diff --git a/lib/reducers/entry-reducer.js b/lib/reducers/entry-reducer.js
index 1765647f0..ac23e649a 100644
--- a/lib/reducers/entry-reducer.js
+++ b/lib/reducers/entry-reducer.js
@@ -1,728 +1,719 @@
 // @flow
 
 import invariant from 'invariant';
 import _filter from 'lodash/fp/filter.js';
 import _flow from 'lodash/fp/flow.js';
 import _groupBy from 'lodash/fp/groupBy.js';
 import _isEqual from 'lodash/fp/isEqual.js';
 import _map from 'lodash/fp/map.js';
 import _mapKeys from 'lodash/fp/mapKeys.js';
 import _mapValues from 'lodash/fp/mapValues.js';
 import _omitBy from 'lodash/fp/omitBy.js';
 import _pickBy from 'lodash/fp/pickBy.js';
 import _sortBy from 'lodash/fp/sortBy.js';
 import _union from 'lodash/fp/union.js';
 
 import {
   fetchEntriesActionTypes,
   updateCalendarQueryActionTypes,
   createLocalEntryActionType,
   createEntryActionTypes,
   saveEntryActionTypes,
   concurrentModificationResetActionType,
   deleteEntryActionTypes,
   fetchRevisionsForEntryActionTypes,
   restoreEntryActionTypes,
 } from '../actions/entry-actions.js';
 import { siweAuthActionTypes } from '../actions/siwe-actions.js';
 import {
   deleteThreadActionTypes,
   leaveThreadActionTypes,
   joinThreadActionTypes,
   changeThreadSettingsActionTypes,
   removeUsersFromThreadActionTypes,
   changeThreadMemberRolesActionTypes,
   newThreadActionTypes,
 } from '../actions/thread-actions.js';
 import {
   logOutActionTypes,
   deleteAccountActionTypes,
   logInActionTypes,
 } from '../actions/user-actions.js';
 import {
   entryID,
   filterRawEntryInfosByCalendarQuery,
   serverEntryInfosObject,
 } from '../shared/entry-utils.js';
 import { threadInFilterList } from '../shared/thread-utils.js';
+import { updateSpecs } from '../shared/updates/update-specs.js';
 import type {
   RawEntryInfo,
   EntryStore,
   CalendarQuery,
 } from '../types/entry-types.js';
 import type { BaseAction } from '../types/redux-types.js';
 import {
   type ClientEntryInconsistencyReportCreationRequest,
   reportTypes,
 } from '../types/report-types.js';
 import {
   serverRequestTypes,
   processServerRequestsActionType,
 } from '../types/request-types.js';
 import {
   fullStateSyncActionType,
   incrementalStateSyncActionType,
 } from '../types/socket-types.js';
 import { type RawThreadInfo } from '../types/thread-types.js';
-import { updateTypes } from '../types/update-types-enum.js';
 import {
   type ClientUpdateInfo,
   processUpdatesActionType,
 } from '../types/update-types.js';
 import { actionLogger } from '../utils/action-logger.js';
 import { setNewSessionActionType } from '../utils/action-utils.js';
 import { getConfig } from '../utils/config.js';
 import { dateString } from '../utils/date-utils.js';
 import { values } from '../utils/objects.js';
 import { generateReportID } from '../utils/report-utils.js';
 import { sanitizeActionSecrets } from '../utils/sanitization.js';
 
 function daysToEntriesFromEntryInfos(
   entryInfos: $ReadOnlyArray<RawEntryInfo>,
 ): { [day: string]: string[] } {
   return _flow(
     _sortBy((['id', 'localID']: $ReadOnlyArray<string>)),
     _groupBy((entryInfo: RawEntryInfo) =>
       dateString(entryInfo.year, entryInfo.month, entryInfo.day),
     ),
     _mapValues((entryInfoGroup: $ReadOnlyArray<RawEntryInfo>) =>
       _map(entryID)(entryInfoGroup),
     ),
   )([...entryInfos]);
 }
 
 function filterExistingDaysToEntriesWithNewEntryInfos(
   oldDaysToEntries: { +[id: string]: string[] },
   newEntryInfos: { +[id: string]: RawEntryInfo },
 ) {
   return _mapValues((entryIDs: string[]) =>
     _filter((id: string) => newEntryInfos[id])(entryIDs),
   )(oldDaysToEntries);
 }
 
