diff --git a/app/admin/plans.rb b/app/admin/plans.rb
index dd4be1f..14aa310 100644
--- a/app/admin/plans.rb
+++ b/app/admin/plans.rb
@@ -25,7 +25,7 @@
       row :status
       row :current_revision
       row :created_by_user
-      row :tags
+      row(:tags) { |plan| plan.tag_names.join(", ") }
       row :metadata
       row :created_at
       row :updated_at
diff --git a/app/admin/tags.rb b/app/admin/tags.rb
new file mode 100644
index 0000000..a65cecb
--- /dev/null
+++ b/app/admin/tags.rb
@@ -0,0 +1,31 @@
+ActiveAdmin.register CoPlan::Tag, as: "Tag" do
+  permit_params :name
+
+  index do
+    selectable_column
+    id_column
+    column :name
+    column :plans_count
+    column :created_at
+    actions
+  end
+
+  show do
+    attributes_table do
+      row :id
+      row :name
+      row :plans_count
+      row :created_at
+      row :updated_at
+    end
+
+    panel "Plans" do
+      table_for resource.plans.order(updated_at: :desc) do
+        column :id
+        column :title
+        column :status
+        column :updated_at
+      end
+    end
+  end
+end
diff --git a/db/migrate/20260403213228_create_coplan_plan_types.co_plan.rb b/db/migrate/20260403213228_create_coplan_plan_types.co_plan.rb
new file mode 100644
index 0000000..013b280
--- /dev/null
+++ b/db/migrate/20260403213228_create_coplan_plan_types.co_plan.rb
@@ -0,0 +1,19 @@
+# This migration comes from co_plan (originally 20260403000000)
+class CreateCoplanPlanTypes < ActiveRecord::Migration[8.1]
+  def change
+    create_table :coplan_plan_types, id: { type: :string, limit: 36 } do |t|
+      t.string :name, null: false
+      t.text :description
+      t.json :default_tags
+      t.text :template_content
+      t.json :metadata
+      t.timestamps
+    end
+
+    add_index :coplan_plan_types, :name, unique: true
+
+    add_column :coplan_plans, :plan_type_id, :string, limit: 36
+    add_index :coplan_plans, :plan_type_id
+    add_foreign_key :coplan_plans, :coplan_plan_types, column: :plan_type_id
+  end
+end
diff --git a/db/migrate/20260403213229_seed_general_plan_type.co_plan.rb b/db/migrate/20260403213229_seed_general_plan_type.co_plan.rb
new file mode 100644
index 0000000..08f3019
--- /dev/null
+++ b/db/migrate/20260403213229_seed_general_plan_type.co_plan.rb
@@ -0,0 +1,23 @@
+# This migration comes from co_plan (originally 20260403100000)
+class SeedGeneralPlanType < ActiveRecord::Migration[8.1]
+  def up
+    general_id = SecureRandom.uuid_v7
+    execute <<~SQL
+      INSERT INTO coplan_plan_types (id, name, description, default_tags, template_content, metadata, created_at, updated_at)
+      VALUES (#{quote(general_id)}, 'General', 'General-purpose plan', '[]', NULL, '{}', NOW(), NOW())
+    SQL
+
+    execute <<~SQL
+      UPDATE coplan_plans SET plan_type_id = #{quote(general_id)} WHERE plan_type_id IS NULL
+    SQL
+  end
+
+  def down
+    general = execute("SELECT id FROM coplan_plan_types WHERE name = 'General' LIMIT 1")
+    if general.any?
