diff --git a/keyserver/src/responders/message-responders.js b/keyserver/src/responders/message-responders.js
index 5e00c1904..e26658b92 100644
--- a/keyserver/src/responders/message-responders.js
+++ b/keyserver/src/responders/message-responders.js
@@ -1,186 +1,255 @@
 // @flow
 
 import invariant from 'invariant';
 import t from 'tcomb';
 
 import { createMediaMessageData, trimMessage } from 'lib/shared/message-utils';
+import { relationshipBlockedInEitherDirection } from 'lib/shared/relationship-utils';
 import type { Media } from 'lib/types/media-types.js';
 import {
   messageTypes,
   type SendTextMessageRequest,
   type SendMultimediaMessageRequest,
+  type SendReactionMessageRequest,
   type FetchMessageInfosResponse,
   type FetchMessageInfosRequest,
   defaultNumberPerThread,
   type SendMessageResponse,
+  type ReactionMessageData,
 } from 'lib/types/message-types';
 import type { TextMessageData } from 'lib/types/messages/text';
 import { threadPermissions } from 'lib/types/thread-types';
 import { ServerError } from 'lib/utils/errors';
-import { tShape, tMediaMessageMedia } from 'lib/utils/validation-utils';
+import {
+  tString,
+  tShape,
+  tMediaMessageMedia,
+} from 'lib/utils/validation-utils';
 
 import createMessages from '../creators/message-creator';
 import {
   fetchMessageInfos,
   fetchMessageInfoForLocalID,
+  fetchMessageInfoByID,
 } from '../fetchers/message-fetchers';
 import { checkThreadPermission } from '../fetchers/thread-permission-fetchers';
 import {
   fetchMedia,
   fetchMediaFromMediaMessageContent,
 } from '../fetchers/upload-fetchers';
+import { fetchKnownUserInfos } from '../fetchers/user-fetchers';
 import type { Viewer } from '../session/viewer';
 import {
   assignMedia,
   assignMessageContainerToMedia,
 } from '../updaters/upload-updaters';
 import { validateInput } from '../utils/validation-utils';
 
 const sendTextMessageRequestInputValidator = tShape({
   threadID: t.String,
   localID: t.maybe(t.String),
   text: t.String,
 });
 async function textMessageCreationResponder(
   viewer: Viewer,
   input: any,
 ): Promise<SendMessageResponse> {
   const request: SendTextMessageRequest = input;
   await validateInput(viewer, sendTextMessageRequestInputValidator, request);
 
   const { threadID, localID, text: rawText } = request;
   const text = trimMessage(rawText);
   if (!text) {
     throw new ServerError('invalid_parameters');
   }
 
   const hasPermission = await checkThreadPermission(
     viewer,
     threadID,
     threadPermissions.VOICED,
   );
   if (!hasPermission) {
     throw new ServerError('invalid_parameters');
   }
 
   const messageData: TextMessageData = {
     type: messageTypes.TEXT,
     threadID,
     creatorID: viewer.id,
     time: Date.now(),
     text,
   };
   if (localID) {
     messageData.localID = localID;
   }
   const rawMessageInfos = await createMessages(viewer, [messageData]);
 
   return { newMessageInfo: rawMessageInfos[0] };
 }
 
 const fetchMessageInfosRequestInputValidator = tShape({
   cursors: t.dict(t.String, t.maybe(t.String)),
   numberPerThread: t.maybe(t.Number),
 });
 async function messageFetchResponder(
   viewer: Viewer,
   input: any,
 ): Promise<FetchMessageInfosResponse> {
   const request: FetchMessageInfosRequest = input;
   await validateInput(viewer, fetchMessageInfosRequestInputValidator, request);
   const response = await fetchMessageInfos(
     viewer,
     { threadCursors: request.cursors },
     request.numberPerThread ? request.numberPerThread : defaultNumberPerThread,
   );
   return { ...response, userInfos: {} };
 }
 
 const sendMultimediaMessageRequestInputValidator = t.union([
   tShape({
     threadID: t.String,
     localID: t.String,
     mediaIDs: t.list(t.String),
   }),
   tShape({
     threadID: t.String,
     localID: t.String,
     mediaMessageContents: t.list(tMediaMessageMedia),
   }),
 ]);
 async function multimediaMessageCreationResponder(
   viewer: Viewer,
   input: any,
 ): Promise<SendMessageResponse> {
   const request: SendMultimediaMessageRequest = input;
   await validateInput(
     viewer,
     sendMultimediaMessageRequestInputValidator,
     request,
   );
 
