diff --git a/app/channels/asset_sync_channel.rb b/app/channels/asset_sync_channel.rb
new file mode 100644
index 0000000000..24d5b159c3
--- /dev/null
+++ b/app/channels/asset_sync_channel.rb
@@ -0,0 +1,20 @@
+# frozen_string_literal: true
+
+class AssetSyncChannel < ApplicationCable::Channel
+  include Canaid::Helpers::PermissionsHelper
+
+  def subscribed
+    asset = Asset.find_by(id: params[:asset_id])
+
+    return reject unless asset&.file&.attached?
+    return reject unless can_manage_asset?(current_user, asset) && can_open_asset_locally?(current_user, asset)
+
+    stream_for asset
+
+    transmit({ checksum: asset.file.checksum })
+  end
+
+  def unsubscribed
+    stop_all_streams
+  end
+end
diff --git a/app/controllers/asset_sync_controller.rb b/app/controllers/asset_sync_controller.rb
index 79912834ba..7566979deb 100644
--- a/app/controllers/asset_sync_controller.rb
+++ b/app/controllers/asset_sync_controller.rb
@@ -44,6 +44,8 @@ def update
       log_activity(:edit)
     end
 
+    AssetSyncChannel.broadcast_to(@asset, checksum: @asset.file.checksum)
+
     if asset_conflicts
       ActiveRecord::Base.transaction do
         conflict_response = AssetSyncTokenSerializer.new(@asset_sync_token).as_json
diff --git a/app/controllers/assets_controller.rb b/app/controllers/assets_controller.rb
index 5445a5648a..f81aca6bf7 100644
--- a/app/controllers/assets_controller.rb
+++ b/app/controllers/assets_controller.rb
@@ -442,10 +442,6 @@ def duplicate
     end
   end
 