+      general_id = general.first[0]
+      execute("UPDATE coplan_plans SET plan_type_id = NULL WHERE plan_type_id = #{quote(general_id)}")
+      execute("DELETE FROM coplan_plan_types WHERE id = #{quote(general_id)}")
+    end
+  end
+end
diff --git a/db/migrate/20260403220000_create_coplan_tags.co_plan.rb b/db/migrate/20260403220000_create_coplan_tags.co_plan.rb
new file mode 100644
index 0000000..90b429f
--- /dev/null
+++ b/db/migrate/20260403220000_create_coplan_tags.co_plan.rb
@@ -0,0 +1,23 @@
+# This migration comes from co_plan (originally 20260403000000)
+class CreateCoplanTags < ActiveRecord::Migration[8.1]
+  def change
+    create_table :coplan_tags, id: { type: :string, limit: 36 } do |t|
+      t.string :name, null: false
+      t.integer :plans_count, default: 0, null: false
+      t.timestamps
+    end
+
+    add_index :coplan_tags, :name, unique: true
+
+    create_table :coplan_plan_tags, id: { type: :string, limit: 36 } do |t|
+      t.string :plan_id, limit: 36, null: false
+      t.string :tag_id, limit: 36, null: false
+      t.timestamps
+    end
+
+    add_index :coplan_plan_tags, [:plan_id, :tag_id], unique: true
+    add_index :coplan_plan_tags, :tag_id
+    add_foreign_key :coplan_plan_tags, :coplan_plans, column: :plan_id
+    add_foreign_key :coplan_plan_tags, :coplan_tags, column: :tag_id
+  end
+end
diff --git a/db/migrate/20260403220001_backfill_structured_tags.co_plan.rb b/db/migrate/20260403220001_backfill_structured_tags.co_plan.rb
new file mode 100644
index 0000000..af0a737
--- /dev/null
+++ b/db/migrate/20260403220001_backfill_structured_tags.co_plan.rb
@@ -0,0 +1,40 @@
+# This migration comes from co_plan (originally 20260403100000)
+class BackfillStructuredTags < ActiveRecord::Migration[8.1]
+  def up
+    # Backfill Tag + PlanTag records from JSON tags column
+    execute(<<~SQL).each do |row|
+      SELECT id, tags FROM coplan_plans WHERE tags IS NOT NULL
+    SQL
+      plan_id = row[0] || row["id"]
+      raw = row[1] || row["tags"]
+      next if raw.blank?
+
+      parsed = raw.is_a?(String) ? JSON.parse(raw) : raw
+      next unless parsed.is_a?(Array)
+
+      parsed.each do |name|
+        name = name.to_s.strip
+        next if name.blank?
+
+        tag_id = SecureRandom.uuid_v7
+        execute "INSERT IGNORE INTO coplan_tags (id, name, plans_count, created_at, updated_at) VALUES (#{quote(tag_id)}, #{quote(name)}, 0, NOW(), NOW())"
+        actual_tag_id = select_value("SELECT id FROM coplan_tags WHERE name = #{quote(name)}")
+        pt_id = SecureRandom.uuid_v7
+        execute "INSERT IGNORE INTO coplan_plan_tags (id, plan_id, tag_id, created_at, updated_at) VALUES (#{quote(pt_id)}, #{quote(plan_id)}, #{quote(actual_tag_id)}, NOW(), NOW())"
+      end
+    end
+
+    # Reset counter caches
+    execute <<~SQL
+      UPDATE coplan_tags SET plans_count = (
+        SELECT COUNT(*) FROM coplan_plan_tags WHERE coplan_plan_tags.tag_id = coplan_tags.id
+      )
