diff --git a/lib/actions/activity-actions.js b/lib/actions/activity-actions.js
--- a/lib/actions/activity-actions.js
+++ b/lib/actions/activity-actions.js
@@ -25,7 +25,7 @@
   ): ((input: UpdateActivityInput) => Promise<ActivityUpdateSuccessPayload>) =>
   async input => {
     const { activityUpdates } = input;
-    const requests = {};
+    const requests: { [string]: { +updates: ActivityUpdate[] } } = {};
     for (const update of activityUpdates) {
       const keyserverID = extractKeyserverIDFromID(update.threadID);
       if (!requests[keyserverID]) {
@@ -36,7 +36,7 @@
 
     const responses = await callKeyserverEndpoint('update_activity', requests);
 
-    let unfocusedToUnread = [];
+    let unfocusedToUnread: $ReadOnlyArray<string> = [];
     for (const keyserverID in responses) {
       unfocusedToUnread = unfocusedToUnread.concat(
         responses[keyserverID].unfocusedToUnread,
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
@@ -39,13 +39,13 @@
       calendarQuery,
       allKeyserverIDs,
     );
-    const requests = {};
+    const requests: { [string]: CalendarQuery } = {};
     for (const keyserverID of allKeyserverIDs) {
       requests[keyserverID] = calendarQueries[keyserverID];
     }
 
     const responses = await callKeyserverEndpoint('fetch_entries', requests);
-    let rawEntryInfos = [];
+    let rawEntryInfos: $ReadOnlyArray<RawEntryInfo> = [];
     for (const keyserverID in responses) {
       rawEntryInfos = rawEntryInfos.concat(
         responses[keyserverID].rawEntryInfos,
@@ -87,7 +87,7 @@
       allKeyserverIDs,
     );
 
-    const requests = {};
+    const requests: { [string]: CalendarQuery } = {};
     for (const keyserverID of allKeyserverIDs) {
       requests[keyserverID] = calendarQueries[keyserverID];
     }
@@ -96,8 +96,8 @@
       'update_calendar_query',
       requests,
     );
-    let rawEntryInfos = [];
-    let deletedEntryIDs = [];
+    let rawEntryInfos: $ReadOnlyArray<RawEntryInfo> = [];
+    let deletedEntryIDs: $ReadOnlyArray<string> = [];
     for (const keyserverID in responses) {
       rawEntryInfos = rawEntryInfos.concat(
         responses[keyserverID].rawEntryInfos,
diff --git a/lib/actions/link-actions.js b/lib/actions/link-actions.js
--- a/lib/actions/link-actions.js
+++ b/lib/actions/link-actions.js
@@ -49,7 +49,7 @@
     allKeyserverIDs: $ReadOnlyArray<string>,
   ): (() => Promise<FetchInviteLinksResponse>) =>
   async () => {
-    const requests = {};
+    const requests: { [string]: void } = {};
     for (const keyserverID of allKeyserverIDs) {
       requests[keyserverID] = undefined;
     }
@@ -57,7 +57,7 @@
       'fetch_primary_invite_links',
       requests,
     );
-    let links = [];
+    let links: $ReadOnlyArray<InviteLink> = [];
     for (const keyserverID in responses) {
       links = links.concat(responses[keyserverID].links);
     }
diff --git a/lib/actions/message-actions.js b/lib/actions/message-actions.js
--- a/lib/actions/message-actions.js
+++ b/lib/actions/message-actions.js
@@ -13,6 +13,9 @@
   FetchPinnedMessagesResult,
   SearchMessagesRequest,
   SearchMessagesResponse,
+  FetchMessageInfosRequest,
+  RawMessageInfo,
+  MessageTruncationStatuses,
 } from '../types/message-types.js';
 import type { MediaMessageServerDBContent } from '../types/messages/media.js';
 import type {
@@ -121,7 +124,7 @@
   async threadIDs => {
     const sortedThreadIDs = sortThreadIDsPerKeyserver(threadIDs);
 
-    const requests = {};
+    const requests: { [string]: FetchMessageInfosRequest } = {};
     for (const keyserverID in sortedThreadIDs) {
       const cursors = Object.fromEntries(
         sortedThreadIDs[keyserverID].map(threadID => [threadID, null]),
@@ -133,8 +136,8 @@
     }
 
     const responses = await callKeyserverEndpoint('fetch_messages', requests);
-    let rawMessageInfos = [];
-    let truncationStatuses = {};
+    let rawMessageInfos: $ReadOnlyArray<RawMessageInfo> = [];
+    let truncationStatuses: MessageTruncationStatuses = {};
     for (const keyserverID in responses) {
       rawMessageInfos = rawMessageInfos.concat(
         responses[keyserverID].rawMessageInfos,
diff --git a/lib/actions/user-actions.js b/lib/actions/user-actions.js
--- a/lib/actions/user-actions.js
+++ b/lib/actions/user-actions.js
@@ -296,7 +296,7 @@
     allKeyserverIDs: $ReadOnlyArray<string>,
   ): ((input: UpdateUserSettingsRequest) => Promise<void>) =>
   async input => {
-    const requests = {};
+    const requests: { [string]: UpdateUserSettingsRequest } = {};
     for (const keyserverID of allKeyserverIDs) {
       requests[keyserverID] = input;
     }
diff --git a/lib/types/filter-types.js b/lib/types/filter-types.js
--- a/lib/types/filter-types.js
+++ b/lib/types/filter-types.js
@@ -17,7 +17,7 @@
   +type: 'threads',
   +threadIDs: $ReadOnlyArray<string>,
 };
-type NotDeletedFilter = { +type: 'not_deleted' };
+export type NotDeletedFilter = { +type: 'not_deleted' };
 export type CalendarFilter = NotDeletedFilter | CalendarThreadFilter;
 