-  def checksum
-    render json: { checksum: @asset.file.blob.checksum }
-  end
-
   def versions
     blobs =
       [@asset.file.blob] +
diff --git a/app/controllers/global_constants_controller.rb b/app/controllers/global_constants_controller.rb
index da8cccbd37..d42d43ab7a 100644
--- a/app/controllers/global_constants_controller.rb
+++ b/app/controllers/global_constants_controller.rb
@@ -28,7 +28,6 @@ def load_global_constants
       FILENAME_MAX_LENGTH: Constants::FILENAME_MAX_LENGTH,
       FAST_STATUS_POLLING_INTERVAL: Constants::FAST_STATUS_POLLING_INTERVAL,
       SLOW_STATUS_POLLING_INTERVAL: Constants::SLOW_STATUS_POLLING_INTERVAL,
-      ASSET_POLLING_INTERVAL: Constants::ASSET_POLLING_INTERVAL,
       ASSET_SYNC_URL: Constants::ASSET_SYNC_URL,
       GLOBAL_SEARCH_PREVIEW_LIMIT: Constants::GLOBAL_SEARCH_PREVIEW_LIMIT,
       SEARCH_LIMIT: Constants::SEARCH_LIMIT,
diff --git a/app/javascript/vue/protocol/step.vue b/app/javascript/vue/protocol/step.vue
index bd8817e839..0d2814b8fe 100644
--- a/app/javascript/vue/protocol/step.vue
+++ b/app/javascript/vue/protocol/step.vue
@@ -203,7 +203,6 @@
                     @attachment:deleted="attachmentDeleted"
                     @attachment:update="updateAttachment"
                     @attachment:uploaded="loadAttachments"
-                    @attachment:changed="reloadAttachment"
                     @attachments:order="changeAttachmentsOrder"
                     @attachment:moved="moveAttachment"
                     @attachments:viewMode="changeAttachmentsViewMode"
@@ -596,25 +595,6 @@
           this.$emit('step:drag_enter', this.step.id);
         }
       },
-      reloadAttachment(attachmentId) {
-        const index = this.attachments.findIndex(attachment => attachment.id === attachmentId);
-        const attachmentUrl = this.attachments[index].attributes.urls.asset_show
-
-        axios.get(attachmentUrl)
-          .then((response) => {
-            const updatedAttachment = response.data.data;
-            const index = this.attachments.findIndex(attachment => attachment.id === attachmentId);
-
-            if (index !== -1) {
-              this.attachments[index] = updatedAttachment;
-            }
-          })
-          .catch((error) => {
-            console.error("Failed to reload attachment:", error);
-          });
-
-        this.showFileModal = false;
-      },
       showStorageUsage() {
         return (this.elements.length || this.attachments.length) && !this.isCollapsed && this.step.attributes.storage_limit;
       },
diff --git a/app/javascript/vue/shared/content/attachments.vue b/app/javascript/vue/shared/content/attachments.vue
index 20b96f63a5..2a8347e395 100644
--- a/app/javascript/vue/shared/content/attachments.vue
+++ b/app/javascript/vue/shared/content/attachments.vue
@@ -30,7 +30,7 @@
       <div class="attachments" :data-parent-id="parent.id">
         <component
           v-for="(attachment, index) in attachmentsOrdered"
-          :key="attachment.id"
+          :key="`${attachment.id}-${attachment.attributes.checksum}`"
           :is="attachment_view_mode(attachmentsOrdered[index])"
           :attachment="attachment"
           :parentId="parseInt(parent.id)"
@@ -42,7 +42,6 @@
           @attachment:archive="deleteAttachment(attachment.id)"
           @attachment:moved="attachmentMoved"
           @attachment:uploaded="$emit('attachment:uploaded')"
-          @attachment:changed="$emit('attachment:changed', $event)"
           @attachment:update="$emit('attachment:update', $event)"
         />
       </div>
diff --git a/app/javascript/vue/shared/content/attachments/attachment_actions.vue b/app/javascript/vue/shared/content/attachments/attachment_actions.vue
index d27107ad87..c157b214f9 100644
--- a/app/javascript/vue/shared/content/attachments/attachment_actions.vue
+++ b/app/javascript/vue/shared/content/attachments/attachment_actions.vue
@@ -49,7 +49,6 @@
       @attachment:delete="$emit('attachment:delete', $event)"
       @attachment:moved="$emit('attachment:moved', $event)"
       @attachment:uploaded="$emit('attachment:uploaded', $event)"
-      @attachment:changed="$emit('attachment:changed', $event)"
       @attachment:update="$emit('attachment:update', $event)"
       @menu-toggle="$emit('attachment:toggle_menu', $event)"
       @attachment:versionRestored="$emit('attachment:versionRestored', $event)"
@@ -72,7 +71,6 @@
 </template>
 
 <script>
-import OpenLocallyMixin from './mixins/open_locally.js';
 import OpenMenu from './open_menu.vue';
 import ContextMenu from './context_menu.vue';
 import deleteAttachmentModal from './delete_modal.vue';
@@ -92,7 +90,6 @@ export default {
       default: ''
     }
   },
-  mixins: [OpenLocallyMixin],
   data() {
     return {
       deleteModal: false,
diff --git a/app/javascript/vue/shared/content/attachments/inline.vue b/app/javascript/vue/shared/content/attachments/inline.vue
index 42e6a701a6..1c85bd26c9 100644
--- a/app/javascript/vue/shared/content/attachments/inline.vue
+++ b/app/javascript/vue/shared/content/attachments/inline.vue
@@ -49,7 +49,6 @@
           @attachment:moved="attachmentMoved"
           @attachment:uploaded="reloadAttachments"
           @attachment:versionRestored="reloadAttachments"
-          @attachment:changed="$emit('attachment:changed', $event)"
           @attachment:update="$emit('attachment:update', $event)"
           @attachment:toggle_menu="toggleMenuDropdown"
           @attachment:move_modal="showMoveModal"
@@ -110,19 +109,16 @@ import ContextMenu from './context_menu.vue';
 import PdfViewer from '../../pdf_viewer.vue';
 import MoveAssetModal from '../modal/move.vue';
 import MoveMixin from './mixins/move.js';
-import OpenLocallyMixin from './mixins/open_locally.js';
 import AttachmentActions from './attachment_actions.vue';
-import OpenMenu from './open_menu.vue';
 import LockedTag from '../../snippets/locked_tag.vue';
 
