diff --git a/lib/actions/entry-actions.js b/lib/actions/entry-actions.js
--- a/lib/actions/entry-actions.js
+++ b/lib/actions/entry-actions.js
@@ -1,12 +1,22 @@
 // @flow
 
+import invariant from 'invariant';
+import * as React from 'react';
+import uuid from 'uuid';
+
 import {
   extractKeyserverIDFromID,
   sortCalendarQueryPerKeyserver,
 } from '../keyserver-conn/keyserver-call-utils.js';
 import { useKeyserverCall } from '../keyserver-conn/keyserver-call.js';
 import type { CallKeyserverEndpoint } from '../keyserver-conn/keyserver-conn-types.js';
+import {
+  dmOperationSpecificationTypes,
+  type OutboundDMOperationSpecification,
+} from '../shared/dm-ops/dm-op-utils.js';
+import { useProcessAndSendDMOperation } from '../shared/dm-ops/process-dm-ops.js';
 import { getNextLocalID } from '../shared/message-utils.js';
+import { type DMCreateEntryOperation } from '../types/dm-ops.js';
 import type {
   RawEntryInfo,
   CalendarQuery,
@@ -22,7 +32,10 @@
   CalendarQueryUpdateResult,
 } from '../types/entry-types.js';
 import type { HistoryRevisionInfo } from '../types/history-types.js';
+import type { ThreadInfo } from '../types/minimally-encoded-thread-permissions-types';
+import { thickThreadTypes } from '../types/thread-types-enum.js';
 import { dateFromString } from '../utils/date-utils.js';
+import { useSelector } from '../utils/redux-utils.js';
 
 const fetchEntriesActionTypes = Object.freeze({
   started: 'FETCH_ENTRIES_STARTED',
@@ -178,10 +191,76 @@
     };
   };
 
+export type UseCreateEntryInput = $ReadOnly<
+  | {
+      +thick: false,
+      +createEntryInfo: CreateEntryInfo,
+    }
+  | {
+      +thick: true,
+      +threadInfo: ThreadInfo,
+      +createEntryInfo: CreateEntryInfo,
+    },
+>;
 function useCreateEntry(): (
-  request: CreateEntryInfo,
+  input: UseCreateEntryInput,
 ) => Promise<CreateEntryPayload> {
-  return useKeyserverCall(createEntry);
+  const viewerID = useSelector(
+    state => state.currentUserInfo && state.currentUserInfo.id,
+  );
+
+  const processAndSendDMOperation = useProcessAndSendDMOperation();
+  const keyserverCall = useKeyserverCall(createEntry);
+
+  return React.useCallback(
+    async (input: UseCreateEntryInput) => {
+      if (!input.thick) {
+        const result = await keyserverCall(input.createEntryInfo);
+        return result;
+      }
+
+      invariant(viewerID, 'viewerID must be set');
+      const entryID = uuid.v4();
+
+      const { createEntryInfo, threadInfo } = input;
+      const op: DMCreateEntryOperation = {
+        type: 'create_entry',
+        threadID: threadInfo.id,
+        creatorID: viewerID,
+        time: createEntryInfo.timestamp,
+        entryID: uuid.v4(),
+        entryDate: createEntryInfo.date,
+        text: createEntryInfo.text,
+        messageID: uuid.v4(),
+      };
+      const opSpecification: OutboundDMOperationSpecification = {
+        type: dmOperationSpecificationTypes.OUTBOUND,
+        op,
+        recipients: {
+          type: 'all_thread_members',
+          threadID:
+            threadInfo.type === thickThreadTypes.THICK_SIDEBAR &&
+            threadInfo.parentThreadID
+              ? threadInfo.parentThreadID
+              : threadInfo.id,
+        },
+      };
+
+      await processAndSendDMOperation(opSpecification);
+
+      return {
+        entryID,
+        newMessageInfos: [],
+        threadID: createEntryInfo.threadID,
+        localID: createEntryInfo.localID,
+        updatesResult: {
+          viewerUpdates: [],
+          userInfos: [],
+        },
+      };
+    },
+    [keyserverCall, processAndSendDMOperation, viewerID],
+  );
 }
 