 function mergeNewEntryInfos(
   currentEntryInfos: { +[id: string]: RawEntryInfo },
   currentDaysToEntries: ?{ +[day: string]: string[] },
   newEntryInfos: $ReadOnlyArray<RawEntryInfo>,
   threadInfos: { +[id: string]: RawThreadInfo },
 ) {
   const mergedEntryInfos = {};
   let someEntryUpdated = false;
 
   for (const rawEntryInfo of newEntryInfos) {
     const serverID = rawEntryInfo.id;
     invariant(serverID, 'new entryInfos should have serverID');
     const currentEntryInfo = currentEntryInfos[serverID];
     let newEntryInfo;
     if (currentEntryInfo && currentEntryInfo.localID) {
       newEntryInfo = {
         id: serverID,
         // Try to preserve localIDs. This is because we use them as React
         // keys and changing React keys leads to loss of component state.
         localID: currentEntryInfo.localID,
         threadID: rawEntryInfo.threadID,
         text: rawEntryInfo.text,
         year: rawEntryInfo.year,
         month: rawEntryInfo.month,
         day: rawEntryInfo.day,
         creationTime: rawEntryInfo.creationTime,
         creatorID: rawEntryInfo.creatorID,
         deleted: rawEntryInfo.deleted,
       };
     } else {
       newEntryInfo = {
         id: serverID,
         threadID: rawEntryInfo.threadID,
         text: rawEntryInfo.text,
         year: rawEntryInfo.year,
         month: rawEntryInfo.month,
         day: rawEntryInfo.day,
         creationTime: rawEntryInfo.creationTime,
         creatorID: rawEntryInfo.creatorID,
         deleted: rawEntryInfo.deleted,
       };
     }
 
     if (_isEqual(currentEntryInfo)(newEntryInfo)) {
       mergedEntryInfos[serverID] = currentEntryInfo;
     } else {
       mergedEntryInfos[serverID] = newEntryInfo;
       someEntryUpdated = true;
     }
   }
 
   for (const id in currentEntryInfos) {
     const newEntryInfo = mergedEntryInfos[id];
     if (!newEntryInfo) {
       mergedEntryInfos[id] = currentEntryInfos[id];
     }
   }
 
   for (const id in mergedEntryInfos) {
     const entryInfo = mergedEntryInfos[id];
     if (!threadInFilterList(threadInfos[entryInfo.threadID])) {
       someEntryUpdated = true;
       delete mergedEntryInfos[id];
     }
   }
 
   const daysToEntries =
     !currentDaysToEntries || someEntryUpdated
       ? daysToEntriesFromEntryInfos(values(mergedEntryInfos))
       : currentDaysToEntries;
   const entryInfos = someEntryUpdated ? mergedEntryInfos : currentEntryInfos;
   return [entryInfos, daysToEntries];
 }
 
