diff --git a/lib/selectors/user-selectors.js b/lib/selectors/user-selectors.js
--- a/lib/selectors/user-selectors.js
+++ b/lib/selectors/user-selectors.js
@@ -9,6 +9,7 @@
   getRandomDefaultEmojiAvatar,
 } from '../shared/avatar-utils.js';
 import { getSingleOtherUser } from '../shared/thread-utils.js';
+import { type P2PMessageRecipient } from '../tunnelbroker/peer-to-peer-context.js';
 import {
   type AuxUserInfos,
   type AuxUserInfo,
@@ -305,11 +306,9 @@
 
 const getAllPeerUserIDAndDeviceIDs: (
   state: BaseAppState<>,
-) => $ReadOnlyArray<{ +userID: string, +deviceID: string }> = createSelector(
+) => $ReadOnlyArray<P2PMessageRecipient> = createSelector(
   (state: BaseAppState<>) => state.auxUserStore.auxUserInfos,
-  (
-    auxUserInfos: AuxUserInfos,
-  ): $ReadOnlyArray<{ +userID: string, +deviceID: string }> =>
+  (auxUserInfos: AuxUserInfos): $ReadOnlyArray<P2PMessageRecipient> =>
     entries(auxUserInfos).flatMap(
       ([userID, { deviceList }]: [string, AuxUserInfo]) =>
         deviceList?.devices.map(deviceID => ({
diff --git a/lib/shared/dm-ops/dm-op-utils.js b/lib/shared/dm-ops/dm-op-utils.js
--- a/lib/shared/dm-ops/dm-op-utils.js
+++ b/lib/shared/dm-ops/dm-op-utils.js
@@ -11,8 +11,10 @@
 import { useFindUserIdentities } from '../../actions/user-actions.js';
 import { useLoggedInUserInfo } from '../../hooks/account-hooks.js';
 import { useGetLatestMessageEdit } from '../../hooks/latest-message-edit.js';
+import { useGetAndUpdateDeviceListsForUsers } from '../../hooks/peer-list-hooks.js';
 import { mergeUpdatesWithMessageInfos } from '../../reducers/message-reducer.js';
 import { getAllPeerUserIDAndDeviceIDs } from '../../selectors/user-selectors.js';
+import { type P2PMessageRecipient } from '../../tunnelbroker/peer-to-peer-context.js';
 import type {
   CreateThickRawThreadInfoInput,
   DMAddMembersOperation,
@@ -45,6 +47,7 @@
 import { getContentSigningKey } from '../../utils/crypto-utils.js';
 import { useSelector } from '../../utils/redux-utils.js';
 import { messageSpecs } from '../messages/message-specs.js';
+import { userSupportsThickThreads } from '../thread-utils.js';
 import { updateSpecs } from '../updates/update-specs.js';
 
 function generateMessagesToPeers(
@@ -124,6 +127,55 @@
 ) => Promise<$ReadOnlyArray<OutboundP2PMessage>> {
   const allPeerUserIDAndDeviceIDs = useSelector(getAllPeerUserIDAndDeviceIDs);
   const utilities = useSendDMOperationUtils();
+  const auxUserInfos = useSelector(state => state.auxUserStore.auxUserInfos);
+  const getAndUpdateDeviceListsForUsers = useGetAndUpdateDeviceListsForUsers();
+
+  const getUsersWithoutDeviceList = React.useCallback(
+    (userIDs: $ReadOnlyArray<string>) => {
+      const missingDeviceListsUserIDs: Array<string> = [];
+      for (const userID of userIDs) {
+        const supportsThickThreads = userSupportsThickThreads(
+          userID,
+          auxUserInfos,
+        );
+        if (!supportsThickThreads) {
+          missingDeviceListsUserIDs.push(userID);
+        }
+      }
+      return missingDeviceListsUserIDs;
+    },
+    [auxUserInfos],
+  );
+
+  const getMissingPeers = React.useCallback(
+    async (
+      userIDs: $ReadOnlyArray<string>,
+    ): Promise<$ReadOnlyArray<P2PMessageRecipient>> => {
+      const missingDeviceListsUserIDs = getUsersWithoutDeviceList(userIDs);
+      if (missingDeviceListsUserIDs.length === 0) {
+        return [];
+      }
+
+      const deviceLists = await getAndUpdateDeviceListsForUsers(
+        missingDeviceListsUserIDs,
+        true,
+      );
+
+      const updatedPeers: Array<P2PMessageRecipient> = [];
+      for (const userID of missingDeviceListsUserIDs) {
+        if (deviceLists[userID] && deviceLists[userID].devices.length > 0) {
+          updatedPeers.push(
+            ...deviceLists[userID].devices.map(deviceID => ({
+              deviceID,
+              userID,
+            })),
+          );
+        }
+      }
+      return updatedPeers;
+    },
+    [getAndUpdateDeviceListsForUsers, getUsersWithoutDeviceList],
+  );
 
