diff --git a/apps/api/Gemfile.lock b/apps/api/Gemfile.lock
index 54c462d..4690bb5 100644
--- a/apps/api/Gemfile.lock
+++ b/apps/api/Gemfile.lock
@@ -1,7 +1,7 @@
 PATH
   remote: ../libs/ruby/warp_engine
   specs:
-    warp_engine (0.4.0)
+    warp_engine (0.5.0)
       apipie-rails
       blueprinter
       rails (>= 8.0)
diff --git a/libs/ruby/warp_engine/README.md b/libs/ruby/warp_engine/README.md
index 3d30fa8..8dc0104 100644
--- a/libs/ruby/warp_engine/README.md
+++ b/libs/ruby/warp_engine/README.md
@@ -5,7 +5,7 @@ software catalog: catalog models, a CI-pipeline-callable release updater, a
 public read-only JSON API, and optional ActiveAdmin resources that plug into
 your app's existing admin.
 
-Repository: `https://git.teletypegames.org/tools/warp_engine`
+Repository: `https://git.teletypegames.org/engines/warp_engine`
 
 ## Features
 
@@ -18,6 +18,12 @@ Repository: `https://git.teletypegames.org/tools/warp_engine`
   box: TIC-80, Ebitengine, LÖVE, C64, Godot, Bevy, Phaser. Authenticated by
   a shared secret or by per-owner database tokens with expiry and scopes
   (`ApplicationToken`, managed in the admin).
+- **Pluggable access**: a host supplies a policy and the catalog gains prices,
+  entitlements and gated downloads — and *says so* in its API, so clients can
+  show a paid title as paid instead of failing at the download. Default `:open`
+  is the catalog as it always was.
+- **Client sign-in**: an RFC 8628 device authorization grant for clients with no
+  browser of their own, over the host's own user model. Off unless configured.
 - **Pluggable storage**: artifacts are served through a storage adapter
   (`:local` by default); a host can serve them from an object store without
   patching the engine.
@@ -152,7 +158,7 @@ From the git repository:
 
 ```ruby
 # Gemfile
-gem "warp_engine", git: "https://git.teletypegames.org/tools/warp_engine.git"
+gem "warp_engine", git: "https://git.teletypegames.org/engines/warp_engine.git"
 ```
 
 Or from the Forgejo rubygems registry (tagged releases):
@@ -206,6 +212,15 @@ Rails.application.config.to_prepare do
     # after backfilling owners — ownerless softwares are claimable by anyone.
     # c.enforce_software_ownership = true
 
+    # Who may see a title and who may download it (see "Access" below).
+    # :open (default) lists everything and serves everything.
+    # c.access_policy = MyStore::AccessPolicy.new
+
+    # Client sign-in (see "Client sign-in" below). nil (default) means there is
+    # none: /api/auth/* answers 404 and GET /api/service reports auth: null.
+    # c.access_token_owner_class  = "User"
+    # c.identity_verification_url = "/devices"
+
     # If your app's own models reference catalog images, register them so the
     # admin Images page counts them as "in use":
     # c.image_owners = [
@@ -369,6 +384,91 @@ signing adapter turns them into redirects without any further change.
 the admin file manager — still writes to the local disk. A remote adapter
 needs its own upload path today.
 
+## Access
+
+Who may see a title, and who may download it. The default answers "everyone" to
+both — every software listed, every artifact served, no prices — which is the
+catalog the engine always had:
+
+```ruby
+c.access_policy = :open   # default
+```
+
+A host that sells supplies a policy instead. The contract is three methods:
+
+```ruby
+class MyStore::AccessPolicy
+  # Which titles GET /api/software lists at all.
+  def visible_software_scope(subject: nil) = ...   # an ActiveRecord scope
+
+  # What a client is told about one title.
+  def access_for(software:, subject: nil)
+    WarpEngine::Access.new(
+      gated: true, entitled: false,           # needs an entitlement; this caller has none
+      price_cents: 1490, currency: "EUR",
+      purchase_url: "https://shop.example/games/slug",
+      web_url: "https://shop.example/play/slug"   # nil keeps the engine's own /file/ path
+    )
+  end
+
+  # nil refuses the download; a Grant allows it.
+  def authorize_download(asset:, subject:, request:) = WarpEngine::Access::Grant.new
+end
+
+c.access_policy = MyStore::AccessPolicy.new
+```
+
+`subject` is whoever the request authenticated as, or `nil` for an anonymous
+caller — deliberately untyped, because the engine has no user model and whose
+object this is belongs to the host.
+
+Every catalog entry carries an `access` block, **including under the open
+policy**, so a client never has to tell "this catalog says nothing" from "this
+title is not gated":
+
+```json
+"access": { "gated": false, "entitled": true, "price": null,
+            "purchaseUrl": null, "webUrl": null }
+```
+
+The vocabulary is generic on purpose. A client reads more than one store, and a
+word from any one host's domain would make it specific to that host.
+
+**A policy that raises is treated as a refusal**: an empty catalog and a denied
+download, logged. An artifact served because the gatekeeper crashed is the one
+failure mode this engine must not have.
+
+## Client sign-in
+
+A desktop client has no cookie jar and no browser session, so it cannot host a
+login form without asking somebody to type a password into a window that is not
+a browser. The engine implements the device authorization grant (RFC 8628)
+instead — but only where a host has said whose tokens these are:
+
+```ruby
+c.access_token_owner_class  = "User"      # nil (default): no sign-in at all
+c.identity_verification_url = "/devices"  # your page where a person types the code
+c.device_code_ttl           = 600
+c.device_code_interval      = 5
+```
+
+With `access_token_owner_class` unset, `/api/auth/*` answers 404 and
+`GET /api/service` reports `auth: null`, so a client offers no sign-in.
+
+The flow:
+
+1. the client `POST`s `/api/auth/device` and shows the `userCode` it gets back;
+2. the person opens `verificationUrl` in a browser and types that code;
+3. **your page** calls `WarpEngine::DeviceGrantService#approve(user_code:, subject:)`
+   with the signed-in user — approving needs a session and HTML, neither of
+   which is the engine's business;
+4. the client's next `POST /api/auth/device/token` carries the token away. It is
+   handed over exactly once and never stored in the clear afterwards.
+
+The token is a `WarpEngine::ApplicationToken` with the `catalog` scope, sent as
+`Authorization: Bearer …`. `DELETE /api/auth/token` revokes it (signing out),
+and the admin lists both kinds of token and the sign-ins behind them.
+
 ## Publish events
 
