diff --git a/src/__mocks__/clipboardSubsection.js b/src/__mocks__/clipboardSubsection.js
new file mode 100644
index 000000000..541e95f97
--- /dev/null
+++ b/src/__mocks__/clipboardSubsection.js
@@ -0,0 +1,16 @@
+export default {
+  content: {
+    id: 67,
+    userId: 3,
+    created: '2024-01-16T13:09:11.540615Z',
+    purpose: 'clipboard',
+    status: 'ready',
+    blockType: 'sequential',
+    blockTypeDisplay: 'Subsection',
+    olxUrl: 'http://localhost:18010/api/content-staging/v1/staged-content/67/olx',
+    displayName: 'Sequences',
+  },
+  sourceUsageKey: 'block-v1:edX+DemoX+Demo_Course+type@sequential+block@sequential_0270f6de40fc',
+  sourceContextTitle: 'Demonstration Course',
+  sourceEditUrl: 'http://localhost:18010/container/block-v1:edX+DemoX+Demo_Course+type@sequential+block@sequential_0270f6de40fc',
+};
diff --git a/src/__mocks__/index.js b/src/__mocks__/index.js
index b3b5984d3..abacaea4a 100644
--- a/src/__mocks__/index.js
+++ b/src/__mocks__/index.js
@@ -1,2 +1,3 @@
 export { default as clipboardUnit } from './clipboardUnit';
+export { default as clipboardSubsection } from './clipboardSubsection';
 export { default as clipboardXBlock } from './clipboardXBlock';
diff --git a/src/generic/clipboard/hooks/useClipboard.test.tsx b/src/generic/clipboard/hooks/useClipboard.test.tsx
index 0b73ef8b4..74a0fb1e0 100644
--- a/src/generic/clipboard/hooks/useClipboard.test.tsx
+++ b/src/generic/clipboard/hooks/useClipboard.test.tsx
@@ -2,6 +2,7 @@ import { renderHook } from '@testing-library/react';
 import MockAdapter from 'axios-mock-adapter';
 
 import {
+  clipboardSubsection,
   clipboardUnit,
   clipboardXBlock,
 } from '../../../__mocks__';
@@ -42,7 +43,7 @@ describe('useClipboard', () => {
 
       axiosMock
         .onPost(getClipboardUrl())
-        .reply(200, clipboardUnit);
+        .reply(200, clipboardSubsection);
 
       await result.current.copyToClipboard(unitId);
 
@@ -89,6 +90,7 @@ describe('useClipboard', () => {
   describe('broadcast channel message handling', () => {
     it('updates states correctly on receiving a broadcast message', async () => {
       const { result, rerender } = renderHook(() => useClipboard(true), { wrapper: makeWrapper() });
+      // Subsections cannot be pasted:
       clipboardBroadcastChannelMock.postMessage({ data: clipboardUnit });
 
       rerender();
diff --git a/src/library-authoring/add-content/AddContent.test.tsx b/src/library-authoring/add-content/AddContent.test.tsx
index b761b3e92..d6c1ea1b8 100644
--- a/src/library-authoring/add-content/AddContent.test.tsx
+++ b/src/library-authoring/add-content/AddContent.test.tsx
@@ -207,6 +207,23 @@ describe('<AddContent />', () => {
     await waitFor(() => expect(axiosMock.history.post[0].url).toEqual(pasteUrl));
   });
 
+  it('should show error toast on paste failure', async () => {
+    // Simulate having an HTML block in the clipboard:
+    mockClipboardHtml.applyMock();
+
+    const pasteUrl = getLibraryPasteClipboardUrl(libraryId);
+    axiosMock.onPost(pasteUrl).reply(500, { block_type: 'Unsupported block type.' });
+
+    render();
+    const pasteButton = await screen.findByRole('button', { name: /paste from clipboard/i });
+    fireEvent.click(pasteButton);
+
+    await waitFor(() => expect(axiosMock.history.post[0].url).toEqual(pasteUrl));
+    expect(mockShowToast).toHaveBeenCalledWith(
+      'There was an error pasting the content: {"block_type":"Unsupported block type."}',
+    );
+  });
+
   it('should paste content inside a collection', async () => {
     // Simulate having an HTML block in the clipboard:
     const getClipboardSpy = mockClipboardHtml.applyMock();
diff --git a/src/library-authoring/add-content/AddContent.tsx b/src/library-authoring/add-content/AddContent.tsx
index 08b924d7a..7610b743f 100644
--- a/src/library-authoring/add-content/AddContent.tsx
+++ b/src/library-authoring/add-content/AddContent.tsx
@@ -191,7 +191,15 @@ export const parseErrorMsg = (
 ) => {
   try {
     const { response: { data } } = error;
-    const detail = data && (Array.isArray(data) ? data.join() : String(data));
+    let detail = '';
+    if (Array.isArray(data)) {
+      detail = data.join(', ');
+    } else if (typeof data === 'string') {
+      /* istanbul ignore next */
+      detail = data.substring(0, 400); // In case this is a giant HTML response, only show the first little bit.
+    } else if (data) {
+      detail = JSON.stringify(data);
+    }
     if (detail) {
       return intl.formatMessage(detailedMessage, { detail });
     }
@@ -217,7 +225,7 @@ const AddContent = () => {
   const pasteClipboardMutation = useLibraryPasteClipboard();
   const { showToast } = useContext(ToastContext);
   const canEdit = useSelector(getCanEdit);
-  const { showPasteXBlock, sharedClipboardData } = useClipboard(canEdit);
+  const { showPasteUnit, showPasteXBlock, sharedClipboardData } = useClipboard(canEdit);
 