+    SQL
+
+    remove_column :coplan_plans, :tags
+  end
+
+  def down
+    add_column :coplan_plans, :tags, :json
+  end
+end
diff --git a/db/schema.rb b/db/schema.rb
index 2d39b1b..b94a5cd 100644
--- a/db/schema.rb
+++ b/db/schema.rb
@@ -10,7 +10,7 @@
 #
 # It's strongly recommended that you check this file into your version control system.
 
-ActiveRecord::Schema[8.1].define(version: 2026_04_03_202530) do
+ActiveRecord::Schema[8.1].define(version: 2026_04_03_220001) do
   create_table "active_admin_comments", id: { type: :string, limit: 36 }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.bigint "author_id"
     t.string "author_type"
@@ -148,6 +148,15 @@
     t.index ["user_id"], name: "index_coplan_plan_collaborators_on_user_id"
   end
 
+  create_table "coplan_plan_tags", id: { type: :string, limit: 36 }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.datetime "created_at", null: false
+    t.string "plan_id", limit: 36, null: false
+    t.string "tag_id", limit: 36, null: false
+    t.datetime "updated_at", null: false
+    t.index ["plan_id", "tag_id"], name: "index_coplan_plan_tags_on_plan_id_and_tag_id", unique: true
+    t.index ["tag_id"], name: "index_coplan_plan_tags_on_tag_id"
+  end
+
   create_table "coplan_plan_types", id: { type: :string, limit: 36 }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.datetime "created_at", null: false
     t.json "default_tags"
@@ -199,7 +208,6 @@
     t.json "metadata"
     t.string "plan_type_id", limit: 36
     t.string "status", default: "brainstorm", null: false
-    t.json "tags"
     t.string "title", null: false
     t.datetime "updated_at", null: false
     t.index ["created_by_user_id"], name: "index_coplan_plans_on_created_by_user_id"
@@ -209,6 +217,14 @@
     t.index ["updated_at"], name: "index_coplan_plans_on_updated_at"
   end
 
+  create_table "coplan_tags", id: { type: :string, limit: 36 }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.datetime "created_at", null: false
+    t.string "name", null: false
+    t.integer "plans_count", default: 0, null: false
+    t.datetime "updated_at", null: false
+    t.index ["name"], name: "index_coplan_tags_on_name", unique: true
+  end
+
   create_table "coplan_users", id: { type: :string, limit: 36 }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.boolean "admin", default: false, null: false
     t.string "avatar_url"
@@ -243,6 +259,8 @@
   add_foreign_key "coplan_plan_collaborators", "coplan_plans", column: "plan_id"
   add_foreign_key "coplan_plan_collaborators", "coplan_users", column: "added_by_user_id"
   add_foreign_key "coplan_plan_collaborators", "coplan_users", column: "user_id"
+  add_foreign_key "coplan_plan_tags", "coplan_plans", column: "plan_id"
+  add_foreign_key "coplan_plan_tags", "coplan_tags", column: "tag_id"
   add_foreign_key "coplan_plan_versions", "coplan_plans", column: "plan_id"
   add_foreign_key "coplan_plan_viewers", "coplan_plans", column: "plan_id"
   add_foreign_key "coplan_plan_viewers", "coplan_users", column: "user_id"
diff --git a/engine/app/controllers/coplan/api/v1/plans_controller.rb b/engine/app/controllers/coplan/api/v1/plans_controller.rb
index 5947158..3b4c67e 100644
--- a/engine/app/controllers/coplan/api/v1/plans_controller.rb
+++ b/engine/app/controllers/coplan/api/v1/plans_controller.rb
@@ -48,8 +48,8 @@ def update
           permitted = {}
           permitted[:title] = params[:title] if params.key?(:title)
           permitted[:status] = params[:status] if params.key?(:status)
-          permitted[:tags] = params[:tags] if params.key?(:tags)
 
+          @plan.tag_names = params[:tags] if params.key?(:tags)
           @plan.update!(permitted)
 
           if @plan.saved_changes?
@@ -86,7 +86,7 @@ def plan_json(plan)
             title: plan.title,
             status: plan.status,
             current_revision: plan.current_revision,
-            tags: plan.tags,
+            tags: plan.tag_names,
             plan_type_id: plan.plan_type_id,
             plan_type_name: plan.plan_type&.name,
             created_by: plan.created_by_user.name,
diff --git a/engine/app/controllers/coplan/api/v1/tags_controller.rb b/engine/app/controllers/coplan/api/v1/tags_controller.rb
new file mode 100644
index 0000000..0c6521b
--- /dev/null
+++ b/engine/app/controllers/coplan/api/v1/tags_controller.rb
@@ -0,0 +1,29 @@
+module CoPlan