 Publishing a release emits an `ActiveSupport::Notifications` event, so a host
@@ -387,17 +487,29 @@ end
 Hosts that must support older engine versions can feature-detect with
 `WarpEngine.respond_to?(:instruments_publish?) && WarpEngine.instruments_publish?`.
 
+Downloads emit one too — `warp_engine.download`, with `path`, `asset`,
+`release`, `software`, `subject` and the `Download` record — so a host can keep
+its own account of who fetched what without reaching into `DownloadService`.
+
 ## Public API
 
 | Endpoint | Purpose |
 | --- | --- |
-| `GET /api/software` | Full catalog with releases, assets, links, download counts; `?owner_id=` filters to one publisher |
+| `GET /api/service` | What this deployment is: version, whether the catalog gates, and how to sign in (or that you cannot) |
+| `GET /api/software` | Full catalog with releases, assets, links, download counts and an `access` block; `?owner_id=` filters to one publisher |
 | `GET /api/software/highlighted` | The currently highlighted title |
 | `GET /api/builds` | Expected asset kinds per platform (build matrix) |
 | `GET /api/softwares/:name/builds` | Actual vs. missing build assets per release |
 | `GET /api/image/:id` | Serves catalog images |
 | `GET /api/download?path=` | Serves an artifact and logs a download record |
 | `GET /file/*path` | Serves static build output (web-playable games, docs) |
+| `POST /api/auth/device` | Starts a device sign-in; returns the code pair (404 without a client identity) |
+| `POST /api/auth/device/token` | Polls a device sign-in for its token |
+| `DELETE /api/auth/token` | Revokes the bearer token on the request (signing out) |
+
+Every read endpoint accepts an optional `Authorization: Bearer …`; none requires
+one. What it changes is what the access policy is asked about — an anonymous
+caller is a normal, supported caller.
 
 ### `WarpEngine-Version`
 
@@ -406,7 +518,7 @@ client can branch on the engine's age without a round trip to ask:
 
 ```
 $ curl -sI https://teletypegames.org/api/software | grep -i warpengine
-WarpEngine-Version: 0.4.0
+WarpEngine-Version: 0.5.0
 ```
 
 Set before the action runs rather than after, which means an error response carries it
diff --git a/libs/ruby/warp_engine/app/admin/application_tokens.rb b/libs/ruby/warp_engine/app/admin/application_tokens.rb
index dcc9a66..cc24fba 100644
--- a/libs/ruby/warp_engine/app/admin/application_tokens.rb
+++ b/libs/ruby/warp_engine/app/admin/application_tokens.rb
@@ -10,6 +10,12 @@ ActiveAdmin.register WarpEngine::ApplicationToken, as: "Application Token" do
   scope :all, default: true
   scope("Active")  { |scope| scope.where("expires_at IS NULL OR expires_at > ?", Time.current) }
   scope("Expired") { |scope| scope.where("expires_at <= ?", Time.current) }
+  # Two kinds of token share this table: one publishes software, the other reads the
+  # catalog from somebody's desktop client. They are told apart by scope, and an admin
+  # looking for one is rarely looking for the other.
+  CATALOG_SCOPE_SQL = %(JSON_CONTAINS(COALESCE(scopes, '[]'), '"catalog"')).freeze
+  scope("Publishing") { |scope| scope.where("NOT #{CATALOG_SCOPE_SQL}") }
+  scope("Clients")    { |scope| scope.where(CATALOG_SCOPE_SQL) }
 