   const [isAddLibraryContentModalOpen, showAddLibraryContentModal, closeAddLibraryContentModal] = useToggle();
   const [isAdvancedListOpen, showAdvancedList, closeAdvancedList] = useToggle();
@@ -281,7 +289,7 @@ const AddContent = () => {
 
   // Include the 'Paste from Clipboard' button if there is an Xblock in the clipboard
   // that can be pasted
-  if (showPasteXBlock) {
+  if (showPasteXBlock || showPasteUnit) {
     const pasteButton = {
       name: intl.formatMessage(messages.pasteButton),
       disabled: false,
@@ -317,7 +325,6 @@ const AddContent = () => {
     }
     pasteClipboardMutation.mutateAsync({
       libraryId,
-      blockId: `${uuid4()}`,
     }).then((data) => {
       linkComponent(data.id);
       showToast(intl.formatMessage(messages.successPasteClipboardMessage));
diff --git a/src/library-authoring/data/api.mocks.ts b/src/library-authoring/data/api.mocks.ts
index 5b5ba4348..9a71c8aa8 100644
--- a/src/library-authoring/data/api.mocks.ts
+++ b/src/library-authoring/data/api.mocks.ts
@@ -186,7 +186,6 @@ export async function mockCreateLibraryBlock(
 }
 mockCreateLibraryBlock.newHtmlData = {
   id: 'lb:Axim:TEST:html:123',
-  defKey: '123',
   blockType: 'html',
   displayName: 'New Text Component',
   hasUnpublishedChanges: true,
@@ -201,7 +200,6 @@ mockCreateLibraryBlock.newHtmlData = {
 } satisfies api.LibraryBlockMetadata;
 mockCreateLibraryBlock.newProblemData = {
   id: 'lb:Axim:TEST:problem:prob1',
-  defKey: 'prob1',
   blockType: 'problem',
   displayName: 'New Problem',
   hasUnpublishedChanges: true,
@@ -216,7 +214,6 @@ mockCreateLibraryBlock.newProblemData = {
 } satisfies api.LibraryBlockMetadata;
 mockCreateLibraryBlock.newVideoData = {
   id: 'lb:Axim:TEST:video:vid1',
-  defKey: 'vid1',
   blockType: 'video',
   displayName: 'New Video',
   hasUnpublishedChanges: true,
@@ -349,7 +346,6 @@ mockLibraryBlockMetadata.usageKeyError404 = 'lb:Axim:error404:html:123';
 mockLibraryBlockMetadata.usageKeyNeverPublished = 'lb:Axim:TEST1:html:571fe018-f3ce-45c9-8f53-5dafcb422fd1';
 mockLibraryBlockMetadata.dataNeverPublished = {
   id: 'lb:Axim:TEST1:html:571fe018-f3ce-45c9-8f53-5dafcb422fd1',
-  defKey: null,
   blockType: 'html',
   displayName: 'Introduction to Testing 1',
   lastPublished: null,
@@ -365,7 +361,6 @@ mockLibraryBlockMetadata.dataNeverPublished = {
 mockLibraryBlockMetadata.usageKeyPublished = 'lb:Axim:TEST2:html:571fe018-f3ce-45c9-8f53-5dafcb422fd2';
 mockLibraryBlockMetadata.dataPublished = {
   id: 'lb:Axim:TEST2:html:571fe018-f3ce-45c9-8f53-5dafcb422fd2',
-  defKey: null,
   blockType: 'html',
   displayName: 'Introduction to Testing 2',
   lastPublished: '2024-06-22T00:00:00',
@@ -394,7 +389,6 @@ mockLibraryBlockMetadata.usageKeyForTags = mockContentTaxonomyTagsData.largeTags
 mockLibraryBlockMetadata.usageKeyWithCollections = 'lb:Axim:TEST:html:571fe018-f3ce-45c9-8f53-5dafcb422fdd';
 mockLibraryBlockMetadata.dataWithCollections = {
   id: 'lb:Axim:TEST:html:571fe018-f3ce-45c9-8f53-5dafcb422fdd',
-  defKey: null,
   blockType: 'html',
   displayName: 'Introduction to Testing 2',
   lastPublished: '2024-06-21T00:00:00',
@@ -411,7 +405,6 @@ mockLibraryBlockMetadata.usageKeyPublishedWithChanges = 'lb:Axim:TEST:html:571fe
 mockLibraryBlockMetadata.usageKeyPublishedWithChangesV2 = 'lb:Axim:TEST:html:571fe018-f3ce-45c9-8f53-5dafcb422fv2';
 mockLibraryBlockMetadata.dataPublishedWithChanges = {
   id: 'lb:Axim:TEST2:html:571fe018-f3ce-45c9-8f53-5dafcb422fvv',
-  defKey: null,
   blockType: 'html',
   displayName: 'Introduction to Testing 2',
   lastPublished: '2024-06-22T00:00:00',
@@ -492,7 +485,7 @@ mockGetContainerMetadata.containerIdLoading = 'lct:org:lib:unit:container_loadin
 mockGetContainerMetadata.containerIdForTags = mockContentTaxonomyTagsData.largeTagsId;
 mockGetContainerMetadata.containerIdWithCollections = 'lct:org:lib:unit:container_collections';
 mockGetContainerMetadata.containerData = {
-  containerKey: 'lct:org:lib:unit:test-unit-9a2072',
+  id: 'lct:org:lib:unit:test-unit-9a2072',
   containerType: 'unit',
   displayName: 'Test Unit',
   created: '2024-09-19T10:00:00Z',
@@ -552,7 +545,6 @@ mockGetContainerChildren.sixChildren = 'lct:org1:Demo_Course:unit:unit-6';
 mockGetContainerChildren.childTemplate = {
   id: 'lb:org1:Demo_course:html:text',
   blockType: 'html',
-  defKey: 'def_key',
   displayName: 'text block',
   lastPublished: null,
   publishedBy: null,
diff --git a/src/library-authoring/data/api.ts b/src/library-authoring/data/api.ts
index 30feaa9bc..b4a7bca67 100644
--- a/src/library-authoring/data/api.ts
+++ b/src/library-authoring/data/api.ts
@@ -245,7 +245,6 @@ export interface CollectionMetadata {
 export interface LibraryBlockMetadata {
   id: string;
   blockType: string;
-  defKey: string | null;
   displayName: string;
   lastPublished: string | null;
   publishedBy: string | null;
@@ -270,7 +269,6 @@ export interface UpdateLibraryDataRequest {
 
