diff --git a/backend/spec/requests/posts_spec.rb b/backend/spec/requests/posts_spec.rb
index aa5f7c1..e13036a 100644
--- a/backend/spec/requests/posts_spec.rb
+++ b/backend/spec/requests/posts_spec.rb
@@ -206,6 +206,25 @@ RSpec.describe 'Posts API', type: :request do
     end
 
     context 'when q is provided' do
+      ['SPEC_TAG', 'SpEc_TaG', 'spec_tág', 'MANKO', 'MaNkO', 'mánko'].each do |name|
+        it "matches #{ name } using database collation" do
+          get '/posts', params: { locale: 'ja', tags: name }
+
+          expect(response).to have_http_status(:ok)
+          expect(json.fetch('count')).to eq(2)
+          expect(json.fetch('posts').map { _1.fetch('id') })
+              .to contain_exactly(post_record.id, hit_post.id)
+        end
+
+        it "excludes #{ name } using database collation" do
+          get '/posts', params: { locale: 'ja', tags: "not:#{ name }" }
+
+          expect(response).to have_http_status(:ok)
+          expect(json.fetch('count')).to eq(1)
+          expect(json.fetch('posts').map { _1.fetch('id') }).to eq([miss_post.id])
+        end
+      end
+
       it 'filters posts by q (hit case)' do
         get '/posts', params: { tags: 'spec_tag' }
 
@@ -266,6 +285,26 @@ RSpec.describe 'Posts API', type: :request do
         PostExternalTag.create!(post: miss_post, external_tag:)
       end
 
+      ['NICO:SEARCH_EXTERNAL', 'NiCo:SeArCh_ExTeRnAl', 'nico:séarch_external'].each do |name|
+        it "matches #{ name } using database collation" do
+          get '/posts', params: { locale: 'ja', tags: name }
+
+          expect(response).to have_http_status(:ok)
+          expect(json.fetch('count')).to eq(2)
+          expect(json.fetch('posts').map { _1.fetch('id') })
+              .to contain_exactly(miss_post.id, both_post.id)
+        end
+
+        it "excludes #{ name } using database collation" do
+          get '/posts', params: { locale: 'ja', tags: "not:#{ name }" }
+
+          expect(response).to have_http_status(:ok)
+          expect(json.fetch('count')).to eq(2)
+          expect(json.fetch('posts').map { _1.fetch('id') })
+              .to contain_exactly(post_record.id, hit_post.id)
+        end
+      end
+
       it 'keeps internal name searches independent of colliding external ids' do
         get '/posts', params: { tags: primary_tag_name_for(tag, 'ja').name }