 function reduceEntryInfos(
   entryStore: EntryStore,
   action: BaseAction,
   newThreadInfos: { +[id: string]: RawThreadInfo },
 ): [EntryStore, $ReadOnlyArray<ClientEntryInconsistencyReportCreationRequest>] {
   const { entryInfos, daysToEntries, lastUserInteractionCalendar } = entryStore;
   if (
     action.type === logOutActionTypes.success ||
     action.type === deleteAccountActionTypes.success ||
     action.type === deleteThreadActionTypes.success ||
     action.type === leaveThreadActionTypes.success
   ) {
     const authorizedThreadInfos = _pickBy(threadInFilterList)(newThreadInfos);
     const newEntryInfos = _pickBy(
       (entry: RawEntryInfo) => authorizedThreadInfos[entry.threadID],
     )(entryInfos);
     const newLastUserInteractionCalendar =
       action.type === logOutActionTypes.success ||
       action.type === deleteAccountActionTypes.success
         ? 0
         : lastUserInteractionCalendar;
     if (Object.keys(newEntryInfos).length === Object.keys(entryInfos).length) {
       return [
         {
           entryInfos,
           daysToEntries,
           lastUserInteractionCalendar: newLastUserInteractionCalendar,
         },
         [],
       ];
     }
     const newDaysToEntries = filterExistingDaysToEntriesWithNewEntryInfos(
       daysToEntries,
       newEntryInfos,
     );
     return [
       {
         entryInfos: newEntryInfos,
         daysToEntries: newDaysToEntries,
         lastUserInteractionCalendar: newLastUserInteractionCalendar,
       },
       [],
     ];
   } else if (action.type === setNewSessionActionType) {
     const authorizedThreadInfos = _pickBy(threadInFilterList)(newThreadInfos);
     const newEntryInfos = _pickBy(
       (entry: RawEntryInfo) => authorizedThreadInfos[entry.threadID],
     )(entryInfos);
     const newDaysToEntries = filterExistingDaysToEntriesWithNewEntryInfos(
       daysToEntries,
       newEntryInfos,
     );
     const newLastUserInteractionCalendar = action.payload.sessionChange
       .cookieInvalidated
       ? 0
       : lastUserInteractionCalendar;
     if (Object.keys(newEntryInfos).length === Object.keys(entryInfos).length) {
       return [
         {
           entryInfos,
           daysToEntries,
           lastUserInteractionCalendar: newLastUserInteractionCalendar,
         },
         [],
       ];
     }
     return [
       {
         entryInfos: newEntryInfos,
         daysToEntries: newDaysToEntries,
         lastUserInteractionCalendar: newLastUserInteractionCalendar,
       },
       [],
     ];
   } else if (action.type === fetchEntriesActionTypes.success) {
     const [updatedEntryInfos, updatedDaysToEntries] = mergeNewEntryInfos(
       entryInfos,
       daysToEntries,
       action.payload.rawEntryInfos,
       newThreadInfos,
     );
     return [
       {
         entryInfos: updatedEntryInfos,
         daysToEntries: updatedDaysToEntries,
         lastUserInteractionCalendar,
       },
       [],
     ];
   } else if (
     action.type === updateCalendarQueryActionTypes.started &&
     action.payload &&
     action.payload.calendarQuery
   ) {
     return [
       {
         entryInfos,
         daysToEntries,
         lastUserInteractionCalendar: Date.now(),
       },
       [],
     ];
   } else if (action.type === updateCalendarQueryActionTypes.success) {
     const newLastUserInteractionCalendar = action.payload.calendarQuery
       ? Date.now()
       : lastUserInteractionCalendar;
     const [updatedEntryInfos, updatedDaysToEntries] = mergeNewEntryInfos(
       entryInfos,
       daysToEntries,
       action.payload.rawEntryInfos,
       newThreadInfos,
     );
     const deletionMarkedEntryInfos = markDeletedEntries(
       updatedEntryInfos,
       action.payload.deletedEntryIDs,
     );
     return [
       {
         entryInfos: deletionMarkedEntryInfos,
         daysToEntries: updatedDaysToEntries,
         lastUserInteractionCalendar: newLastUserInteractionCalendar,
       },
       [],
     ];
   } else if (action.type === createLocalEntryActionType) {
     const entryInfo = action.payload;
     const localID = entryInfo.localID;
     invariant(localID, 'localID should be set in CREATE_LOCAL_ENTRY');
     const newEntryInfos = {
       ...entryInfos,
       [localID]: entryInfo,
     };
     const dayString = dateString(
       entryInfo.year,
       entryInfo.month,
       entryInfo.day,
     );
     const newDaysToEntries = {
       ...daysToEntries,
       [dayString]: _union([localID])(daysToEntries[dayString]),
     };
     return [
       {
         entryInfos: newEntryInfos,
         daysToEntries: newDaysToEntries,
         lastUserInteractionCalendar: Date.now(),
       },
       [],
     ];
   } else if (action.type === createEntryActionTypes.success) {
     const localID = action.payload.localID;
     const serverID = action.payload.entryID;
     // If an entry with this serverID already got into the store somehow
     // (likely through an unrelated request), we need to dedup them.
     let rekeyedEntryInfos;
     if (entryInfos[serverID]) {
       // It's fair to assume the serverID entry is newer than the localID
       // entry, and this probably won't happen often, so for now we can just
       // keep the serverID entry.
       rekeyedEntryInfos = _omitBy(
         (candidate: RawEntryInfo) =>
           !candidate.id && candidate.localID === localID,
       )(entryInfos);
     } else if (entryInfos[localID]) {
       rekeyedEntryInfos = _mapKeys((oldKey: string) =>
         entryInfos[oldKey].localID === localID ? serverID : oldKey,
       )(entryInfos);
     } else {
       // This happens if the entry is deauthorized before it's saved
       return [entryStore, []];
     }
 
