diff --git a/apps/scratch/src/blocks/make-toolbox-xml.ts b/apps/scratch/src/blocks/make-toolbox-xml.ts
index cdc05b6..b77a6e9 100644
--- a/apps/scratch/src/blocks/make-toolbox-xml.ts
+++ b/apps/scratch/src/blocks/make-toolbox-xml.ts
@@ -60,14 +60,35 @@ const createDictionaryWithAllValuesInfinite = (
 ): Record<string, number> =>
   Object.values(object).reduce((acc, key) => ({ ...acc, [key]: -1 }), {});
 
-export const allowAllBlocks: BlockLimits = {
-  ...createDictionaryWithAllValuesInfinite(MotionOpCode),
-  ...createDictionaryWithAllValuesInfinite(LooksOpCode),
-  ...createDictionaryWithAllValuesInfinite(SoundOpCode),
-  ...createDictionaryWithAllValuesInfinite(EventOpCode),
-  ...createDictionaryWithAllValuesInfinite(ControlOpCode),
-  ...createDictionaryWithAllValuesInfinite(SensingOpCode),
-  ...createDictionaryWithAllValuesInfinite(OperatorsOpCode),
+const allowAllMotionBlocks: BlockLimits =
+  createDictionaryWithAllValuesInfinite(MotionOpCode);
+
+const allowAllLooksBlocks: BlockLimits =
+  createDictionaryWithAllValuesInfinite(LooksOpCode);
+
+const allowAllSoundBlocks: BlockLimits =
+  createDictionaryWithAllValuesInfinite(SoundOpCode);
+
+const allowAllEventBlocks: BlockLimits =
+  createDictionaryWithAllValuesInfinite(EventOpCode);
+
+const allowAllControlBlocks: BlockLimits =
+  createDictionaryWithAllValuesInfinite(ControlOpCode);
+
+const allowAllSensingBlocks: BlockLimits =
+  createDictionaryWithAllValuesInfinite(SensingOpCode);
+
+const allowAllOperatorsBlocks: BlockLimits =
+  createDictionaryWithAllValuesInfinite(OperatorsOpCode);
+
+export const allowAllStandardBlocks: BlockLimits = {
+  ...allowAllMotionBlocks,
+  ...allowAllLooksBlocks,
+  ...allowAllSoundBlocks,
+  ...allowAllEventBlocks,
+  ...allowAllControlBlocks,
+  ...allowAllSensingBlocks,
+  ...allowAllOperatorsBlocks,
   variables: true,
   customBlocks: true,
 };
@@ -88,6 +109,7 @@ export const allowNoBlocks: BlockLimits = {};
  * @param costumeName - The name of the default selected costume dropdown.
  * @param backdropName - The name of the default selected backdrop dropdown.
  * @param soundName -  The name of the default selected sound dropdown.
+ * @param blockLimits - The number of blocks that are allowed to be used for a given task. Null means all blocks are allowed.
  * @returns- a ScratchBlocks-style XML document for the contents of the toolbox.
  */
 const makeToolboxXML = function (
@@ -99,7 +121,7 @@ const makeToolboxXML = function (
   costumeName: string = "",
   backdropName: string = "",
   soundName: string = "",
-  blockLimits: BlockLimits = allowAllBlocks,
+  blockLimits: BlockLimits | null = null,
 ): string {
   isStage = isInitialSetup || isStage;
   const gap = categorySeparator;
@@ -127,7 +149,7 @@ const makeToolboxXML = function (
       isStage,
       targetId,
       colors.motion,
-      blockLimits,
+      blockLimits ?? allowAllMotionBlocks,
     );
   const looksXML =
     moveCategory("looks") ||
@@ -138,7 +160,7 @@ const makeToolboxXML = function (
       costumeName,
       backdropName,
       colors.looks,
-      blockLimits,
+      blockLimits ?? allowAllLooksBlocks,
     );
   const soundXML =
     moveCategory("sound") ||
@@ -148,11 +170,17 @@ const makeToolboxXML = function (
       targetId,
       soundName,
       colors.sounds,
-      blockLimits,
+      blockLimits ?? allowAllSoundBlocks,
     );
   const eventsXML =
     moveCategory("event") ||
-    buildEventXml(isInitialSetup, isStage, targetId, colors.event, blockLimits);
+    buildEventXml(
+      isInitialSetup,
+      isStage,
+      targetId,
+      colors.event,
+      blockLimits ?? allowAllEventBlocks,
+    );
   const controlXML =
     moveCategory("control") ||
     buildControlXml(
@@ -160,7 +188,7 @@ const makeToolboxXML = function (
       isStage,
       targetId,
       colors.control,
-      blockLimits,
+      blockLimits ?? allowAllControlBlocks,
     );
   const sensingXML =
     moveCategory("sensing") ||
@@ -169,7 +197,7 @@ const makeToolboxXML = function (
       isStage,
       targetId,
       colors.sensing,
-      blockLimits,
+      blockLimits ?? allowAllSensingBlocks,
     );
   const operatorsXML =
     moveCategory("operators") ||
@@ -178,7 +206,7 @@ const makeToolboxXML = function (
       isStage,
       targetId,
       colors.operators,
-      blockLimits,
+      blockLimits ?? allowAllOperatorsBlocks,
     );
   const variablesXML =
     moveCategory("data") ||
@@ -203,20 +231,46 @@ const makeToolboxXML = function (
     gap,
     operatorsXML,
     gap,
-    blockLimits.variables ? variablesXML : "",
+    blockLimits == null || blockLimits.variables ? variablesXML : "",
     gap,
-    blockLimits.customBlocks ? myBlocksXML : "",
+    blockLimits == null || blockLimits.customBlocks ? myBlocksXML : "",
   ];
 
+  for (const extensionCategory of categoriesXML) {
+    if (blockLimits !== null) {
+      // find all <block type="{opcode}"> elements and check them against blockLimits
+      extensionCategory.xml = extensionCategory.xml.replace(
+        /<block type="([^"]+)">.*?<\/block>/g,
+        (match, opcode) => {
+          // if the opcode is not in any of the blockLimits, return the match
+          if (opcode in blockLimits) {
+            const limit = blockLimits[opcode];
+            if (limit === 0) {
+              // if the limit is 0, don't show the block
+              return "";
+            }
+          } else {
+            // if the opcode is not in blockLimits, don't show the block
+            return "";
+          }
+
+          return match;
+        },
+      );
+    }
+
+    // check if the xml still contains some <block> elements
+    if (extensionCategory.xml.includes("<block")) {
+      // if so, add the category to the toolbox
+      everything.push(extensionCategory.xml);
+    }
+  }
+
   if (!everything.find((xml) => xml.includes("<category"))) {
     // if all blocks are disabled, show a message
     everything.push(allBlocksAreDisabled);
   }
 
-  for (const extensionCategory of categoriesXML) {
-    everything.push(gap, extensionCategory.xml);
-  }
-
   everything.push(xmlClose);
   return everything.join("\n");
 };
diff --git a/apps/scratch/src/components/TaskConfig.tsx b/apps/scratch/src/components/TaskConfig.tsx
index 51fd97a..f7598e9 100644
--- a/apps/scratch/src/components/TaskConfig.tsx
+++ b/apps/scratch/src/components/TaskConfig.tsx
@@ -1,9 +1,16 @@
-import React, { FormEvent, useCallback, useEffect } from "react";
+import React, { FormEvent, useCallback, useEffect, useState } from "react";
 
-import VM, { ScratchCrtConfig } from "scratch-vm";
+import VM from "scratch-vm";
 import Modal from "./modal/Modal";
-import { allowAllBlocks, allowNoBlocks } from "../blocks/make-toolbox-xml";
+import {
+  allowAllStandardBlocks,
+  allowNoBlocks,
+} from "../blocks/make-toolbox-xml";
 import { UpdateBlockToolboxEvent } from "../events/update-block-toolbox";
+import { useAssertionsEnabled } from "../hooks/useAssertionsEnabled";
+import { ExtensionId } from "../extensions";
+import { ScratchCrtConfig } from "../types/scratch-vm-custom";
+import { FormattedMessage } from "react-intl";
 
 const TaskConfig = ({
   vm,
@@ -14,6 +21,12 @@ const TaskConfig = ({
   isShown?: boolean;
   hideModal: () => void;
 }) => {
+  const assertionsEnabled = useAssertionsEnabled(vm);
+  const [isAssertionsExtensionEnabled, setIsAssertionsExtensionEnabled] =
+    useState(false);
+
+  const [enableAssertions, setEnableAssertions] = useState(false);
+
   const updateConfig = useCallback(
     (
       e: React.SyntheticEvent,
@@ -37,9 +50,12 @@ const TaskConfig = ({
     [vm],
   );
 
-  const onAllowAllBlocks = useCallback(
+  const onAllowAllStandardBlocks = useCallback(
     (e: React.MouseEvent<HTMLButtonElement, MouseEvent>) => {
-      updateConfig(e, (config) => (config.allowedBlocks = allowAllBlocks));
+      updateConfig(
+        e,
+        (config) => (config.allowedBlocks = allowAllStandardBlocks),
+      );
     },
     [updateConfig],
   );
@@ -55,32 +71,83 @@ const TaskConfig = ({
       e.preventDefault();
       e.stopPropagation();
 
-      updateConfig(e, () => {});
+      updateConfig(e, () => {
+        // update assertions
+        if (isAssertionsExtensionEnabled) {
+          vm.runtime.emit(
+            enableAssertions ? "ENABLE_ASSERTIONS" : "DISABLE_ASSERTIONS",
+          );
+        }
+      });
 
       hideModal();
     },
-    [vm],
+    [
+      vm,
+      isAssertionsExtensionEnabled,
+      enableAssertions,
+      updateConfig,
+      hideModal,
+    ],
   );
 
   useEffect(() => {
     // every time the modal is opened, load the form values based on the config
+    setIsAssertionsExtensionEnabled(
+      vm.extensionManager.isExtensionLoaded(ExtensionId.Assertions),
+    );
+
+    setEnableAssertions(assertionsEnabled);
   }, [isShown]);
 
   return (
     <Modal isShown={isShown}>
-      <h1>Task Config</h1>
+      <h1>
+        <FormattedMessage
+          defaultMessage="Task Config"
+          description="Heading of the task config modal."
+          id="crt.taskConfig.heading"
+        />
+      </h1>
 
       <form onSubmit={onSubmit} data-testid="task-config-form">
         <button
-          onClick={onAllowAllBlocks}
-          data-testid="allow-all-blocks-button"
+          onClick={onAllowAllStandardBlocks}
+          data-testid="allow-all-standard-blocks-button"
         >
-          Allow all blocks to be used
+          <FormattedMessage
+            defaultMessage="Allow all standard blocks to be used"
+            description="Label shown on the button that, when clicked, allows all standard blocks to be used by students."
+            id="crt.taskConfig.heading"
+          />
         </button>
         <button onClick={onAllowNoBlocks} data-testid="allow-no-blocks-button">
-          Disallow any block to be used
+          <FormattedMessage
+            defaultMessage="Disallow all blocks"
+            description="Label shown on the button that, when clicked, disallows all blocks from being used by students."
+            id="crt.taskConfig.heading"
+          />
         </button>
 
+        {isAssertionsExtensionEnabled && (
+          <label>
+            <span>
+              <FormattedMessage
+                defaultMessage="Enable assertions simulating a student solving the task."
+                description="Label shown next to the checkbox that allows a teacher to simulate the assertion mode when editing."
+                id="crt.taskConfig.heading"
+              />
+            </span>
+            <input
+              type="checkbox"
+              min="0"
+              checked={enableAssertions}
+              onChange={(e) => setEnableAssertions(e.target.checked)}
+              data-testid="enable-assertions-checkbox"
+            />
+          </label>
+        )}
+
         <input
           type="submit"
           value="Save"
diff --git a/apps/scratch/src/components/assertions-state/AssertionsState.tsx b/apps/scratch/src/components/assertions-state/AssertionsState.tsx
new file mode 100644
index 0000000..0f4adeb
--- /dev/null
+++ b/apps/scratch/src/components/assertions-state/AssertionsState.tsx
@@ -0,0 +1,40 @@
+import VM from "scratch-vm";
+import styles from "./assertions-state.css";
+import icon from "../../extensions/assertions/test-icon-white.svg";
+import { useAssertionsEnabled } from "../../hooks/useAssertionsEnabled";
+import { useAssertionsState } from "../../hooks/useAssertionsState";
+import classNames from "classnames";
+
+const AssertionsState = ({ vm }: { vm: VM }) => {
+  const assertionsEnabled = useAssertionsEnabled(vm);
+  const assertionsState = useAssertionsState(vm);
+
+  if (!assertionsEnabled) {
+    return null;
+  }
+
+  const isSuccessful =
+    assertionsState.total > 0 &&
+    assertionsState.passed >= assertionsState.total;
+
+  return (
+    <div
+      className={classNames(
+        styles.state,
+        isSuccessful ? styles.success : styles.noSuccess,
+      )}
+      data-testid="assertion-state"
+    >
+      {assertionsState.total > 0 && (
+        <span>
+          <span data-testid="passed">{assertionsState.passed}</span>
+          {" / "}
+          <span data-testid="total">{assertionsState.total}</span>
+        </span>
+      )}
+      <img src={icon} />
+    </div>
+  );
+};
+
+export default AssertionsState;
diff --git a/apps/scratch/src/components/assertions-state/assertions-state.css b/apps/scratch/src/components/assertions-state/assertions-state.css
new file mode 100644
index 0000000..80dfe72
--- /dev/null
+++ b/apps/scratch/src/components/assertions-state/assertions-state.css
@@ -0,0 +1,25 @@
+.state {
+  padding: 0.25rem;
+  border-radius: 0.25rem;
+  background-color: var(--assertions-default-color);
+  color: #fff;
+
+  display: flex;
+  flex-direction: row;
+  justify-content: center;
+
+  gap: 0.5rem;
+
+  img {
+    height: 20px;
+    width: auto;
+  }
+}
+
+.success {
+  background-color: var(--assertions-success-color);
+}
+
+.no-success {
+  background-color: var(--assertions-default-color);
+}
diff --git a/apps/scratch/src/components/block-config/BlockConfig.tsx b/apps/scratch/src/components/block-config/BlockConfig.tsx
index 0ddc968..ce82b83 100644
--- a/apps/scratch/src/components/block-config/BlockConfig.tsx
+++ b/apps/scratch/src/components/block-config/BlockConfig.tsx
@@ -4,6 +4,7 @@ import { ModifyBlockConfigEvent } from "../../events/modify-block-config";
 import VM from "scratch-vm";
 import { UpdateBlockToolboxEvent } from "../../events/update-block-toolbox";
 import Modal from "../modal/Modal";
+import { FormattedMessage } from "react-intl";
 
 const cannotBeUsed = 0;
 const infiniteUses = -1;
@@ -96,12 +97,23 @@ const BlockConfig = ({ vm }: { vm: VM }) => {
   return (
     <Modal isShown={blockId !== null}>
       <h1>
-        Block Config <small>({blockId})</small>
+        <FormattedMessage
+          defaultMessage="Block Config"
+          description="Heading of the block config modal shown when configuring whether a given block can be used by students."
+          id="crt.blockConfig.heading"
+        />
+        <small>({blockId})</small>
       </h1>
 
       <form onSubmit={onSubmit} data-testid="block-config-form">
         <label>
-          <span>Can this block be used?</span>
+          <span>
+            <FormattedMessage
+              defaultMessage="Can this block be used by students?"
+              description="Label shown next to the checkbox for configuring whether a given block can be used by students."
+              id="crt.blockConfig.canBlockBeUsed"
+            />
+          </span>
           <input
             type="checkbox"
             min="0"
@@ -114,7 +126,11 @@ const BlockConfig = ({ vm }: { vm: VM }) => {
         {canBeUsed && (
           <label>
             <span>
-              Is there a limit to how many times this block can be used?
+              <FormattedMessage
+                defaultMessage="Is there a limit to how many times this block can be used?"
+                description="Label shown next to the checkbox for configuring whether a there is a limit to how often a block can be used by students."
+                id="crt.blockConfig.hasBlockUsageLimit"
+              />
             </span>
             <input
               type="checkbox"
@@ -128,7 +144,13 @@ const BlockConfig = ({ vm }: { vm: VM }) => {
 
         {hasBlockLimit && (
           <label>
-            <span>How many times can this block be used?</span>
+            <span>
+              <FormattedMessage
+                defaultMessage="How many times can this block be used?"
+                description="Label shown next to the input field for configuring how often a block can be used by students."
+                id="crt.blockConfig.hasBlockUsageLimit"
+              />
+            </span>
             <input
               type="number"
               min="1"
diff --git a/apps/scratch/src/components/gui/Gui.tsx b/apps/scratch/src/components/customized-scratch-components/gui/Gui.tsx
similarity index 90%
rename from apps/scratch/src/components/gui/Gui.tsx
rename to apps/scratch/src/components/customized-scratch-components/gui/Gui.tsx
index 3275c40..34b02e6 100644
--- a/apps/scratch/src/components/gui/Gui.tsx
+++ b/apps/scratch/src/components/customized-scratch-components/gui/Gui.tsx
@@ -48,10 +48,10 @@ import addExtensionIcon from "@scratch-submodule/scratch-gui/src/components/gui/
 import codeIcon from "@scratch-submodule/scratch-gui/src/components/gui/icon--code.svg";
 import costumesIcon from "@scratch-submodule/scratch-gui/src/components/gui/icon--costumes.svg";
 import soundsIcon from "@scratch-submodule/scratch-gui/src/components/gui/icon--sounds.svg";
-import { ReactNode } from "react";
+import { useEffect } from "react";
 
-import Blocks from "../../containers/Blocks";
-import TargetPane from "../../containers/TargetPane";
+import Blocks from "../../../containers/customized-scratch-containers/Blocks";
+import TargetPane from "../../../containers/customized-scratch-containers/TargetPane";
 import StageWrapper from "../stage-wrapper/StageWrapper";
 
 const messages = defineMessages({
@@ -73,7 +73,6 @@ const GUIComponent = (props: {
 
   // optional
   assetHost?: string;
-  children?: ReactNode;
   cloudHost?: string;
   fetchingProject?: boolean;
   isLoading?: boolean;
@@ -114,6 +113,7 @@ const GUIComponent = (props: {
   onRequestCloseCostumeLibrary?: () => void;
   onShowPrivacyPolicy?: () => void;
   onTabSelect?: () => void;
+  onTaskProgress?: (totalAssertions: number, passedAssertions: number) => void;
   soundsTabVisible?: boolean;
   // see https://github.com/scratchfoundation/scratch-gui/blob/d678d609e182ccc5ab557d7d45a3cc3e6430b056/src/lib/layout-constants.js#L7
   stageSizeMode: StageSizeMode;
@@ -134,7 +134,6 @@ const GUIComponent = (props: {
     canEditTask,
     isCostumesTabEnabled,
     isSoundsTabEnabled,
-    children,
     connectionModalVisible,
     costumeLibraryVisible,
     costumesTabVisible,
@@ -156,13 +155,10 @@ const GUIComponent = (props: {
     theme,
     tipsLibraryVisible,
     vm,
+    onTaskProgress,
     ...componentProps
   } = omit(props, "dispatch");
 
-  if (children) {
-    return <Box {...componentProps}>{children}</Box>;
-  }
-
   const tabClassNames = {
     tabs: styles.tabs,
     tab: classNames(tabStyles.reactTabsTab, styles.tab),
@@ -179,6 +175,44 @@ const GUIComponent = (props: {
     isRendererSupported = Renderer.isSupported();
   }
 