   index do
     id_column
@@ -45,7 +51,7 @@ ActiveAdmin.register WarpEngine::ApplicationToken, as: "Application Token" do
       end
       f.input :name
       f.input :scopes_string, label: "Scopes (comma separated)",
-              hint: %(The "update" scope is required for /build/publish, the "upload" scope for /build/upload.)
+              hint: %(The "update" scope is required for /build/publish, the "upload" scope for /build/upload, the "catalog" scope for a client reading the API. Client tokens are normally issued by device sign-in rather than created here.)
       f.input :unrestricted, hint: "Internal token: exempt from owner isolation (enforce_software_ownership)."
       f.input :expires_at, hint: "Leave empty for a token that never expires."
     end
diff --git a/libs/ruby/warp_engine/app/admin/device_grants.rb b/libs/ruby/warp_engine/app/admin/device_grants.rb
new file mode 100644
index 0000000..8a8a28d
--- /dev/null
+++ b/libs/ruby/warp_engine/app/admin/device_grants.rb
@@ -0,0 +1,62 @@
+ActiveAdmin.register WarpEngine::DeviceGrant, as: "Device Sign-in" do
+  # Read-only on purpose. A grant is created by a client and answered by a person on the
+  # host's own page; an admin creating one by hand would be issuing somebody else a
+  # credential, which is not a thing this page should make easy.
+  actions :index, :show
+
+  menu parent: "🌀 WarpEngine", priority: 10, label: "📱 Device Sign-ins",
+       if: proc { WarpEngine.identity_configured? }
+
+  config.sort_order = "created_at_desc"
+  config.batch_actions = false
+
+  scope :all, default: true
+  scope("Pending")  { |scope| scope.where(approved_at: nil, denied_at: nil).where(expires_at: Time.current..) }
+  scope("Approved") { |scope| scope.where.not(approved_at: nil) }
+  scope("Denied")   { |scope| scope.where.not(denied_at: nil) }
+
+  index do
+    id_column
+    column("Code") { |g| code g.formatted_user_code, style: "font-family:monospace;" }
+    column("Device") { |g| g.client_name }
+    column("State") { |g| status_tag g.state.to_s }
+    column("Who") do |g|
+      next "—" if g.subject_id.blank?
+
+      subject = g.subject
+      subject.try(:email) || subject.try(:name) || "#{g.subject_type} ##{g.subject_id}"
+    end
+    column :expires_at
+    column :created_at
+  end
+
+  filter :client_name_cont, label: "Device"
+  filter :created_at
+  filter :expires_at
+
+  show do
+    attributes_table do
+      row("User code") { |g| code g.formatted_user_code, style: "font-family:monospace;" }
+      row("Device") { |g| g.client_name }
+      row("State") { |g| status_tag g.state.to_s }
+      row("Who") do |g|
+        next "—" if g.subject_id.blank?
+
+        subject = g.subject
+        subject.try(:email) || subject.try(:name) || "#{g.subject_type} ##{g.subject_id}"
+      end
+      # The device code itself is never shown: it is the client's live credential for as
+      # long as the grant is pending, and this page is not where it should leak from.
+      row("Token") do |g|
+        token = g.application_token
+        next "—" if token.nil?
+
+        link_to "#{token.token_prefix}… (#{token.name})", admin_application_token_path(token)
+      end
+      row :approved_at
+      row :denied_at
+      row :expires_at
+      row :created_at
+    end
+  end
+end
diff --git a/libs/ruby/warp_engine/app/controllers/concerns/warp_engine/subject_authentication.rb b/libs/ruby/warp_engine/app/controllers/concerns/warp_engine/subject_authentication.rb
new file mode 100644
index 0000000..0de8e4b
--- /dev/null
+++ b/libs/ruby/warp_engine/app/controllers/concerns/warp_engine/subject_authentication.rb
@@ -0,0 +1,51 @@
+module WarpEngine
+  # Who the caller is, on the read-only side of the API.
+  #
+  # Distinct from UpdateAuthentication, which guards publishing: that one asks "may this
+  # pipeline write to the catalog", this one asks "whose library am I looking at". The
+  # answer is allowed to be nobody — an anonymous caller is a normal, supported caller,
+  # and a catalog with no policy configured never needs one.
+  #
+  # The credential is a bearer token, because that is what a client can carry: it has no
+  # cookie jar and no browser session.
+  module SubjectAuthentication
+    extend ActiveSupport::Concern
+
+    private
+
+    # The ApplicationToken behind the request, or nil.
+    def current_access_token
+      return @current_access_token if defined?(@current_access_token)
+
+      @current_access_token = resolve_access_token
+    end
+
+    # Whoever that token belongs to — the host's own object. nil when the request is
+    # anonymous, when the token is unknown, or when no subject class is configured.
+    def current_subject
+      current_access_token&.owner
+    end
+
+    def resolve_access_token
+      return nil unless WarpEngine.identity_configured?
+
+      token = bearer_token
+      return nil if token.blank?
+
+      record = WarpEngine::ApplicationToken.authenticate(
+        token, required_scope: WarpEngine::ApplicationToken::CATALOG_SCOPE
+      )
+      return nil if record.nil?
+
+      record.touch_last_used!
+      record
+    end
+
+    def bearer_token
+      header = request.headers["Authorization"].to_s
+      return nil unless header.start_with?("Bearer ")
+
+      header.delete_prefix("Bearer ").strip.presence
+    end
+  end
+end
diff --git a/libs/ruby/warp_engine/app/controllers/warp_engine/api/auth/devices_controller.rb b/libs/ruby/warp_engine/app/controllers/warp_engine/api/auth/devices_controller.rb
new file mode 100644
index 0000000..20aebaa
--- /dev/null
+++ b/libs/ruby/warp_engine/app/controllers/warp_engine/api/auth/devices_controller.rb
@@ -0,0 +1,61 @@
+module WarpEngine
+  # The device authorization grant, client side (RFC 8628).
+  #
+  # Both actions are unauthenticated, and have to be: the whole point of the flow is
+  # that the caller has no credential yet. What protects it is that a device code is
+  # useless until a signed-in person approves it on the host's own page.
+  class Api::Auth::DevicesController < ApiController
+    before_action :ensure_identity_configured
+
+    resource_description do
+      short "Device sign-in"
+    end
+
+    api :POST, "/api/auth/device", "Start a device sign-in and get a code pair"
+    param :client_name, String, required: false, desc: "What to call this device in the person's account"
+    returns code: 200, desc: "The code pair and where to take it"
+    error code: 404, desc: "This deployment has no client sign-in"
+    def create
+      grant = service.request(client_name: params[:client_name])
+
+      render json: {
+        deviceCode:      grant.device_code,
+        userCode:        grant.formatted_user_code,
+        verificationUrl: service.verification_url(base_url: request.base_url),
+        interval:        WarpEngine.config.device_code_interval.to_i,
+        expiresIn:       (grant.expires_at - Time.current).to_i
+      }
+    end
+
+    api :POST, "/api/auth/device/token", "Poll a device sign-in for its token"
+    param :device_code, String, required: true, desc: "The device code from POST /api/auth/device"
+    returns code: 200, desc: "state is one of pending, approved, denied, expired"
+    error code: 404, desc: "No such device code, or no client sign-in here"
+    def token
+      state, plain = service.poll(device_code: params[:device_code])
+
+      # The token rides on the one poll that finds the grant newly approved; a client
+      # that loses it starts the flow again. Keeping a plain token around to hand out
+      # twice would mean storing it, which is the thing this design avoids.
+      body = { state: state.to_s }
+      body[:token] = plain if plain.present?
+      render json: body
+    rescue WarpEngine::DeviceGrantService::UnknownCode
+      render json: { error: "Not found" }, status: :not_found
+    end
+
+    private
+
+    def service
+      @service ||= WarpEngine::DeviceGrantService.new
+    end
+
+    # A deployment with no configured subject class has no sign-in at all, and says so
+    # the same way GET /api/service does — by not offering it.
+    def ensure_identity_configured
+      return if WarpEngine.identity_configured?
+
+      render json: { error: "Not found" }, status: :not_found
+    end
+  end
+end
diff --git a/libs/ruby/warp_engine/app/controllers/warp_engine/api/auth/tokens_controller.rb b/libs/ruby/warp_engine/app/controllers/warp_engine/api/auth/tokens_controller.rb
new file mode 100644
index 0000000..5c1914f
--- /dev/null
+++ b/libs/ruby/warp_engine/app/controllers/warp_engine/api/auth/tokens_controller.rb
@@ -0,0 +1,24 @@
+module WarpEngine
+  # Signing out: a client throws away its own token.
+  #
+  # Revocation is a soft delete on the ApplicationToken, so the record of which device
+  # signed in and when survives it. The person's own list of devices — where somebody
+  # revokes a token for a laptop they no longer have — is the host's page, because it
+  # needs a session and a browser.
+  class Api::Auth::TokensController < ApiController
+    resource_description do
+      short "Client tokens"
+    end
+
+    api :DELETE, "/api/auth/token", "Revoke the bearer token this request carries"
+    returns code: 204, desc: "Revoked"
+    error code: 401, desc: "No usable bearer token on the request"
+    def destroy
+      token = current_access_token
+      return head(:unauthorized) if token.nil?
+
+      WarpEngine::DeviceGrantService.new.revoke(token: token)
+      head :no_content
+    end
+  end
+end
diff --git a/libs/ruby/warp_engine/app/controllers/warp_engine/api/downloads_controller.rb b/libs/ruby/warp_engine/app/controllers/warp_engine/api/downloads_controller.rb
index 950169d..821daa8 100644
--- a/libs/ruby/warp_engine/app/controllers/warp_engine/api/downloads_controller.rb
+++ b/libs/ruby/warp_engine/app/controllers/warp_engine/api/downloads_controller.rb
@@ -10,6 +10,7 @@ module WarpEngine
     returns code: 200, desc: "File binary data"
     returns code: 302, desc: "Redirect to the storage location (non-local storage adapter)"
     error code: 400, desc: "Path is blank"
+    error code: 403, desc: "The access policy refused this caller"
     error code: 404, desc: "File not found"
     def show
       path = params[:path]
@@ -19,7 +20,9 @@ module WarpEngine
         path: path,
         ip: request.remote_ip,
         user_agent: request.user_agent,
-        referer: request.referer
+        referer: request.referer,
+        subject: current_subject,
+        request: request
       )
 