+  module Api
+    module V1
+      class TagsController < BaseController
+        def index
+          visible_plans = Plan.where.not(status: "brainstorm")
+            .or(Plan.where(created_by_user: current_user))
+
+          tags = Tag
+            .joins(:plan_tags)
+            .where(plan_tags: { plan_id: visible_plans.select(:id) })
+            .select("coplan_tags.*, COUNT(plan_tags.plan_id) AS visible_plans_count")
+            .group("coplan_tags.id")
+            .order("visible_plans_count DESC, coplan_tags.name ASC")
+
+          render json: tags.map { |t|
+            {
+              id: t.id,
+              name: t.name,
+              plans_count: t.visible_plans_count,
+              created_at: t.created_at,
+              updated_at: t.updated_at
+            }
+          }
+        end
+      end
+    end
+  end
+end
diff --git a/engine/app/models/coplan/plan.rb b/engine/app/models/coplan/plan.rb
index c1e3015..eac9e91 100644
--- a/engine/app/models/coplan/plan.rb
+++ b/engine/app/models/coplan/plan.rb
@@ -11,10 +11,11 @@ class Plan < ApplicationRecord
     has_many :comment_threads, dependent: :destroy
     has_many :edit_sessions, dependent: :destroy
     has_one :edit_lease, dependent: :destroy
+    has_many :plan_tags, dependent: :destroy
+    has_many :tags, through: :plan_tags, source: :tag
     has_many :plan_viewers, dependent: :destroy
     has_many :notifications, dependent: :destroy
 
-    after_initialize { self.tags ||= [] }
     after_initialize { self.metadata ||= {} }
 
     validates :title, presence: true
@@ -35,5 +36,14 @@ def to_param
     def current_content
       current_plan_version&.content_markdown
     end
+
+    def tag_names
+      tags.pluck(:name)
+    end
+
+    def tag_names=(names)
+      desired = Array(names).map(&:strip).reject(&:blank?).uniq
+      self.tags = desired.map { |name| Tag.find_or_create_by!(name: name) }
+    end
   end
 end
diff --git a/engine/app/models/coplan/plan_tag.rb b/engine/app/models/coplan/plan_tag.rb
new file mode 100644
index 0000000..210fa69
--- /dev/null
+++ b/engine/app/models/coplan/plan_tag.rb
@@ -0,0 +1,16 @@
+module CoPlan
+  class PlanTag < ApplicationRecord
+    belongs_to :plan
+    belongs_to :tag, counter_cache: :plans_count
+
+    validates :tag_id, uniqueness: { scope: :plan_id }
+
+    def self.ransackable_attributes(auth_object = nil)
+      %w[id plan_id tag_id created_at updated_at]
+    end
+
+    def self.ransackable_associations(auth_object = nil)
+      %w[plan tag]
+    end
+  end
+end
diff --git a/engine/app/models/coplan/tag.rb b/engine/app/models/coplan/tag.rb
new file mode 100644
index 0000000..0ed238b
--- /dev/null
+++ b/engine/app/models/coplan/tag.rb
@@ -0,0 +1,16 @@
+module CoPlan
+  class Tag < ApplicationRecord
+    has_many :plan_tags, dependent: :destroy
+    has_many :plans, through: :plan_tags
+
+    validates :name, presence: true, uniqueness: true
+
+    def self.ransackable_attributes(auth_object = nil)
+      %w[id name plans_count created_at updated_at]
+    end
+
+    def self.ransackable_associations(auth_object = nil)
+      %w[plan_tags plans]
+    end
+  end
+end
diff --git a/engine/app/views/coplan/agent_instructions/show.text.erb b/engine/app/views/coplan/agent_instructions/show.text.erb
index 095052f..e90123b 100644
--- a/engine/app/views/coplan/agent_instructions/show.text.erb
+++ b/engine/app/views/coplan/agent_instructions/show.text.erb
@@ -46,6 +46,26 @@ Update plan metadata (title, status, tags). Only fields included in the request
 
 Allowed fields: `title` (string), `status` (string), `tags` (array of strings).
 
+### List Tags
+
+```bash
+<%= @curl %> \
+  "<%= @base %>/api/v1/tags" | jq .
+```
+
+Returns all tags sorted by usage frequency (most-used first). Each tag includes `id`, `name`, `plans_count`.
+
+### Tags
+
+Tags categorize plans for discovery. Use them to help people find related plans.
+
+**Guidelines:**
+- Check existing tags with `GET /api/v1/tags` before creating new ones — reuse over invention.
+- Use lowercase, hyphenated names (e.g., `infrastructure`, `api-design`, `cost-reduction`).
+- Apply 1–3 tags per plan. Too many tags dilute meaning.
+- Update tags via `PATCH /api/v1/plans/:id` with `{"tags": ["tag-1", "tag-2"]}`.
+- Good tags describe the *domain* or *concern* (e.g., `security`, `onboarding`), not the status or team.