 export const calendarFilterValidator: TUnion<CalendarFilter> = t.union([
diff --git a/lib/utils/action-utils.js b/lib/utils/action-utils.js
--- a/lib/utils/action-utils.js
+++ b/lib/utils/action-utils.js
@@ -22,6 +22,7 @@
 import type { PlatformDetails } from '../types/device-types.js';
 import type { Endpoint, SocketAPIHandler } from '../types/endpoints.js';
 import type { CalendarQuery } from '../types/entry-types.js';
+import type { NotDeletedFilter } from '../types/filter-types.js';
 import type { LoadingOptions, LoadingInfo } from '../types/loading-types.js';
 import type {
   ActionPayload,
@@ -466,7 +467,7 @@
 function sortThreadIDsPerKeyserver(threadIDs: $ReadOnlyArray<string>): {
   +[keyserverID: string]: $ReadOnlyArray<string>,
 } {
-  const results = {};
+  const results: { [string]: string[] } = {};
   for (const threadID of threadIDs) {
     const keyserverID = extractKeyserverIDFromID(threadID);
     invariant(keyserverID, 'keyserver data missing from thread id');
@@ -478,6 +479,18 @@
   return results;
 }
 
+type CalendarThreadFilterWithWritableThreadIDs = {
+  +type: 'threads',
+  +threadIDs: string[],
+};
+type CalendarFilterWithWritableThreadIDs =
+  | NotDeletedFilter
+  | CalendarThreadFilterWithWritableThreadIDs;
+type CalendarQueryWithWritableFilters = {
+  +startDate: string,
+  +endDate: string,
+  +filters: CalendarFilterWithWritableThreadIDs[],
+};
 function sortCalendarQueryPerKeyserver(
   calendarQuery: CalendarQuery,
   keyserverIDs: $ReadOnlyArray<string>,
@@ -485,7 +498,7 @@
   +[keyserverID: string]: CalendarQuery,
 } {
   const { startDate, endDate, filters } = calendarQuery;
-  const results = {};
+  const results: { [string]: CalendarQueryWithWritableFilters } = {};
 
   for (const keyserverID of keyserverIDs) {
     results[keyserverID] = {
@@ -509,6 +522,10 @@
         let threadFilter = results[keyserverID].filters.find(
           flt => flt.type === 'threads',
         );
+        invariant(
+          !threadFilter || threadFilter.type === 'threads',
+          'should only match CalendarThreadFilter',
+        );
         if (!threadFilter) {
           threadFilter = { type: 'threads', threadIDs: [] };
           results[keyserverID].filters.push(threadFilter);
diff --git a/lib/utils/keyserver-call.js b/lib/utils/keyserver-call.js
--- a/lib/utils/keyserver-call.js
+++ b/lib/utils/keyserver-call.js
@@ -2,7 +2,6 @@
 
 import _memoize from 'lodash/memoize.js';
 import * as React from 'react';
-import { useDispatch } from 'react-redux';
 import { createSelector } from 'reselect';
 
 import { bindCookieAndUtilsIntoCallServerEndpoint } from './action-utils.js';
@@ -12,7 +11,7 @@
   CallServerEndpointOptions,
 } from './call-server-endpoint.js';
 import { promiseAll } from './promises.js';
-import { useSelector } from './redux-utils.js';
+import { useSelector, useDispatch } from './redux-utils.js';
 import type { PlatformDetails } from '../types/device-types.js';
 import type { Endpoint } from '../types/endpoints.js';
 import type { KeyserverInfo } from '../types/keyserver-types.js';
@@ -80,7 +79,12 @@
   +keyserverInfos: { +[keyserverID: string]: KeyserverInfo },
 };
 
-const bindCallKeyserverEndpointSelector = createSelector(
+const bindCallKeyserverEndpointSelector: BindKeyserverCallParams => <
+  Args: mixed,
+  Return,
+>(
+  keyserverCall: ActionFunc<Args, Return>,
+) => Args => Promise<Return> = createSelector(
   (state: BindKeyserverCallParams) => state.dispatch,
   (state: BindKeyserverCallParams) => state.currentUserInfo,
   (state: BindKeyserverCallParams) => state.keyserverInfos,
@@ -126,7 +130,7 @@
             );
           };
 
-          const promises = {};
+          const promises: { [string]: Promise<CallServerEndpoint> } = {};
           for (const keyserverID in requests) {
             promises[keyserverID] = bindCallKeyserverEndpoint(keyserverID);
           }