       if location.nil?
diff --git a/libs/ruby/warp_engine/app/controllers/warp_engine/api/service_controller.rb b/libs/ruby/warp_engine/app/controllers/warp_engine/api/service_controller.rb
new file mode 100644
index 0000000..09b01e3
--- /dev/null
+++ b/libs/ruby/warp_engine/app/controllers/warp_engine/api/service_controller.rb
@@ -0,0 +1,62 @@
+module WarpEngine
+  # What this deployment is and what it can do, in one unauthenticated request.
+  #
+  # This is how a client stops guessing. Before it existed, everything a client knew
+  # about a store was compiled into the client — which endpoints to call, whether
+  # signing in was a thing here, where to send somebody who wanted to buy something.
+  # Every one of those is a property of the *server*, and a client that carries them
+  # can only ever serve the one store it was built for.
+  class Api::ServiceController < ApiController
+    resource_description do
+      short "Service descriptor"
+    end
+
+    api :GET, "/api/service", "What this WarpEngine deployment offers"
+    desc <<~DESC
+      Public on purpose: a client reads this *before* it can have a credential.
+
+      `auth` is null where the host has configured no client identity — the catalog is
+      open, there is nobody to sign in as, and a client should not offer to. Where it is
+      present, `auth.device` describes the device authorization grant a client with no
+      browser of its own uses to sign in.
+
+      `catalog.gated` says whether any title here can require an entitlement. A client
+      can render a store that never gates differently from one that sometimes does,
+      without having to read the whole catalog first to find out.
+    DESC
+    returns code: 200, desc: "The descriptor" do
+      property :engine, String, desc: "Always 'warp_engine'"
+      property :version, String, desc: "Engine version, same value as the WarpEngine-Version header"
+      property :catalog, Hash, desc: "Catalog properties" do
+        property :gated, :boolean, desc: "Whether titles here can require an entitlement"
+      end
+      property :auth, Hash, desc: "How to sign in, or null where there is no sign-in"
+    end
+    def show
+      render json: {
+        engine:  "warp_engine",
+        version: WarpEngine::VERSION,
+        catalog: { gated: !WarpEngine::AccessPolicy.open? },
+        auth:    auth_descriptor
+      }
+    end
+
+    private
+
+    def auth_descriptor
+      return nil unless WarpEngine.identity_configured?
+
+      service = WarpEngine::DeviceGrantService.new
+      {
+        schemes: [ "bearer" ],
+        device: {
+          authorizeUrl:    warp_engine.api_auth_device_url,
+          tokenUrl:        warp_engine.api_auth_device_token_url,
+          revokeUrl:       warp_engine.api_auth_token_url,
+          verificationUrl: service.verification_url(base_url: request.base_url),
+          interval:        WarpEngine.config.device_code_interval.to_i
+        }
+      }
+    end
+  end
+end
diff --git a/libs/ruby/warp_engine/app/controllers/warp_engine/api/software_controller.rb b/libs/ruby/warp_engine/app/controllers/warp_engine/api/software_controller.rb
index b45d038..1322e8e 100644
--- a/libs/ruby/warp_engine/app/controllers/warp_engine/api/software_controller.rb
+++ b/libs/ruby/warp_engine/app/controllers/warp_engine/api/software_controller.rb
@@ -56,7 +56,7 @@ module WarpEngine
       end
     end
     def index
-      render json: WarpEngine::SoftwareService.new.index(owner_id: params[:owner_id])
+      render json: WarpEngine::SoftwareService.new.index(owner_id: params[:owner_id], subject: current_subject)
     end
   end
 end
diff --git a/libs/ruby/warp_engine/app/controllers/warp_engine/api/software_highlighted_controller.rb b/libs/ruby/warp_engine/app/controllers/warp_engine/api/software_highlighted_controller.rb
index 9e0640f..903c5f8 100644
--- a/libs/ruby/warp_engine/app/controllers/warp_engine/api/software_highlighted_controller.rb
+++ b/libs/ruby/warp_engine/app/controllers/warp_engine/api/software_highlighted_controller.rb
@@ -34,7 +34,7 @@ module WarpEngine
     end
     error code: 404, desc: "No highlighted software found"
     def index
-      result = WarpEngine::SoftwareHighlightedService.new.index
+      result = WarpEngine::SoftwareHighlightedService.new.index(subject: current_subject)
       if result
         render json: result
       else
diff --git a/libs/ruby/warp_engine/app/controllers/warp_engine/api_controller.rb b/libs/ruby/warp_engine/app/controllers/warp_engine/api_controller.rb
index 78a93af..78923d2 100644
--- a/libs/ruby/warp_engine/app/controllers/warp_engine/api_controller.rb
+++ b/libs/ruby/warp_engine/app/controllers/warp_engine/api_controller.rb
@@ -11,6 +11,10 @@ module WarpEngine
     # client needs the version most when something came back wrong.
     before_action :set_version_header
 