 export default {
   name: 'inlineAttachment',
-  mixins: [ContextMenuMixin, AttachmentMovedMixin, MoveMixin, OpenLocallyMixin],
+  mixins: [ContextMenuMixin, AttachmentMovedMixin, MoveMixin],
   components: {
     ContextMenu,
     PdfViewer,
     MoveAssetModal,
-    OpenMenu,
     AttachmentActions,
     LockedTag
   },
diff --git a/app/javascript/vue/shared/content/attachments/list.vue b/app/javascript/vue/shared/content/attachments/list.vue
index cde5068b59..f20698386a 100644
--- a/app/javascript/vue/shared/content/attachments/list.vue
+++ b/app/javascript/vue/shared/content/attachments/list.vue
@@ -54,7 +54,6 @@
           @attachment:moved="attachmentMoved"
           @attachment:uploaded="reloadAttachments"
           @attachment:versionRestored="reloadAttachments"
-          @attachment:changed="$emit('attachment:changed', $event)"
           @attachment:update="$emit('attachment:update', $event)"
           @attachment:toggle_menu="toggleMenuDropdown"
           @attachment:move_modal="showMoveModal"
@@ -79,7 +78,6 @@ import ContextMenu from './context_menu.vue';
 import MoveMixin from './mixins/move.js';
 import MoveAssetModal from '../modal/move.vue';
 import AttachmentActions from './attachment_actions.vue';
-import OpenMenu from './open_menu.vue';
 import LockedTag from '../../snippets/locked_tag.vue';
 
 export default {
@@ -88,7 +86,6 @@ export default {
   components: {
     ContextMenu,
     MoveAssetModal,
-    OpenMenu,
     AttachmentActions,
     LockedTag
   },
diff --git a/app/javascript/vue/shared/content/attachments/mixins/open_locally.js b/app/javascript/vue/shared/content/attachments/mixins/open_locally.js
index eef60aaa80..98420d1a93 100644
--- a/app/javascript/vue/shared/content/attachments/mixins/open_locally.js
+++ b/app/javascript/vue/shared/content/attachments/mixins/open_locally.js
@@ -15,8 +15,7 @@ export default {
       showNoPredefinedAppModal: false,
       showRestrictedExtensionModal: false,
       showUpdateVersionModal: false,
-      editAppModal: false,
-      pollingInterval: null
+      editAppModal: false
     };
   },
   components: {
@@ -36,9 +35,6 @@ export default {
              && this.attributes.asset_type !== 'marvinjs';
     }
   },
-  beforeUnmount() {
-    this.stopPolling();
-  },
   methods: {
     async checkScinoteEditRunning() {
       // responses will be cached on window, so the requests only run once per page load
@@ -119,7 +115,6 @@ export default {
 
       this.editAppModal = true;
       try {
-        this.startPolling();
         const { data } = await axios.get(this.attributes.urls.open_locally);
         await axios.post(`${this.attributes.urls.open_locally_api}/download`, data);
       } catch (error) {
@@ -129,33 +124,6 @@ export default {
     isWrongVersion(version) {
       const { min, max } = this.attributes.edit_version_range;
       return !satisfies(version, `${min} - ${max}`);
-    },
-    async pollForChanges() {
-      try {
-        const checksumResponse = await axios.get(this.attributes.urls.asset_checksum);
-
-        if (checksumResponse.status === 200) {
-          const currentChecksum = checksumResponse.data.checksum;
-
-          if (currentChecksum !== this.attributes.checksum) {
-            this.$emit('attachment:changed', this.attachment.id);
-          }
-        }
-      } catch (error) {
-        console.error('Error polling for changes:', error);
-        this.stopPolling();
-      }
-    },
-    startPolling() {
-      if (this.pollingInterval === null) {
-        this.pollingInterval = setInterval(this.pollForChanges, GLOBAL_CONSTANTS.ASSET_POLLING_INTERVAL);
-      }
-    },
-    stopPolling() {
-      if (this.pollingInterval !== null) {
-        clearInterval(this.pollingInterval);
-        this.pollingInterval = null;
-      }
     }
   }
 }
diff --git a/app/javascript/vue/shared/content/attachments/open_locally_menu.vue b/app/javascript/vue/shared/content/attachments/open_locally_menu.vue
index add560936d..1b471bbf85 100644
--- a/app/javascript/vue/shared/content/attachments/open_locally_menu.vue
+++ b/app/javascript/vue/shared/content/attachments/open_locally_menu.vue
@@ -62,6 +62,8 @@
 </template>
 
