diff --git a/keyserver/src/responders/message-responders.js b/keyserver/src/responders/message-responders.js
--- a/keyserver/src/responders/message-responders.js
+++ b/keyserver/src/responders/message-responders.js
@@ -3,6 +3,7 @@
 import invariant from 'invariant';
 import t from 'tcomb';
 
+import { onlyOneEmojiRegex } from 'lib/shared/emojis';
 import { createMediaMessageData, trimMessage } from 'lib/shared/message-utils';
 import { relationshipBlockedInEitherDirection } from 'lib/shared/relationship-utils';
 import type { Media } from 'lib/types/media-types.js';
@@ -20,11 +21,7 @@
 import type { TextMessageData } from 'lib/types/messages/text';
 import { threadPermissions } from 'lib/types/thread-types';
 import { ServerError } from 'lib/utils/errors';
-import {
-  tString,
-  tShape,
-  tMediaMessageMedia,
-} from 'lib/utils/validation-utils';
+import { tRegex, tShape, tMediaMessageMedia } from 'lib/utils/validation-utils';
 
 import createMessages from '../creators/message-creator';
 import { SQL } from '../database/database';
@@ -194,7 +191,7 @@
   threadID: t.String,
   localID: t.maybe(t.String),
   targetMessageID: t.String,
-  reaction: tString('👍'),
+  reaction: tRegex(onlyOneEmojiRegex),
   action: t.enums.of(['add_reaction', 'remove_reaction']),
 });
 async function reactionMessageCreationResponder(
diff --git a/lib/shared/emojis.js b/lib/shared/emojis.js
--- a/lib/shared/emojis.js
+++ b/lib/shared/emojis.js
@@ -9,6 +9,8 @@
   'could not extract innerEmojiRegex from emoji-regex.js',
 );
 const innerEmojiRegexString = emojiRegexMatches[1];
+
 const onlyEmojiRegex: RegExp = new RegExp(`^(${innerEmojiRegexString})+$`);
+const onlyOneEmojiRegex: RegExp = new RegExp(`^(${innerEmojiRegexString})$`);
 
-export { onlyEmojiRegex };
+export { onlyEmojiRegex, onlyOneEmojiRegex };