+
 ### Plan Statuses
 
 Plans move through a lifecycle. **Keep the status current** — update it as the plan progresses.
diff --git a/engine/config/routes.rb b/engine/config/routes.rb
index 617e1b5..7abfbe8 100644
--- a/engine/config/routes.rb
+++ b/engine/config/routes.rb
@@ -24,6 +24,7 @@
       resources :users, only: [] do
         get :search, on: :collection
       end
+      resources :tags, only: [:index]
       resources :plans, only: [:index, :show, :create, :update] do
         get :versions, on: :member
         get :comments, on: :member
diff --git a/engine/db/migrate/20260403000000_create_coplan_tags.rb b/engine/db/migrate/20260403000000_create_coplan_tags.rb
new file mode 100644
index 0000000..1166007
--- /dev/null
+++ b/engine/db/migrate/20260403000000_create_coplan_tags.rb
@@ -0,0 +1,22 @@
+class CreateCoplanTags < ActiveRecord::Migration[8.1]
+  def change
+    create_table :coplan_tags, id: { type: :string, limit: 36 } do |t|
+      t.string :name, null: false
+      t.integer :plans_count, default: 0, null: false
+      t.timestamps
+    end
+
+    add_index :coplan_tags, :name, unique: true
+
+    create_table :coplan_plan_tags, id: { type: :string, limit: 36 } do |t|
+      t.string :plan_id, limit: 36, null: false
+      t.string :tag_id, limit: 36, null: false
+      t.timestamps
+    end
+
+    add_index :coplan_plan_tags, [:plan_id, :tag_id], unique: true
+    add_index :coplan_plan_tags, :tag_id
+    add_foreign_key :coplan_plan_tags, :coplan_plans, column: :plan_id
+    add_foreign_key :coplan_plan_tags, :coplan_tags, column: :tag_id
+  end
+end
diff --git a/engine/db/migrate/20260403100000_backfill_structured_tags.rb b/engine/db/migrate/20260403100000_backfill_structured_tags.rb
new file mode 100644
index 0000000..dc84c51
--- /dev/null
+++ b/engine/db/migrate/20260403100000_backfill_structured_tags.rb
@@ -0,0 +1,39 @@
+class BackfillStructuredTags < ActiveRecord::Migration[8.1]
+  def up
+    # Backfill Tag + PlanTag records from JSON tags column
+    execute(<<~SQL).each do |row|
+      SELECT id, tags FROM coplan_plans WHERE tags IS NOT NULL
+    SQL
+      plan_id = row[0] || row["id"]
+      raw = row[1] || row["tags"]
+      next if raw.blank?
+
+      parsed = raw.is_a?(String) ? JSON.parse(raw) : raw
+      next unless parsed.is_a?(Array)
+
+      parsed.each do |name|
+        name = name.to_s.strip
+        next if name.blank?
+
+        tag_id = SecureRandom.uuid_v7
+        execute "INSERT IGNORE INTO coplan_tags (id, name, plans_count, created_at, updated_at) VALUES (#{quote(tag_id)}, #{quote(name)}, 0, NOW(), NOW())"
+        actual_tag_id = select_value("SELECT id FROM coplan_tags WHERE name = #{quote(name)}")
+        pt_id = SecureRandom.uuid_v7
+        execute "INSERT IGNORE INTO coplan_plan_tags (id, plan_id, tag_id, created_at, updated_at) VALUES (#{quote(pt_id)}, #{quote(plan_id)}, #{quote(actual_tag_id)}, NOW(), NOW())"
+      end
+    end
+
+    # Reset counter caches
+    execute <<~SQL
+      UPDATE coplan_tags SET plans_count = (
+        SELECT COUNT(*) FROM coplan_plan_tags WHERE coplan_plan_tags.tag_id = coplan_tags.id
+      )
+    SQL
+
+    remove_column :coplan_plans, :tags
+  end
+
+  def down
+    add_column :coplan_plans, :tags, :json
+  end
+end
diff --git a/spec/factories/plan_tags.rb b/spec/factories/plan_tags.rb
new file mode 100644
index 0000000..cd5ee41
--- /dev/null
+++ b/spec/factories/plan_tags.rb
@@ -0,0 +1,6 @@
+FactoryBot.define do
+  factory :plan_tag, class: "CoPlan::PlanTag" do