     const [updatedEntryInfos, updatedDaysToEntries] = mergeNewEntryInfos(
       rekeyedEntryInfos,
       null,
       mergeUpdateEntryInfos([], action.payload.updatesResult.viewerUpdates),
       newThreadInfos,
     );
 
     return [
       {
         entryInfos: updatedEntryInfos,
         daysToEntries: updatedDaysToEntries,
         lastUserInteractionCalendar: Date.now(),
       },
       [],
     ];
   } else if (action.type === saveEntryActionTypes.success) {
     const serverID = action.payload.entryID;
     if (
       !entryInfos[serverID] ||
       !threadInFilterList(newThreadInfos[entryInfos[serverID].threadID])
     ) {
       // This happens if the entry is deauthorized before it's saved
       return [entryStore, []];
     }
 
     const [updatedEntryInfos, updatedDaysToEntries] = mergeNewEntryInfos(
       entryInfos,
       daysToEntries,
       mergeUpdateEntryInfos([], action.payload.updatesResult.viewerUpdates),
       newThreadInfos,
     );
 
     return [
       {
         entryInfos: updatedEntryInfos,
         daysToEntries: updatedDaysToEntries,
         lastUserInteractionCalendar: Date.now(),
       },
       [],
     ];
   } else if (action.type === concurrentModificationResetActionType) {
     const { payload } = action;
     if (
       !entryInfos[payload.id] ||
       !threadInFilterList(newThreadInfos[entryInfos[payload.id].threadID])
     ) {
       // This happens if the entry is deauthorized before it's restored
       return [entryStore, []];
     }
     const newEntryInfos = {
       ...entryInfos,
       [payload.id]: {
         ...entryInfos[payload.id],
         text: payload.dbText,
       },
     };
     return [
       {
         entryInfos: newEntryInfos,
         daysToEntries,
         lastUserInteractionCalendar,
       },
       [],
     ];
   } else if (action.type === deleteEntryActionTypes.started) {
     const payload = action.payload;
     const id =
       payload.serverID && entryInfos[payload.serverID]
         ? payload.serverID
         : payload.localID;
     invariant(id, 'either serverID or localID should be set');
     const newEntryInfos = {
       ...entryInfos,
       [id]: {
         ...entryInfos[id],
         deleted: true,
       },
     };
     return [
       {
         entryInfos: newEntryInfos,
         daysToEntries,
         lastUserInteractionCalendar: Date.now(),
       },
       [],
     ];
   } else if (action.type === deleteEntryActionTypes.success && action.payload) {
     const { payload } = action;
     const [updatedEntryInfos, updatedDaysToEntries] = mergeNewEntryInfos(
       entryInfos,
       daysToEntries,
       mergeUpdateEntryInfos([], payload.updatesResult.viewerUpdates),
       newThreadInfos,
     );
     return [
       {
         entryInfos: updatedEntryInfos,
         daysToEntries: updatedDaysToEntries,
         lastUserInteractionCalendar,
       },
       [],
     ];
   } else if (action.type === fetchRevisionsForEntryActionTypes.success) {
     const id = action.payload.entryID;
     if (
       !entryInfos[id] ||
       !threadInFilterList(newThreadInfos[entryInfos[id].threadID])
     ) {
       // This happens if the entry is deauthorized before it's restored
       return [entryStore, []];
     }
     // Make sure the entry is in sync with its latest revision
     const newEntryInfos = {
       ...entryInfos,
       [id]: {
         ...entryInfos[id],
         text: action.payload.text,
         deleted: action.payload.deleted,
       },
     };
     return [
       {
         entryInfos: newEntryInfos,
         daysToEntries,
         lastUserInteractionCalendar,
       },
       [],
     ];
   } else if (action.type === restoreEntryActionTypes.success) {
     const [updatedEntryInfos, updatedDaysToEntries] = mergeNewEntryInfos(
       entryInfos,
       daysToEntries,