 <script>
+import { markRaw } from 'vue';
+import consumer from '../../../../channels/consumer';
 import OpenLocallyMixin from './mixins/open_locally.js';
 import MenuDropdown from '../../menu_dropdown.vue';
 import UpdateVersionModal from '../modal/update_version_modal.vue';
@@ -77,14 +79,19 @@ export default {
   },
   data() {
     return {
-      fileVersionsModal: false
+      fileVersionsModal: false,
+      fileChangesSubscription: null
     };
   },
   created() {
     this.fetchLocalAppInfo();
     window.openLocallyMenu = this;
   },
+  mounted() {
+    this.subscribeFileChanges();
+  },
   beforeUnmount() {
+    this.unsubscribeFileChanges();
     delete window.openLocallyMenuComponent;
   },
   computed: {
@@ -133,6 +140,28 @@ export default {
     openImageEditor() {
       document.getElementById('editImageButton').click();
     },
+    subscribeFileChanges() {
+      if (this.fileChangesSubscription || !this.attachment.attributes.urls.open_locally) return;
+
+      // markRaw, because ActionCable forgets a subscription by identity and would
+      // never match a reactive proxy of it - so unsubscribing would silently no-op.
+      this.fileChangesSubscription = markRaw(consumer.subscriptions.create(
+        { channel: 'AssetSyncChannel', asset_id: this.attachment.attributes.id },
+        {
+          received: (data) => {
+            if (data.checksum === this.attachment.attributes.checksum) return;
+            this.refreshPreview();
+          },
+          rejected: () => this.unsubscribeFileChanges()
+        }
+      ));
+    },
+    unsubscribeFileChanges() {
+      if (!this.fileChangesSubscription) return;
+
+      consumer.subscriptions.remove(this.fileChangesSubscription);
+      this.fileChangesSubscription = null;
+    },
     refreshPreview() {
       const filePreview = document.querySelector('.file-preview-container');
 
diff --git a/app/javascript/vue/shared/content/attachments/thumbnail.vue b/app/javascript/vue/shared/content/attachments/thumbnail.vue
index 1002ca6105..67f5e1eeda 100644
--- a/app/javascript/vue/shared/content/attachments/thumbnail.vue
+++ b/app/javascript/vue/shared/content/attachments/thumbnail.vue
@@ -74,7 +74,6 @@
           @attachment:moved="attachmentMoved"
           @attachment:uploaded="reloadAttachments"
           @attachment:versionRestored="reloadAttachments"
-          @attachment:changed="$emit('attachment:changed', $event)"
           @attachment:update="$emit('attachment:update', $event)"
           @attachment:toggle_menu="toggleMenu"
           @attachment:move_modal="showMoveModal"
@@ -119,7 +118,6 @@ import MenuDropdown from '../../../shared/menu_dropdown.vue';
 import MoveAssetModal from '../modal/move.vue';
 import MoveMixin from './mixins/move.js';
 import PreviewStatusMixin from './mixins/preview_status.js';
-import OpenMenu from './open_menu.vue';
 import AttachmentActions from './attachment_actions.vue';
 import { vOnClickOutside } from '@vueuse/components';
 import LockedTag from '../../snippets/locked_tag.vue';
@@ -132,7 +130,6 @@ export default {
     deleteAttachmentModal,
     MoveAssetModal,
     MenuDropdown,
-    OpenMenu,
     AttachmentActions,
     LockedTag
   },
diff --git a/app/javascript/vue/shared/content/mixins/attachments.js b/app/javascript/vue/shared/content/mixins/attachments.js
index 2a001e1320..45c9d07195 100644
--- a/app/javascript/vue/shared/content/mixins/attachments.js
+++ b/app/javascript/vue/shared/content/mixins/attachments.js
@@ -1,5 +1,9 @@
 /* global ActiveStorage GLOBAL_CONSTANTS Promise I18n */
 