+    plan
+    tag
+  end
+end
diff --git a/spec/factories/tags.rb b/spec/factories/tags.rb
new file mode 100644
index 0000000..a98694f
--- /dev/null
+++ b/spec/factories/tags.rb
@@ -0,0 +1,5 @@
+FactoryBot.define do
+  factory :tag, class: "CoPlan::Tag" do
+    sequence(:name) { |n| "tag-#{n}" }
+  end
+end
diff --git a/spec/models/plan_tag_spec.rb b/spec/models/plan_tag_spec.rb
new file mode 100644
index 0000000..2cce923
--- /dev/null
+++ b/spec/models/plan_tag_spec.rb
@@ -0,0 +1,27 @@
+require "rails_helper"
+
+RSpec.describe CoPlan::PlanTag, type: :model do
+  it "is valid with valid attributes" do
+    plan_tag = create(:plan_tag)
+    expect(plan_tag).to be_valid
+  end
+
+  it "requires unique tag per plan" do
+    plan = create(:plan)
+    tag = create(:tag)
+    create(:plan_tag, plan: plan, tag: tag)
+    duplicate = build(:plan_tag, plan: plan, tag: tag)
+    expect(duplicate).not_to be_valid
+    expect(duplicate.errors[:tag_id]).to include("has already been taken")
+  end
+
+  it "belongs to plan" do
+    plan_tag = create(:plan_tag)
+    expect(plan_tag.plan).to be_a(CoPlan::Plan)
+  end
+
+  it "belongs to tag" do
+    plan_tag = create(:plan_tag)
+    expect(plan_tag.tag).to be_a(CoPlan::Tag)
+  end
+end
diff --git a/spec/models/tag_spec.rb b/spec/models/tag_spec.rb
new file mode 100644
index 0000000..b53f99b
--- /dev/null
+++ b/spec/models/tag_spec.rb
@@ -0,0 +1,63 @@
+require "rails_helper"
+
+RSpec.describe CoPlan::Tag, type: :model do
+  it "is valid with valid attributes" do
+    tag = create(:tag)
+    expect(tag).to be_valid
+  end
+
+  it "requires name" do
+    tag = build(:tag, name: nil)
+    expect(tag).not_to be_valid
+    expect(tag.errors[:name]).to include("can't be blank")
+  end
+
+  it "requires unique name" do
+    create(:tag, name: "infrastructure")
+    tag = build(:tag, name: "infrastructure")
+    expect(tag).not_to be_valid
+    expect(tag.errors[:name]).to include("has already been taken")
+  end
+
+  it "has many plans through plan_tags" do
+    tag = create(:tag)
+    plan = create(:plan)
+    create(:plan_tag, plan: plan, tag: tag)
+    expect(tag.plans).to include(plan)
+  end
+
+  it "updates plans_count via counter cache" do
+    tag = create(:tag)
+    plan = create(:plan)
+    expect { create(:plan_tag, plan: plan, tag: tag) }.to change { tag.reload.plans_count }.from(0).to(1)
+  end
+
+  describe "Plan#tag_names=" do
+    it "creates Tag and PlanTag records" do
+      plan = create(:plan)
+      plan.tag_names = ["infrastructure", "api-design"]
+      expect(plan.tag_names).to match_array(["infrastructure", "api-design"])
+      expect(CoPlan::Tag.where(name: "infrastructure")).to exist
+    end
+
+    it "reuses existing Tag records" do
+      create(:tag, name: "security")
+      plan = create(:plan)
+      expect { plan.tag_names = ["security"] }.not_to change(CoPlan::Tag, :count)
+      expect(plan.tag_names).to eq(["security"])
+    end
+
+    it "removes old associations when tags change" do
+      plan = create(:plan)
+      plan.tag_names = ["alpha", "beta"]
+      plan.tag_names = ["beta", "gamma"]
+      expect(plan.tag_names).to match_array(["beta", "gamma"])
+    end
+
+    it "handles blank and duplicate names" do
+      plan = create(:plan)
+      plan.tag_names = ["  infra  ", "infra", "", "api"]
+      expect(plan.tag_names).to match_array(["infra", "api"])
+    end
+  end
+end
diff --git a/spec/requests/api/v1/plans_spec.rb b/spec/requests/api/v1/plans_spec.rb