       mergeUpdateEntryInfos([], action.payload.updatesResult.viewerUpdates),
       newThreadInfos,
     );
     return [
       {
         entryInfos: updatedEntryInfos,
         daysToEntries: updatedDaysToEntries,
         lastUserInteractionCalendar: Date.now(),
       },
       [],
     ];
   } else if (
     action.type === logInActionTypes.success ||
     action.type === siweAuthActionTypes.success
   ) {
     const { calendarResult } = action.payload;
     if (calendarResult) {
       const [updatedEntryInfos, updatedDaysToEntries] = mergeNewEntryInfos(
         entryInfos,
         daysToEntries,
         calendarResult.rawEntryInfos,
         newThreadInfos,
       );
       return [
         {
           entryInfos: updatedEntryInfos,
           daysToEntries: updatedDaysToEntries,
           lastUserInteractionCalendar,
         },
         [],
       ];
     }
   } else if (action.type === incrementalStateSyncActionType) {
     const [updatedEntryInfos, updatedDaysToEntries] = mergeNewEntryInfos(
       entryInfos,
       daysToEntries,
       mergeUpdateEntryInfos(
         action.payload.deltaEntryInfos,
         action.payload.updatesResult.newUpdates,
       ),
       newThreadInfos,
     );
     const deletionMarkedEntryInfos = markDeletedEntries(
       updatedEntryInfos,
       action.payload.deletedEntryIDs,
     );
     return [
       {
         entryInfos: deletionMarkedEntryInfos,
         daysToEntries: updatedDaysToEntries,
         lastUserInteractionCalendar,
       },
       [],
     ];
   } else if (
     action.type === processUpdatesActionType ||
     action.type === joinThreadActionTypes.success ||
     action.type === newThreadActionTypes.success
   ) {
     const [updatedEntryInfos, updatedDaysToEntries] = mergeNewEntryInfos(
       entryInfos,
       daysToEntries,
       mergeUpdateEntryInfos([], action.payload.updatesResult.newUpdates),
       newThreadInfos,
     );
     return [
       {
         entryInfos: updatedEntryInfos,
         daysToEntries: updatedDaysToEntries,
         lastUserInteractionCalendar,
       },
       [],
     ];
   } else if (action.type === fullStateSyncActionType) {
     const [updatedEntryInfos, updatedDaysToEntries] = mergeNewEntryInfos(
       entryInfos,
       daysToEntries,
       action.payload.rawEntryInfos,
       newThreadInfos,
     );
     return [
       {
         entryInfos: updatedEntryInfos,
         daysToEntries: updatedDaysToEntries,
         lastUserInteractionCalendar,
       },
       [],
     ];
   } else if (
     action.type === changeThreadSettingsActionTypes.success ||
     action.type === removeUsersFromThreadActionTypes.success ||
     action.type === changeThreadMemberRolesActionTypes.success
   ) {
     const authorizedThreadInfos = _pickBy(threadInFilterList)(newThreadInfos);
     const newEntryInfos = _pickBy(
       (entry: RawEntryInfo) => authorizedThreadInfos[entry.threadID],
     )(entryInfos);
     if (Object.keys(newEntryInfos).length === Object.keys(entryInfos).length) {
       return [entryStore, []];
     }
     const newDaysToEntries = filterExistingDaysToEntriesWithNewEntryInfos(
       daysToEntries,
       newEntryInfos,
     );
     return [
       {
         entryInfos: newEntryInfos,
         daysToEntries: newDaysToEntries,
         lastUserInteractionCalendar,
       },
       [],
     ];
   } else if (action.type === processServerRequestsActionType) {
     const checkStateRequest = action.payload.serverRequests.find(
       candidate => candidate.type === serverRequestTypes.CHECK_STATE,
     );
     if (!checkStateRequest || !checkStateRequest.stateChanges) {
       return [entryStore, []];
     }
     const { rawEntryInfos, deleteEntryIDs } = checkStateRequest.stateChanges;
     if (!rawEntryInfos && !deleteEntryIDs) {
       return [entryStore, []];
     }
 
