diff --git a/backend/app/controllers/posts_controller.rb b/backend/app/controllers/posts_controller.rb
index fb0d8b5..e4dbb04 100644
--- a/backend/app/controllers/posts_controller.rb
+++ b/backend/app/controllers/posts_controller.rb
@@ -44,7 +44,11 @@ class PostsController < ApplicationController
               nil
             end
       end
-    }, count: filtered_posts.count(:id), next_cursor: }
+    }, count: if filtered_posts.group_values.present?
+      filtered_posts.count.size
+    else
+      filtered_posts.count
+    end, next_cursor: }
   end
 
   def random
@@ -181,7 +185,7 @@ class PostsController < ApplicationController
   private
 
   def filtered_posts
-    tag_names = params[:tags]&.split(' ')
+    tag_names = params[:tags].to_s.split(' ')
     match_type = params[:match]
     if tag_names.present?
       filter_posts_by_tags(tag_names, match_type)
diff --git a/backend/spec/requests/posts_spec.rb b/backend/spec/requests/posts_spec.rb
index 7480812..4e0cf11 100644
--- a/backend/spec/requests/posts_spec.rb
+++ b/backend/spec/requests/posts_spec.rb
@@ -23,23 +23,68 @@ RSpec.describe 'Posts API', type: :request do
     end
   end
 
-  describe 'GET /posts' do
-    it 'returns posts with tag name in JSON' do
-      get '/posts'
+  describe "GET /posts" do
+    let!(:user) { create_member_user! }
+
+    let!(:tag_name) { TagName.create!(name: "spec_tag") }
+    let!(:tag) { Tag.create!(tag_name:, category: "general") }
+    let!(:tag_name2) { TagName.create!(name: 'unko') }
+    let!(:tag2) { Tag.create!(tag_name: tag_name2, category: 'deerjikist') }
+
+    let!(:hit_post) do
+      Post.create!(uploaded_user: user, title: "hello spec world",
+                   url: 'https://example.com/spec').tap do |p|
+        PostTag.create!(post: p, tag:)
+      end
+    end
+
+    let!(:miss_post) do
+      Post.create!(uploaded_user: user, title: "unrelated title",
+                   url: 'https://example.com/spec2').tap do |p|
+        PostTag.create!(post: p, tag: tag2)
+      end
+    end
+
+    it "returns posts with tag name in JSON" do
+      get "/posts"
 
       expect(response).to have_http_status(:ok)
-      expect(json).to have_key('posts')
-      expect(json['posts']).to be_an(Array)
-      expect(json['posts']).not_to be_empty
+      posts = json.fetch("posts")
+
+      # 全postの全tagが name を含むこと
+      expect(posts).not_to be_empty
+      posts.each do |p|
+        expect(p["tags"]).to be_an(Array)
+        p["tags"].each do |t|
+          expect(t).to include("name", "category")
+        end
+      end
+      expect(json['count']).to be_an(Integer)
+
+      # spec_tag を含む投稿が存在すること
+      all_tag_names = posts.flat_map { |p| p["tags"].map { |t| t["name"] } }
+      expect(all_tag_names).to include("spec_tag")
+    end
+
+    context "when q is provided" do
+      it "filters posts by q (hit case)" do
+        get "/posts", params: { tags: "spec_tag" }
+
+        expect(response).to have_http_status(:ok)
+        ids = json.fetch("posts").map { |p| p["id"] }
 
-      tags = json['posts'][0]['tags']
-      expect(tags).to be_an(Array)
-      expect(tags).not_to be_empty
+        expect(ids).to include(hit_post.id)
+        expect(ids).not_to include(miss_post.id)
+        expect(json['count']).to be_an(Integer)
+      end
+
+      it "returns empty posts when nothing matches" do
+        get "/posts", params: { tags: "no_such_keyword_12345" }
 
-      # Tag は name カラムを持たないので、API 側が methods: [:name] 等で出す必要がある
-      expect(tags[0]).to have_key('name')
-      expect(tags.map { |t| t['name'] }).to include('spec_tag')
-      expect(tags[0]).to include('category')
+        expect(response).to have_http_status(:ok)
+        expect(json.fetch("posts")).to eq([])
+        expect(json.fetch('count')).to eq(0)
+      end
     end
   end
 
@@ -211,7 +256,7 @@ RSpec.describe 'Posts API', type: :request do
     end
   end
 
-  describe 'DELETE /posts/:id/unviewed' do
+  describe 'DELETE /posts/:id/viewed' do
     let(:user) { create(:user) }
 
     it '401 when not logged in' do