+import { markRaw } from 'vue';
+import axios from '../../../../packs/custom_axios.js';
+import consumer from '../../../../channels/consumer';
+
 export default {
   data() {
     return {
@@ -8,6 +12,7 @@ export default {
         thumbnail: 1,
         list: 2
       },
+      assetSyncSubscriptions: {}
     };
   },
   computed: {
@@ -19,8 +24,27 @@ export default {
     },
     cantUploadFiles() {
       return false;
+    },
+    // A primitive, so the watcher only fires when the set of ids changes - not when
+    // reloadAttachment replaces an object in the array.
+    assetSyncIds() {
+      return this.attachments
+        .filter((attachment) => attachment?.id && attachment.attributes?.urls?.open_locally)
+        .map((attachment) => String(attachment.id))
+        .join(',');
+    }
+  },
+  watch: {
+    assetSyncIds() {
+      this.syncAssetSubscriptions();
     }
   },
+  mounted() {
+    this.syncAssetSubscriptions();
+  },
+  beforeUnmount() {
+    this.unsubscribeAllAssetSync();
+  },
   methods: {
     dropFile(e) {
       if (this.cantUploadFiles) return;
@@ -155,6 +179,67 @@ export default {
       const attachment = this.attachments.find(e => e.id === id);
       attachment.attributes['view_mode'] = viewMode;
       attachment.attributes['asset_order'] = this.viewModeOrder[viewMode];
+    },
+    syncAssetSubscriptions() {
+      const watchedIds = this.assetSyncIds ? this.assetSyncIds.split(',') : [];
+
+      Object.keys(this.assetSyncSubscriptions)
+        .filter((assetId) => !watchedIds.includes(assetId))
+        .forEach((assetId) => this.unsubscribeAssetSync(assetId));
+
+      watchedIds.forEach((assetId) => this.subscribeAssetSync(assetId));
+    },
+    subscribeAssetSync(assetId) {
+      if (assetId in this.assetSyncSubscriptions) return;
+      this.assetSyncSubscriptions[assetId] = null;
+
+      // markRaw, because ActionCable forgets a subscription and would
+      // never match its reactive proxy; so unsubscribing would silently do nothing.
+      this.assetSyncSubscriptions[assetId] = markRaw(consumer.subscriptions.create(
+        { channel: 'AssetSyncChannel', asset_id: assetId },
+        {
+          received: (data) => {
+            const attachment = this.attachments.find((a) => String(a?.id) === assetId);
+
+            if (!attachment || data.checksum === attachment.attributes.checksum) return;
+
+            this.reloadAttachment(attachment.id);
+          },
+          rejected: () => this.unsubscribeAssetSync(assetId)
+        }
+      ));
+    },
+    unsubscribeAssetSync(assetId) {
+      if (!(assetId in this.assetSyncSubscriptions)) return;
+
+      const subscription = this.assetSyncSubscriptions[assetId];
+
+      if (subscription) consumer.subscriptions.remove(subscription);
+
+      delete this.assetSyncSubscriptions[assetId];
+    },
+    unsubscribeAllAssetSync() {
+      Object.keys(this.assetSyncSubscriptions).forEach((assetId) => this.unsubscribeAssetSync(assetId));
+    },
+    reloadAttachment(attachmentId) {
+      const index = this.attachments.findIndex(attachment => attachment?.id === attachmentId);
+      if (index === -1) return;
+
+      const attachmentUrl = this.attachments[index].attributes.urls.asset_show;
+      if (!attachmentUrl) return;
+
+      axios.get(attachmentUrl)
+        .then((response) => {
+          const updatedAttachment = response.data?.data;
+          const updatedIndex = this.attachments.findIndex(attachment => attachment?.id === attachmentId);
+
+          if (updatedAttachment && updatedIndex !== -1) {
+            this.attachments[updatedIndex] = updatedAttachment;
+          }
+        })
+        .catch((error) => {
+          console.error('Failed to reload attachment:', error);
+        });
     }
   }
 };
diff --git a/app/serializers/asset_serializer.rb b/app/serializers/asset_serializer.rb
index 9c28b6baaa..c12ececb70 100644
--- a/app/serializers/asset_serializer.rb
+++ b/app/serializers/asset_serializer.rb
@@ -8,7 +8,7 @@ class AssetSerializer < ActiveModel::Serializer
   include InputSanitizeHelper
   include ApplicationHelper
 