     let updatedEntryInfos = { ...entryInfos };
     if (deleteEntryIDs) {
       for (const deleteEntryID of deleteEntryIDs) {
         delete updatedEntryInfos[deleteEntryID];
       }
     }
 
     let updatedDaysToEntries;
     if (rawEntryInfos) {
       [updatedEntryInfos, updatedDaysToEntries] = mergeNewEntryInfos(
         updatedEntryInfos,
         null,
         rawEntryInfos,
         newThreadInfos,
       );
     } else {
       updatedDaysToEntries = daysToEntriesFromEntryInfos(
         values(updatedEntryInfos),
       );
     }
 
     const newInconsistencies = findInconsistencies(
       action,
       entryInfos,
       updatedEntryInfos,
       action.payload.calendarQuery,
     );
 
     return [
       {
         entryInfos: updatedEntryInfos,
         daysToEntries: updatedDaysToEntries,
         lastUserInteractionCalendar,
       },
       newInconsistencies,
     ];
   }
   return [entryStore, []];
 }
 
 function mergeUpdateEntryInfos(
   entryInfos: $ReadOnlyArray<RawEntryInfo>,
   newUpdates: $ReadOnlyArray<ClientUpdateInfo>,
 ): RawEntryInfo[] {
-  const entryIDs = new Set(entryInfos.map(entryInfo => entryInfo.id));
+  const entryIDs = new Set(
+    entryInfos.map(entryInfo => entryInfo.id).filter(Boolean),
+  );
   const mergedEntryInfos = [...entryInfos];
   for (const updateInfo of newUpdates) {
-    if (updateInfo.type === updateTypes.JOIN_THREAD) {
-      for (const entryInfo of updateInfo.rawEntryInfos) {
-        if (entryIDs.has(entryInfo.id)) {
-          continue;
-        }
-        mergedEntryInfos.push(entryInfo);
-        entryIDs.add(entryInfo.id);
-      }
-    } else if (updateInfo.type === updateTypes.UPDATE_ENTRY) {
-      const { entryInfo } = updateInfo;
-      if (entryIDs.has(entryInfo.id)) {
-        continue;
-      }
-      mergedEntryInfos.push(entryInfo);
-      entryIDs.add(entryInfo.id);
-    }
+    updateSpecs[updateInfo.type].mergeEntryInfos?.(
+      entryIDs,
+      mergedEntryInfos,
+      updateInfo,
+    );
   }
   return mergedEntryInfos;
 }
 
 const emptyArray = [];
 function findInconsistencies(
   action: BaseAction,
   beforeStateCheck: { +[id: string]: RawEntryInfo },
   afterStateCheck: { +[id: string]: RawEntryInfo },
   calendarQuery: CalendarQuery,
 ): ClientEntryInconsistencyReportCreationRequest[] {
   // We don't want to bother reporting an inconsistency if it's just because of
   // extraneous EntryInfos (not within the current calendarQuery) on either side
   const filteredBeforeResult = filterRawEntryInfosByCalendarQuery(
     serverEntryInfosObject(values(beforeStateCheck)),
     calendarQuery,
   );
   const filteredAfterResult = filterRawEntryInfosByCalendarQuery(
     serverEntryInfosObject(values(afterStateCheck)),
     calendarQuery,
   );
   if (_isEqual(filteredBeforeResult)(filteredAfterResult)) {
     return emptyArray;
   }
   return [
     {
       type: reportTypes.ENTRY_INCONSISTENCY,
       platformDetails: getConfig().platformDetails,
       beforeAction: beforeStateCheck,
       action: sanitizeActionSecrets(action),
       calendarQuery,
       pushResult: afterStateCheck,
       lastActions: actionLogger.interestingActionSummaries,
       time: Date.now(),
       id: generateReportID(),
     },
   ];
 }
 
 function markDeletedEntries(
   entryInfos: { +[id: string]: RawEntryInfo },
   deletedEntryIDs: $ReadOnlyArray<string>,
 ): { +[id: string]: RawEntryInfo } {
   let result = entryInfos;
   for (const deletedEntryID of deletedEntryIDs) {
     const entryInfo = entryInfos[deletedEntryID];
     if (!entryInfo || entryInfo.deleted) {
       continue;
     }
     result = {
       ...result,
       [deletedEntryID]: {
         ...entryInfo,
         deleted: true,
       },
     };
   }
   return result;
 }
 