   if (
     (request.mediaIDs && request.mediaIDs.length === 0) ||
     (request.mediaMessageContents && request.mediaMessageContents.length === 0)
   ) {
     throw new ServerError('invalid_parameters');
   }
 
   const { threadID, localID } = request;
   const hasPermission = await checkThreadPermission(
     viewer,
     threadID,
     threadPermissions.VOICED,
   );
   if (!hasPermission) {
     throw new ServerError('invalid_parameters');
   }
 
   const existingMessageInfoPromise = fetchMessageInfoForLocalID(
     viewer,
     localID,
   );
   const mediaPromise: Promise<$ReadOnlyArray<Media>> = request.mediaIDs
     ? fetchMedia(viewer, request.mediaIDs)
     : fetchMediaFromMediaMessageContent(viewer, request.mediaMessageContents);
 
   const [existingMessageInfo, media] = await Promise.all([
     existingMessageInfoPromise,
     mediaPromise,
   ]);
 
   if (media.length === 0 && !existingMessageInfo) {
     throw new ServerError('invalid_parameters');
   }
 
   const messageData = createMediaMessageData({
     localID,
     threadID,
     creatorID: viewer.id,
     media,
   });
   const [newMessageInfo] = await createMessages(viewer, [messageData]);
   const { id } = newMessageInfo;
   invariant(
     id !== null && id !== undefined,
     'serverID should be set in createMessages result',
   );
 
   if (request.mediaIDs) {
     await assignMedia(viewer, request.mediaIDs, id);
   } else {
     await assignMessageContainerToMedia(
       viewer,
       request.mediaMessageContents,
       id,
     );
   }
 
   return { newMessageInfo };
 }
 
+const sendReactionMessageRequestInputValidator = tShape({
+  threadID: t.String,
+  targetMessageID: t.String,
+  reaction: tString('👍'),
+  action: t.enums.of(['add_reaction', 'remove_reaction']),
+});
+async function reactionMessageCreationResponder(
+  viewer: Viewer,
+  input: any,
+): Promise<SendMessageResponse> {
+  const request: SendReactionMessageRequest = input;
+  await validateInput(viewer, sendReactionMessageRequestInputValidator, input);
+
+  const { threadID, targetMessageID, reaction, action } = request;
+
+  if (!targetMessageID || !reaction) {
+    throw new ServerError('invalid_parameters');
+  }
+
+  const targetMessageInfo = await fetchMessageInfoByID(viewer, targetMessageID);
+
+  if (!targetMessageInfo || !targetMessageInfo.id) {
+    throw new ServerError('invalid_parameters');
+  }
+
+  const [hasPermission, targetMessageUserInfos] = await Promise.all([
+    checkThreadPermission(viewer, threadID, threadPermissions.VOICED),
+    fetchKnownUserInfos(viewer, [targetMessageInfo.creatorID]),
+  ]);
+
+  const targetMessageCreator =
+    targetMessageUserInfos[targetMessageInfo.creatorID];
+
+  const targetMessageCreatorRelationship =
+    targetMessageCreator?.relationshipStatus;
+
+  const creatorRelationshipHasBlock =
+    targetMessageCreatorRelationship &&
+    relationshipBlockedInEitherDirection(targetMessageCreatorRelationship);
+
+  if (!hasPermission || creatorRelationshipHasBlock) {
+    throw new ServerError('invalid_parameters');
+  }
+
+  const messageData: ReactionMessageData = {
+    type: messageTypes.REACTION,
+    threadID,
+    creatorID: viewer.id,
+    time: Date.now(),
+    targetMessageID,
+    reaction,
+    action,
+  };
+
+  const rawMessageInfos = await createMessages(viewer, [messageData]);
+
+  return { newMessageInfo: rawMessageInfos[0] };
+}
+
 export {
   textMessageCreationResponder,
   messageFetchResponder,
   multimediaMessageCreationResponder,
+  reactionMessageCreationResponder,
 };