-  attributes :file_name, :file_extension, :view_mode, :icon, :urls, :updated_at_formatted,
+  attributes :id, :file_name, :file_extension, :view_mode, :icon, :urls, :updated_at_formatted,
              :file_size, :medium_preview, :large_preview, :asset_type, :wopi, :file_name_without_extension,
              :wopi_context, :pdf_previewable, :file_size_formatted, :asset_order,
              :updated_at, :metadata, :image_editable, :image_context, :pdf, :attached, :parent_type,
@@ -188,7 +188,6 @@ def urls
       urls[:open_locally] = asset_sync_show_path(object)
       urls[:open_locally_api] = Constants::ASSET_SYNC_URL
       urls[:asset_show] = asset_show_path(object)
-      urls[:asset_checksum] = asset_checksum_path(object)
     end
 
     urls[:wopi_action] = object.get_action_url(user, 'embedview') if wopi && can_read_asset?(user, object)
diff --git a/config/initializers/constants.rb b/config/initializers/constants.rb
index 8a9a8b9b2b..8694c84af6 100644
--- a/config/initializers/constants.rb
+++ b/config/initializers/constants.rb
@@ -454,9 +454,6 @@ class Constants
   # Infinite date, for use in sorting
   INFINITE_DATE = Date.new(9999, 1, 1).freeze
 
-  # Interval time for polling asset changes when editing with SciNote Edit
-  ASSET_POLLING_INTERVAL = 5000
-
   ASSET_SYNC_TOKEN_EXPIRATION = 1.year
   ASSET_SYNC_URL = ENV['ASSET_SYNC_URL'].freeze
 
