diff --git a/backend/app/controllers/post_versions_controller.rb b/backend/app/controllers/post_versions_controller.rb
index 0c6f632..545b8e0 100644
--- a/backend/app/controllers/post_versions_controller.rb
+++ b/backend/app/controllers/post_versions_controller.rb
@@ -49,10 +49,10 @@ class PostVersionsController < ApplicationController
 
     rows.map do |row|
       cur_tags =
-          Array(normalise_json(row.tags_json))
+          normalise_json(row.tags_json)
           .map { Post.tag_snapshot_literal(_1) }
       prev_tags =
-          Array(normalise_json(row.attributes['prev_tags_json']))
+          (normalise_json(row.attributes['prev_tags_json']) || [])
           .map { Post.tag_snapshot_literal(_1) }
 
       { post_id: row.post_id,
diff --git a/backend/db/migrate/20260727123600_add_tags_json_to_post_versions.rb b/backend/db/migrate/20260727123600_add_tags_json_to_post_versions.rb
index d306715..7bbf3c8 100644
--- a/backend/db/migrate/20260727123600_add_tags_json_to_post_versions.rb
+++ b/backend/db/migrate/20260727123600_add_tags_json_to_post_versions.rb
@@ -30,19 +30,46 @@ class AddTagsJsonToPostVersions < ActiveRecord::Migration[8.0]
     intervals_by_name = build_intervals_by_name
 
     say_with_time 'Backfilling post_versions.tags_json' do
-      MigrationPostVersion
-          .where(tags_json: nil)
-          .find_each(batch_size: 500) do |version|
-        version.update_columns(
-            tags_json: build_tags_json(version, intervals_by_name))
+      MigrationPostVersion.where(tags_json: nil).find_each(batch_size: 500) do |version|
+        version.update_columns(tags_json: build_tags_json(version, intervals_by_name))
       end
     end
 
-    assert_backfill_complete!
     change_column_null :post_versions, :tags_json, false
+
+    schema = connection.quote(JSON.generate({
+        type: 'array',
+        items: { type: 'object',
+                 properties: { id: { type: 'integer', minimum: 1 },
+                               version_no: { type: 'integer', minimum: 1 },
+                               name: { type: 'string', minLength: 1 },
+                               category: { type: 'string', enum: ['deerjikist',
+                                                                  'meme',
+                                                                  'character',
+                                                                  'general',
+                                                                  'material',
+                                                                  'meta',
+                                                                  'nico'] },
+                               sections: { type: 'array',
+                                           items: { type: 'object',
+                                                    properties: { begin_ms: { type: 'integer',
+                                                                              minimum: 0 },
+                                                                  end_ms: { type: ['integer',
+                                                                                   'null'],
+                                                                            minimum: 0 } },
+                                                    required: ['begin_ms', 'end_ms'],
+                                                    additionalProperties: false } } },
+                 required: ['id', 'version_no', 'name', 'category', 'sections'],
+                 additionalProperties: false } }))
+
+    add_check_constraint :post_versions,
+                         "JSON_SCHEMA_VALID(#{ schema }, tags_json)",
+                         name: 'chk_post_versions_tags_json_schema'
   end
 
   def down
+    remove_check_constraint :post_versions, name: 'chk_post_versions_tags_json_schema'
+
     remove_column :post_versions, :tags_json
   end
 
@@ -269,11 +296,4 @@ class AddTagsJsonToPostVersions < ActiveRecord::Migration[8.0]
 
     raise "Duplicate tag IDs: #{ details }"
   end
-
-  def assert_backfill_complete!
-    missing_count = MigrationPostVersion.where(tags_json: nil).count
-    return if missing_count.zero?
-
-    raise "#{ missing_count } post versions were not backfilled"
-  end
 end
diff --git a/backend/db/schema.rb b/backend/db/schema.rb
index d4a82c7..5ebbcc5 100644
--- a/backend/db/schema.rb
+++ b/backend/db/schema.rb
@@ -363,6 +363,7 @@ ActiveRecord::Schema[8.0].define(version: 2026_07_27_123600) do
     t.check_constraint "(`video_ms` is null) or (`video_ms` > 0)", name: "chk_post_versions_video_ms_positive"
     t.check_constraint "`event_type` in (_utf8mb4'create',_utf8mb4'update',_utf8mb4'discard',_utf8mb4'restore')", name: "post_versions_event_type_valid"
     t.check_constraint "`version_no` > 0", name: "post_versions_version_no_positive"
+    t.check_constraint "json_schema_valid(_utf8mb4'{\"type\":\"array\",\"items\":{\"type\":\"object\",\"properties\":{\"id\":{\"type\":\"integer\",\"minimum\":1},\"version_no\":{\"type\":\"integer\",\"minimum\":1},\"name\":{\"type\":\"string\",\"minLength\":1},\"category\":{\"type\":\"string\",\"enum\":[\"deerjikist\",\"meme\",\"character\",\"general\",\"material\",\"meta\",\"nico\"]},\"sections\":{\"type\":\"array\",\"items\":{\"type\":\"object\",\"properties\":{\"begin_ms\":{\"type\":\"integer\",\"minimum\":0},\"end_ms\":{\"type\":[\"integer\",\"null\"],\"minimum\":0}},\"required\":[\"begin_ms\",\"end_ms\"],\"additionalProperties\":false}}},\"required\":[\"id\",\"version_no\",\"name\",\"category\",\"sections\"],\"additionalProperties\":false}}',`tags_json`)", name: "chk_post_versions_tags_json_schema"
   end
 
   create_table "posts", charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|