3.tgz#cac2f24b4739c344c42f928a4c1af1aab245364e"
-  integrity sha512-F+LPHVwVWp6bkhRq7sMdW0JsQMXd5ZuPbjwU/X6HaAtvIjRmYL2z4BCIopnBRcrzZLorVFf8gxY/GR7szLuPEw==
-
 "@types/eslint-visitor-keys@^1.0.0":
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/@types/eslint-visitor-keys/-/eslint-visitor-keys-1.0.0.tgz#1ee30d79544ca84d68d4b3cdb0af4f205663dd2d"