+    # Every read-only endpoint may be called with a bearer token; none of them requires
+    # one. See WarpEngine::SubjectAuthentication.
+    include WarpEngine::SubjectAuthentication
+
     rescue_from StandardError do |e|
       Rails.logger.error("[#{self.class.name}] #{e.class}: #{e.message}")
       render json: { error: "Internal server error" }, status: :internal_server_error
@@ -28,6 +32,10 @@ module WarpEngine
       render json: { error: e.message }, status: :bad_request
     end
 
+    rescue_from WarpEngine::DownloadService::Denied do
+      render json: { error: "Forbidden" }, status: :forbidden
+    end
+
     private
 
     def set_version_header
diff --git a/libs/ruby/warp_engine/app/controllers/warp_engine/files_controller.rb b/libs/ruby/warp_engine/app/controllers/warp_engine/files_controller.rb
index 17cd56d..c5cc320 100644
--- a/libs/ruby/warp_engine/app/controllers/warp_engine/files_controller.rb
+++ b/libs/ruby/warp_engine/app/controllers/warp_engine/files_controller.rb
@@ -9,9 +9,12 @@ module WarpEngine
     param :path, String, required: true, desc: "File path"
     returns code: 200, desc: "File binary data"
     returns code: 301, desc: "Redirect to file URL"
+    error code: 403, desc: "The access policy refused this caller"
     error code: 404, desc: "File not found"
     def show
-      result = WarpEngine::FileService.new.show(WarpEngine::FileShowInputDto.new(path: params[:path]))
+      result = WarpEngine::FileService.new.show(
+        WarpEngine::FileShowInputDto.new(path: params[:path]), subject: current_subject
+      )
       case result.type
       when :redirect  then redirect_to result.url, status: :moved_permanently
       when :file      then send_file result.path, disposition: "inline", type: resolve_mime(result.path)
diff --git a/libs/ruby/warp_engine/app/models/warp_engine/application_token.rb b/libs/ruby/warp_engine/app/models/warp_engine/application_token.rb
index a9a4d86..8ea307e 100644
--- a/libs/ruby/warp_engine/app/models/warp_engine/application_token.rb
+++ b/libs/ruby/warp_engine/app/models/warp_engine/application_token.rb
@@ -6,6 +6,9 @@ module WarpEngine
 
     UPDATE_SCOPE = "update".freeze
     UPLOAD_SCOPE = "upload".freeze
+    # A token held by a *client* rather than a publisher: it reads the catalog and
+    # downloads artifacts, and it never publishes anything.
+    CATALOG_SCOPE = "catalog".freeze
 
     # The generated token is only available in memory at creation time — the DB
     # stores nothing but the SHA256 digest and the non-secret prefix.
@@ -78,6 +81,15 @@ module WarpEngine
       self.owner_type = WarpEngine.config.application_token_owner_class if owner_type.blank?
     end
 
+    # Publishing tokens and client tokens share this table but not their owners: one
+    # belongs to whoever ships software, the other to whoever buys it. Both classes are
+    # the host's to name, and either is acceptable here — which of the two a given token
+    # may do is decided by its scopes, not by its owner.
+    def self.permitted_owner_types
+      [ WarpEngine.config.application_token_owner_class,
+        WarpEngine.config.access_token_owner_class ].compact_blank
+    end
+
     def generate_token
       return if token_digest.present?
 