 const saveEntryActionTypes = Object.freeze({
diff --git a/native/calendar/entry.react.js b/native/calendar/entry.react.js
--- a/native/calendar/entry.react.js
+++ b/native/calendar/entry.react.js
@@ -25,6 +25,7 @@
   useCreateEntry,
   useDeleteEntry,
   useSaveEntry,
+  type UseCreateEntryInput,
 } from 'lib/actions/entry-actions.js';
 import { extractKeyserverIDFromIDOptional } from 'lib/keyserver-conn/keyserver-call-utils.js';
 import { registerFetchKey } from 'lib/reducers/loading-reducer.js';
@@ -34,7 +35,6 @@
 import { useThreadHasPermission } from 'lib/shared/thread-utils.js';
 import type {
   CalendarQuery,
-  CreateEntryInfo,
   CreateEntryPayload,
   DeleteEntryInfo,
   DeleteEntryResult,
@@ -49,6 +49,7 @@
 } from 'lib/types/minimally-encoded-thread-permissions-types.js';
 import type { Dispatch } from 'lib/types/redux-types.js';
 import { threadPermissions } from 'lib/types/thread-permission-types.js';
+import { threadTypeIsThick } from 'lib/types/thread-types-enum.js';
 import { dateString } from 'lib/utils/date-utils.js';
 import { useResolvedThreadInfo } from 'lib/utils/entity-helpers.js';
 import { ServerError } from 'lib/utils/errors.js';
@@ -206,7 +207,7 @@
   +dispatch: Dispatch,
   +dispatchActionPromise: DispatchActionPromise,
   // async functions that hit server APIs
-  +createEntry: (info: CreateEntryInfo) => Promise<CreateEntryPayload>,
+  +createEntry: (input: UseCreateEntryInput) => Promise<CreateEntryPayload>,
   +saveEntry: (info: SaveEntryInfo) => Promise<SaveEntryResult>,
   +deleteEntry: (info: DeleteEntryInfo) => Promise<DeleteEntryResult>,
   +canEditEntry: boolean,
@@ -637,7 +638,7 @@
     invariant(localID, "if there's no serverID, there should be a localID");
     const curSaveAttempt = this.nextSaveAttemptIndex++;
     try {
-      const response = await this.props.createEntry({
+      const createEntryInfo = {
         text,
         timestamp: this.props.entryInfo.creationTime,
         date: dateString(
@@ -648,7 +649,21 @@
         threadID: this.props.entryInfo.threadID,
         localID,
         calendarQuery: this.props.calendarQuery(),
-      });
+      };
+
+      const useCreateEntryInput = threadTypeIsThick(this.props.threadInfo.type)
+        ? {
+            thick: true,
+            threadInfo: this.props.threadInfo,
+            createEntryInfo,
+          }
+        : {
+            thick: false,
+            createEntryInfo,
+          };
+
+      const response = await this.props.createEntry(useCreateEntryInput);
+
       if (curSaveAttempt + 1 === this.nextSaveAttemptIndex) {
         this.guardedSetState({ loadingStatus: 'inactive' });
       }
diff --git a/web/calendar/entry.react.js b/web/calendar/entry.react.js
--- a/web/calendar/entry.react.js
+++ b/web/calendar/entry.react.js
@@ -12,6 +12,7 @@
   deleteEntryActionTypes,
   useDeleteEntry,
   concurrentModificationResetActionType,
+  type UseCreateEntryInput,
 } from 'lib/actions/entry-actions.js';
 import {
   type PushModal,
@@ -25,7 +26,6 @@
 import { useThreadHasPermission } from 'lib/shared/thread-utils.js';
 import {
   type EntryInfo,
-  type CreateEntryInfo,
   type SaveEntryInfo,
   type SaveEntryResult,
   type SaveEntryPayload,
@@ -38,6 +38,7 @@
 import type { ResolvedThreadInfo } from 'lib/types/minimally-encoded-thread-permissions-types.js';
 import type { Dispatch } from 'lib/types/redux-types.js';
 import { threadPermissions } from 'lib/types/thread-permission-types.js';
+import { threadTypeIsThick } from 'lib/types/thread-types-enum.js';
 import { dateString } from 'lib/utils/date-utils.js';
 import { useResolvedThreadInfo } from 'lib/utils/entity-helpers.js';
 import { ServerError } from 'lib/utils/errors.js';
@@ -71,7 +72,7 @@
   +online: boolean,
   +dispatch: Dispatch,
   +dispatchActionPromise: DispatchActionPromise,
-  +createEntry: (info: CreateEntryInfo) => Promise<CreateEntryPayload>,
+  +createEntry: (input: UseCreateEntryInput) => Promise<CreateEntryPayload>,
   +saveEntry: (info: SaveEntryInfo) => Promise<SaveEntryResult>,
   +deleteEntry: (info: DeleteEntryInfo) => Promise<DeleteEntryResult>,
   +pushModal: PushModal,
@@ -322,7 +323,7 @@
     const curSaveAttempt = this.nextSaveAttemptIndex++;
     this.guardedSetState({ loadingStatus: 'loading' });
     try {
-      const response = await this.props.createEntry({
+      const createEntryInfo = {
         text,
         timestamp: this.props.entryInfo.creationTime,
         date: dateString(
@@ -333,7 +334,20 @@
         threadID: this.props.entryInfo.threadID,
         localID,
         calendarQuery: this.props.calendarQuery(),
-      });
+      };
+
+      const useCreateEntryInput = threadTypeIsThick(this.props.threadInfo.type)
+        ? {
+            thick: true,
+            threadInfo: this.props.threadInfo,
+            createEntryInfo,
+          }
+        : {
+            thick: false,
+            createEntryInfo,
+          };
+
+      const response = await this.props.createEntry(useCreateEntryInput);
       if (curSaveAttempt + 1 === this.nextSaveAttemptIndex) {
         this.guardedSetState({ loadingStatus: 'inactive' });
       }