diff --git a/packages/local-sync/src/models/thread.js b/packages/local-sync/src/models/thread.js
index 7342c3142..6b588ee6a 100644
--- a/packages/local-sync/src/models/thread.js
+++ b/packages/local-sync/src/models/thread.js
@@ -2,6 +2,7 @@ const {DatabaseTypes: {JSONARRAYType}} = require('isomorphic-core');
 
 module.exports = (sequelize, Sequelize) => {
   return sequelize.define('thread', {
+    id: { type: Sequelize.STRING(65), primaryKey: true },
     accountId: { type: Sequelize.STRING, allowNull: false },
     version: Sequelize.INTEGER,
     remoteThreadId: Sequelize.STRING,
diff --git a/packages/local-sync/src/new-message-processor/detect-thread.js b/packages/local-sync/src/new-message-processor/detect-thread.js
index e5ed3b42c..8cf15d8fb 100644
--- a/packages/local-sync/src/new-message-processor/detect-thread.js
+++ b/packages/local-sync/src/new-message-processor/detect-thread.js
@@ -95,6 +95,11 @@ function detectThread({db, message}) {
 
     // update the basic properties of the thread
     thread.accountId = message.accountId;
+    // Threads may, locally, have the ID of any message within the thread
+    // (message IDs are globally unique, even across accounts!)
+    if (!thread.id) {
+      thread.id = `t:${message.id}`
+    }
 
     // update the participants on the thread
     const threadParticipants = [].concat(thread.participants);