 export { daysToEntriesFromEntryInfos, reduceEntryInfos };
diff --git a/lib/shared/updates/join-thread-spec.js b/lib/shared/updates/join-thread-spec.js
index 692412f71..37d842227 100644
--- a/lib/shared/updates/join-thread-spec.js
+++ b/lib/shared/updates/join-thread-spec.js
@@ -1,27 +1,42 @@
 // @flow
 
 import _isEqual from 'lodash/fp/isEqual.js';
 
 import type { UpdateSpec } from './update-spec.js';
+import type { RawEntryInfo } from '../../types/entry-types.js';
 import type { RawThreadInfos } from '../../types/thread-types.js';
 import type { ThreadJoinUpdateInfo } from '../../types/update-types.js';
 
 export const joinThreadSpec: UpdateSpec<ThreadJoinUpdateInfo> = Object.freeze({
   generateOpsForThreadUpdates(
     storeThreadInfos: RawThreadInfos,
     update: ThreadJoinUpdateInfo,
   ) {
     if (_isEqual(storeThreadInfos[update.threadInfo.id])(update.threadInfo)) {
       return null;
     }
     return [
       {
         type: 'replace',
         payload: {
           id: update.threadInfo.id,
           threadInfo: update.threadInfo,
         },
       },
     ];
   },
+  mergeEntryInfos(
+    entryIDs: Set<string>,
+    mergedEntryInfos: Array<RawEntryInfo>,
+    update: ThreadJoinUpdateInfo,
+  ) {
+    for (const entryInfo of update.rawEntryInfos) {
+      const entryID = entryInfo.id;
+      if (!entryID || entryIDs.has(entryID)) {
+        continue;
+      }
+      mergedEntryInfos.push(entryInfo);
+      entryIDs.add(entryID);
+    }
+  },
 });
diff --git a/lib/shared/updates/update-entry-spec.js b/lib/shared/updates/update-entry-spec.js
index c19ef485b..c76c2c4d4 100644
--- a/lib/shared/updates/update-entry-spec.js
+++ b/lib/shared/updates/update-entry-spec.js
@@ -1,6 +1,21 @@
 // @flow
 
 import type { UpdateSpec } from './update-spec.js';
+import type { RawEntryInfo } from '../../types/entry-types.js';
 import type { EntryUpdateInfo } from '../../types/update-types.js';
 
-export const updateEntrySpec: UpdateSpec<EntryUpdateInfo> = Object.freeze({});
+export const updateEntrySpec: UpdateSpec<EntryUpdateInfo> = Object.freeze({
+  mergeEntryInfos(
+    entryIDs: Set<string>,
+    mergedEntryInfos: Array<RawEntryInfo>,
+    update: EntryUpdateInfo,
+  ) {
+    const { entryInfo } = update;
+    const entryID = entryInfo.id;
+    if (!entryID || entryIDs.has(entryID)) {
+      return;
+    }
+    mergedEntryInfos.push(entryInfo);
+    entryIDs.add(entryID);
+  },
+});
diff --git a/lib/shared/updates/update-spec.js b/lib/shared/updates/update-spec.js
index 0ad06f093..1b29a5d16 100644
--- a/lib/shared/updates/update-spec.js
+++ b/lib/shared/updates/update-spec.js
@@ -1,12 +1,18 @@
 // @flow
 
 import type { ThreadStoreOperation } from '../../ops/thread-store-ops.js';
+import type { RawEntryInfo } from '../../types/entry-types.js';
 import type { RawThreadInfos } from '../../types/thread-types.js';
 import type { ClientUpdateInfo } from '../../types/update-types.js';
 
 export type UpdateSpec<UpdateInfo: ClientUpdateInfo> = {
   +generateOpsForThreadUpdates?: (
     storeThreadInfos: RawThreadInfos,
     update: UpdateInfo,
   ) => ?$ReadOnlyArray<ThreadStoreOperation>,
+  +mergeEntryInfos?: (
+    entryIDs: Set<string>,
+    mergedEntryInfos: Array<RawEntryInfo>,
+    update: UpdateInfo,
+  ) => void,
 };