+  useEffect(() => {
+    const enableAssertions = () => vm.runtime.emit("ENABLE_ASSERTIONS");
+
+    if (!canEditTask) {
+      // if we cannot edit the task, we always enable assertions
+      // if the extension is loaded
+
+      const onAssertionsChecked = (
+        totalAssertions: number,
+        passedAssertions: number,
+      ) => onTaskProgress && onTaskProgress(totalAssertions, passedAssertions);
+
+      // enable assertions as soon as the extension is loaded
+      vm.runtime.on("ASSERTIONS_EXTENSION_LOADED", enableAssertions);
+      vm.runtime.on("ASSERTIONS_CHECKED", onAssertionsChecked);
+
+      // or if the extension is already loaded, enable them now
+      enableAssertions();
+
+      return () => {
+        // when effect is cleaned up, remove event listeners
+        vm.runtime.removeListener(
+          "ASSERTIONS_EXTENSION_LOADED",
+          enableAssertions,
+        );
+
+        vm.runtime.off("ASSERTIONS_CHECKED", onAssertionsChecked);
+
+        // and disable assertions that were enabled automatically
+        vm.runtime.emit("DISABLE_ASSERTIONS");
+      };
+    }
+
+    // otherwise we only enable assertions if the user has enabled them
+    // return no-op cleanup function
+    return () => {};
+  }, [vm.runtime, canEditTask, onTaskProgress]);
+
   return (
     <MediaQuery minWidth={layout.fullSizeMinWidth}>
       {(isFullSize) => {
diff --git a/apps/scratch/src/components/gui/gui.css b/apps/scratch/src/components/customized-scratch-components/gui/gui.css
similarity index 100%
rename from apps/scratch/src/components/gui/gui.css
rename to apps/scratch/src/components/customized-scratch-components/gui/gui.css
diff --git a/apps/scratch/src/components/sprite-selector/SpriteList.tsx b/apps/scratch/src/components/customized-scratch-components/sprite-selector/SpriteList.tsx
similarity index 100%
rename from apps/scratch/src/components/sprite-selector/SpriteList.tsx
rename to apps/scratch/src/components/customized-scratch-components/sprite-selector/SpriteList.tsx
diff --git a/apps/scratch/src/components/sprite-selector/SpriteSelector.tsx b/apps/scratch/src/components/customized-scratch-components/sprite-selector/SpriteSelector.tsx
similarity index 99%
rename from apps/scratch/src/components/sprite-selector/SpriteSelector.tsx
rename to apps/scratch/src/components/customized-scratch-components/sprite-selector/SpriteSelector.tsx
index cd80d37..a00d19f 100644
--- a/apps/scratch/src/components/sprite-selector/SpriteSelector.tsx
+++ b/apps/scratch/src/components/customized-scratch-components/sprite-selector/SpriteSelector.tsx
@@ -18,7 +18,7 @@ import surpriseIcon from "@scratch-submodule/scratch-gui/src/components/action-m
 import searchIcon from "@scratch-submodule/scratch-gui/src/components/action-menu/icon--search.svg";
 import { STAGE_DISPLAY_SIZES } from "@scratch-submodule/scratch-gui/src/lib/layout-constants";
 import { Sprite } from "../target-pane/TargetPane";
-import { WithoutInteraction } from "../WithoutInteraction";
+import { WithoutInteraction } from "../../WithoutInteraction";
 
 const messages = defineMessages({
   addSpriteFromLibrary: {
diff --git a/apps/scratch/src/components/stage-wrapper/StageWrapper.tsx b/apps/scratch/src/components/customized-scratch-components/stage-wrapper/StageWrapper.tsx
similarity index 86%
rename from apps/scratch/src/components/stage-wrapper/StageWrapper.tsx
rename to apps/scratch/src/components/customized-scratch-components/stage-wrapper/StageWrapper.tsx
index 0a6efe5..43ffeaf 100644
--- a/apps/scratch/src/components/stage-wrapper/StageWrapper.tsx
+++ b/apps/scratch/src/components/customized-scratch-components/stage-wrapper/StageWrapper.tsx
@@ -22,11 +22,12 @@ import smallStageIcon from "@scratch-submodule/scratch-gui/src/components/stage-
 import unFullScreenIcon from "@scratch-submodule/scratch-gui/src/components/stage-header/icon--unfullscreen.svg";
 import settingsIcon from "@scratch-submodule/scratch-gui/src/components/menu-bar/icon--settings.svg";
 import { getStageDimensions } from "@scratch-submodule/scratch-gui/src/lib/screen-utils";
-import Controls from "../../scratch/scratch-gui/src/containers/controls";
-import ToggleButtons from "../../scratch/scratch-gui/src/components/toggle-buttons/toggle-buttons";
+import Controls from "../../../scratch/scratch-gui/src/containers/controls";
+import ToggleButtons from "../../../scratch/scratch-gui/src/components/toggle-buttons/toggle-buttons";
 import { useSelector, useDispatch } from "react-redux";
 import { useState } from "react";
-import TaskConfig from "../TaskConfig";
+import TaskConfig from "../../TaskConfig";
+import AssertionsState from "../../assertions-state/AssertionsState";
 
 const messages = defineMessages({
   largeStageSizeMessage: {
@@ -108,7 +109,9 @@ const StageWrapper = function (props: Props) {
             className={headerStyles.stageMenuWrapper}
             style={{ width: getStageDimensions(null, true).width }}
           >
-            <Controls vm={vm} />
+            <div className="d-contents" data-testid="stage-controls">
+              <Controls vm={vm} />
+            </div>
             <div className={headerStyles.unselectWrapper}>
               <Button
                 className={headerStyles.stageButton}
@@ -132,21 +135,25 @@ const StageWrapper = function (props: Props) {
       ) : (
         <Box className={headerStyles.stageHeaderWrapper}>
           <Box className={headerStyles.stageMenuWrapper}>
-            <Controls vm={vm} />
+            <div className="d-contents" data-testid="stage-controls">
+              <Controls vm={vm} />
+            </div>
             <div>
-              {canEditTask && (
-                <button
-                  className={crtStyles.settingsButton}
-                  onClick={() => setShowTaskConfig(true)}
-                  data-testid="open-taskconfig-button"
-                >
-                  <img
-                    className={headerStyles.stageButtonIcon}
-                    draggable={false}
-                    src={settingsIcon}
-                  />
-                </button>
-              )}
+              <div className={crtStyles.buttons}>
+                <AssertionsState vm={vm} />
+                {canEditTask && (
+                  <button
+                    onClick={() => setShowTaskConfig(true)}
+                    data-testid="open-taskconfig-button"
+                  >
+                    <img
+                      className={headerStyles.stageButtonIcon}
+                      draggable={false}
+                      src={settingsIcon}
+                    />
+                  </button>
+                )}
+              </div>
             </div>
             <div className={headerStyles.stageSizeRow}>
               <div className={headerStyles.stageSizeToggleGroup}>
diff --git a/apps/scratch/src/components/customized-scratch-components/stage-wrapper/stage-wrapper.css b/apps/scratch/src/components/customized-scratch-components/stage-wrapper/stage-wrapper.css
new file mode 100644
index 0000000..c819189
--- /dev/null
+++ b/apps/scratch/src/components/customized-scratch-components/stage-wrapper/stage-wrapper.css
@@ -0,0 +1,24 @@
+@import "../../../scratch/scratch-gui/src/css/units.css";
+@import "../../../scratch/scratch-gui/src/css/colors.css";
+@import "../../../scratch/scratch-gui/src/css/z-index.css";
+
+.non-interactive {
+    pointer-events: none;
+}
+
+.buttons {
+    display: flex;
+    flex-direction: row;
+    align-items: center;
+    gap: 0.5rem;
+}
+
+.buttons button {
+    background-color: #000;
+    border: #000 1px solid;
+
+    padding: 0.25rem;
+    border-radius: 0.25rem;
+
+    cursor: pointer;
+}
\ No newline at end of file
diff --git a/apps/scratch/src/components/target-pane/TargetPane.tsx b/apps/scratch/src/components/customized-scratch-components/target-pane/TargetPane.tsx
similarity index 98%
rename from apps/scratch/src/components/target-pane/TargetPane.tsx
rename to apps/scratch/src/components/customized-scratch-components/target-pane/TargetPane.tsx
index 74b845a..6961e96 100644
--- a/apps/scratch/src/components/target-pane/TargetPane.tsx
+++ b/apps/scratch/src/components/customized-scratch-components/target-pane/TargetPane.tsx
@@ -6,7 +6,7 @@ import StageSelector from "@scratch-submodule/scratch-gui/src/containers/stage-s
 import { STAGE_DISPLAY_SIZES } from "@scratch-submodule/scratch-gui/src/lib/layout-constants";
 
 import styles from "@scratch-submodule/scratch-gui/src/components/target-pane/target-pane.css";
-import MinimalStageSelector from "../MinimalStageSelector";
+import MinimalStageSelector from "../../MinimalStageSelector";
 import React from "react";
 
 export type Sprite = {
diff --git a/apps/scratch/src/components/stage-wrapper/stage-wrapper.css b/apps/scratch/src/components/stage-wrapper/stage-wrapper.css
deleted file mode 100644
index cf6a72c..0000000
--- a/apps/scratch/src/components/stage-wrapper/stage-wrapper.css
+++ /dev/null
@@ -1,17 +0,0 @@
-@import "../../scratch/scratch-gui/src/css/units.css";
-@import "../../scratch/scratch-gui/src/css/colors.css";
-@import "../../scratch/scratch-gui/src/css/z-index.css";
-
-.non-interactive {
-    pointer-events: none;
-}
-
-.settings-button {
-    background-color: #000;
-    border: #000 1px solid;
-
-    padding: 0.25rem;
-    border-radius: 0.25rem;
-
-    cursor: pointer;
-}
\ No newline at end of file
diff --git a/apps/scratch/src/containers/Blocks.tsx b/apps/scratch/src/containers/customized-scratch-containers/Blocks.tsx
similarity index 98%
rename from apps/scratch/src/containers/Blocks.tsx
rename to apps/scratch/src/containers/customized-scratch-containers/Blocks.tsx
index 62d6750..dfa8c18 100644
--- a/apps/scratch/src/containers/Blocks.tsx
+++ b/apps/scratch/src/containers/customized-scratch-containers/Blocks.tsx
@@ -49,29 +49,29 @@ import {
 } from "@scratch-submodule/scratch-gui/src/reducers/editor-tab";
 import { StageDisplaySize } from "@scratch-submodule/scratch-gui/src/lib/screen-utils";
 import ScratchBlocks, { Flyout, Workspace } from "scratch-blocks";
-import makeToolboxXML, { allowAllBlocks } from "../blocks/make-toolbox-xml";
+import makeToolboxXML from "../../blocks/make-toolbox-xml";
 import { Action, Dispatch } from "redux";
 import VMScratchBlocks from "@scratch-submodule/scratch-gui/src/lib/blocks";
 import ExtensionLibrary from "./ExtensionLibrary";
 import {
   addBlockConfigButtons,
   updateSingleBlockConfigButton,
-} from "../blocks/block-config";
-import BlockConfig from "../components/block-config/BlockConfig";
-import { UpdateBlockToolboxEvent } from "../events/update-block-toolbox";
-import { filterNonNull } from "../utilities/filter-non-null";
+} from "../../blocks/block-config";
+import BlockConfig from "../../components/block-config/BlockConfig";
+import { UpdateBlockToolboxEvent } from "../../events/update-block-toolbox";
+import { filterNonNull } from "../../utilities/filter-non-null";
 import {
   addFreezeButtonsToStack,
   freezeTaskBlocks,
   removeFreezeButtons,
-} from "../blocks/freeze-task-blocks";
+} from "../../blocks/freeze-task-blocks";
 import {
   isBlockBeingDragged,
   isBlockInsertionMarker,
   isBlocksCanvas,
   isWithinStack,
   isVisualTopOfStack,
-} from "../utilities/scratch-selectors";
+} from "../../utilities/scratch-selectors";
 
 // reverse engineered from https://github.com/scratchfoundation/scratch-vm/blob/613399e9a9a333eef5c8fb5e846d5c8f4f9536c6/src/engine/blocks.js#L312
 interface WorkspaceChangeEvent {
@@ -661,9 +661,7 @@ class Blocks extends React.Component<Props, State> {
         targetSounds.length > 0
           ? targetSounds[targetSounds.length - 1].name
           : "",
-        this.props.canEditTask
-          ? allowAllBlocks
-          : this.props.vm.crtConfig?.allowedBlocks,
+        this.props.canEditTask ? null : this.props.vm.crtConfig?.allowedBlocks,
       );
     } catch {
       return null;
diff --git a/apps/scratch/src/containers/ExtensionLibrary.tsx b/apps/scratch/src/containers/customized-scratch-containers/ExtensionLibrary.tsx
similarity index 96%
rename from apps/scratch/src/containers/ExtensionLibrary.tsx
rename to apps/scratch/src/containers/customized-scratch-containers/ExtensionLibrary.tsx
index 6a07f7c..75f93d4 100644
--- a/apps/scratch/src/containers/ExtensionLibrary.tsx
+++ b/apps/scratch/src/containers/customized-scratch-containers/ExtensionLibrary.tsx
@@ -3,7 +3,7 @@ import React from "react";
 import VM from "scratch-vm";
 import { defineMessages, InjectedIntl, injectIntl } from "react-intl";
 
-import extensionLibraryContent, { ExtensionId } from "../extensions";
+import extensionLibraryContent, { ExtensionId } from "../../extensions";
 
 import LibraryComponent from "@scratch-submodule/scratch-gui/src/components/library/library.jsx";
 import extensionIcon from "@scratch-submodule/scratch-gui/src/components/action-menu/icon--sprite.svg";
diff --git a/apps/scratch/src/containers/Gui.tsx b/apps/scratch/src/containers/customized-scratch-containers/Gui.tsx
similarity index 98%
rename from apps/scratch/src/containers/Gui.tsx
rename to apps/scratch/src/containers/customized-scratch-containers/Gui.tsx
index 7ffdcea..507ff28 100644
--- a/apps/scratch/src/containers/Gui.tsx
+++ b/apps/scratch/src/containers/customized-scratch-containers/Gui.tsx
@@ -4,7 +4,7 @@ import { connect } from "react-redux";
 import VM from "scratch-vm";
 import { InjectedIntl, injectIntl } from "react-intl";
 
-import GUIComponent from "../components/gui/Gui";
+import GUIComponent from "../../components/customized-scratch-components/gui/Gui";
 import ErrorBoundaryHOC from "@scratch-submodule/scratch-gui/src/lib/error-boundary-hoc.jsx";
 import {
   getIsError,
@@ -36,7 +36,7 @@ import { setIsScratchDesktop } from "@scratch-submodule/scratch-gui/src/lib/isSc
 import { StageSizeMode } from "@scratch-submodule/scratch-gui/src/lib/screen-utils";
 import { AppStateHOC } from "@scratch-submodule/scratch-gui/src";
 import HashParserHOC from "@scratch-submodule/scratch-gui/src/lib/hash-parser-hoc";
-import { loadCrtProject } from "../vm/load-crt-project";
+import { loadCrtProject } from "../../vm/load-crt-project";
 
 const { RequestMetadata, setMetadata, unsetMetadata } = storage.scratchFetch;
 