@@ -87,11 +99,11 @@ module WarpEngine
     end
 
     def owner_type_matches_configuration
-      expected = WarpEngine.config.application_token_owner_class
-      if expected.blank?
+      permitted = self.class.permitted_owner_types
+      if permitted.empty?
         errors.add(:base, "application_token_owner_class is not configured")
-      elsif owner_type != expected
-        errors.add(:owner_type, "must be #{expected}")
+      elsif !permitted.include?(owner_type)
+        errors.add(:owner_type, "must be #{permitted.join(' or ')}")
       end
     end
 
diff --git a/libs/ruby/warp_engine/app/models/warp_engine/device_grant.rb b/libs/ruby/warp_engine/app/models/warp_engine/device_grant.rb
new file mode 100644
index 0000000..fb7050d
--- /dev/null
+++ b/libs/ruby/warp_engine/app/models/warp_engine/device_grant.rb
@@ -0,0 +1,97 @@
+module WarpEngine
+  # One pending sign-in from a client that has no browser of its own.
+  #
+  # The shape is RFC 8628's device authorization grant, and the reason for it is that a
+  # desktop client cannot host a login form without asking a person to type a password
+  # into a window that is not a browser. So the client asks for a pair of codes, sends
+  # the person to the host's own page with the short one, and polls with the long one
+  # until somebody approves it.
+  #
+  # Short-lived by design: this row exists for the minute or two between "the client
+  # asked" and "the person answered". What survives it is the ApplicationToken.
+  class DeviceGrant < ApplicationRecord
+    self.table_name = "device_grants"
+
+    # No I, O, 0 or 1: this alphabet is read off one screen and typed into another, and
+    # those four are where that goes wrong.
+    USER_CODE_ALPHABET = "ABCDEFGHJKLMNPQRSTUVWXYZ23456789".freeze
+    USER_CODE_LENGTH   = 8
+
+    belongs_to :application_token, class_name: "WarpEngine::ApplicationToken", optional: true
+    belongs_to :subject, polymorphic: true, optional: true
+
+    validates :device_code, presence: true, uniqueness: true
+    validates :user_code, presence: true, uniqueness: true
+    validates :expires_at, presence: true
+
+    scope :pending, -> { where(approved_at: nil, denied_at: nil).where(expires_at: Time.current..) }
+
+    before_validation :generate_codes, on: :create
+    before_validation :set_expiry, on: :create
+
+    def self.find_pending_by_user_code(code)
+      pending.find_by(user_code: normalize_user_code(code))
+    end
+
+    # Typed by a person, so it arrives with whatever case and separators they used.
+    def self.normalize_user_code(code)
+      code.to_s.upcase.gsub(/[^A-Z0-9]/, "")
+    end
+
+    def expired? = expires_at <= Time.current
+    def approved? = approved_at.present?
+    def denied? = denied_at.present?
+
+    # What the polling client is told. Order matters: a denied grant is denied even
+    # after it expires, because "somebody said no" is the more useful answer.
+    def state
+      return :denied if denied?
+      return :approved if approved?
+      return :expired if expired?
+
+      :pending
+    end
+
+    # Grouped for reading aloud and for typing: WARP-K7M2.
+    def formatted_user_code
+      user_code.to_s.scan(/.{1,4}/).join("-")
+    end
+
+    # Housekeeping for a host that wants it: an expired grant has nothing left to give,
+    # and its issued_token would be a live secret nobody is waiting for.
+    def self.sweep_expired!
+      where(expires_at: ...Time.current).where.not(issued_token: nil).update_all(issued_token: nil)
+    end
+
+    def self.ransackable_attributes(auth_object = nil)
+      %w[approved_at client_name created_at denied_at expires_at id subject_id subject_type updated_at user_code]
+    end
+
+    def self.ransackable_associations(auth_object = nil)
+      []
+    end
+
+    private
+
+    def generate_codes
+      self.device_code = SecureRandom.hex(32) if device_code.blank?
+      self.user_code = self.class.generate_user_code if user_code.blank?
+    end
+
+    def self.generate_user_code
+      # Retried rather than trusted: the alphabet is small enough that a collision is
+      # a real, if rare, event, and a unique index would turn it into a 500.
+      10.times do
+        candidate = Array.new(USER_CODE_LENGTH) { USER_CODE_ALPHABET.chars.sample }.join
+        return candidate unless exists?(user_code: candidate)
+      end
+      raise "could not generate a free device user code"
+    end
+
+    def set_expiry
+      self.expires_at ||= WarpEngine.config.device_code_ttl.to_i.seconds.from_now
+    end
+
+    ActiveSupport.run_load_hooks(:warp_engine_device_grant, self)
+  end
+end
diff --git a/libs/ruby/warp_engine/app/serializers/warp_engine/software_detail_serializer.rb b/libs/ruby/warp_engine/app/serializers/warp_engine/software_detail_serializer.rb
index d553be4..e7c97fa 100644
--- a/libs/ruby/warp_engine/app/serializers/warp_engine/software_detail_serializer.rb
+++ b/libs/ruby/warp_engine/app/serializers/warp_engine/software_detail_serializer.rb
@@ -5,5 +5,8 @@ module WarpEngine
     field(:latestRelease)      { |_, opts|  opts[:latest]       ? ReleaseSerializer.render_as_hash(opts[:latest], download_counts: opts[:download_counts])       : nil }
     field(:webPlayableRelease) { |_, opts|  opts[:web_playable] ? ReleaseSerializer.render_as_hash(opts[:web_playable], download_counts: opts[:download_counts]) : nil }
     field(:totalDownloads)     { |_, opts|  opts[:total_downloads] || 0 }
+    # Whether this title is gated, what it costs and where to get it. Always present —
+    # see WarpEngine::Access. Under the open policy it is the constant OPEN answer.
+    field(:access)             { |_, opts| (opts[:access] || WarpEngine::Access::OPEN).as_json }
   end
 end
diff --git a/libs/ruby/warp_engine/app/services/warp_engine/device_grant_service.rb b/libs/ruby/warp_engine/app/services/warp_engine/device_grant_service.rb
new file mode 100644
index 0000000..6722820
--- /dev/null
+++ b/libs/ruby/warp_engine/app/services/warp_engine/device_grant_service.rb
@@ -0,0 +1,102 @@
+module WarpEngine
+  # The device authorization grant, from both ends.
+  #
+  # The client's end is #request, #poll and #revoke, all reachable over /api/auth/*.
+  # The person's end is #approve and #deny, which the *host* calls from its own page —
+  # approving needs a session and a logged-in human, and the engine has neither.
+  class DeviceGrantService
+    class NotConfigured < StandardError; end
+    class UnknownCode < StandardError; end
+
+    # A client asks for a code pair. Deliberately unauthenticated: there is nobody to
+    # authenticate as yet, which is the whole reason this flow exists.
+    def request(client_name:)
+      ensure_configured!
+
+      WarpEngine::DeviceGrant.create!(client_name: client_name.presence&.truncate(128))
+    end
+
+    # The client polls with the device code. Returns [state, token], where the token is
+    # the plain string and is available exactly once — on the poll that finds the grant
+    # newly approved. A second poll gets :approved with no token, which is the honest
+    # answer: the secret was handed over and is not kept.
+    def poll(device_code:)
+      ensure_configured!
+
+      grant = WarpEngine::DeviceGrant.find_by(device_code: device_code.to_s)
+      raise UnknownCode if grant.nil?
+
+      return [ grant.state, nil ] unless grant.state == :approved
+
+      # Read once, then gone: the column exists only to carry the secret across the gap
+      # between the browser that approved it and the client that is polling for it.
+      plain = grant.issued_token
+      grant.update_columns(issued_token: nil) if plain.present?
+      [ :approved, plain ]
+    end
+
+    # The host's approval page calls this with the code a person typed and the subject
+    # they are signed in as. Issuing the token here rather than on the next poll keeps
+    # the decision and its consequence in one transaction.
+    def approve(user_code:, subject:)
+      ensure_configured!
+
+      grant = WarpEngine::DeviceGrant.find_pending_by_user_code(user_code)
+      raise UnknownCode if grant.nil?
+
+      ActiveRecord::Base.transaction do
+        token = WarpEngine::ApplicationToken.create!(
+          name: grant.client_name.presence || "Client",
+          owner_type: WarpEngine.config.access_token_owner_class,
+          owner_id: subject.id,
+          scopes: [ WarpEngine::ApplicationToken::CATALOG_SCOPE ]
+        )
+        grant.update!(
+          subject_type: WarpEngine.config.access_token_owner_class,
+          subject_id: subject.id,
+          application_token: token,
+          issued_token: token.plain_token,
+          approved_at: Time.current
+        )
+      end
+
+      grant
+    end
+
+    def deny(user_code:)
+      ensure_configured!
+
+      grant = WarpEngine::DeviceGrant.find_pending_by_user_code(user_code)
+      raise UnknownCode if grant.nil?
+
+      grant.update!(denied_at: Time.current)
+      grant
+    end
+
+    # Signing out: the client throws its own token away. Revocation is a soft delete on
+    # the token, so the audit trail of who signed in from where survives it.
+    def revoke(token:)
+      return false if token.nil?
+
+      token.revoke!
+      true
+    end
+
+    # Where a person goes to type the user code. A path is made absolute against the
+    # request's own base, so a host that configured "/devices" does not have to know its
+    # own hostname.
+    def verification_url(base_url: nil)
+      configured = WarpEngine.config.identity_verification_url.presence || "/devices"
+      return configured if configured.start_with?("http://", "https://")
+      return configured if base_url.blank?
+
+      "#{base_url.to_s.chomp('/')}/#{configured.delete_prefix('/')}"
+    end
+
+    private
+
+    def ensure_configured!
+      raise NotConfigured unless WarpEngine.identity_configured?
+    end
+  end
+end
diff --git a/libs/ruby/warp_engine/app/services/warp_engine/download_service.rb b/libs/ruby/warp_engine/app/services/warp_engine/download_service.rb
index cfcfb2c..fca42ea 100644
--- a/libs/ruby/warp_engine/app/services/warp_engine/download_service.rb
+++ b/libs/ruby/warp_engine/app/services/warp_engine/download_service.rb
@@ -1,5 +1,10 @@
 module WarpEngine
   class DownloadService
+    # The access policy said no. The controller turns this into a 403 — distinct from
+    # the nil that means "no such file", because telling a person their file is missing
+    # when it is merely locked sends them looking for the wrong problem.
+    class Denied < StandardError; end
+
     def self.container_base
       WarpEngine.config.file_container_path
     end
@@ -12,19 +17,28 @@ module WarpEngine
     # A letöltés helyét adja vissza (fájl vagy aláírt URL) és naplózza a
     # letöltést. A hely feloldása a storage adapteren megy — alapból :local,
     # tehát változatlanul lemezről.
-    def locate(path:, ip:, user_agent:, referer:)
+    #
+    # `subject` is whoever the request authenticated as, or nil. Under the open policy
+    # it is ignored and every file is served, exactly as before.
+    def locate(path:, ip:, user_agent:, referer:, subject: nil, request: nil)
       relative = path.to_s
       return nil unless storage.file?(relative)
 