 export interface LibraryPasteClipboardRequest {
   libraryId: string;
-  blockId: string;
 }
 
 export interface UpdateXBlockFieldsRequest {
@@ -426,16 +424,10 @@ export async function getBlockTypes(libraryId: string): Promise<BlockTypeMetadat
  */
 export async function libraryPasteClipboard({
   libraryId,
-  blockId,
 }: LibraryPasteClipboardRequest): Promise<LibraryBlockMetadata> {
   const client = getAuthenticatedHttpClient();
-  const { data } = await client.post(
-    getLibraryPasteClipboardUrl(libraryId),
-    {
-      block_id: blockId,
-    },
-  );
-  return data;
+  const { data } = await client.post(getLibraryPasteClipboardUrl(libraryId), {});
+  return camelCaseObject(data);
 }
 
 /**
@@ -597,7 +589,7 @@ export async function createLibraryContainer(
 }
 
 export interface Container {
-  containerKey: string;
+  id: string;
   containerType: 'unit';
   displayName: string;
   lastPublished: string | null;
diff --git a/src/library-authoring/data/apiHooks.test.tsx b/src/library-authoring/data/apiHooks.test.tsx
index e4c6a7dba..57e0a8fa7 100644
--- a/src/library-authoring/data/apiHooks.test.tsx
+++ b/src/library-authoring/data/apiHooks.test.tsx
@@ -191,7 +191,6 @@ describe('library api hooks', () => {
       {
         id: 'lb:org1:Demo_course:html:text',
         block_type: 'html',
-        def_key: 'def_key',
         display_name: 'text block',
         last_published: null,
         published_by: null,
@@ -206,7 +205,6 @@ describe('library api hooks', () => {
       {
         id: 'lb:org1:Demo_course:video:video1',
         block_type: 'video',
-        def_key: 'def_key',
         display_name: 'video block',
         last_published: null,
         published_by: null,
@@ -227,7 +225,6 @@ describe('library api hooks', () => {
       {
         id: 'lb:org1:Demo_course:html:text',
         blockType: 'html',
-        defKey: 'def_key',
         displayName: 'text block',
         lastPublished: null,
         publishedBy: null,
@@ -242,7 +239,6 @@ describe('library api hooks', () => {
       {
         id: 'lb:org1:Demo_course:video:video1',
         blockType: 'video',
-        defKey: 'def_key',
         displayName: 'video block',
         lastPublished: null,
         publishedBy: null,