diff --git a/apps/scratch/src/containers/TargetPane.tsx b/apps/scratch/src/containers/customized-scratch-containers/TargetPane.tsx
similarity index 99%
rename from apps/scratch/src/containers/TargetPane.tsx
rename to apps/scratch/src/containers/customized-scratch-containers/TargetPane.tsx
index 77d164f..f3036dd 100644
--- a/apps/scratch/src/containers/TargetPane.tsx
+++ b/apps/scratch/src/containers/customized-scratch-containers/TargetPane.tsx
@@ -23,7 +23,7 @@ import DragConstants from "@scratch-submodule/scratch-gui/src/lib/drag-constants
 import TargetPaneComponent, {
   Sprite,
   Props as TargetPaneProps,
-} from "../components/target-pane/TargetPane";
+} from "../../components/customized-scratch-components/target-pane/TargetPane";
 import { BLOCKS_DEFAULT_SCALE } from "@scratch-submodule/scratch-gui/src/lib/layout-constants";
 import spriteLibraryContent from "@scratch-submodule/scratch-gui/src/lib/libraries/sprites.json";
 import {
diff --git a/apps/scratch/src/containers/__mocks__/Blocks.tsx b/apps/scratch/src/containers/customized-scratch-containers/__mocks__/Blocks.tsx
similarity index 100%
rename from apps/scratch/src/containers/__mocks__/Blocks.tsx
rename to apps/scratch/src/containers/customized-scratch-containers/__mocks__/Blocks.tsx
diff --git a/apps/scratch/src/extensions/assertions/assertions.svg b/apps/scratch/src/extensions/assertions/assertions.svg
new file mode 100644
index 0000000..01c412d
--- /dev/null
+++ b/apps/scratch/src/extensions/assertions/assertions.svg
@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" height="50" width="50"></svg>
\ No newline at end of file
diff --git a/apps/scratch/src/extensions/assertions/index.ts b/apps/scratch/src/extensions/assertions/index.ts
new file mode 100644
index 0000000..4b85930
--- /dev/null
+++ b/apps/scratch/src/extensions/assertions/index.ts
@@ -0,0 +1,436 @@
+// adapted from https://github.com/scratchfoundation/scratch-vm/blob/766c767c7a2f3da432480ade515de0a9f98804ba/src/extensions/scratch3_music/index.js
+
+import VM from "scratch-vm";
+import formatMessage from "format-message";
+import { ExtensionId, ExtensionMetadata, ExtensionUtilType } from "..";
+import { BlockType } from "../../blocks/block-type";
+import { ArgumentType } from "../../blocks/argument-type";
+import testIconWhite from "./test-icon-white.svg";
+import testIconBlack from "./test-icon-black.svg";
+
+/**
+ * Icon svg to be displayed at the left edge of each extension block, encoded as a data URI.
+ */
+const blockIconURI = testIconWhite;
+
+/**
+ * Icon svg to be displayed in the category menu, encoded as a data URI.
+ */
+const menuIconURI = testIconBlack;
+
+type CustomState = {
+  totalNumberOfAssertions: number;
+  numberOfPassedAssertions: number;
+};
+
+const EXTENSION_ID = ExtensionId.Assertions;
+
+/**
+ * Get the custom state for a target.
+ * @param {Target} target
+ * @returns the mutable custom state associated with that target. This will be created if necessary.
+ */
+const getCustomState = (target: VM.TargetExtended): CustomState => {
+  let state = (target as VM.TargetWithCustomState<CustomState>).getCustomState(
+    AssertionExtension.STATE_KEY,
+  );
+
+  if (!state) {
+    state = { ...AssertionExtension.DEFAULT_STATE };
+
+    (target as VM.TargetWithCustomState<CustomState>).setCustomState(
+      AssertionExtension.STATE_KEY,
+      state,
+    );
+  }
+
+  return state;
+};
+
+/**
+ * Set the custom state for a target.
+ */
+const setCustomState = (
+  target: VM.TargetExtended,
+  state: CustomState,
+): void => {
+  (target as VM.TargetWithCustomState<CustomState>).setCustomState(
+    AssertionExtension.STATE_KEY,
+    state,
+  );
+};
+
+interface RememberedTargetState {
+  currentCostume: number;
+  volume: number;
+  customState: Partial<VM.CustomState>;
+}
+
+interface RememberedStageState extends RememberedTargetState {
+  tempo: number;
+  videoTransparency: number;
+}
+
+interface RememberedSpriteState extends RememberedTargetState {
+  name: string;
+  x: number;
+  y: number;
+  size: number;
+  direction: number;
+  draggable: boolean;
+  rotationStyle: VM.RotationStyle;
+  layerOrder: number;
+}
+
+/**
+ * Adds assertions to the Scratch VM.
+ * See https://github.com/scratchfoundation/scratch-vm/blob/develop/docs/extensions.md for a documentation of the Scratch VM extensions.
+ * @param {Runtime} runtime - the runtime instantiating this block package.
+ * @constructor
+ */
+class AssertionExtension {
+  static readonly STATE_KEY = EXTENSION_ID;
+  static readonly DEFAULT_STATE: CustomState = {
+    numberOfPassedAssertions: 0,
+    totalNumberOfAssertions: 0,
+  };
+
+  /**
+   * The runtime instantiating this block package.
+   * @type {Runtime}
+   */
+  private readonly runtime: VM.RuntimeExtended;
+
+  private rememberedStageState: RememberedStageState | null = null;
+  private rememberedSpriteState: {
+    [targetId: string]: RememberedSpriteState;
+  } = {};
+
+  /**
+   * Whether assertions are enabled.
+   */
+  private enabled: boolean = false;
+
+  /**
+   * Whether assertions are currently being run.
+   */
+  private runningAssertions: boolean = false;
+
+  constructor(runtime: VM.RuntimeExtended) {
+    this.runtime = runtime;
+
+    this.onEnableAssertions = this.onEnableAssertions.bind(this);
+    this.onDisableAssertions = this.onDisableAssertions.bind(this);
+    this.onTargetCreated = this.onTargetCreated.bind(this);
+    this.onProjectStart = this.onProjectStart.bind(this);
+    this.onProjectStop = this.onProjectStop.bind(this);
+    this.onProjectStopAll = this.onProjectStopAll.bind(this);
+    this.onProjectLoaded = this.onProjectLoaded.bind(this);
+
+    this.runtime.on("ENABLE_ASSERTIONS", this.onEnableAssertions);
+    this.runtime.on("DISABLE_ASSERTIONS", this.onDisableAssertions);
+
+    // reply to queries about whether assertions are enabled
+    this.runtime.on("ARE_ASSERTIONS_ENABLED_QUERY", () =>
+      this.runtime.emit("ARE_ASSERTIONS_ENABLED_RESPONSE", this.enabled),
+    );
+
+    // emit an event to signal that the extension has been loaded
+    this.runtime.emit("ASSERTIONS_EXTENSION_LOADED");
+  }
+
+  /**
+   * Called when assertions are to be enabled.
+   * Remembers the initial state of all targets,
+   * resets them when the project starts running and
+   * checks the assertions when the project stops running.
+   */
+  onEnableAssertions = (): void => {
+    this.rememberTargetStates();
+
+    this.runtime.on("PROJECT_LOADED", this.onProjectLoaded);
+    this.runtime.on("targetWasCreated", this.onTargetCreated);
+    this.runtime.on("PROJECT_START", this.onProjectStart);
+    this.runtime.on("PROJECT_RUN_STOP", this.onProjectStop);
+    this.runtime.on("PROJECT_STOP_ALL", this.onProjectStopAll);
+
+    this.enabled = true;
+    this.runtime.emit("ASSERTIONS_ENABLED");
+  };
+
+  /**
+   * Called when the project is loaded while assertions are enabled.
+   * This requires us to remember the initial state of all targets.
+   */
+  onProjectLoaded = (): void => {
+    this.rememberTargetStates();
+  };
+
+  /**
+   * Called when assertions are to be disabled.
+   * Stops resetting targets when the project starts running and
+   * no longer checks the assertions when the project stops running.
+   */
+  onDisableAssertions = (): void => {
+    this.runtime.off("targetWasCreated", this.onTargetCreated);
+    this.runtime.off("PROJECT_START", this.onProjectStart);
+    this.runtime.off("PROJECT_RUN_STOP", this.onProjectStop);
+    this.runtime.off("PROJECT_STOP_ALL", this.onProjectStopAll);
+
+    this.enabled = false;
+    this.runtime.emit("ASSERTIONS_DISABLED");
+  };
+
+  /**
+   * When a Target is cloned, clone the custom state.
+   * @param newTarget - the newly created target.
+   * @param sourceTarget - the target used as a source for the new clone, if any.
+   * @listens Runtime#event:targetWasCreated
+   * @private
+   */
+  onTargetCreated(
+    newTarget: VM.TargetExtended,
+    sourceTarget?: VM.TargetExtended,
+  ): void {
+    if (sourceTarget) {
+      const state = (
+        sourceTarget as VM.TargetWithCustomState<CustomState>
+      ).getCustomState(AssertionExtension.STATE_KEY);
+
+      if (state) {
+        (newTarget as VM.TargetWithCustomState<CustomState>).setCustomState(
+          AssertionExtension.STATE_KEY,
+          {
+            ...state,
+          },
+        );
+      }
+    }
+  }
+
+  /**
+   * Callback for when the project starts running.
+   */
+  onProjectStart = (): void => {
+    this.resetAllTargets();
+  };
+
+  /**
+   * Remembers the state of all targets so they can be reset later.
+   */
+  rememberTargetStates = (): void => {
+    this.rememberedSpriteState = {};
+    this.rememberedStageState = null;
+
+    for (const target of this.runtime.targets) {
+      if (target.isStage) {
+        this.rememberedStageState = {
+          currentCostume: target.currentCostume,
+          volume: target.volume,
+          tempo: target.tempo,
+          videoTransparency: target.videoTransparency,
+          customState: target._customState,
+        };
+      } else if (target.isSprite()) {
+        this.rememberedSpriteState[target.id] = {
+          currentCostume: target.currentCostume,
+          volume: target.volume,
+          name: target.getName(),
+          x: target.x,
+          y: target.y,
+          size: target.size,
+          direction: target.direction,
+          draggable: target.draggable,
+          rotationStyle: target.rotationStyle,
+          layerOrder: target.getLayerOrder(),
+          customState: target._customState,
+        };
+      }
+    }
+  };
+
+  /**
+   * Reset all targets to their initial state, i.e. the state they were in when assertions were enabled.
+   */
+  resetAllTargets = (): void => {
+    this.runtime.targets.forEach((target) => {
+      // reset the state for each target
+
+      if (target.isStage && this.rememberedStageState) {
+        target.setCostume(this.rememberedStageState.currentCostume);
+        target.volume = this.rememberedStageState.volume;
+        target.tempo = this.rememberedStageState.tempo;
+        target.videoTransparency = this.rememberedStageState.videoTransparency;
+
+        target._customState = this.rememberedStageState.customState;
+      } else if (target.isSprite() && this.rememberedSpriteState[target.id]) {
+        const state = this.rememberedSpriteState[target.id];
+
+        target.setCostume(state.currentCostume);
+        target.volume = state.volume;
+        target.sprite.name = state.name;
+        target.setXY(state.x, state.y);
+        target.setSize(state.size);
+        target.setDirection(state.direction);
+        target.setDraggable(state.draggable);
+        target.setRotationStyle(state.rotationStyle);
+        // see https://github.com/scratchfoundation/scratch-vm/blob/bb1659e1f42de5bd28d7233c8e418c4e536a2bf0/src/sprites/rendered-target.js#L850C71-L850C97
+        target.renderer.setDrawableOrder(
+          target.drawableID,
+          state.layerOrder,
+          "sprite",
+        );
+
+        target._customState = state.customState;
+      }
+
+      // reset the assertions state for each target
+      const state = getCustomState(target);
+
+      setCustomState(target, {
+        ...state,
+        numberOfPassedAssertions: 0,
+        totalNumberOfAssertions: 0,
+      });
+    });
+  };
+
+  /**
+   * Callback for when the project stops running.
+   */
+  onProjectStop = (): void => {
+    const afterRunOpcode = `${EXTENSION_ID}_event_whenTaskFinishedRunning`;
+
+    let projectHasAfterRunAssertions: boolean = false;
+    this.runtime.allScriptsByOpcodeDo(
+      afterRunOpcode,
+      () => (projectHasAfterRunAssertions = true),
+    );
+
+    if (projectHasAfterRunAssertions && !this.runningAssertions) {
+      // once the project has stopped running, check the after-run assertions
+      // note that after running them, we'll land in this method again
+      this.runningAssertions = true;
+
+      // see https://github.com/scratchfoundation/scratch-vm/blob/3867903e65ed9ed03cfeeb80a785457df7c2f099/src/blocks/scratch3_event.js#L12
+      this.runtime.startHats(afterRunOpcode);
+    } else {
+      // we just ran the after-run assertions
+      this.runningAssertions = false;
+
+      this.onProjectAssertionsRan();
+    }
+  };
+
+  /**
+   * Callback for when the project assertions were run.
+   */
+  onProjectAssertionsRan = (): void => {
+    const { totalNumberOfAssertions, numberOfPassedAssertions } =
+      this.runtime.targets.reduce(
+        ({ totalNumberOfAssertions, numberOfPassedAssertions }, target) => {
+          const state = getCustomState(target);
+
+          return {
+            totalNumberOfAssertions:
+              totalNumberOfAssertions + state.totalNumberOfAssertions,
+            numberOfPassedAssertions:
+              numberOfPassedAssertions + state.numberOfPassedAssertions,
+          };
+        },
+        {
+          totalNumberOfAssertions: 0,
+          numberOfPassedAssertions: 0,
+        },
+      );
+
+    this.runtime.emit(
+      "ASSERTIONS_CHECKED",
+      totalNumberOfAssertions,
+      numberOfPassedAssertions,
+    );
+  };
+
+  /**
+   * Callback for when the red stop button is pressed.
+   */
+  onProjectStopAll = (): void => {
+    this.resetAllTargets();
+  };
+
+  /**
+   * @returns {object} metadata for this extension and its blocks.
+   */
+  getInfo(): ExtensionMetadata {
+    return {
+      id: EXTENSION_ID,
+      name: formatMessage({
+        id: "crt.extensions.assertions.categoryName",
+        default: "Assertions",
+        description: "Label for the Assertions extension category",
+      }),
+      menuIconURI,
+      blockIconURI,
+      blocks: [
+        {
+          opcode: "event_whenTaskFinishedRunning",
+          text: formatMessage({
+            id: "crt.extensions.assertions.event_whenTaskFinishedRunning",
+            default: "when task finished running",
+            description:
+              "The name displayed on the scratch block that runs after the project has stopped running",
+          }),
+          blockType: BlockType.hat,
+          isEdgeActivated: false,
+          arguments: {},
+        },
+        {
+          // Note: In order to avoid hardcoding all extension opcodes into the collimator backend, we tell
+          // collimator via a opcode prefix what the block should be translated to.
+          opcode: "noop_assert",
+          blockType: BlockType.command,
+          text: formatMessage({
+            id: "crt.extensions.assertions.assert",
+            default: "Assert [CONDITION] is true",
+          }),
+          arguments: {
+            CONDITION: {
+              type: ArgumentType.boolean,
+            },
+          },
+        },
+      ],
+      menus: {},
+    };
+  }
+
+  /**
+   * A scratch hat block that is triggered when the task has finished running.
+   */
+  event_whenTaskFinishedRunning(
+    _args: unknown,
+    _util: ExtensionUtilType,
+  ): boolean {
+    // always run the children when this block is triggered
+    return true;
+  }
+
+  /**
+   * Assert a condition is true.
+   */
+  noop_assert(
+    args: { CONDITION: boolean },
+    { target }: ExtensionUtilType,
+  ): void {
+    const state = getCustomState(target);
+
+    setCustomState(target, {
+      ...state,
+      totalNumberOfAssertions: state.totalNumberOfAssertions + 1,
+      numberOfPassedAssertions:
+        state.numberOfPassedAssertions + (args.CONDITION ? 1 : 0),
+    });
+  }
+}
+
+export default AssertionExtension;
diff --git a/apps/scratch/src/extensions/assertions/test-icon-black.svg b/apps/scratch/src/extensions/assertions/test-icon-black.svg
new file mode 100644
index 0000000..eea8234
--- /dev/null
+++ b/apps/scratch/src/extensions/assertions/test-icon-black.svg
@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 448 512">
+  <!--!Font Awesome Free 6.6.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free Copyright 2024 Fonticons, Inc.-->
+  <path fill="#000" d="M288 0L160 0 128 0C110.3 0 96 14.3 96 32s14.3 32 32 32l0 132.8c0 11.8-3.3 23.5-9.5 33.5L10.3 406.2C3.6 417.2 0 429.7 0 442.6C0 480.9 31.1 512 69.4 512l309.2 0c38.3 0 69.4-31.1 69.4-69.4c0-12.8-3.6-25.4-10.3-36.4L329.5 230.4c-6.2-10.1-9.5-21.7-9.5-33.5L320 64c17.7 0 32-14.3 32-32s-14.3-32-32-32L288 0zM192 196.8L192 64l64 0 0 132.8c0 23.7 6.6 46.9 19 67.1L309.5 320l-171 0L173 263.9c12.4-20.2 19-43.4 19-67.1z"/>
+</svg>
\ No newline at end of file
diff --git a/apps/scratch/src/extensions/assertions/test-icon-white.svg b/apps/scratch/src/extensions/assertions/test-icon-white.svg
new file mode 100644
index 0000000..cb19e87
--- /dev/null
+++ b/apps/scratch/src/extensions/assertions/test-icon-white.svg
@@ -0,0 +1,4 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 448 512">
+  <!--!Font Awesome Free 6.6.0 by @fontawesome - https://fontawesome.com License - https://fontawesome.com/license/free Copyright 2024 Fonticons, Inc.-->
+  <path fill="#fff" d="M288 0L160 0 128 0C110.3 0 96 14.3 96 32s14.3 32 32 32l0 132.8c0 11.8-3.3 23.5-9.5 33.5L10.3 406.2C3.6 417.2 0 429.7 0 442.6C0 480.9 31.1 512 69.4 512l309.2 0c38.3 0 69.4-31.1 69.4-69.4c0-12.8-3.6-25.4-10.3-36.4L329.5 230.4c-6.2-10.1-9.5-21.7-9.5-33.5L320 64c17.7 0 32-14.3 32-32s-14.3-32-32-32L288 0zM192 196.8L192 64l64 0 0 132.8c0 23.7 6.6 46.9 19 67.1L309.5 320l-171 0L173 263.9c12.4-20.2 19-43.4 19-67.1z"/>
+</svg>
\ No newline at end of file
diff --git a/apps/scratch/src/extensions/example/index.ts b/apps/scratch/src/extensions/example/index.ts
deleted file mode 100644
index 7059c5d..0000000
--- a/apps/scratch/src/extensions/example/index.ts
+++ /dev/null
@@ -1,160 +0,0 @@
-// adapted from https://github.com/scratchfoundation/scratch-vm/blob/766c767c7a2f3da432480ade515de0a9f98804ba/src/extensions/scratch3_music/index.js
-
-import VM from "scratch-vm";
-import formatMessage from "format-message";
-import { ExtensionId, ExtensionMetadata, ExtensionUtilType } from "..";
-import { BlockType } from "../../blocks/block-type";
-import { ArgumentType } from "../../blocks/argument-type";
-
-/**
- * Icon svg to be displayed at the left edge of each extension block, encoded as a data URI.
- */
-const blockIconURI: string =
-  "data:image/svg+xml;base64,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";
-
-/**
- * Icon svg to be displayed in the category menu, encoded as a data URI.
- */
-const menuIconURI: string =
-  "data:image/svg+xml;base64,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";
-
-type CustomState = {
-  test: string;
-};
-
-const EXTENSION_ID = ExtensionId.Example;
-
-/**
- * Class for an example Scratch 3.0 extension.
- * @param {Runtime} runtime - the runtime instantiating this block package.
- * @constructor
- */
-class ExampleExtension {
-  static readonly STATE_KEY = EXTENSION_ID;
-  static readonly DEFAULT_STATE: CustomState = {
-    test: "value",
-  };
-
-  /**
-   * The runtime instantiating this block package.
-   * @type {Runtime}
-   */
-  private readonly runtime: VM.RuntimeExtended;
-
-  constructor(runtime: VM.RuntimeExtended) {
-    this.runtime = runtime;
-
-    this.onTargetCreated = this.onTargetCreated.bind(this);
-
-    // eslint-disable-next-line @typescript-eslint/no-explicit-any
-    this.runtime.on("targetWasCreated", this.onTargetCreated as any);
-  }
-  /**
-   * Create data for a menu in scratch-blocks format, consisting of an array of objects with text and
-   * value properties. The text is a translated string, and the value is one-indexed.
-   * @param  {object[]} info - An array of info objects each having a name property.
-   * @return {array} - An array of objects with text and value properties.
-   * @private
-   */
-  buildMenu(info: { name: string }[]): { text: string; value: string }[] {
-    return info.map((entry, index) => ({
-      text: entry.name,
-      value: String(index + 1),
-    }));
-  }
-
-  /**
-   * Gte the custom state for a target.
-   * @param {Target} target
-   * @returns the mutable custom state associated with that target. This will be created if necessary.
-   * @private
-   */
-  getCustomState(target: VM.TargetExtended): CustomState {
-    let state = (
-      target as VM.TargetWithCustomState<CustomState>
-    ).getCustomState(ExampleExtension.STATE_KEY);
-
-    if (!state) {
-      state = { ...ExampleExtension.DEFAULT_STATE };
-
-      (target as VM.TargetWithCustomState<CustomState>).setCustomState(
-        ExampleExtension.STATE_KEY,
-        state,
-      );
-    }
-
-    return state;
-  }
-
-  /**
-   * When a Target is cloned, clone the custom state.
-   * @param newTarget - the newly created target.
-   * @param sourceTarget - the target used as a source for the new clone, if any.
-   * @listens Runtime#event:targetWasCreated
-   * @private
-   */
-  onTargetCreated(
-    newTarget: VM.TargetExtended,
-    sourceTarget?: VM.TargetExtended,
-  ): void {
-    if (sourceTarget) {
-      const state = (
-        sourceTarget as VM.TargetWithCustomState<CustomState>
-      ).getCustomState(ExampleExtension.STATE_KEY);
-
-      if (state) {
-        (newTarget as VM.TargetWithCustomState<CustomState>).setCustomState(
-          ExampleExtension.STATE_KEY,
-          {
-            ...state,
-          },
-        );
-      }
-    }
-  }
-
-  /**
-   * @returns {object} metadata for this extension and its blocks.
-   */
-  getInfo(): ExtensionMetadata {
-    return {
-      id: EXTENSION_ID,
-      name: formatMessage({
-        id: "music.categoryName",
-        default: "Music",
-        description: "Label for the Music extension category",
-      }),
-      menuIconURI,
-      blockIconURI,
-      blocks: [
-        {
-          // Note: In order to avoid hardcoding all extension opcodes into collimator, we tell
-          // collimator via a opcode prefix what the block should be translated to.
-          opcode: "functionCall_setX",
-          blockType: BlockType.command,
-          text: formatMessage({
-            id: "music.setTempo",
-            default: "set tempo to [TEMPO]",
-            description: "set tempo (speed) for notes, drums, and rests played",
-          }),
-          arguments: {
-            TEMPO: {
-              type: ArgumentType.number,
-              defaultValue: 60,
-            },
-          },
-        },
-      ],
-      menus: {},
-    };
-  }
-
-  /**
-   * Set the target's x position.
-   */
-  setX(args: { TEMPO: string | number }, { target }: ExtensionUtilType): void {
-    target.setXY(parseInt(args.TEMPO.toString()), target.y);
-  }
-}
-
-export default ExampleExtension;
diff --git a/apps/scratch/src/extensions/example/music-small.svg b/apps/scratch/src/extensions/example/music-small.svg
deleted file mode 100644
index 7ffde85..0000000
--- a/apps/scratch/src/extensions/example/music-small.svg
+++ /dev/null
@@ -1 +0,0 @@
-<svg width="40" height="40" viewBox="0 0 40 40" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink"><title>music-block-icon</title><defs><path d="M32.18 25.874C32.636 28.157 30.512 30 27.433 30c-3.07 0-5.923-1.843-6.372-4.126-.458-2.285 1.665-4.136 4.743-4.136.647 0 1.283.084 1.89.234.338.086.637.18.938.302.87-.02-.104-2.294-1.835-12.23-2.134-12.302 3.06-1.87 8.768-2.752 5.708-.885.076 4.82-3.65 3.844-3.724-.987-4.65-7.153.263 14.738zm-16.998 5.99C15.63 34.148 13.507 36 10.44 36c-3.07 0-5.922-1.852-6.38-4.136-.448-2.284 1.674-4.135 4.75-4.135 1.003 0 1.975.196 2.855.543.822-.055-.15-2.377-1.862-12.228-2.133-12.303 3.06-1.87 8.764-2.753 5.706-.894.076 4.82-3.648 3.834-3.724-.987-4.65-7.152.262 14.738z" id="a"/></defs><g fill="none" fill-rule="evenodd"><use fill="#FFF" xlink:href="#a"/><path stroke-opacity=".1" stroke="#000" d="M28.456 21.675c-.01-.312-.087-.825-.256-1.702-.096-.495-.612-3.022-.753-3.73-.395-1.98-.76-3.92-1.142-6.113-.732-4.223-.693-6.05.344-6.527.5-.23 1.06-.08 1.84.35.414.227 2.182 1.365 2.07 1.296 1.994 1.242 3.464 1.774 4.93 1.548 1.526-.237 2.504-.06 2.876.618.348.635.015 1.416-.73 2.18-1.472 1.516-3.975 2.514-5.848 2.023-.822-.22-1.238-.465-2.38-1.267l-.095-.066c.047.593.264 1.74.717 3.803.294 1.336 2.08 9.187 2.637 11.674l.002.012c.528 2.637-1.873 4.724-5.236 4.724-3.29 0-6.363-1.988-6.862-4.528-.53-2.64 1.873-4.734 5.233-4.734.672 0 1.347.085 2.014.25.227.057.436.118.636.187zm-16.996 5.99c-.01-.318-.09-.838-.266-1.737-.09-.46-.595-2.937-.753-3.727-.39-1.96-.75-3.89-1.13-6.07-.732-4.223-.692-6.05.344-6.526.502-.23 1.06-.082 1.84.35.415.227 2.182 1.364 2.07 1.295 1.993 1.242 3.462 1.774 4.926 1.548 1.525-.24 2.504-.064 2.876.614.348.635.015 1.415-.728 2.18-1.474 1.517-3.977 2.513-5.847 2.017-.82-.22-1.236-.464-2.378-1.267l-.095-.065c.047.593.264 1.74.717 3.802.294 1.337 2.078 9.19 2.636 11.675l.003.013c.517 2.638-1.884 4.732-5.234 4.732-3.287 0-6.36-1.993-6.87-4.54-.52-2.64 1.884-4.73 5.24-4.73.905 0 1.803.15 2.65.436z"/></g></svg>
\ No newline at end of file
diff --git a/apps/scratch/src/extensions/example/music.png b/apps/scratch/src/extensions/example/music.png
deleted file mode 100644
index f8fa9aa..0000000
Binary files a/apps/scratch/src/extensions/example/music.png and /dev/null differ
diff --git a/apps/scratch/src/extensions/index.tsx b/apps/scratch/src/extensions/index.tsx
index 12c39a6..1a31b13 100644
--- a/apps/scratch/src/extensions/index.tsx
+++ b/apps/scratch/src/extensions/index.tsx
@@ -1,14 +1,14 @@
 import { FormattedMessage } from "react-intl";
-
-import musicIconURL from "./example/music.png";
-import musicInsetIconURL from "./example/music-small.svg";
 import React from "react";
 import VM from "scratch-vm";
 import { ArgumentType } from "../blocks/argument-type";
 import { BlockType } from "../blocks/block-type";
 