diff --git a/spec/channels/asset_sync_channel_spec.rb b/spec/channels/asset_sync_channel_spec.rb
new file mode 100644
index 0000000000..e9e7a207cf
--- /dev/null
+++ b/spec/channels/asset_sync_channel_spec.rb
@@ -0,0 +1,92 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+RSpec.describe AssetSyncChannel, type: :channel do
+  let(:user) { create(:user) }
+  let(:asset) { create(:asset) }
+
+  # This matches Connection#find_verified_user (env['warden'].user must return the current user)
+  let(:warden) { instance_double('Warden::Proxy') }
+
+  # Canaid implements can_*? through method_missing, so any_instance verification
+  # (which checks method_defined?) cannot see it.
+  def stub_permissions(manage: true, open_locally: true)
+    without_partial_double_verification do
+      allow_any_instance_of(described_class).to receive(:can_manage_asset?).and_return(manage)
+      allow_any_instance_of(described_class).to receive(:can_open_asset_locally?).and_return(open_locally)
+    end
+  end
+
+  before do
+    allow(warden).to receive(:user).and_return(user)
+
+    stub_connection current_user: user, env: { 'warden' => warden }
+  end
+
+  context 'when the user can manage the asset and SciNote Edit is enabled' do
+    before { stub_permissions }
+
+    it 'subscribes and streams for the asset' do
+      subscribe(asset_id: asset.id)
+
+      expect(subscription).to be_confirmed
+      expect(subscription).to have_stream_for(asset)
+    end
+
+    it 'transmits the current checksum on subscribe' do
+      subscribe(asset_id: asset.id)
+
+      expect(transmissions.last['checksum']).to eq(asset.file.checksum)
+    end
+
+    it 'receives broadcasts for the subscribed asset' do
+      subscribe(asset_id: asset.id)
+
+      expect do
+        described_class.broadcast_to(asset, checksum: 'new-checksum')
+      end.to have_broadcasted_to(asset).from_channel(described_class).with(checksum: 'new-checksum')
+    end
+
+    it 'does not stream for another asset' do
+      other_asset = create(:asset)
+      subscribe(asset_id: asset.id)
+
+      expect(subscription).not_to have_stream_for(other_asset)
+    end
+
+    it 'rejects the subscription when the asset does not exist' do
+      subscribe(asset_id: -1)
+
+      expect(subscription).to be_rejected
+    end
+
+    it 'rejects the subscription when the asset has no file attached' do
+      asset.file.purge
+
+      subscribe(asset_id: asset.id)
+
+      expect(subscription).to be_rejected
+    end
+  end
+
+  context 'when the user cannot manage the asset' do
+    before { stub_permissions(manage: false) }
+
+    it 'rejects the subscription' do
+      subscribe(asset_id: asset.id)
+
+      expect(subscription).to be_rejected
+    end
+  end
+
+  context 'when SciNote Edit is not enabled' do
+    before { stub_permissions(open_locally: false) }
+
+    it 'rejects the subscription' do
+      subscribe(asset_id: asset.id)
+
+      expect(subscription).to be_rejected
+    end
+  end
+end
diff --git a/spec/controllers/asset_sync_controller_spec.rb b/spec/controllers/asset_sync_controller_spec.rb
new file mode 100644
index 0000000000..476aa0b53e
--- /dev/null
+++ b/spec/controllers/asset_sync_controller_spec.rb
@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+require 'rails_helper'
+
+describe AssetSyncController, type: :controller do
+  login_user
+
+  include_context 'reference_project_structure', {
+    team_role: :owner,
+    step: true,
+    step_asset: true
+  }
+
+  let(:asset) { step_asset.asset }
+  let(:asset_sync_token) { user.asset_sync_tokens.create!(asset: asset) }
+
+  before do
+    stub_const('ENV', ENV.to_h.merge('ASSET_SYNC_URL' => 'http://localhost:9999'))
+
+    # authenticate_asset_sync_token! is a prepend_before_action, so it checks
+    # permissions before any team context is set up for the request - permission_team
+    # falls back to the user's current team, which real users always have.
+    user.update!(current_team_id: team.id)
+  end
+
+  describe 'PUT #update' do
+    let(:action) do
+      request.headers['Authentication'] = asset_sync_token.token
+      request.headers['VersionToken'] = asset_sync_token.version_token
+      request.env['RAW_POST_DATA'] = 'updated file contents'
+
+      put :update
+    end
+
+    it 'broadcasts the new checksum to the asset' do
+      expect { action }
+        .to have_broadcasted_to(asset)
+        .from_channel(AssetSyncChannel)
+        .with { |data| expect(data[:checksum]).to eq(asset.reload.file.checksum) }
+    end
+  end
+end
diff --git a/spec/controllers/assets_controller_spec.rb b/spec/controllers/assets_controller_spec.rb
index e07c51d8c5..0ed6c08e5e 100644
--- a/spec/controllers/assets_controller_spec.rb
+++ b/spec/controllers/assets_controller_spec.rb
@@ -268,15 +268,6 @@ def expect_unprocessable_json
     end
   end
 
-  describe 'GET #checksum' do
-    let(:action) { get :checksum, params: { id: step_asset.asset.id } }
-
-    it 'checksum returned' do
-      action
-      expect_success_json
-    end
-  end
-
   describe 'POST #duplicate' do
     let(:action) { post :duplicate, params: { id: element_id } }