-      log_download(relative, ip: ip, user_agent: user_agent, referer: referer)
+      asset = find_asset(relative)
+      grant = authorize!(asset, subject, request)
 
-      storage.locate(relative, filename: File.basename(relative))
+      log_download(relative, asset: asset, ip: ip, user_agent: user_agent, referer: referer, subject: subject)
+
+      storage.locate(relative,
+                     filename: grant.filename.presence || File.basename(relative),
+                     expires_in: grant.expires_in)
     end
 
     # Visszafelé kompatibilis felület: az abszolút fájlútvonalat adja vissza
     # (vagy nil-t). Nem lemezes adapternél nincs útvonal — ott a #locate való.
-    def create(path:, ip:, user_agent:, referer:)
-      location = locate(path: path, ip: ip, user_agent: user_agent, referer: referer)
+    def create(path:, ip:, user_agent:, referer:, subject: nil, request: nil)
+      location = locate(path: path, ip: ip, user_agent: user_agent, referer: referer,
+                        subject: subject, request: request)
       return nil if location.nil?
 
       location.file? ? location.path : nil
@@ -36,18 +50,41 @@ module WarpEngine
       WarpEngine.storage
     end
 
-    def log_download(relative, ip:, user_agent:, referer:)
-      escaped = relative.gsub("%", "\\%").gsub("_", "\\_")
-      asset = WarpEngine::ReleaseAsset.find_by(path: File.join(self.class.container_base, relative)) ||
-              WarpEngine::ReleaseAsset.where("path LIKE ?", "%#{escaped}%").first
+    # A policy that refuses returns nil; one that raises is treated as a refusal too.
+    # An artifact served because the gatekeeper crashed is the one failure mode this
+    # engine must not have.
+    def authorize!(asset, subject, request)
+      grant = WarpEngine.access_policy.authorize_download(asset: asset, subject: subject, request: request)
+      raise Denied if grant.nil?
 