+import assertionsIconURL from "./assertions/assertions.svg";
+import assertionsInsetIconURL from "./assertions/test-icon-white.svg";
+
 export enum ExtensionId {
-  Example = "example",
+  Assertions = "assertions",
 }
 
 export interface Extension {
@@ -22,7 +22,7 @@ export interface Extension {
 export interface ExtensionMetdataBlockArgument {
   type: ArgumentType;
   menu?: string;
-  defaultValue: number;
+  defaultValue?: unknown;
 }
 
 export interface ExtensionMetdataBlock {
@@ -33,6 +33,12 @@ export interface ExtensionMetdataBlock {
     [key: string]: ExtensionMetdataBlockArgument;
   };
   hideFromPalette?: boolean;
+  restartExistingThreads?: boolean;
+
+  // If true, scratch will continously run the block and starts the stack if the return value changes from false to true.
+  // If undefined it is assumed to be true.
+  // Note however, that this causes some UI bugs such as https://github.com/scratchfoundation/scratch-vm/issues/1948
+  isEdgeActivated?: boolean;
 }
 
 export interface ExtensionMetadataMenu {
@@ -56,21 +62,21 @@ export interface ExtensionUtilType {
 }
 
 export default {
-  [ExtensionId.Example]: {
+  [ExtensionId.Assertions]: {
     name: (
       <FormattedMessage
-        defaultMessage="Music"
-        description="Name for the 'Music' extension"
-        id="gui.extension.music.name"
+        defaultMessage="Assertions"
+        description="Name for the 'Assertions' extension"
+        id="crt.extension.assertions.name"
       />
     ),
-    iconURL: musicIconURL,
-    insetIconURL: musicInsetIconURL,
+    iconURL: assertionsIconURL,
+    insetIconURL: assertionsInsetIconURL,
     description: (
       <FormattedMessage
-        defaultMessage="Play instruments and drums."
-        description="Description for the 'Music' extension"
-        id="gui.extension.music.description"
+        defaultMessage="Assert conditions on students' code"
+        description="Description for the 'Assertions' extension"
+        id="crt.extension.assertions.description"
       />
     ),
     featured: true,
diff --git a/apps/scratch/src/hooks/useAssertionsEnabled.ts b/apps/scratch/src/hooks/useAssertionsEnabled.ts
new file mode 100644
index 0000000..9a43070
--- /dev/null
+++ b/apps/scratch/src/hooks/useAssertionsEnabled.ts
@@ -0,0 +1,27 @@
+import { useEffect, useState } from "react";
+import VM from "scratch-vm";
+
+export const useAssertionsEnabled = (vm: VM): boolean => {
+  const [assertionsEnabled, setAssertionsEnabled] = useState(false);
+
+  useEffect(() => {
+    const onAssertionsEnabled = (): void => setAssertionsEnabled(true);
+    const onAssertionsDisabled = (): void => setAssertionsEnabled(false);
+
+    vm.runtime.on("ASSERTIONS_ENABLED", onAssertionsEnabled);
+    vm.runtime.on("ASSERTIONS_DISABLED", onAssertionsDisabled);
+    vm.runtime.on("ARE_ASSERTIONS_ENABLED_RESPONSE", setAssertionsEnabled);
+
+    // query for the current state. if we don't get a response, we'll assume assertions are disabled
+    // which is the default state initialized above.
+    vm.runtime.emit("ARE_ASSERTIONS_ENABLED_QUERY");
+
+    return (): void => {
+      vm.runtime.off("ASSERTIONS_ENABLED", onAssertionsEnabled);
+      vm.runtime.off("ASSERTIONS_DISABLED", onAssertionsDisabled);
+      vm.runtime.off("ARE_ASSERTIONS_ENABLED_RESPONSE", setAssertionsEnabled);
+    };
+  }, [vm, setAssertionsEnabled]);
+
+  return assertionsEnabled;
+};
diff --git a/apps/scratch/src/hooks/useAssertionsState.ts b/apps/scratch/src/hooks/useAssertionsState.ts
new file mode 100644
index 0000000..82baae1
--- /dev/null
+++ b/apps/scratch/src/hooks/useAssertionsState.ts
@@ -0,0 +1,24 @@
+import { useEffect, useState } from "react";
+import VM from "scratch-vm";
+
+export const useAssertionsState = (
+  vm: VM,
+): { total: number; passed: number } => {
+  const [assertionsSatisfied, setAssertionsSatisfied] = useState({
+    total: 0,
+    passed: 0,
+  });
+
+  useEffect(() => {
+    const fn = (total: number, passed: number): void =>
+      setAssertionsSatisfied({ total, passed });
+
+    vm.runtime.on("ASSERTIONS_CHECKED", fn);
+
+    return (): void => {
+      vm.runtime.off("ASSERTIONS_CHECKED", fn);
+    };
+  }, [vm, setAssertionsSatisfied]);
+
+  return assertionsSatisfied;
+};
diff --git a/apps/scratch/src/hooks/useEmbeddedScratch.ts b/apps/scratch/src/hooks/useEmbeddedScratch.ts
new file mode 100644
index 0000000..063ade7
--- /dev/null
+++ b/apps/scratch/src/hooks/useEmbeddedScratch.ts
@@ -0,0 +1,58 @@
+import VM from "scratch-vm";
+import { useIframeParent } from "./useIframeParent";
+import { useCallback } from "react";
+import { saveCrtProject } from "../vm/save-crt-project";
+import { loadCrtProject } from "../vm/load-crt-project";
+
+export const useEmbeddedScratch = (
+  vm: VM | null,
+): ReturnType<typeof useIframeParent> => {
+  const handleRequest = useCallback(
+    async (request, respondToMessageEvent) => {
+      switch (request.procedure) {
+        case "getHeight":
+          respondToMessageEvent({
+            procedure: "getHeight",
+            result: document.body.scrollHeight,
+          });
+          break;
+        case "getSubmission":
+          if (vm) {
+            respondToMessageEvent({
+              procedure: "getSubmission",
+              result: new Blob([vm.toJSON()], {
+                type: "application/json",
+              }),
+            });
+          }
+          break;
+        case "getTask":
+          if (vm) {
+            saveCrtProject(vm).then((content) => {
+              respondToMessageEvent({
+                procedure: "getTask",
+                result: content,
+              });
+            });
+          }
+          break;
+        case "loadTask":
+          if (vm) {
+            const sb3Project = await request.arguments.arrayBuffer();
+
+            loadCrtProject(vm, sb3Project).then(() => {
+              respondToMessageEvent({
+                procedure: "loadTask",
+              });
+            });
+          }
+          break;
+        default:
+          break;
+      }
+    },
+    [vm],
+  );
+
+  return useIframeParent(handleRequest);
+};
diff --git a/apps/scratch/src/hooks/useIframeParent.ts b/apps/scratch/src/hooks/useIframeParent.ts
new file mode 100644
index 0000000..224a1a7
--- /dev/null
+++ b/apps/scratch/src/hooks/useIframeParent.ts
@@ -0,0 +1,172 @@
+import { useCallback, useEffect, useRef, useState } from "react";
+import {
+  AppIFrameMessage,
+  AppIFrameRequest,
+  AppIFrameResponse,
+} from "../../../../frontend/src/types/app-iframe-message";
+
+const MAX_COUNTER = 1000000;
+
+const sendMessage = (
+  window: Window | MessagePort | ServiceWorker,
+  message: AppIFrameMessage,
+  targetOrigin: string,
+): void =>
+  window.postMessage(message, {
+    targetOrigin: targetOrigin,
+  });
+
+const respondToMessageEvent = (
+  event: MessageEvent,
+  message: Omit<AppIFrameResponse, "id" | "type">,
+): void => {
+  if (!event.source) {
+    console.error("Cannot respond to event without source:", event);
+    return;
+  }
+
+  return sendMessage(
+    event.source,
+    {
+      id: event.data.id,
+      type: "response",
+      ...message,
+    } as AppIFrameMessage,
+    event.origin,
+  );
+};
+
+export const useIframeParent = (
+  handleRequest: (
+    request: AppIFrameRequest,
+    respondToMessageEvent: (
+      message: Omit<AppIFrameResponse, "id" | "type">,
+    ) => void,
+  ) => Promise<void>,
+): {
+  isInIframe: boolean;
+  hasLoaded: boolean;
+
+  sendRequest: <ProcedureName extends AppIFrameMessage["procedure"]>(
+    request: Omit<AppIFrameRequest, "id" | "type"> & {
+      procedure: ProcedureName;
+    },
+  ) => Promise<AppIFrameResponse & { procedure: ProcedureName }>;
+} => {
+  const [hasLoaded, setHasLoaded] = useState(false);
+  const [isInIframe, setIsInIframe] = useState(false);
+  const [taskOrigin, setTaskOrigin] = useState<string | null>(null);
+
+  const counter = useRef<number>(0);
+  const pendingRequests = useRef<{
+    [key: number]: (response: AppIFrameResponse) => void;
+  }>({});
+
+  const sendRequest = useCallback(
+    <ProcedureName extends AppIFrameMessage["procedure"]>(
+      request: Omit<AppIFrameRequest, "id" | "type"> & {
+        procedure: ProcedureName;
+      },
+    ): Promise<AppIFrameResponse & { procedure: ProcedureName }> => {
+      if (!taskOrigin) {
+        return Promise.reject(
+          new Error(
+            "Cannot send messages (yet) because no task origin has been set (i.e. no task has been loaded so far)",
+          ),
+        );
+      }
+
+      return new Promise((resolve) => {
+        // store the resolve function in the pendingRequests object
+        pendingRequests.current[counter.current] = (
+          response: AppIFrameResponse,
+        ): void => {
+          if (response.procedure !== request.procedure) {
+            console.error("Invalid response procedure", response, request);
+            return;
+          }
+
+          resolve(response as AppIFrameResponse & { procedure: ProcedureName });
+        };
+
+        // send the message to the iframe
+        sendMessage(
+          window.parent,
+          // add a unique id to the message
+          {
+            id: counter.current,
+            type: "request",
+            ...request,
+          } as AppIFrameMessage,
+          taskOrigin,
+        );
+
+        // increment the counter
+        counter.current = (counter.current + 1) % MAX_COUNTER;
+      });
+    },
+    [taskOrigin],
+  );
+
+  // a callback function that listens for messages from the parent window
+  const handleParentWindowMessage = useCallback(
+    async (event: MessageEvent) => {
+      if (event.source !== window.parent) {
+        return;
+      }
+
+      const message = event.data as AppIFrameMessage;
+
+      if (message.type === "response") {
+        // get the resolve function from the pendingRequests object
+        const resolve = pendingRequests.current[message.id];
+        if (!resolve) {
+          console.error("No resolve function found for message", message);
+        }
+
+        // call the resolve function with the message
+        resolve(message);
+        // remove the resolve function from the pendingRequests object
+        delete pendingRequests.current[message.id];
+
+        return;
+      }
+
+      if (message.procedure === "loadTask") {
+        setTaskOrigin(event.origin);
+      }
+
+      return handleRequest(message, (response) =>
+        respondToMessageEvent(event, response),
+      );
+    },
+    [handleRequest],
+  );
+
+  // when mounting, check whether we are in an iframe
+  useEffect(() => {
+    if (window.parent && window.parent !== window) {
+      setIsInIframe(true);
+    }
+
+    // also only set the hasLoaded variable after the rendering finished
+    setHasLoaded(true);
+  }, []);
+
+  // add an event listener to listen for messages from the iFrame
+  useEffect(() => {
+    if (isInIframe) {
+      window.addEventListener("message", handleParentWindowMessage);
+
+      return (): void => {
+        window.removeEventListener("message", handleParentWindowMessage);
+      };
+    }
+  }, [handleParentWindowMessage, isInIframe]);
+
+  return {
+    isInIframe,
+    hasLoaded,
+    sendRequest,
+  };
+};
diff --git a/apps/scratch/src/index.css b/apps/scratch/src/index.css
index dc6c234..13025f3 100644
--- a/apps/scratch/src/index.css
+++ b/apps/scratch/src/index.css
@@ -13,6 +13,9 @@
   --input-border-color: #000;
   --input-background-color: #fff;
   --input-background-color-hover: #ddd;
+
+  --assertions-default-color: #ff3355;
+  --assertions-success-color: #0b8e69;
 }
 
 body {
@@ -121,8 +124,10 @@ body,
 
 .frozen-block-frozen > .blocklyBlockBackground,
 .frozen-block-frozen > *:not(.blocklyDraggable,[data-argument-type^="text"]) > .blocklyBlockBackground,
+.frozen-block-frozen > *:not(.blocklyDraggable,[data-argument-type^="text"]) > line,
 .frozen-block-appendable > .blocklyBlockBackground,
-.frozen-block-appendable > *:not(.blocklyDraggable,[data-argument-type^="text"]) > .blocklyBlockBackground
+.frozen-block-appendable > *:not(.blocklyDraggable,[data-argument-type^="text"]) > .blocklyBlockBackground,
+.frozen-block-appendable > *:not(.blocklyDraggable,[data-argument-type^="text"]) > line
 {
   fill: var(--disabled-block-background-color);
   stroke: var(--disabled-block-background-color);
@@ -142,3 +147,11 @@ body,
 .blocklyFlyout rect:not(.interactive) {
   pointer-events: none;
 }
+
+/*
+  reduce the size of the custom image of extension blocks
+*/
+.blocklyDraggable > g > image {
+  transform: scale(0.9);
+  transform-origin: 20px 20px;
+}
\ No newline at end of file
diff --git a/apps/scratch/src/pages/Edit.tsx b/apps/scratch/src/pages/Edit.tsx
index 08da68c..e891186 100644
--- a/apps/scratch/src/pages/Edit.tsx
+++ b/apps/scratch/src/pages/Edit.tsx
@@ -1,132 +1,20 @@
 import { useParams } from "react-router-dom";
-import Gui from "../containers/Gui";
+import Gui from "../containers/customized-scratch-containers/Gui";
 import VM from "scratch-vm";
-import { useCallback, useEffect, useState } from "react";
-import {
-  AppIFrameMessage,
-  AppIFrameResponse,
-} from "../../../../frontend/src/types/app-iframe-message";
+import { useState } from "react";
 import { patchScratchVm } from "../vm";
-import { saveCrtProject } from "../vm/save-crt-project";
-import { loadCrtProject } from "../vm/load-crt-project";
-
-const respondToMessageEvent = (
-  event: MessageEvent,
-  message: Omit<AppIFrameResponse, "id" | "type">,
-) => {
-  if (!event.source) {
-    console.error("Cannot respond to event without source:", event);
-    return;
-  }
-
-  event.source.postMessage(
-    {
-      id: event.data.id,
-      type: "response",
-      ...message,
-    } as AppIFrameMessage,
-    {
-      targetOrigin: event.origin,
-    },
-  );
-};
+import { useEmbeddedScratch } from "../hooks/useEmbeddedScratch";
 
 const Edit = () => {
   const { _sessionId, _taskId } = useParams();
-  const [hasLoaded, setHasLoaded] = useState(false);
-  const [isInIframe, setIsInIframe] = useState(false);
   const [vm, setVm] = useState<VM | null>(null);
 
-  useEffect(() => {
-    if (window.parent && window.parent !== window) {
-      setIsInIframe(true);
-    }
-
-    setHasLoaded(true);
-  }, []);
-
-  // a callback function that listens for messages from the parent window
-  const handleParentWindowMessage = useCallback(
-    async (event: MessageEvent) => {
-      if (event.source !== window.parent) {
-        return;
-      }
-
-      const message = event.data as AppIFrameMessage;
-
-      if (message.type === "response") {
-        console.error(
-          `Response in AppIFrameMessage message: '${JSON.stringify(message)}'`,
-        );
-        return;
-      }
-
-      switch (message.procedure) {
-        case "getHeight":
-          respondToMessageEvent(event, {
-            procedure: "getHeight",
-            result: document.body.scrollHeight,
-          });
-          break;
-        case "getSubmission":
-          if (vm) {
-            respondToMessageEvent(event, {
-              procedure: "getSubmission",
-              result: new Blob([vm.toJSON()], {
-                type: "application/json",
-              }),
-            });
-          }
-          break;
-        case "getTask":
-          if (vm) {
-            saveCrtProject(vm).then((content) => {
-              respondToMessageEvent(event, {
-                procedure: "getTask",
-                result: content,
-              });
-            });
-          }
-          break;
-        case "loadTask":
-          if (vm) {
-            const sb3Project = await message.arguments.arrayBuffer();
-
-            loadCrtProject(vm, sb3Project).then(() => {
-              respondToMessageEvent(event, {
-                procedure: "loadTask",
-              });
-            });
-          }
-          break;
-        default:
-          break;
-      }
-    },
-    [vm],
-  );
-
-  // add an event listener to listen for messages from the iFrame
-  useEffect(() => {
-    if (isInIframe) {
-      window.addEventListener("message", handleParentWindowMessage);
-
-      return () => {
-        window.removeEventListener("message", handleParentWindowMessage);
-      };
-    }
-  }, [handleParentWindowMessage, isInIframe]);
+  const { hasLoaded } = useEmbeddedScratch(vm);
 
   if (!hasLoaded) {
     return null;
   }
 
-  /*if (!isInIframe) {
-    return (
-      <div>Tasks can only be solved in the context of the CRT application.</div>
-    );
-  }*/
-
   return (
     <Gui
       canEditTask={true}
diff --git a/apps/scratch/src/pages/Solve.tsx b/apps/scratch/src/pages/Solve.tsx
index 29da48f..2955d1e 100644
--- a/apps/scratch/src/pages/Solve.tsx
+++ b/apps/scratch/src/pages/Solve.tsx
@@ -1,122 +1,29 @@
 import { useParams } from "react-router-dom";
-import Gui from "../containers/Gui";
+import Gui from "../containers/customized-scratch-containers/Gui";
 import VM from "scratch-vm";
-import { useCallback, useEffect, useState } from "react";
-import {
-  AppIFrameMessage,
-  AppIFrameResponse,
-} from "../../../../frontend/src/types/app-iframe-message";
+import { useCallback, useState } from "react";
 import { patchScratchVm } from "../vm";
-import { saveCrtProject } from "../vm/save-crt-project";
-import { loadCrtProject } from "../vm/load-crt-project";
-
-const respondToMessageEvent = (
-  event: MessageEvent,
-  message: Omit<AppIFrameResponse, "id" | "type">,
-) => {
-  if (!event.source) {
-    console.error("Cannot respond to event without source:", event);
-    return;
-  }
-
-  event.source.postMessage(
-    {
-      id: event.data.id,
-      type: "response",
-      ...message,
-    } as AppIFrameMessage,
-    {
-      targetOrigin: event.origin,
-    },
-  );
-};
+import { useEmbeddedScratch } from "../hooks/useEmbeddedScratch";
 
 const Solve = () => {
   const { _sessionId, _taskId } = useParams();
-  const [hasLoaded, setHasLoaded] = useState(false);
-  const [isInIframe, setIsInIframe] = useState(false);
-  const [vm, setVm] = useState<VM | null>(null);
-
-  useEffect(() => {
-    if (window.parent && window.parent !== window) {
-      setIsInIframe(true);
-    }
 
-    setHasLoaded(true);
-  }, []);
-
-  // a callback function that listens for messages from the parent window
-  const handleParentWindowMessage = useCallback(
-    async (event: MessageEvent) => {
-      if (event.source !== window.parent) {
-        return;
-      }
-
-      const message = event.data as AppIFrameMessage;
-
-      if (message.type === "response") {
-        console.error(
-          `Response in AppIFrameMessage message: '${JSON.stringify(message)}'`,
-        );
-        return;
-      }
-
-      switch (message.procedure) {
-        case "getHeight":
-          respondToMessageEvent(event, {
-            procedure: "getHeight",
-            result: document.body.scrollHeight,
-          });
-          break;
-        case "getSubmission":
-          if (vm) {
-            respondToMessageEvent(event, {
-              procedure: "getSubmission",
-              result: new Blob([vm.toJSON()], {
-                type: "application/json",
-              }),
-            });
-          }
-          break;
-        case "getTask":
-          if (vm) {
-            saveCrtProject(vm).then((content) => {
-              respondToMessageEvent(event, {
-                procedure: "getTask",
-                result: content,
-              });
-            });
-          }
-          break;
-        case "loadTask":
-          if (vm) {
-            const sb3Project = await message.arguments.arrayBuffer();
+  const [vm, setVm] = useState<VM | null>(null);
 