   return React.useCallback(
     async (
@@ -141,8 +193,11 @@
           peer => peer.userID === viewerID,
         );
       } else if (recipients.type === 'some_users') {
+        const missingPeers = await getMissingPeers(recipients.userIDs);
+        const updatedPeers = [...allPeerUserIDAndDeviceIDs, ...missingPeers];
+
         const userIDs = new Set(recipients.userIDs);
-        peerUserIDAndDeviceIDs = allPeerUserIDAndDeviceIDs.filter(peer =>
+        peerUserIDAndDeviceIDs = updatedPeers.filter(peer =>
           userIDs.has(peer.userID),
         );
       } else if (recipients.type === 'all_thread_members') {
@@ -157,8 +212,11 @@
         const members = threadInfos[recipients.threadID]?.members ?? [];
         const memberIDs = members.map(member => member.id);
 
+        const missingPeers = await getMissingPeers(memberIDs);
+        const updatedPeers = [...allPeerUserIDAndDeviceIDs, ...missingPeers];
+
         const userIDs = new Set(memberIDs);
-        peerUserIDAndDeviceIDs = allPeerUserIDAndDeviceIDs.filter(peer =>
+        peerUserIDAndDeviceIDs = updatedPeers.filter(peer =>
           userIDs.has(peer.userID),
         );
       } else if (recipients.type === 'some_devices') {
@@ -174,7 +232,7 @@
       );
       return generateMessagesToPeers(operation, targetPeers);
     },
-    [allPeerUserIDAndDeviceIDs, utilities],
+    [allPeerUserIDAndDeviceIDs, getMissingPeers, utilities],
   );
 }
 
diff --git a/lib/tunnelbroker/peer-to-peer-context.js b/lib/tunnelbroker/peer-to-peer-context.js
--- a/lib/tunnelbroker/peer-to-peer-context.js
+++ b/lib/tunnelbroker/peer-to-peer-context.js
@@ -29,6 +29,8 @@
 } from '../utils/peer-to-peer-communication-utils.js';
 import { useSelector } from '../utils/redux-utils.js';
 
+export type P2PMessageRecipient = { +userID: string, +deviceID: string };
+
 type PeerToPeerContextType = {
   +processOutboundMessages: (
     outboundMessageIDs: ?$ReadOnlyArray<string>,
@@ -41,7 +43,7 @@
   },
   +broadcastEphemeralMessage: (
     contentPayload: string,
-    recipients: $ReadOnlyArray<{ +userID: string, +deviceID: string }>,
+    recipients: $ReadOnlyArray<P2PMessageRecipient>,
     authMetadata: AuthMetadata,
   ) => Promise<void>,
 };
@@ -354,7 +356,7 @@
   const broadcastEphemeralMessage = React.useCallback(
     async (
       contentPayload: string,
-      recipients: $ReadOnlyArray<{ +userID: string, +deviceID: string }>,
+      recipients: $ReadOnlyArray<P2PMessageRecipient>,
       authMetadata: AuthMetadata,
     ) => {
       const { userID: thisUserID, deviceID: thisDeviceID } = authMetadata;
@@ -380,8 +382,7 @@
       // 2. Analyze results to retrieve all devices that need session creation
       // and map by userID.
       const userDevicesWithoutSession: { [userID: string]: Array<string> } = {};
-      const recipientsToRetry: Array<{ +userID: string, +deviceID: string }> =
-        [];
+      const recipientsToRetry: Array<P2PMessageRecipient> = [];
       for (const result of messagesResults) {
         if (result.status === 'missing_session') {
           recipientsToRetry.push(result.recipient);
diff --git a/lib/utils/peer-to-peer-communication-utils.js b/lib/utils/peer-to-peer-communication-utils.js
--- a/lib/utils/peer-to-peer-communication-utils.js
+++ b/lib/utils/peer-to-peer-communication-utils.js
@@ -3,6 +3,7 @@
 import { getConfig } from './config.js';
 import { olmSessionErrors } from './olm-utils.js';
 import { type AuthMetadata } from '../shared/identity-client-context.js';
+import type { P2PMessageRecipient } from '../tunnelbroker/peer-to-peer-context.js';
 import type { TunnelbrokerClientMessageToDevice } from '../tunnelbroker/tunnelbroker-context.js';
 import {
   outboundP2PMessageStatuses,
@@ -139,11 +140,11 @@
 
 export type EphemeralEncryptAndSendMessageToPeerResult = {
   +status: 'success' | 'failure' | 'missing_session',
-  +recipient: { +userID: string, +deviceID: string },
+  +recipient: P2PMessageRecipient,
 };
 async function ephemeralEncryptAndSendMessageToPeer(
   contentPayload: string,
-  recipient: { +userID: string, +deviceID: string },
+  recipient: P2PMessageRecipient,
   authMetadata: ?AuthMetadata,
   sendMessage: (
     message: TunnelbrokerClientMessageToDevice,