diff --git a/common/comment.ts b/common/comment.ts
index 77b211d3..c1b721cc 100644
--- a/common/comment.ts
+++ b/common/comment.ts
@@ -4,6 +4,8 @@ import type { JSONContent } from '@tiptap/core'
 // They're uniquely identified by the pair contractId/betId.
 export type Comment = {
   id: string
+  commentType: 'contract' | 'group'
+
   contractId?: string
   groupId?: string
   betId?: string
diff --git a/functions/src/scripts/backfill-comment-types.ts b/functions/src/scripts/backfill-comment-types.ts
new file mode 100644
index 00000000..cce1c85d
--- /dev/null
+++ b/functions/src/scripts/backfill-comment-types.ts
@@ -0,0 +1,31 @@
+// Comment types were introduced in August 2022.
+
+import { initAdmin } from './script-init'
+import { log, writeAsync } from '../utils'
+
+if (require.main === module) {
+  const app = initAdmin()
+  const firestore = app.firestore()
+  const commentsRef = firestore.collectionGroup('comments')
+  commentsRef.get().then(async (commentsSnaps) => {
+    log(`Loaded ${commentsSnaps.size} contracts.`)
+    const needsFilling = commentsSnaps.docs.filter((ct) => {
+      return !('commentType' in ct.data())
+    })
+    log(`Found ${needsFilling.length} comments to update.`)
+    const updates = needsFilling.map((d) => {
+      const comment = d.data()
+      const fields: { [k: string]: unknown } = {}
+      if (comment.contractId != null && comment.groupId == null) {
+        fields.commentType = 'contract'
+      } else if (comment.groupId != null && comment.contractId == null) {
+        fields.commentType = 'group'
+      } else {
+        log(`Invalid comment ${comment}; not touching it.`)
+      }
+      return { doc: d.ref, fields, info: comment }
+    })
+    await writeAsync(firestore, updates)
+    log(`Updated all comments.`)
+  })
+}
diff --git a/web/lib/firebase/comments.ts b/web/lib/firebase/comments.ts
index e82c6d45..f0678367 100644
--- a/web/lib/firebase/comments.ts
+++ b/web/lib/firebase/comments.ts
@@ -33,6 +33,7 @@ export async function createCommentOnContract(
     : doc(getCommentsCollection(contractId))
   const comment: Comment = removeUndefinedProps({
     id: ref.id,
+    commentType: 'contract',
     contractId,
     userId: commenter.id,
     content: content,
@@ -61,6 +62,7 @@ export async function createCommentOnGroup(
   const ref = doc(getCommentsOnGroupCollection(groupId))
   const comment: Comment = removeUndefinedProps({
     id: ref.id,
+    commentType: 'group',
     groupId,
     userId: user.id,
     content: content,