-            loadCrtProject(vm, sb3Project).then(() => {
-              respondToMessageEvent(event, {
-                procedure: "loadTask",
-              });
-            });
-          }
-          break;
-        default:
-          break;
-      }
-    },
-    [vm],
+  const { hasLoaded, sendRequest } = useEmbeddedScratch(vm);
+
+  const reportProgress = useCallback(
+    (totalAssertions: number, passedAssertions: number) =>
+      sendRequest({
+        procedure: "reportProgress",
+        arguments: {
+          totalTests: totalAssertions,
+          passedTests: passedAssertions,
+        },
+      }),
+    [sendRequest],
   );
 
-  // add an event listener to listen for messages from the iFrame
-  useEffect(() => {
-    if (isInIframe) {
-      window.addEventListener("message", handleParentWindowMessage);
-
-      return () => {
-        window.removeEventListener("message", handleParentWindowMessage);
-      };
-    }
-  }, [handleParentWindowMessage, isInIframe]);
-
   if (!hasLoaded) {
     return null;
   }
@@ -134,6 +41,7 @@ const Solve = () => {
         setVm(vm);
         patchScratchVm(vm);
       }}
+      onTaskProgress={reportProgress}
     />
   );
 };
diff --git a/apps/scratch/src/pages/__tests__/Edit.spec.tsx b/apps/scratch/src/pages/__tests__/Edit.spec.tsx
index 49f97ad..89f45fb 100644
--- a/apps/scratch/src/pages/__tests__/Edit.spec.tsx
+++ b/apps/scratch/src/pages/__tests__/Edit.spec.tsx
@@ -4,10 +4,9 @@ import {
   MockMessageEvent,
 } from "./mock-message-event";
 import { TestTaskPage } from "./page-objects/test-task";
-import { EditTaskPage } from "./page-objects/edit-task";
-import { Page } from "playwright/test";
-import tasks from "./tasks";
+import { EditTaskPage, Extension } from "./page-objects/edit-task";
 import { getExpectedBlockConfigButtonLabel } from "./helpers";
+import { AssertionTaskPage } from "./page-objects/assertion-task";
 
 declare global {
   interface Window {
@@ -17,48 +16,8 @@ declare global {
   }
 }
 
-const task = tasks.testTask;
-
-const loadTestTask = async (pwPage: Page) => {
-  await pwPage.evaluate(async () => {
-    const task = await fetch("https://example.com/test-task.sb3").then(
-      (response) => response.blob(),
-    );
-
-    const event = new window.MockMessageEvent(window.parent, {
-      id: 0,
-      type: "request",
-      procedure: "loadTask",
-      arguments: task,
-    });
-
-    window.dispatchEvent(event);
-  });
-
-  await pwPage.waitForFunction(() => window.postedMessages.length > 0);
-
-  const messages = await pwPage.evaluate(() => window.postedMessages);
-
-  expect(messages).toHaveLength(1);
-
-  expect(messages[0].message).toEqual({
-    id: 0,
-    type: "response",
-    procedure: "loadTask",
-    result: undefined,
-  });
-};
-
 test.describe("/edit/taskId", () => {
   test.beforeEach(async ({ page, baseURL }) => {
-    page.route(/test-task.sb3$/, async (route) =>
-      route.fulfill({
-        body: await task.file,
-        contentType: "application/x.scratch.sb3",
-        status: 200,
-      }),
-    );
-
     page.on(
       "framenavigated",
       async () =>
@@ -132,9 +91,7 @@ test.describe("/edit/taskId", () => {
   });
 
   test("can load task via window.postMessage", async ({ page: pwPage }) => {
-    await loadTestTask(pwPage);
-
-    const page = new TestTaskPage(pwPage);
+    const { page, task } = await TestTaskPage.load(pwPage);
 
     // check the block config buttons
     const { moveSteps, turnRight, goto } = page.enabledBlockConfigButtons;
@@ -163,7 +120,7 @@ test.describe("/edit/taskId", () => {
 
     // check the freeze buttons
     const editableStackButton = page.getBlockFreezeButton(
-      page.taskBlocks.catActor.topOfEditableStack,
+      page.taskBlocks.catActor.visualTopOfEditableStack,
     );
     const frozenStackButton = page.getBlockFreezeButton(
       page.taskBlocks.catActor.topOfFrozenStack,
@@ -243,35 +200,37 @@ test.describe("/edit/taskId", () => {
     await expect(configButton).toHaveText("0");
   });
 
-  test("can add extension", async ({ page: pwPage }) => {
+  test("can load assertions extension", async ({ page: pwPage }) => {
     const page = new EditTaskPage(pwPage);
 
     await page.addExtensionButton.click();
 
-    // load the first extension
-    await page.loadExtension(1);
+    await page.loadExtension(Extension.Assertions);
 
-    // ensure the custom block is added
+    // ensure the extension blocks are added
     await expect(
-      page.getBlockInToolbox("example_functionCall_setX"),
+      page.getBlockInToolbox("assertions_event_whenTaskFinishedRunning"),
     ).toHaveCount(1);
+
+    await expect(page.getBlockInToolbox("assertions_noop_assert")).toHaveCount(
+      1,
+    );
   });
 
-  test("can add extension twice", async ({ page: pwPage }) => {
+  test("can load assertion extension twice", async ({ page: pwPage }) => {
     const page = new EditTaskPage(pwPage);
 
     await page.addExtensionButton.click();
 
-    // load the first extension
-    await page.loadExtension(1);
+    await page.loadExtension(Extension.Assertions);
 
     // add the extension again
     await page.addExtensionButton.click();
     await page.loadExtension(1);
 
-    // ensure the custom block is added just once
+    // ensure the custom blocks are added just once
     await expect(
-      page.getBlockInToolbox("example_functionCall_setX"),
+      page.getBlockInToolbox("assertions_event_whenTaskFinishedRunning"),
     ).toHaveCount(1);
   });
 
@@ -340,11 +299,10 @@ test.describe("/edit/taskId", () => {
   });
 
   test("can toggle freeze mode of task blocks", async ({ page: pwPage }) => {
-    await loadTestTask(pwPage);
-    const page = new TestTaskPage(pwPage);
+    const { page } = await TestTaskPage.load(pwPage);
 
     const editableStackButton = page.getBlockFreezeButton(
-      page.taskBlocks.catActor.topOfEditableStack,
+      page.taskBlocks.catActor.visualTopOfEditableStack,
     );
 
     await expect(editableStackButton).toHaveText("✎");
@@ -357,11 +315,10 @@ test.describe("/edit/taskId", () => {
   });
 
   test("can prepend blocks to all stacks", async ({ page: pwPage }) => {
-    await loadTestTask(pwPage);
-    const page = new TestTaskPage(pwPage);
+    const { page } = await TestTaskPage.load(pwPage);
 
     const stacks = [
-      page.taskBlocks.catActor.topOfEditableStack,
+      page.taskBlocks.catActor.visualTopOfEditableStack,
       page.taskBlocks.catActor.topOfFrozenStack,
       page.taskBlocks.catActor.visualTopOfAppendableStack,
     ];
@@ -374,4 +331,130 @@ test.describe("/edit/taskId", () => {
       expect(await page.countBlocksInParentStack(stack)).toBe(blockCount + 1);
     }
   });
+
+  test("does not reset to the initial state when running", async ({
+    page: pwPage,
+  }) => {
+    const editPage = new EditTaskPage(pwPage);
+    const { page } = await AssertionTaskPage.load(pwPage);
+
+    await expect(page.assertionState.passed).toHaveCount(0);
+
+    expect(await pwPage.evaluate(() => window.postedMessages)).toHaveLength(1);
+
+    // solve task
+    for (let i = 0; i < 5; i++) {
+      await page.appendNewBlockToBottomOfStack(
+        "motion_movesteps",
+        page.taskBlocks.catActor.visualTopOfEditableStack,
+      );
+    }
+
+    // run the project once with assertions disabled
+    await page.pressGreenFlag();
+
+    // then, enable assertions
+    await editPage.enableAssertions();
+
+    // and run the project again. we expect the first run to persist and therefore
+    // the assertions to fail
+    await page.pressGreenFlag();
+
+    await expect(page.assertionState.passed).toHaveText("0");
+    await expect(page.assertionState.total).toHaveText("1");
+
+    // no progress should be reported in edit mode
+    expect(await pwPage.evaluate(() => window.postedMessages)).toHaveLength(1);
+  });
+
+  test("resets to the initial state when running in assertion mode", async ({
+    page: pwPage,
+  }) => {
+    const editPage = new EditTaskPage(pwPage);
+    const { page } = await AssertionTaskPage.load(pwPage);
+
+    await expect(page.assertionState.passed).toHaveCount(0);
+
+    expect(await pwPage.evaluate(() => window.postedMessages)).toHaveLength(1);
+
+    // solve task
+    for (let i = 0; i < 4; i++) {
+      await page.appendNewBlockToBottomOfStack(
+        "motion_movesteps",
+        page.taskBlocks.catActor.visualTopOfEditableStack,
+      );
+    }
+
+    // enable assertions
+    await editPage.enableAssertions();
+
+    // and run the project.
+    await page.pressGreenFlag();
+
+    // since we are missing one move steps, this should fail
+    await expect(page.assertionState.passed).toHaveText("0");
+    await expect(page.assertionState.total).toHaveText("1");
+
+    // however, after adding one more it should pass.
+    // note that without a reset, it will fail.
+    await page.appendNewBlockToBottomOfStack(
+      "motion_movesteps",
+      page.taskBlocks.catActor.visualTopOfEditableStack,
+    );
+
+    await page.pressGreenFlag();
+
+    await expect(page.assertionState.passed).toHaveText("1");
+    await expect(page.assertionState.total).toHaveText("1");
+
+    // no progress should be reported in edit mode
+    expect(await pwPage.evaluate(() => window.postedMessages)).toHaveLength(1);
+
+    // when disabling assertions, the state should disappear
+    await editPage.disableAssertions();
+
+    await expect(page.assertionState.passed).toHaveCount(0);
+    await expect(page.assertionState.total).toHaveCount(0);
+  });
+
+  test("can disable assertion mode", async ({ page: pwPage }) => {
+    const editPage = new EditTaskPage(pwPage);
+    const { page } = await AssertionTaskPage.load(pwPage);
+
+    await expect(page.assertionState.passed).toHaveCount(0);
+
+    // enable assertions
+    await editPage.enableAssertions();
+
+    // and run the project.
+    await page.pressGreenFlag();
+
+    // since we did not solve the task, this should fail
+    await expect(page.assertionState.passed).toHaveText("0");
+    await expect(page.assertionState.total).toHaveText("1");
+
+    // disable assertions should make the assertion state disappear
+    await editPage.disableAssertions();
+    await expect(page.assertionState.passed).toHaveCount(0);
+    await expect(page.assertionState.total).toHaveCount(0);
+
+    // when adding a block and running the project four times
+    // we should be in the asserted state after a firth and final run
+    await page.appendNewBlockToBottomOfStack(
+      "motion_movesteps",
+      page.taskBlocks.catActor.visualTopOfEditableStack,
+    );
+
+    for (let i = 0; i < 4; i++) {
+      await page.pressGreenFlag();
+    }
+
+    // hence, if we enable assertions now and run the project,
+    // we should succeed
+    await editPage.enableAssertions();
+    await page.pressGreenFlag();
+
+    await expect(page.assertionState.passed).toHaveText("1");
+    await expect(page.assertionState.total).toHaveText("1");
+  });
 });
diff --git a/apps/scratch/src/pages/__tests__/Solve.spec.tsx b/apps/scratch/src/pages/__tests__/Solve.spec.tsx
index 72ccef3..14dd970 100644
--- a/apps/scratch/src/pages/__tests__/Solve.spec.tsx
+++ b/apps/scratch/src/pages/__tests__/Solve.spec.tsx
@@ -5,8 +5,8 @@ import {
 } from "./mock-message-event";
 import { SolveTaskPage } from "./page-objects/solve-task";
 import { TestTaskPage } from "./page-objects/test-task";
-import tasks from "./tasks";
 import { getExpectedBlockConfigButtonLabel } from "./helpers";
+import { AssertionTaskPage } from "./page-objects/assertion-task";
 
 declare global {
   interface Window {
@@ -16,8 +16,6 @@ declare global {
   }
 }
 
-const task = tasks.testTask;
-
 test.describe("/solve/sessionId/taskId", () => {
   test.beforeEach(async ({ page, baseURL }) => {
     page.on("framenavigated", async () =>
@@ -38,42 +36,6 @@ test.describe("/solve/sessionId/taskId", () => {
     await page.waitForSelector("#root");
 
     await defineCustomMessageEvent(page);
-
-    page.route(/test-task.sb3$/, async (route) =>
-      route.fulfill({
-        body: await task.file,
-        contentType: "application/zip",
-        status: 200,
-      }),
-    );
-
-    await page.evaluate(async () => {
-      const task = await fetch("https://example.com/test-task.sb3").then(
-        (response) => response.blob(),
-      );
-
-      const event = new window.MockMessageEvent(window.parent, {
-        id: 0,
-        type: "request",
-        procedure: "loadTask",
-        arguments: task,
-      });
-
-      window.dispatchEvent(event);
-    });
-
-    await page.waitForFunction(() => window.postedMessages.length > 0);
-
-    const messages = await page.evaluate(() => window.postedMessages);
-
-    expect(messages).toHaveLength(1);
-
-    expect(messages[0].message).toEqual({
-      id: 0,
-      type: "response",
-      procedure: "loadTask",
-      result: undefined,
-    });
   });
 
   test("can select the stage", async ({ page: pwPage }) => {
@@ -112,9 +74,9 @@ test.describe("/solve/sessionId/taskId", () => {
 
     const messages = await page.evaluate(() => window.postedMessages);
 
-    expect(messages).toHaveLength(2);
+    expect(messages).toHaveLength(1);
 
-    expect(messages[1].message).toEqual({
+    expect(messages[0].message).toEqual({
       id: 0,
       type: "response",
       procedure: "getHeight",
@@ -137,9 +99,9 @@ test.describe("/solve/sessionId/taskId", () => {
 
     const messages = await page.evaluate(() => window.postedMessages);
 
-    expect(messages).toHaveLength(2);
+    expect(messages).toHaveLength(1);
 
-    expect(messages[1].message).toEqual({
+    expect(messages[0].message).toEqual({
       id: 0,
       type: "response",
       procedure: "getSubmission",
@@ -149,7 +111,7 @@ test.describe("/solve/sessionId/taskId", () => {
   });
 
   test("loads the initial task blocks", async ({ page: pwPage }) => {
-    const page = new SolveTaskPage(pwPage);
+    const { page, task } = await TestTaskPage.load(pwPage);
 
     await expect(page.blocksOfCurrentTarget).toHaveCount(
       task.blocksOfMainTarget,
@@ -157,7 +119,7 @@ test.describe("/solve/sessionId/taskId", () => {
   });
 
   test("loads the allowed blocks correctly", async ({ page: pwPage }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page, task } = await TestTaskPage.load(pwPage);
 
     const { moveSteps, turnRight, goto } = page.enabledBlockConfigButtons;
     const { turnLeft } = page.disabledBlockConfigButtons;
@@ -181,7 +143,7 @@ test.describe("/solve/sessionId/taskId", () => {
   });
 
   test("cannot open block config menu", async ({ page: pwPage }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page } = await TestTaskPage.load(pwPage);
 
     await page.openBlockConfig("motion_movesteps", { force: true });
 
@@ -189,7 +151,7 @@ test.describe("/solve/sessionId/taskId", () => {
   });
 
   test("reduces number of allowed blocks", async ({ page: pwPage }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page, task } = await TestTaskPage.load(pwPage);
     const moveStepsAllowedCount =
       task.crtConfig.allowedBlocks["motion_movesteps"]!;
 
@@ -238,7 +200,7 @@ test.describe("/solve/sessionId/taskId", () => {
   test("removing student-added blocks increases the limit", async ({
     page: pwPage,
   }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page, task } = await TestTaskPage.load(pwPage);
     const moveStepsAllowedCount =
       task.crtConfig.allowedBlocks["motion_movesteps"]!;
 
@@ -269,7 +231,7 @@ test.describe("/solve/sessionId/taskId", () => {
   });
 
   test("cannot remove frozen blocks", async ({ page: pwPage }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page } = await TestTaskPage.load(pwPage);
 
     await expect(page.taskBlocks.catActor.frozenBlock).toHaveCount(1);
 
@@ -281,7 +243,7 @@ test.describe("/solve/sessionId/taskId", () => {
   test("cannot remove frozen but appendable blocks", async ({
     page: pwPage,
   }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page } = await TestTaskPage.load(pwPage);
 
     await expect(
       page.taskBlocks.catActor.visualTopOfAppendableStack,
@@ -295,7 +257,7 @@ test.describe("/solve/sessionId/taskId", () => {
   });
 
   test("can prepend to editable stack", async ({ page: pwPage }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page } = await TestTaskPage.load(pwPage);
 
     const initialBlocksInStack = await page.countBlocksInSubStack(
       page.taskBlocks.catActor.editableBlock,
@@ -313,7 +275,7 @@ test.describe("/solve/sessionId/taskId", () => {
   });
 
   test("can append to editable stack", async ({ page: pwPage }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page } = await TestTaskPage.load(pwPage);
 
     const initialBlocksInStack = await page.countBlocksInSubStack(
       page.taskBlocks.catActor.editableBlock,
@@ -333,7 +295,7 @@ test.describe("/solve/sessionId/taskId", () => {
   test("cannot prepend to top of frozen but appendable stack", async ({
     page: pwPage,
   }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page } = await TestTaskPage.load(pwPage);
 
     const initialBlocksInStack = await page.countBlocksInSubStack(
       page.taskBlocks.catActor.visualTopOfAppendableStack,
@@ -354,7 +316,7 @@ test.describe("/solve/sessionId/taskId", () => {
   test("cannot append to top of frozen but appendable stack", async ({
     page: pwPage,
   }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page } = await TestTaskPage.load(pwPage);
 
     const initialBlocksInStack = await page.countBlocksInSubStack(
       page.taskBlocks.catActor.visualTopOfAppendableStack,
@@ -375,7 +337,7 @@ test.describe("/solve/sessionId/taskId", () => {
   test("can append to bottom of frozen but appendable stack", async ({
     page: pwPage,
   }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page } = await TestTaskPage.load(pwPage);
 
     const initialBlocksInStack = await page.countBlocksInSubStack(
       page.taskBlocks.catActor.visualTopOfAppendableStack,
@@ -396,7 +358,7 @@ test.describe("/solve/sessionId/taskId", () => {
   test("can append to empty slot in appendable stack", async ({
     page: pwPage,
   }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page } = await TestTaskPage.load(pwPage);
 
     const initialBlocksInStack = await page.countBlocksInSubStack(
       page.taskBlocks.catActor.insertableSlot,
@@ -413,7 +375,7 @@ test.describe("/solve/sessionId/taskId", () => {
   });
 
   test("cannot open task config", async ({ page: pwPage }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page } = await TestTaskPage.load(pwPage);
 
     await expect(page.openTaskConfigButton).toHaveCount(0);
 
@@ -423,7 +385,7 @@ test.describe("/solve/sessionId/taskId", () => {
   test("removing initial blocks does not increase the limit", async ({
     page: pwPage,
   }) => {
-    const page = new TestTaskPage(pwPage);
+    const { page, task } = await TestTaskPage.load(pwPage);
 
     const { moveSteps, turnRight } = page.enabledBlockConfigButtons;
 