index 947cc0f..561ef9c 100644
--- a/spec/requests/api/v1/plans_spec.rb
+++ b/spec/requests/api/v1/plans_spec.rb
@@ -107,8 +107,8 @@
       patch api_v1_plan_path(plan), params: { tags: ["infra", "api"] }, headers: headers, as: :json
       expect(response).to have_http_status(:success)
       body = JSON.parse(response.body)
-      expect(body["tags"]).to eq(["infra", "api"])
-      expect(plan.reload.tags).to eq(["infra", "api"])
+      expect(body["tags"]).to match_array(["infra", "api"])
+      expect(plan.reload.tag_names).to match_array(["infra", "api"])
     end
 
     it "updates multiple fields at once" do
diff --git a/spec/requests/api/v1/tags_spec.rb b/spec/requests/api/v1/tags_spec.rb
new file mode 100644
index 0000000..a5a634d
--- /dev/null
+++ b/spec/requests/api/v1/tags_spec.rb
@@ -0,0 +1,68 @@
+require "rails_helper"
+
+RSpec.describe "Api::V1::Tags", type: :request do
+  let(:alice) { create(:coplan_user, :admin) }
+  let(:alice_token) { create(:api_token, user: alice, raw_token: "test-token-alice") }
+  let(:headers) { { "Authorization" => "Bearer test-token-alice" } }
+
+  before do
+    alice_token # ensure token exists
+  end
+
+  it "index returns tags sorted by plans_count descending" do
+    popular = create(:tag, name: "infrastructure")
+    niche = create(:tag, name: "experimental")
+    plan1 = create(:plan, :considering, created_by_user: alice)
+    plan2 = create(:plan, :considering, created_by_user: alice)
+    create(:plan_tag, plan: plan1, tag: popular)
+    create(:plan_tag, plan: plan2, tag: popular)
+    create(:plan_tag, plan: plan1, tag: niche)
+
+    get api_v1_tags_path, headers: headers
+    expect(response).to have_http_status(:success)
+    tags = JSON.parse(response.body)
+    expect(tags.length).to eq(2)
+    expect(tags.first["name"]).to eq("infrastructure")
+    expect(tags.first["plans_count"]).to eq(2)
+    expect(tags.second["name"]).to eq("experimental")
+    expect(tags.second["plans_count"]).to eq(1)
+  end
+
+  it "index returns empty array when no tags exist" do
+    get api_v1_tags_path, headers: headers
+    expect(response).to have_http_status(:success)
+    tags = JSON.parse(response.body)
+    expect(tags).to eq([])
+  end
+
+  it "index requires authentication" do
+    get api_v1_tags_path
+    expect(response).to have_http_status(:unauthorized)
+  end
+
+  it "excludes tags only attached to other users' brainstorm plans" do
+    bob = create(:coplan_user)
+    secret_tag = create(:tag, name: "secret-project")
+    visible_tag = create(:tag, name: "public-project")
+    brainstorm_plan = create(:plan, created_by_user: bob) # brainstorm by default
+    public_plan = create(:plan, :considering, created_by_user: bob)
+    create(:plan_tag, plan: brainstorm_plan, tag: secret_tag)
+    create(:plan_tag, plan: public_plan, tag: visible_tag)
+
+    get api_v1_tags_path, headers: headers
+    tags = JSON.parse(response.body)
+    tag_names = tags.map { |t| t["name"] }
+    expect(tag_names).to include("public-project")
+    expect(tag_names).not_to include("secret-project")
+  end
+
+  it "includes tags from the current user's own brainstorm plans" do
+    my_tag = create(:tag, name: "my-draft")
+    brainstorm_plan = create(:plan, created_by_user: alice)
+    create(:plan_tag, plan: brainstorm_plan, tag: my_tag)
+
+    get api_v1_tags_path, headers: headers
+    tags = JSON.parse(response.body)
+    expect(tags.map { |t| t["name"] }).to include("my-draft")
+  end
+end