@@ -439,4 +401,122 @@ test.describe("/solve/sessionId/taskId", () => {
       getExpectedBlockConfigButtonLabel(task.crtConfig, "motion_turnright"),
     );
   });
+
+  test("loads assertions extension if task contains assertion blocks", async ({
+    page: pwPage,
+  }) => {
+    const { page } = await AssertionTaskPage.load(pwPage);
+
+    await expect(
+      page.taskBlocks.catActor.visualTopOfAssertionStack,
+    ).toHaveCount(1);
+
+    await expect(
+      page.disabledBlockConfigButtons.whenTaskFinishedRunning,
+    ).toHaveCount(0);
+    await expect(page.disabledBlockConfigButtons.assert).toHaveCount(0);
+  });
+
+  test("reports the correct numbers of assertions when failing", async ({
+    page: pwPage,
+  }) => {
+    const { page } = await AssertionTaskPage.load(pwPage);
+
+    await page.pressGreenFlag();
+
+    await expect(page.assertionState.passed).toHaveText("0");
+    await expect(page.assertionState.total).toHaveText("1");
+
+    const messages = await pwPage.evaluate(() => window.postedMessages);
+
+    expect(messages).toHaveLength(2);
+
+    expect(messages[1].message).toEqual({
+      id: 0,
+      type: "request",
+      procedure: "reportProgress",
+      arguments: {
+        totalTests: 1,
+        passedTests: 0,
+      },
+    });
+  });
+
+  test("reports the correct numbers of assertions when passing", async ({
+    page: pwPage,
+  }) => {
+    const { page } = await AssertionTaskPage.load(pwPage);
+
+    await expect(page.assertionState.passed).toHaveCount(0);
+
+    // solve task
+    for (let i = 0; i < 5; i++) {
+      await page.appendNewBlockToBottomOfStack(
+        "motion_movesteps",
+        page.taskBlocks.catActor.visualTopOfEditableStack,
+      );
+    }
+
+    await page.pressGreenFlag();
+
+    await expect(page.assertionState.passed).toHaveText("1");
+    await expect(page.assertionState.total).toHaveText("1");
+
+    const messages = await pwPage.evaluate(() => window.postedMessages);
+
+    expect(messages).toHaveLength(2);
+
+    expect(messages[1].message).toEqual({
+      id: expect.any(Number),
+      type: "request",
+      procedure: "reportProgress",
+      arguments: {
+        totalTests: 1,
+        passedTests: 1,
+      },
+    });
+  });
+
+  test("resets to the initial state when running", async ({ page: pwPage }) => {
+    const { page } = await AssertionTaskPage.load(pwPage);
+
+    expect(await pwPage.evaluate(() => window.postedMessages)).toHaveLength(1);
+
+    // solve task
+    for (let i = 0; i < 5; i++) {
+      await page.appendNewBlockToBottomOfStack(
+        "motion_movesteps",
+        page.taskBlocks.catActor.visualTopOfEditableStack,
+      );
+    }
+
+    // run the project twice - by default scratch would not reset and the cat would
+    // be at position x = 100, violating the assertion
+    await page.pressGreenFlag();
+
+    // wait for results
+    await pwPage.waitForFunction(() => window.postedMessages.length === 2);
+
+    await page.pressGreenFlag();
+
+    // wait for results
+    await pwPage.waitForFunction(() => window.postedMessages.length === 3);
+
+    await expect(page.assertionState.passed).toHaveText("1");
+    await expect(page.assertionState.total).toHaveText("1");
+
+    const messages = await pwPage.evaluate(() => window.postedMessages);
+
+    expect(messages).toHaveLength(3);
+
+    expect(messages[2].message).toEqual({
+      id: expect.any(Number),
+      type: "request",
+      procedure: "reportProgress",
+      arguments: {
+        totalTests: 1,
+        passedTests: 1,
+      },
+    });
+  });
 });
diff --git a/apps/scratch/src/pages/__tests__/helpers.ts b/apps/scratch/src/pages/__tests__/helpers.ts
index 8b6062b..fa076ab 100644
--- a/apps/scratch/src/pages/__tests__/helpers.ts
+++ b/apps/scratch/src/pages/__tests__/helpers.ts
@@ -1,4 +1,7 @@
-import { ScratchCrtConfig } from "scratch-vm";
+import { Page } from "playwright/test";
+import { ScratchCrtConfig } from "../../types/scratch-vm-custom";
+import { TestTask } from "./tasks";
+import { expect } from "playwright-test-coverage";
 
 export const getExpectedBlockConfigButtonLabel = (
   crtConfig: ScratchCrtConfig,
@@ -12,3 +15,46 @@ export const getExpectedBlockConfigButtonLabel = (
 
   return count.toString();
 };
+
+let idx = 0;
+
+export const loadTask = async (pwPage: Page, task: TestTask): Promise<void> => {
+  const filename = idx + ".sb3";
+  idx += 1;
+
+  const url = `https://example.com/${filename}`;
+
+  await pwPage.route(url, async (route) =>
+    route.fulfill({
+      body: await task.file,
+      contentType: "application/x.scratch.sb3",
+      status: 200,
+    }),
+  );
+
+  await pwPage.evaluate(async (url) => {
+    const task = await fetch(url).then((response) => response.blob());
+
+    const event = new window.MockMessageEvent(window.parent, {
+      id: 0,
+      type: "request",
+      procedure: "loadTask",
+      arguments: task,
+    });
+
+    window.dispatchEvent(event);
+  }, url);
+
+  await pwPage.waitForFunction(() => window.postedMessages.length > 0);
+
+  const messages = await pwPage.evaluate(() => window.postedMessages);
+
+  expect(messages).toHaveLength(1);
+
+  expect(messages[0].message).toEqual({
+    id: 0,
+    type: "response",
+    procedure: "loadTask",
+    result: undefined,
+  });
+};
diff --git a/apps/scratch/src/pages/__tests__/mock-message-event.ts b/apps/scratch/src/pages/__tests__/mock-message-event.ts
index 71aa6d2..f9a65fd 100644
--- a/apps/scratch/src/pages/__tests__/mock-message-event.ts
+++ b/apps/scratch/src/pages/__tests__/mock-message-event.ts
@@ -3,6 +3,8 @@
 import { Page } from "playwright/test";
 
 export class MockMessageEvent extends Event {
+  readonly origin = "http://localhost:3101";
+
   source: Window;
   data: unknown;
 
@@ -23,6 +25,7 @@ export const defineCustomMessageEvent = (page: Page): Promise<void> =>
 
         (this as any).source = source;
         (this as any).data = data;
+        (this as any).origin = "http://localhost:3101";
       }
     };
   });
diff --git a/apps/scratch/src/pages/__tests__/page-objects/assertion-task.ts b/apps/scratch/src/pages/__tests__/page-objects/assertion-task.ts
new file mode 100644
index 0000000..8c48ed2
--- /dev/null
+++ b/apps/scratch/src/pages/__tests__/page-objects/assertion-task.ts
@@ -0,0 +1,61 @@
+/* eslint-disable @typescript-eslint/explicit-function-return-type */
+import { Page } from "playwright/test";
+import { getBlockConfigButtonSelector } from "../locators";
+import { ScratchEditorPage } from "./scratch-editor";
+import { loadTask } from "../helpers";
+import tasks, { TestTask } from "../tasks";
+
+export class AssertionTaskPage extends ScratchEditorPage {
+  private constructor(page: Page) {
+    super(page);
+  }
+
+  get enabledBlockConfigButtons() {
+    return {
+      moveSteps: this.page.locator(
+        getBlockConfigButtonSelector("motion_movesteps"),
+      ),
+    };
+  }
+
+  get disabledBlockConfigButtons() {
+    return {
+      whenTaskFinishedRunning: this.page.locator(
+        getBlockConfigButtonSelector(
+          "assertions_event_whenTaskFinishedRunning",
+        ),
+      ),
+      assert: this.page.locator(
+        getBlockConfigButtonSelector("assertions_noop_assert"),
+      ),
+    };
+  }
+
+  get taskBlocks() {
+    return {
+      catActor: {
+        visualTopOfEditableStack: this.page.locator(
+          "[data-id='HN(hP9y9,$[q0=u7VklJ']",
+        ),
+        visualTopOfAssertionStack: this.page.locator(
+          "[data-id='4_e9[GL:|#*oG]X]FdQm']",
+        ),
+      },
+    };
+  }
+
+  get assertionState() {
+    return {
+      passed: this.page.getByTestId("assertion-state").getByTestId("passed"),
+      total: this.page.getByTestId("assertion-state").getByTestId("total"),
+    };
+  }
+
+  static async load(
+    page: Page,
+  ): Promise<{ page: AssertionTaskPage; task: TestTask }> {
+    await loadTask(page, tasks.assertionTask);
+
+    return { page: new AssertionTaskPage(page), task: tasks.assertionTask };
+  }
+}
diff --git a/apps/scratch/src/pages/__tests__/page-objects/edit-task.ts b/apps/scratch/src/pages/__tests__/page-objects/edit-task.ts
index 34b295e..f562a74 100644
--- a/apps/scratch/src/pages/__tests__/page-objects/edit-task.ts
+++ b/apps/scratch/src/pages/__tests__/page-objects/edit-task.ts
@@ -1,4 +1,5 @@
 /* eslint-disable @typescript-eslint/explicit-function-return-type */
+import { expect } from "playwright/test";
 import {
   getBlockConfigBlockLimitInputSelector,
   getBlockConfigCanBeUsedCheckboxSelector,
@@ -7,6 +8,10 @@ import {
 } from "../locators";
 import { ScratchEditorPage } from "./scratch-editor";
 
+export enum Extension {
+  Assertions = 1,
+}
+
 export class EditTaskPage extends ScratchEditorPage {
   get blockConfigFormElements() {
     return {
@@ -29,8 +34,13 @@ export class EditTaskPage extends ScratchEditorPage {
 
   get taskConfigFormElements() {
     return {
-      allowAllBlocksButton: this.page.getByTestId("allow-all-blocks-button"),
+      allowAllBlocksButton: this.page.getByTestId(
+        "allow-all-standard-blocks-button",
+      ),
       allowNoBlocksButton: this.page.getByTestId("allow-no-blocks-button"),
+      enableAssertionsCheckbox: this.page.getByTestId(
+        "enable-assertions-checkbox",
+      ),
       submit: this.page.getByTestId("task-config-form-submit-button"),
     };
   }
@@ -39,10 +49,39 @@ export class EditTaskPage extends ScratchEditorPage {
     return this.page.getByTestId("add-extension-button");
   }
 
-  loadExtension(extensionIndex: number) {
+  loadExtension(extension: Extension) {
     return this.page
       .locator(".ReactModalPortal img")
-      .nth(extensionIndex + 1 /* skip first image */)
+      .nth(extension + 1 /* skip first image */)
       .click();
   }
+
+  async enableAssertions() {
+    await this.openTaskConfig();
+
+    await expect(
+      this.taskConfigFormElements.enableAssertionsCheckbox,
+    ).not.toBeChecked();
+
+    await this.taskConfigFormElements.enableAssertionsCheckbox.click();
+
+    await expect(
+      this.taskConfigFormElements.enableAssertionsCheckbox,
+    ).toBeChecked();
+    await this.taskConfigFormElements.submit.click();
+  }
+
+  async disableAssertions() {
+    await this.openTaskConfig();
+
+    await expect(
+      this.taskConfigFormElements.enableAssertionsCheckbox,
+    ).toBeChecked();
+    await this.taskConfigFormElements.enableAssertionsCheckbox.click();
+    await expect(
+      this.taskConfigFormElements.enableAssertionsCheckbox,
+    ).not.toBeChecked();
+
+    await this.taskConfigFormElements.submit.click();
+  }
 }
diff --git a/apps/scratch/src/pages/__tests__/page-objects/scratch-editor.ts b/apps/scratch/src/pages/__tests__/page-objects/scratch-editor.ts
index bd100c4..6d635e8 100644
--- a/apps/scratch/src/pages/__tests__/page-objects/scratch-editor.ts
+++ b/apps/scratch/src/pages/__tests__/page-objects/scratch-editor.ts
@@ -1,4 +1,5 @@
 /* eslint-disable @typescript-eslint/explicit-function-return-type */
+import { randomBytes } from "crypto";
 import { Locator, Page } from "playwright/test";
 import {
   getAllTargetBlocksSelector,
@@ -98,6 +99,13 @@ export class ScratchEditorPage {
     return this.openTaskConfigButton.click();
   }
 
+  async pressGreenFlag() {
+    return this.page
+      .getByTestId("stage-controls")
+      .locator("img:first-child")
+      .click();
+  }
+
   async removeAllNonFrozenBlocks() {
     while ((await this.blocksOfCurrentTargetNonFrozen.count()) > 0) {
       await this.blocksOfCurrentTargetNonFrozen.first().dragTo(this.toolbox, {
@@ -137,6 +145,31 @@ export class ScratchEditorPage {
     return block.dragTo(this.toolbox, { force: true });
   }
 
+  async appendNewBlockToBottomOfStack(opcode: string, block: Locator) {
+    const randomClass = "bottom-of-stack-" + randomBytes(20).toString("hex");
+
+    await block.evaluate(
+      (el, { isScratchBlock, randomClass }) =>
+        // get first blocks that does not have any child blocks
+        [el, ...el.querySelectorAll(isScratchBlock)]
+          .find((el) => el.querySelector(isScratchBlock) === null)
+          ?.classList.add(randomClass),
+      { isScratchBlock, randomClass },
+    );
+
+    const lastBlock = this.page.locator(`.${randomClass}`);
+
+    await this.scrollBlockIntoView(lastBlock);
+
+    await this.getBlockInToolbox(opcode).dragTo(lastBlock, {
+      force: true,
+      targetPosition: {
+        x: 50,
+        y: 50,
+      },
+    });
+  }
+
   async appendNewBlockTo(opcode: string, block: Locator) {
     await this.scrollBlockIntoView(block);
 
diff --git a/apps/scratch/src/pages/__tests__/page-objects/test-task.ts b/apps/scratch/src/pages/__tests__/page-objects/test-task.ts
index da67612..1c5229c 100644
--- a/apps/scratch/src/pages/__tests__/page-objects/test-task.ts
+++ b/apps/scratch/src/pages/__tests__/page-objects/test-task.ts
@@ -1,8 +1,15 @@
 /* eslint-disable @typescript-eslint/explicit-function-return-type */
+import { Page } from "playwright/test";
 import { getBlockConfigButtonSelector } from "../locators";
 import { ScratchEditorPage } from "./scratch-editor";
+import { loadTask } from "../helpers";
+import tasks, { TestTask } from "../tasks";
 
 export class TestTaskPage extends ScratchEditorPage {
+  private constructor(page: Page) {
+    super(page);
+  }
+
   get enabledBlockConfigButtons() {
     return {
       moveSteps: this.page.locator(
@@ -26,7 +33,7 @@ export class TestTaskPage extends ScratchEditorPage {
   get taskBlocks() {
     return {
       catActor: {
-        topOfEditableStack: this.page.locator(
+        visualTopOfEditableStack: this.page.locator(
           "[data-id='DMA4vbfmH7Ag=.YTb639']",
         ),
         editableBlock: this.page.locator("[data-id='@Z24?:3gFhIy;D;=NXM*']"),
@@ -42,4 +49,12 @@ export class TestTaskPage extends ScratchEditorPage {
       },
     };
   }
+
+  static async load(
+    page: Page,
+  ): Promise<{ page: TestTaskPage; task: TestTask }> {
+    await loadTask(page, tasks.testTask);
+
+    return { page: new TestTaskPage(page), task: tasks.testTask };
+  }
 }
diff --git a/apps/scratch/src/pages/__tests__/tasks/assertion-task/0fb9be3e8397c983338cb71dc84d0b25.svg b/apps/scratch/src/pages/__tests__/tasks/assertion-task/0fb9be3e8397c983338cb71dc84d0b25.svg
new file mode 100644
index 0000000..5ff997f
--- /dev/null
+++ b/apps/scratch/src/pages/__tests__/tasks/assertion-task/0fb9be3e8397c983338cb71dc84d0b25.svg
@@ -0,0 +1,42 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="91.90780944824219" height="105.20907897949219" viewBox="0.592194938659668 0.3000000357627869 91.90780944824219 105.20907897949219" version="1.1" xml:space="preserve">
+  <!-- Generator: Sketch 52.5 (67469) - http://www.bohemiancoding.com/sketch -->
+  <g>
+    <title>costume2.1</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Page-1" stroke="none" fill-rule="evenodd">
+      <g id="cotsume2">
+        <g id="costume2.1">
+          <g id="tail">
+            <path d="M 12.25 67.3 C 9.15 59.5 10.85 52.5 6.55 52.9 C 2.15 53.3 -2.95 65 6.65 73.6 C 15.55 81.5 24.65 78.6 28.25 76.5 C 31.75 74.3 29.05 73.1 28.45 72.3 C 27.85 71.5 16.65 78.6 12.25 67.3 Z " stroke="#001026" stroke-width="1.2" fill="#FFAB19" stroke-linecap="round" stroke-linejoin="round"/>
+            <path d="M 7.05 53.4 C 4.55 53 2.35 56.6 1.85 60.1 C 1.65 63.7 2.25 65.8 2.95 67.5 C 3.05 66.8 3.95 64.4 6.85 63.9 C 9.25 63.5 10.85 65.5 10.85 65.5 C 10.85 65.5 9.85 62.5 9.35 58.1 C 8.95 55.4 9.05 53.8 7.05 53.4 Z " id="detail" fill="#FFFFFF" stroke-width="1"/>
+          </g>
+          <path d="M 33.15 82.7 C 32.45 83.6 30.45 87.1 25.65 90.1 C 22.35 92.2 13.85 88.7 13.85 88.7 C 14.75 97.9 7.25 97.7 5.85 94.7 C 4.75 92.3 4.95 82 7.15 81.2 C 10.55 80 15.15 82 19.25 82 C 20.45 82 21.75 79.4 24.55 76.6 C 26.55 74.6 29.15 70.9 33.35 73.5 C 36.95 75.8 35.05 80 33.15 82.7 Z " id="leg" stroke="#001026" stroke-width="1.2" fill="#FFAB19" stroke-linecap="round" stroke-linejoin="round"/>
+          <path d="M 50.35 61.5 C 50.65 63.7 50.85 72.1 53.25 73.4 C 55.55 74.6 62.55 72.6 62.95 77.5 C 63.35 82.3 56.85 82.4 51.45 80.9 C 44.45 79 41.25 68.3 45.25 61.7 C 46.75 59.4 50.05 59.2 50.35 61.5 Z " id="arm" stroke="#001026" stroke-width="1.2" fill="#FFAB19" stroke-linecap="round" stroke-linejoin="round"/>
+          <g id="body-and-leg">
+            <path d="M 42.45 77.8 C 43.65 76.9 45.45 74.5 47.05 72.1 C 48.35 70.2 49.75 66.5 49.75 66.5 C 50.65 64 51.25 59.2 47.95 59.3 C 45.75 59.4 43.75 59.1 40.35 58.6 C 34.35 57.4 33.15 56.2 30.75 60.7 C 26.25 69 20.25 74.9 28.45 79.7 C 30.45 80.9 33.65 82.9 36.45 84.9 C 38.45 86.8 35.45 90.9 33.35 93.3 C 32.45 94.4 32.45 96 33.55 97.1 C 40.85 104.2 43.05 106.7 45.85 103.6 C 49.65 99.4 43.65 95.3 43.65 95.3 C 43.65 95.3 48.45 87.8 47.85 84.5 C 47.15 81.3 42.45 77.8 42.45 77.8 Z " id="body" stroke="#001026" stroke-width="1.2" fill="#FFAB19" stroke-linecap="round" stroke-linejoin="round"/>
+            <path d="M 46.9784 71.1029 C 46.9784 71.1029 48.8784 68.6029 44.5784 65.9029 C 40.0784 63.0029 38.3784 66.2029 36.5784 68.6029 C 34.5784 71.7029 36.5784 73.2029 38.5784 75.0029 C 40.1784 76.5029 41.6784 77.7029 41.6784 77.7029 C 41.6784 77.7029 44.7784 75.6029 46.9784 71.1029 Z " id="tummy" fill="#FFFFFF" stroke-width="1"/>
+          </g>
+          <path d="M 32.55 64.7 C 30.35 65.7 26.25 63.9 25.15 64.1 C 24.85 64.2 23.05 69.4 21.95 72.5 C 20.45 76.9 13.65 75.6 13.25 71 C 12.95 66.7 16.85 58.4 18.15 56.9 C 21.05 53.4 30.9092 56.2083 37.4963 57.9565 " id="arm" stroke="#001026" stroke-width="1.2" fill="#FFAB19" stroke-linecap="round" stroke-linejoin="round"/>
+          <g id="head">
+            <path d="M 50.1 9 C 47.8 8.6 45.4 8.4 42.6 8.6 C 37.9 8.8 33.4 10.5 33.4 10.5 L 21.3 2.6 C 20.9 2.4 20.4 2.7 20.5 3.1 L 22.6 21 C 23.2 20.2 12 33.8 19.1 45.2 C 26.2 56.6 41.3 61.7 60.1 58 C 78.9 54.3 83.3 43.5 82.1 37.8 C 80.9 32.1 73.8 30 73.8 30 C 73.8 30 73.7 25.5 70.5 20 C 68.6 16.7 62.2 12 62.2 12 L 59.6 1.3 C 59.5 0.9 59 0.8 58.7 1 L 50.1 9 Z " stroke="#001026" stroke-width="1.2" fill="#FFAB19"/>
+            <path d="M 73.5 30.4 C 73.5 30.4 80.4 32.2 81.6 37.9 C 82.8 43.6 78 53.9 59.4 57.5 C 35.2 62.5 23.7 48.1 30.4 37.5 C 37.1 26.8 48.6 35.9 57 35.3 C 64.2 34.8 65 28.5 73.5 30.4 Z " id="face" fill="#FFFFFF" stroke-width="1"/>
+            <path d="M 42 41.1 C 42 40.7 42.4 40.4 42.8 40.5 C 44.7 41.2 50.1 42.8 56.1 43.2 C 61.5 43.5 64.7 43.2 66.2 42.9 C 66.7 42.8 67.1 43.3 66.9 43.8 C 66 46.5 62.2 54 51.7 53.4 C 42.6 52.4 41.7 46 42 41.1 Z " id="mouth" stroke="#001026" stroke-width="1.2" fill="#FFFFFF" stroke-linecap="round" stroke-linejoin="round"/>
+            <path d="M 80 35.4 C 80 35.4 87.2 35.3 91.9 31.5 " id="whisker" stroke="#001026" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+            <path d="M 80.4 41.3 C 80.4 41.3 84.3 43.2 90.6 42.7 " id="whisker" stroke="#001026" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+            <path d="M 56.6 32.7 C 58.7 32.7 60.9 32.9 61 33.6 C 61.1 35 59.6 37.8 58 37.9 C 56.2 38.1 52 35.6 52 34 C 51.9 32.8 54.6 32.7 56.6 32.7 Z " id="nose" stroke="#001026" stroke-width="1.2" fill="#001026" stroke-linecap="round" stroke-linejoin="round"/>
+            <path d="M 11.6 31.2 C 11.6 31.2 20.2 34 23.7 37.1 " id="whisker" stroke="#001026" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+            <path d="M 12.3 41.2 C 12.3 41.2 19.7 42.3 24 40.6 " id="whisker" stroke="#001026" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+            <g id="eye">
+              <path d="M 68.4 21 C 71.3 25.5 71.4 30.6 68.6 32.4 C 65.8 34.2 61.2 32.1 58.2 27.6 C 55.3 23.1 55.2 18 58 16.2 C 60.8 14.3 65.5 16.5 68.4 21 Z " id="pupil" stroke="#001026" stroke-width="1.2" fill="#FFFFFF" stroke-linecap="round" stroke-linejoin="round"/>
+              <path d="M 68 26.7 C 68 27.8 67.2 28.7 66.2 28.7 C 65.2 28.7 64.4 27.8 64.4 26.7 C 64.4 25.6 65.2 24.7 66.2 24.7 C 67.2 24.7 68 25.6 68 26.7 " id="pupil" fill="#001026" stroke-width="1"/>
+            </g>
+            <g id="eye">
+              <path d="M 43.6 23.8 C 46.6 28.2 46.4 33.6 43.7 35.5 C 40.4 37.4 36 36 33 31.6 C 29.9 27.2 29.7 21.5 32.8 19.3 C 35.9 17 40.6 19.4 43.6 23.8 Z " stroke="#001026" stroke-width="1.2" fill="#FFFFFF" stroke-linecap="round" stroke-linejoin="round"/>
+              <path d="M 43 29.6 C 43 30.7 42.2 31.6 41.2 31.6 C 40.2 31.6 39.4 30.7 39.4 29.6 C 39.4 28.5 40.2 27.6 41.2 27.6 C 42.2 27.7 43 28.5 43 29.6 " id="pupil" fill="#001026" stroke-width="1"/>
+            </g>
+          </g>
+        </g>
+      </g>
+    </g>
+  </g>
+</svg>
\ No newline at end of file
diff --git a/apps/scratch/src/pages/__tests__/tasks/assertion-task/bcf454acf82e4504149f7ffe07081dbc.svg b/apps/scratch/src/pages/__tests__/tasks/assertion-task/bcf454acf82e4504149f7ffe07081dbc.svg
new file mode 100644
index 0000000..03df23e
--- /dev/null
+++ b/apps/scratch/src/pages/__tests__/tasks/assertion-task/bcf454acf82e4504149f7ffe07081dbc.svg
@@ -0,0 +1,42 @@
+<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="95.17898101806641" height="100.04156036376953" viewBox="0.3210171699523926 0.3000000357627869 95.17898101806641 100.04156036376953" version="1.1" xml:space="preserve">
+  <!-- Generator: Sketch 52.5 (67469) - http://www.bohemiancoding.com/sketch -->
+  <g>
+    <title>costume1.1</title>
+    <desc>Created with Sketch.</desc>
+    <g id="Page-1" stroke="none" fill-rule="evenodd">
+      <g id="costume1">
+        <g id="costume1.1">
+          <g id="tail">
+            <path d="M 21.9 73.8 C 19.5 73.3 16.6 72.5 14.2 70.3 C 8.7 65.4 7 57.3 3.2 59.4 C -0.7 61.5 -0.6 74.6 11.6 78.6 C 15.8 80 19.6 80 22.7 79.9 C 23.5 79.9 30.4 79.2 32.8 75.8 C 35.2 72.4 33.5 71.5 32.7 71.1 C 31.8 70.6 25.3 74.4 21.9 73.8 Z " stroke="#001026" stroke-width="1.2" fill="#FFAB19" stroke-linecap="round" stroke-linejoin="round"/>
+            <path d="M 3.8 59.6 C 1.8 60.2 0.8 64.4 1.8 67.9 C 2.8 71.4 4.4 73.2 5.7 74.5 C 5.5 73.8 5.1 71.6 6.8 70.3 C 8.9 68.6 12.6 69.5 12.6 69.5 C 12.6 69.5 9.5 65.7 7.9 63 C 6.3 60.7 5.8 59.2 3.8 59.6 Z " id="detail" fill="#FFFFFF" stroke-width="1"/>
+          </g>
+          <path d="M37.7,81.5 C35.9,82.7 29.7,87.1 21.8,89.6 L21.4,89.7 C21,89.8 20.8,90.3 21,90.7 C22.7,93.1 25.8,97.9 20.3,99.6 C15,101.3 5.1,87.2 9.3,83.5 C11.2,82.1 12.9,82.8 13.8,83.2 C14.3,83.4 14.8,83.4 15.3,83.3 C16.5,82.9 18.7,82.1 20.4,81.2 C24.7,79 25.7,78.1 27.7,76.6 C29.7,75.1 34.3,71.4 38,74.6 C41.2,77.3 39.4,80.3 37.7,81.5 Z" id="leg" stroke="#001026" stroke-width="1.2" fill="#FFAB19" stroke-linecap="round" stroke-linejoin="round"/>
+          <path d="M53.6,60.7 C54.1,61.1 60.2,68.3 62.2,66.5 C64.6,64.4 67.9,60.3 71.5,63.6 C75.1,66.9 68.3,72.5 65.4,74 C58.5,77.1 52.9,71.2 51.7,69.6 C50.5,68 48.4,65.3 48.4,62.7 C48.5,59.9 51.9,59.2 53.6,60.7 Z" id="arm" stroke="#001026" stroke-width="1.2" fill="#FFAB19" stroke-linecap="round" stroke-linejoin="round"/>
+          <g id="body-and-leg">
+            <path d="M 46.2 76.7 C 47.4 75.8 48.6 74.3 50.2 72 C 51.5 70.1 52.9 66.4 52.9 66.4 C 53.8 63.9 54.4 59.1 51.1 59.2 C 48.9 59.3 46.9 59 43.5 58.5 C 37.5 57.3 36.4 56.5 33.9 60.6 C 31.2 65.4 24.3 68.9 32.8 77.2 C 32.8 77.2 37.7 81 43.6 86.8 C 47.6 90.7 53.9 96.3 56.1 98.2 C 56.6 98.6 57.2 98.8 57.8 98.9 C 67.5 99.8 74.7 98.8 74.7 94.5 C 74.7 87.3 60.4 89.8 60.4 89.8 C 60.4 89.8 55.8 85.9 53.7 84 L 46.2 76.7 Z " id="body" stroke="#001026" stroke-width="1.2" fill="#FFAB19" stroke-linecap="round" stroke-linejoin="round"/>
+            <path d="M 50.6 70 C 50.6 70 52.5 67.5 48.2 64.8 C 43.7 61.9 42 65.1 40.2 67.5 C 38.2 70.6 40.2 72.1 42.2 73.9 C 43.8 75.4 45.3 76.6 45.3 76.6 C 45.3 76.6 48.4 74.5 50.6 70 Z " id="tummy" fill="#FFFFFF" stroke-width="1"/>
+          </g>
+          <path d="M30.2,68.4 C32.4,71.2 35.8,74.7 31.5,77.6 C25.6,80.9 20.7,70.9 19.7,67.4 C18.8,64.3 21.4,62.3 23.6,60.6 C27.9,57.5 31.5,54.7 35.5,56.2 C40.5,58 36.9,62 34.4,63.8 C32.9,64.9 31.4,66.1 30.3,66.8 C30,67.3 29.9,67.9 30.2,68.4 Z" id="arm" stroke="#001026" stroke-width="1.2" fill="#FFAB19" stroke-linecap="round" stroke-linejoin="round"/>
+          <g id="head">
+            <path d="M 53.1 9 C 50.8 8.6 48.4 8.4 45.6 8.6 C 40.9 8.8 36.4 10.5 36.4 10.5 L 24.3 2.6 C 23.9 2.4 23.4 2.7 23.5 3.1 L 25.6 21 C 26.2 20.2 15 33.8 22.1 45.2 C 29.2 56.6 44.3 61.7 63.1 58 C 81.9 54.3 86.3 43.5 85.1 37.8 C 83.9 32.1 76.8 30 76.8 30 C 76.8 30 76.7 25.5 73.5 20 C 71.6 16.7 65.2 12 65.2 12 L 62.6 1.3 C 62.5 0.9 62 0.8 61.7 1 L 53.1 9 Z " stroke="#001026" stroke-width="1.2" fill="#FFAB19"/>
+            <path d="M 76.5 30.4 C 76.5 30.4 83.4 32.2 84.6 37.9 C 85.8 43.6 81 53.9 62.4 57.5 C 38.2 62.5 26.7 48.1 33.4 37.5 C 40.1 26.8 51.6 35.9 60 35.3 C 67.2 34.8 68 28.5 76.5 30.4 Z " id="face" fill="#FFFFFF" stroke-width="1"/>
+            <path d="M 45 41.1 C 45 40.7 45.4 40.4 45.8 40.5 C 47.7 41.2 53.1 42.8 59.1 43.2 C 64.5 43.5 67.7 43.2 69.2 42.9 C 69.7 42.8 70.1 43.3 69.9 43.8 C 69 46.5 65.2 54 54.7 53.4 C 45.6 52.4 44.7 46 45 41.1 Z " id="mouth" stroke="#001026" stroke-width="1.2" fill="#FFFFFF" stroke-linecap="round" stroke-linejoin="round"/>
+            <path d="M 83 35.4 C 83 35.4 90.2 35.3 94.9 31.5 " id="whisker" stroke="#001026" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+            <path d="M 83.4 41.3 C 83.4 41.3 87.3 43.2 93.6 42.7 " id="whisker" stroke="#001026" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+            <path d="M 59.6 32.7 C 61.7 32.7 63.9 32.9 64 33.6 C 64.1 35 62.6 37.8 61 37.9 C 59.2 38.1 55 35.6 55 34 C 54.9 32.8 57.6 32.7 59.6 32.7 Z " id="nose" stroke="#001026" stroke-width="1.2" fill="#001026" stroke-linecap="round" stroke-linejoin="round"/>
+            <path d="M 14.6 31.2 C 14.6 31.2 23.2 34 26.7 37.1 " id="whisker" stroke="#001026" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+            <path d="M 15.3 41.2 C 15.3 41.2 22.7 42.3 27 40.6 " id="whisker" stroke="#001026" stroke-width="1.2" stroke-linecap="round" stroke-linejoin="round" fill="none"/>
+            <g id="eye">
+              <path d="M 71.4 21 C 74.3 25.5 74.4 30.6 71.6 32.4 C 68.8 34.2 64.2 32.1 61.2 27.6 C 58.3 23.1 58.2 18 61 16.2 C 63.8 14.3 68.5 16.5 71.4 21 Z " id="pupil" stroke="#001026" stroke-width="1.2" fill="#FFFFFF" stroke-linecap="round" stroke-linejoin="round"/>
+              <path d="M 71 26.7 C 71 27.8 70.2 28.7 69.2 28.7 C 68.2 28.7 67.4 27.8 67.4 26.7 C 67.4 25.6 68.2 24.7 69.2 24.7 C 70.2 24.7 71 25.6 71 26.7 " id="pupil" fill="#001026" stroke-width="1"/>
+            </g>
+            <g id="eye">
+              <path d="M 46.6 23.8 C 49.6 28.2 49.4 33.6 46.7 35.5 C 43.4 37.4 39 36 36 31.6 C 32.9 27.2 32.7 21.5 35.8 19.3 C 38.9 17 43.6 19.4 46.6 23.8 Z " stroke="#001026" stroke-width="1.2" fill="#FFFFFF" stroke-linecap="round" stroke-linejoin="round"/>
+              <path d="M 46 29.6 C 46 30.7 45.2 31.6 44.2 31.6 C 43.2 31.6 42.4 30.7 42.4 29.6 C 42.4 28.5 43.2 27.6 44.2 27.6 C 45.2 27.7 46 28.5 46 29.6 " id="pupil" fill="#001026" stroke-width="1"/>
+            </g>
+          </g>
+        </g>
+      </g>
+    </g>
+  </g>
+</svg>
\ No newline at end of file
diff --git a/apps/scratch/src/pages/__tests__/tasks/assertion-task/cd21514d0531fdffb22204e0ec5ed84a.svg b/apps/scratch/src/pages/__tests__/tasks/assertion-task/cd21514d0531fdffb22204e0ec5ed84a.svg
new file mode 100644
index 0000000..15f7311
--- /dev/null
+++ b/apps/scratch/src/pages/__tests__/tasks/assertion-task/cd21514d0531fdffb22204e0ec5ed84a.svg
@@ -0,0 +1,3 @@
+<svg version="1.1" width="2" height="2" viewBox="-1 -1 2 2" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
+  <!-- Exported by Scratch - http://scratch.mit.edu/ -->
+</svg>
\ No newline at end of file
diff --git a/apps/scratch/src/pages/__tests__/tasks/assertion-task/crt.json b/apps/scratch/src/pages/__tests__/tasks/assertion-task/crt.json
new file mode 100644
index 0000000..98417c3
--- /dev/null
+++ b/apps/scratch/src/pages/__tests__/tasks/assertion-task/crt.json
@@ -0,0 +1,8 @@
+{
+  "allowedBlocks": { "motion_movesteps": -1 },
+  "freezeStateByBlockId": {
+    "4_e9[GL:|#*oG]X]FdQm": "frozen",
+    "HN(hP9y9,$[q0=u7VklJ": "appendable"
+  },
+  "taskBlockIds": { "HN(hP9y9,$[q0=u7VklJ": "appendable" }
+}
diff --git a/apps/scratch/src/pages/__tests__/tasks/assertion-task/project.json b/apps/scratch/src/pages/__tests__/tasks/assertion-task/project.json
new file mode 100644
index 0000000..d657de2
--- /dev/null
+++ b/apps/scratch/src/pages/__tests__/tasks/assertion-task/project.json
@@ -0,0 +1,131 @@
+{
+  "targets": [
+    {
+      "isStage": true,
+      "name": "Stage",
+      "variables": { "`jEk@4|i[#Fk?(8x)AV.-my variable": ["my variable", 0] },
+      "lists": {},
+      "broadcasts": {},
+      "blocks": {},
+      "comments": {},
+      "currentCostume": 0,
+      "costumes": [
+        {
+          "name": "backdrop1",
+          "dataFormat": "svg",
+          "assetId": "cd21514d0531fdffb22204e0ec5ed84a",
+          "md5ext": "cd21514d0531fdffb22204e0ec5ed84a.svg",
+          "rotationCenterX": 240,
+          "rotationCenterY": 180
+        }
+      ],
+      "sounds": [],
+      "volume": 100,
+      "layerOrder": 0,
+      "tempo": 60,
+      "videoTransparency": 50,
+      "videoState": "on",
+      "textToSpeechLanguage": null
+    },
+    {
+      "isStage": false,
+      "name": "Sprite1",
+      "variables": {},
+      "lists": {},
+      "broadcasts": {},
+      "blocks": {
+        "HN(hP9y9,$[q0=u7VklJ": {
+          "opcode": "event_whenflagclicked",
+          "next": null,
+          "parent": null,
+          "inputs": {},
+          "fields": {},
+          "shadow": false,
+          "topLevel": true,
+          "x": 202,
+          "y": 167
+        },
+        "=AZYDuVf2OMO^acD9JwZ": {
+          "opcode": "assertions_noop_assert",
+          "next": null,
+          "parent": "4_e9[GL:|#*oG]X]FdQm",
+          "inputs": { "CONDITION": [2, "f();?%T;l:K-w%Qo!`Dg"] },
+          "fields": {},
+          "shadow": false,
+          "topLevel": false
+        },
+        "f();?%T;l:K-w%Qo!`Dg": {
+          "opcode": "operator_equals",
+          "next": null,
+          "parent": "=AZYDuVf2OMO^acD9JwZ",
+          "inputs": {
+            "OPERAND1": [3, "KMNp~3loiK%UvhdlsKNE", [10, "50"]],
+            "OPERAND2": [1, [10, "50"]]
+          },
+          "fields": {},
+          "shadow": false,
+          "topLevel": false
+        },
+        "KMNp~3loiK%UvhdlsKNE": {
+          "opcode": "motion_xposition",
+          "next": null,
+          "parent": "f();?%T;l:K-w%Qo!`Dg",
+          "inputs": {},
+          "fields": {},
+          "shadow": false,
+          "topLevel": false
+        },
+        "4_e9[GL:|#*oG]X]FdQm": {
+          "opcode": "assertions_event_whenTaskFinishedRunning",
+          "next": "=AZYDuVf2OMO^acD9JwZ",
+          "parent": null,
+          "inputs": {},
+          "fields": {},
+          "shadow": false,
+          "topLevel": true,
+          "x": 572,
+          "y": 184
+        }
+      },
+      "comments": {},
+      "currentCostume": 0,
+      "costumes": [
+        {
+          "name": "costume1",
+          "bitmapResolution": 1,
+          "dataFormat": "svg",
+          "assetId": "bcf454acf82e4504149f7ffe07081dbc",
+          "md5ext": "bcf454acf82e4504149f7ffe07081dbc.svg",
+          "rotationCenterX": 48,
+          "rotationCenterY": 50
+        },
+        {
+          "name": "costume2",
+          "bitmapResolution": 1,
+          "dataFormat": "svg",
+          "assetId": "0fb9be3e8397c983338cb71dc84d0b25",
+          "md5ext": "0fb9be3e8397c983338cb71dc84d0b25.svg",
+          "rotationCenterX": 46,
+          "rotationCenterY": 53
+        }
+      ],
+      "sounds": [],
+      "volume": 100,
+      "layerOrder": 1,
+      "visible": true,
+      "x": 0,
+      "y": 0,
+      "size": 100,
+      "direction": 90,
+      "draggable": false,
+      "rotationStyle": "all around"
+    }
+  ],
+  "monitors": [],
+  "extensions": ["assertions"],
+  "meta": {
+    "semver": "3.0.0",
+    "vm": "4.6.29",
+    "agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:132.0) Gecko/20100101 Firefox/132.0"
+  }
+}
diff --git a/apps/scratch/src/pages/__tests__/tasks/helpers.ts b/apps/scratch/src/pages/__tests__/tasks/helpers.ts
index 2f9caf1..aff338a 100644
--- a/apps/scratch/src/pages/__tests__/tasks/helpers.ts
+++ b/apps/scratch/src/pages/__tests__/tasks/helpers.ts
@@ -19,7 +19,7 @@ export const zipDirectory = (sourceDir: string): Promise<Buffer> => {
     stream.on("close", () => {
       const file = fs.readFileSync(tempFile);
 
-      fs.rmSync(tempFile, { recursive: true });
+      fs.rmSync(tempPath, { recursive: true, force: true });
 
       return resolve(file);
     });
diff --git a/apps/scratch/src/pages/__tests__/tasks/index.ts b/apps/scratch/src/pages/__tests__/tasks/index.ts
index 7905416..e185511 100644
--- a/apps/scratch/src/pages/__tests__/tasks/index.ts
+++ b/apps/scratch/src/pages/__tests__/tasks/index.ts
@@ -3,10 +3,10 @@ import { Buffer } from "buffer";
 import * as path from "path";
 import * as fs from "fs";
 import { zipDirectory } from "./helpers";
-import { ScratchCrtConfig } from "scratch-vm";
 import { BlockFreezeStates } from "../../../blocks/types";
+import { ScratchCrtConfig } from "../../../types/scratch-vm-custom";
 
-type Task = {
+export type TestTask = {
   blocksOfMainTarget: number;
   frozenBlocksOfMainTarget: number;
 
@@ -98,7 +98,7 @@ const getBlocksOfTarget = (
     ),
   );
 
-const getTask = (taskPath: string): Task => {
+const getTask = (taskPath: string): TestTask => {
   const sb3 = JSON.parse(
     fs.readFileSync(path.resolve(taskPath, "project.json"), "utf-8"),
   );
@@ -119,9 +119,12 @@ const getTask = (taskPath: string): Task => {
 };
 
 const tasks = {
-  get testTask(): Task {
+  get testTask(): TestTask {
     return getTask(path.resolve(__dirname, "test-task"));
   },
+  get assertionTask(): TestTask {
+    return getTask(path.resolve(__dirname, "assertion-task"));
+  },
 };
 
 export default tasks;
diff --git a/apps/scratch/src/types/scratch-vm-custom.ts b/apps/scratch/src/types/scratch-vm-custom.ts
new file mode 100644
index 0000000..b7efc94
--- /dev/null
+++ b/apps/scratch/src/types/scratch-vm-custom.ts
@@ -0,0 +1,72 @@
+import { BlockLimits } from "../blocks/make-toolbox-xml";
+import { BlockFreezeStates } from "../blocks/types";
+
+export interface CrtBlock {
+  isTaskBlock?: boolean;
+}
+
+export interface CrtEventMap {
+  /**
+   * Event emitted after the assertions extension has been loaded.
+   */
+  ASSERTIONS_EXTENSION_LOADED: [];
+
+  /**
+   * Event emitted to check if assertions are enabled.
+   */
+  ARE_ASSERTIONS_ENABLED_QUERY: [];
+
+  /**
+   * Event emitted as a response to the query for whether assertions are enabled.
+   */
+  ARE_ASSERTIONS_ENABLED_RESPONSE: [boolean];
+
+  /**
+   * Event emitted to enable assertions.
+   */
+  ENABLE_ASSERTIONS: [];
+
+  /**
+   * Event emitted when assertions are enabled.
+   */
+  ASSERTIONS_ENABLED: [];
+
+  /**
+   * Event emitted to disable assertions.
+   */
+  DISABLE_ASSERTIONS: [];
+
+  /**
+   * Event emitted when assertions are disabled.
+   */
+  ASSERTIONS_DISABLED: [];
+
+  /**
+   * Event emitted after the project's assertions have been evaluated.
+   * Note that there is no guarantee this event is ever emitted.
+   * It will only be emitted if the assertions extension is loaded.
+   * The first argument is a number indicating the total number of checked assertions
+   * and the checked assertions.
+   */
+  ASSERTIONS_CHECKED: [
+    // number of total assertions.
+    number,
+    // number of passed assertions.
+    number,
+  ];
+}
+
+export interface ScratchCrtConfig {
+  /**
+   * A map from scratch opcode to a number that defines how many times
+   * a given block can be used.
+   */
+  allowedBlocks: BlockLimits;
+
+  /**
+   * Whether initial task blocks can be edited.
+   */
+  freezeStateByBlockId: {
+    [taskBlockId: string]: BlockFreezeStates | undefined;
+  };
+}
diff --git a/apps/scratch/src/vm/default-crt-config.ts b/apps/scratch/src/vm/default-crt-config.ts
index c2f8c22..bc6b981 100644
--- a/apps/scratch/src/vm/default-crt-config.ts
+++ b/apps/scratch/src/vm/default-crt-config.ts
@@ -1,4 +1,4 @@
-import { ScratchCrtConfig } from "scratch-vm";
+import { ScratchCrtConfig } from "../types/scratch-vm-custom";
 
 export const defaultCrtConfig: ScratchCrtConfig = {
   allowedBlocks: {},
diff --git a/apps/scratch/src/vm/index.ts b/apps/scratch/src/vm/index.ts
index 3f10da5..435ecf4 100644
--- a/apps/scratch/src/vm/index.ts
+++ b/apps/scratch/src/vm/index.ts
@@ -1,6 +1,6 @@
 import VM from "scratch-vm";
 import { ExtensionId } from "../extensions";
-import ExampleExtension from "../extensions/example";
+import AssertionExtension from "../extensions/assertions";
 
 export const patchScratchVm = (vm: VM): void => {
   // patch extension manager load function with a custom implementation
@@ -10,8 +10,8 @@ export const patchScratchVm = (vm: VM): void => {
     // modified logic from https://github.com/scratchfoundation/scratch-vm/blob/766c767c7a2f3da432480ade515de0a9f98804ba/src/extension-support/extension-manager.js#L142
 
     switch (id) {
-      case ExtensionId.Example: {
-        const extensionInstance = new ExampleExtension(vm.runtime);
+      case ExtensionId.Assertions: {
+        const extensionInstance = new AssertionExtension(vm.runtime);
 
         const serviceName =
           vm.extensionManager._registerInternalExtension(extensionInstance);
@@ -26,11 +26,6 @@ export const patchScratchVm = (vm: VM): void => {
     return Promise.resolve(0);
   };
 
-  // add custom callback to when the greenFlag event is triggered
-  vm.runtime.on("PROJECT_START", () => {
-    console.log("Green flag clicked");
-  });
-
   vm.runtime.on("PROJECT_LOADED", () => {
     // iterate over all the blocks in the runtime
     // and mark them as initial blocks
@@ -39,11 +34,5 @@ export const patchScratchVm = (vm: VM): void => {
         block.isTaskBlock = true;
       }
     }
-
-    console.log("loaded", { ...vm.crtConfig });
-  });
-
-  vm.runtime.on("PROJECT_RUN_STOP", () => {
-    console.log("stopped");
   });
 };
diff --git a/apps/scratch/src/vm/load-crt-project.ts b/apps/scratch/src/vm/load-crt-project.ts
index ca588f0..881addb 100644
--- a/apps/scratch/src/vm/load-crt-project.ts
+++ b/apps/scratch/src/vm/load-crt-project.ts
@@ -1,6 +1,7 @@
-import VM, { ScratchCrtConfig } from "scratch-vm";
+import VM from "scratch-vm";
 import { defaultCrtConfig } from "./default-crt-config";
 import JSZip from "jszip";
+import { ScratchCrtConfig } from "../types/scratch-vm-custom";
 
 /**
  * Load a Scratch project from a .sb, .sb2, .sb3 or json string.
diff --git a/apps/scratch/types/scratch-vm.d.ts b/apps/scratch/types/scratch-vm.d.ts
index 98b0279..239434e 100644
--- a/apps/scratch/types/scratch-vm.d.ts
+++ b/apps/scratch/types/scratch-vm.d.ts
@@ -113,13 +113,12 @@ declare namespace VMExtended {
 
   export type RotationStyle = VM.RotationStyle;
 
-  export interface BlockExtended extends VM.Block {
+  type CrtBlock = import("../src/types/scratch-vm-custom").CrtBlock;
+
+  export interface BlockExtended extends VM.Block, CrtBlock {
     isMonitored: boolean;
     x: number;
     y: number;
-
-    // a property added by the CRT to mark a block as a task block
-    isTaskBlock?: boolean;
   }
 
   export interface BlocksExtended extends VM.Blocks {
@@ -179,7 +178,31 @@ declare namespace VMExtended {
     getId: (string) => string;
   }
 
+  type EventEmitterArgs<Events, K extends keyof Events> = Events[K] extends Array<unknown> ? Events[K] : unknown[];
+  type EventEmitterCallback<Events, K extends keyof Events> = (...args: Events[K] extends Array<unknown> ? Events[K] : unknown[]) => void
+
+  type CrtEventMap = import("../src/types/scratch-vm-custom").CrtEventMap;
+
+  export interface RuntimeEventMapExtended extends VM.RuntimeEventMap, CrtEventMap {
+    targetWasCreated: [
+      // The new target
+      TargetExtended,
+      // The original target, if any. This will be set for clones.
+      TargetExtended?
+    ];
+  }
+
   export interface RuntimeExtended extends VM.Runtime {
+
+    // override event emitter
+    on<K extends keyof RuntimeEventMapExtended>(event: K, callback: EventEmitterCallback<RuntimeEventMapExtended, K>): void;
+    once<K extends keyof RuntimeEventMapExtended>(event: K, callback: EventEmitterCallback<RuntimeEventMapExtended, K>): void;
+    off<K extends keyof RuntimeEventMapExtended>(event: K, callback: EventEmitterCallback<RuntimeEventMapExtended, K>): void;
+    removeListener<K extends keyof RuntimeEventMapExtended>(event: K, callback: EventEmitterCallback<RuntimeEventMapExtended, K>): void;
+    listeners<K extends keyof RuntimeEventMapExtended>(event: K): EventEmitterCallback<RuntimeEventMapExtended, K>[];
+    emit<K extends keyof RuntimeEventMapExtended>(event: K, ...args: EventEmitterArgs<RuntimeEventMapExtended, K>): void;
+    // end override event emitter
+
     monitorBlocks: BlocksExtended;
     monitorBlockInfo: MonitorBlockInfo;
 
@@ -204,6 +227,8 @@ declare namespace VMExtended {
     setCustomState(name: T, value: CustomState): void;
   }
 
+  export interface CustomState extends VM.CustomState {}
+
   export interface Monitor {
     get(name: string): unknown;
   }
@@ -246,19 +271,6 @@ declare namespace VMExtended {
     // https://github.com/scratchfoundation/scratch-vm/blob/e15809697de82760a6f13e03c502251de5bdd8c7/src/blocks/scratch3_motion.js#L47
     getMonitored?: () => Record<string, MonitorBlockInfo>;
   }
-
-  export interface ScratchCrtConfig {
-    /**
-     * A map from scratch opcode to a number that defines how many times
-     * a given block can be used.
-     */
-    allowedBlocks: import("../src/blocks/make-toolbox-xml").BlockLimits;
-
-    /**
-     * Whether initial task blocks can be edited.
-     */
-    freezeStateByBlockId: {[taskBlockId?: string]: import("../src/blocks/types").BlockFreezeStates | undefined };
-  }
 }
 
 declare class VMExtended extends VM {
@@ -287,7 +299,7 @@ declare class VMExtended extends VM {
   monitorBlockListener: Function;
 
   // add a custom config
-  crtConfig?: VMExtended.ScratchCrtConfig;
+  crtConfig?: import("../src/types/scratch-vm-custom").ScratchCrtConfig;
 }
 
 declare module "scratch-vm" {
diff --git a/backend/src/ast/converters/scratch/helpers.ts b/backend/src/ast/converters/scratch/helpers.ts
index 8c30232..4302e41 100644
--- a/backend/src/ast/converters/scratch/helpers.ts
+++ b/backend/src/ast/converters/scratch/helpers.ts
@@ -18,10 +18,7 @@ import {
 } from "./types";
 import { isWhenBackdropSwitchesToBlock } from "src/ast/types/input/scratch/blocks/event/when-backdrop-switches-to";
 import { isWhenBroadcastReceivedBlock } from "src/ast/types/input/scratch/blocks/event/when-broadcast-received";
-import {
-  isWhenGreaterThanBlock,
-  WhenGreaterThanBlock,
-} from "src/ast/types/input/scratch/blocks/event/when-greater-than";
+import { isWhenGreaterThanBlock } from "src/ast/types/input/scratch/blocks/event/when-greater-than";
 import { isWhenKeyPressedBlock } from "src/ast/types/input/scratch/blocks/event/when-key-pressed";
 import { isWhenFlagClickedBlock } from "src/ast/types/input/scratch/blocks/event/when-flag-clicked";
 import { isWhenStageIsClickedBlock } from "src/ast/types/input/scratch/blocks/event/when-stage-is-clicked";
@@ -78,6 +75,7 @@ import {
   ExtensionExpressionBlock,
   ExtensionStatementBlock,
 } from "src/ast/types/input/scratch/blocks/extensions";
+import { isArbitraryHatBlock } from "src/ast/types/input/scratch/blocks/extensions/arbitrary-hat-block";
 
 export const createLiteralNode = (type: string, value: string): LiteralNode => {
   return {
@@ -178,28 +176,18 @@ export const getEventParameters = (
 ): ExpressionNode[] => {
   return match(block)
     .returnType<ExpressionNode[]>()
-    .with(P.when(isWhenBackdropSwitchesToBlock), (block) => [
-      createLiteralNode("string", block.fields.BACKDROP[0]),
-    ])
-    .with(P.when(isWhenBroadcastReceivedBlock), (block) => [
-      createLiteralNode("string", block.fields.BROADCAST_OPTION[0]),
-    ])
     .with(
+      P.when(isWhenBackdropSwitchesToBlock),
+      P.when(isWhenBroadcastReceivedBlock),
       P.when(isWhenGreaterThanBlock),
-      (block: WhenGreaterThanBlock & BlockTreeWithEventHatRoot) => [
-        convertInputsToExpression(block, "VALUE"),
-        createLiteralNode("string", block.fields.WHENGREATERTHANMENU[0]),
-      ],
-    )
-    .with(P.when(isWhenKeyPressedBlock), (block) => [
-      createLiteralNode("string", block.fields.KEY_OPTION[0]),
-    ])
-    .with(
+      P.when(isWhenKeyPressedBlock),
       P.when(isWhenFlagClickedBlock),
       P.when(isWhenStageIsClickedBlock),
       P.when(iswhenThisSpriteIsClickedBlock),
       P.when(isStartAsCloneBlock),
-      () => [],
+      P.when(isArbitraryHatBlock),
+      (block: BlockTreeWithEventHatRoot) =>
+        createArgumentsFromInputsAndFields(block),
     )
     .exhaustive();
 };
@@ -282,7 +270,8 @@ export const isHatBlock = (
     isWhenStageIsClickedBlock(block) ||
     iswhenThisSpriteIsClickedBlock(block) ||
     isStartAsCloneBlock(block) ||
-    isDefinitionBlock(block)
+    isDefinitionBlock(block) ||
+    isArbitraryHatBlock(block)
   );
 };
 
diff --git a/backend/src/ast/converters/scratch/scratch-extension-block-converter.ts b/backend/src/ast/converters/scratch/scratch-extension-block-converter.ts
index 764fe8e..23b3140 100644
--- a/backend/src/ast/converters/scratch/scratch-extension-block-converter.ts
+++ b/backend/src/ast/converters/scratch/scratch-extension-block-converter.ts
@@ -11,6 +11,10 @@ import {
   ArbitraryFunctionCallBlock,
   isArbitraryStatementBlock,
 } from "src/ast/types/input/scratch/blocks/extensions/arbitrary-function-call-block";
+import {
+  ArbitraryNoOpBlock,
+  isArbitraryNoOpBlock,
+} from "src/ast/types/input/scratch/blocks/extensions/arbitrary-no-op-block";
 
 type ExtensionStatementTreeNode = ExtensionStatementBlock & TreeNode;
 type ExtensionExpressionTreeNode = ExtensionExpressionBlock & TreeNode;
@@ -18,7 +22,7 @@ type ExtensionExpressionTreeNode = ExtensionExpressionBlock & TreeNode;
 export const isExtensionStatementBlock = (
   block: NonHatBlock,
 ): block is ExtensionStatementBlock & NonHatBlock =>
-  isArbitraryStatementBlock(block);
+  isArbitraryStatementBlock(block) || isArbitraryNoOpBlock(block);
 
 export const isExtensionExpressionBlock = (
   block: NonHatBlock,
@@ -39,6 +43,10 @@ export const convertExtensionBlockTreeToStatement = (
         ];
       },
     )
+    .with(
+      P.when(isArbitraryNoOpBlock),
+      (_block: ArbitraryNoOpBlock & ExtensionStatementTreeNode) => [],
+    )
     .exhaustive();
 
 export const convertExtensionBlockTreeToExpression = (
diff --git a/backend/src/ast/converters/scratch/types.ts b/backend/src/ast/converters/scratch/types.ts
index 4573c65..79b67bd 100644
--- a/backend/src/ast/converters/scratch/types.ts
+++ b/backend/src/ast/converters/scratch/types.ts
@@ -3,7 +3,10 @@ import {
   KnownBuiltinScratchExpressionBlock,
   KnownBuiltinScratchHatBlock,
 } from "src/ast/types/input/scratch/blocks";
-import { ExtensionStatementBlock } from "src/ast/types/input/scratch/blocks/extensions";
+import {
+  ExtensionHatBlock,
+  ExtensionStatementBlock,
+} from "src/ast/types/input/scratch/blocks/extensions";
 import { DefinitionBlock } from "src/ast/types/input/scratch/blocks/procedure/definition";
 import {
   Block,
@@ -20,10 +23,9 @@ export type NonHatBlock =
   | ExtensionStatementBlock
   | KnownBuiltinScratchExpressionBlock;
 
-export type EventHatBlock = Exclude<
-  KnownBuiltinScratchHatBlock,
-  DefinitionBlock
->;
+export type EventHatBlock =
+  | Exclude<KnownBuiltinScratchHatBlock, DefinitionBlock>
+  | ExtensionHatBlock;
 
 export type TreeNode = {
   /**
diff --git a/backend/src/ast/types/input/scratch/blocks/extensions/arbitrary-hat-block.ts b/backend/src/ast/types/input/scratch/blocks/extensions/arbitrary-hat-block.ts
new file mode 100644
index 0000000..39a7a0d
--- /dev/null
+++ b/backend/src/ast/types/input/scratch/blocks/extensions/arbitrary-hat-block.ts
@@ -0,0 +1,14 @@
+import { Block } from "../../generated/sb3";
+
+export interface ArbitraryHatBlock extends Block {
+  opcode: string;
+}
+
+export const isArbitraryHatBlock = (
+  block: Block,
+): block is ArbitraryHatBlock => {
+  // extension block opcodes are of the form `extensionName_blockType_<opcode>`
+  const parts = block.opcode.split("_");
+
+  return parts.length === 3 && parts[1] === "event";
+};
diff --git a/backend/src/ast/types/input/scratch/blocks/extensions/arbitrary-no-op-block.ts b/backend/src/ast/types/input/scratch/blocks/extensions/arbitrary-no-op-block.ts
new file mode 100644
index 0000000..bc955a5
--- /dev/null
+++ b/backend/src/ast/types/input/scratch/blocks/extensions/arbitrary-no-op-block.ts
@@ -0,0 +1,15 @@
+import { Block } from "../../generated/sb3";
+
+export interface ArbitraryNoOpBlock extends Block {
+  opcode: string;
+  inputs: Block["inputs"];
+}
+
+export const isArbitraryNoOpBlock = (
+  block: Block,
+): block is ArbitraryNoOpBlock => {
+  // extension block opcodes are of the form `extensionName_blockType_<opcode>`
+  const parts = block.opcode.split("_");
+
+  return parts.length === 3 && parts[1] === "noop";
+};
diff --git a/backend/src/ast/types/input/scratch/blocks/extensions/index.ts b/backend/src/ast/types/input/scratch/blocks/extensions/index.ts
index 277fbd8..f36c2fe 100644
--- a/backend/src/ast/types/input/scratch/blocks/extensions/index.ts
+++ b/backend/src/ast/types/input/scratch/blocks/extensions/index.ts
@@ -1,7 +1,11 @@
 import { ArbitraryFunctionCallBlock } from "./arbitrary-function-call-block";
+import { ArbitraryHatBlock } from "./arbitrary-hat-block";
+import { ArbitraryNoOpBlock } from "./arbitrary-no-op-block";
 
-export type LooksHatBlock = never;
+export type ExtensionHatBlock = ArbitraryHatBlock;
 
-export type ExtensionStatementBlock = ArbitraryFunctionCallBlock;
+export type ExtensionStatementBlock =
+  | ArbitraryFunctionCallBlock
+  | ArbitraryNoOpBlock;
 
 export type ExtensionExpressionBlock = never;
diff --git a/frontend/src/types/app-iframe-message/index.d.ts b/frontend/src/types/app-iframe-message/index.d.ts
index 0997d04..9fc2f9f 100644
--- a/frontend/src/types/app-iframe-message/index.d.ts
+++ b/frontend/src/types/app-iframe-message/index.d.ts
@@ -2,17 +2,23 @@ import { GetHeightRequest, GetHeightReponse } from "./get-height";
 import { GetSubmissionRequest, GetSubmissionResponse } from "./get-submission";
 import { GetTaskRequest, GetTaskResponse } from "./get-task";
 import { LoadTaskRequest, LoadTaskResponse } from "./load-task";
+import {
+  ReportProgressReponse,
+  ReportProgressRequest,
+} from "./report-progress";
 
 export type AppIFrameRequest =
   | GetHeightRequest
   | GetSubmissionRequest
   | GetTaskRequest
-  | LoadTaskRequest;
+  | LoadTaskRequest
+  | ReportProgressRequest;
 
 export type AppIFrameResponse =
   | GetHeightReponse
   | GetSubmissionResponse
   | GetTaskResponse
-  | LoadTaskResponse;
+  | LoadTaskResponse
+  | ReportProgressReponse;
 
 export type AppIFrameMessage = AppIFrameRequest | AppIFrameResponse;
diff --git a/frontend/src/types/app-iframe-message/report-progress.d.ts b/frontend/src/types/app-iframe-message/report-progress.d.ts
new file mode 100644
index 0000000..4f59157
--- /dev/null
+++ b/frontend/src/types/app-iframe-message/report-progress.d.ts
@@ -0,0 +1,16 @@
+import {
+  RemoteProcedureCallRequest,
+  RemoteProcedureCallResponse,
+} from "./remote-procedure-call";
+
+type ProcedureName = "reportProgress";
+
+export type ReportProgressRequest = RemoteProcedureCallRequest<
+  ProcedureName,
+  { totalTests: number; passedTests: number }
+>;
+
+export type ReportProgressReponse = RemoteProcedureCallResponse<
+  ProcedureName,
+  undefined
+>;