-      WarpEngine::Download.create!(
+      grant
+    rescue Denied
+      raise
+    rescue StandardError => e
+      Rails.logger.error("[WarpEngine::AccessPolicy] #{e.class}: #{e.message}")
+      raise Denied
+    end
+
+    def find_asset(relative)
+      escaped = relative.gsub("%", "\\%").gsub("_", "\\_")
+      WarpEngine::ReleaseAsset.find_by(path: File.join(self.class.container_base, relative)) ||
+        WarpEngine::ReleaseAsset.where("path LIKE ?", "%#{escaped}%").first
+    end
+
+    def log_download(relative, asset:, ip:, user_agent:, referer:, subject:)
+      download = WarpEngine::Download.create!(
         file_path: relative,
         release: asset&.release,
         ip_address: ip,
         user_agent: user_agent&.truncate(500),
         referer: referer&.truncate(500)
       )
+
+      # The same seam the publish side has: a host that wants its own record of who
+      # downloaded what subscribes rather than reaching into this class.
+      ActiveSupport::Notifications.instrument("warp_engine.download",
+        path: relative, asset: asset, release: asset&.release,
+        software: asset&.release&.software, subject: subject, download: download, ip: ip)
     end
   end
 end
diff --git a/libs/ruby/warp_engine/app/services/warp_engine/file_service.rb b/libs/ruby/warp_engine/app/services/warp_engine/file_service.rb
index 2198c2c..794a247 100644
--- a/libs/ruby/warp_engine/app/services/warp_engine/file_service.rb
+++ b/libs/ruby/warp_engine/app/services/warp_engine/file_service.rb
@@ -8,7 +8,16 @@ module WarpEngine
     # A fájlok helyét a storage adapter adja (alapból :local, azaz a lemez) —
     # így a host az objektumtárból is kiszolgálhat anélkül, hogy az engine-t
     # patchelné. Lásd WarpEngine::Storage.
-    def show(input)
+    # `subject` is whoever the request authenticated as, or nil. Under the open policy
+    # it is ignored and every file is served, exactly as before.
+    #
+    # A hosted (browser) build reaches this path as hundreds of relative requests for
+    # js, wasm and images, which is why the gate here is the policy's plain yes/no
+    # rather than anything signed: there is nothing to sign per file. A host serving
+    # gated web builds to browsers will usually want its own session-based route in
+    # front of this one — a browser has a session, and a redirect to a login page is a
+    # better answer there than a bare 403.
+    def show(input, subject: nil)
       relative = input.path.to_s
 
       if storage.directory?(relative)
@@ -20,11 +29,34 @@ module WarpEngine
 
       return FileResultDto.not_found unless storage.file?(relative)
 
+      authorize!(relative, subject)
+
       to_result(storage.locate(relative, filename: File.basename(relative)))
     end
 
     private
 
+    # Same rule and same failure mode as DownloadService: a policy that refuses or
+    # raises means no file. An artifact served because the gatekeeper crashed is the
+    # one failure mode this engine must not have.
+    def authorize!(relative, subject)
+      # The open policy authorises everything, and this path serves a browser build as
+      # hundreds of requests for js, wasm and images. Asking it per file would mean a
+      # LIKE query per asset for an answer that is always yes.
+      return WarpEngine::Access::Grant::OPEN if WarpEngine::AccessPolicy.open?
+
+      asset = WarpEngine::ReleaseAsset.where("path LIKE ?", "%#{relative.gsub('%', '\\%').gsub('_', '\\_')}%").first
+      grant = WarpEngine.access_policy.authorize_download(asset: asset, subject: subject, request: nil)
+      raise WarpEngine::DownloadService::Denied if grant.nil?
+
+      grant
+    rescue WarpEngine::DownloadService::Denied
+      raise
+    rescue StandardError => e
+      Rails.logger.error("[WarpEngine::AccessPolicy] #{e.class}: #{e.message}")
+      raise WarpEngine::DownloadService::Denied
+    end
+
     def storage
       WarpEngine.storage
     end
diff --git a/libs/ruby/warp_engine/app/services/warp_engine/software_highlighted_service.rb b/libs/ruby/warp_engine/app/services/warp_engine/software_highlighted_service.rb
index b898109..0947efa 100644
--- a/libs/ruby/warp_engine/app/services/warp_engine/software_highlighted_service.rb
+++ b/libs/ruby/warp_engine/app/services/warp_engine/software_highlighted_service.rb
@@ -2,15 +2,15 @@ module WarpEngine
   class SoftwareHighlightedService
     include SoftwareResponseBuilder
 
-    def index
-      software = WarpEngine::Software.includes(:external_links, :software_images)
+    def index(subject: nil)
+      software = visible_scope(subject).includes(:external_links, :software_images)
                          .where(highlighted: true)
                          .order(id: :desc)
                          .first
       return nil unless software
 
       releases = WarpEngine::Release.includes(:release_assets).where(software_id: software.id).to_a
-      build_response(software, releases, download_counts_for(releases.map(&:id)))
+      build_response(software, releases, download_counts_for(releases.map(&:id)), subject: subject)
     end
   end
 end
diff --git a/libs/ruby/warp_engine/app/services/warp_engine/software_response_builder.rb b/libs/ruby/warp_engine/app/services/warp_engine/software_response_builder.rb
index a895ef1..e62e967 100644
--- a/libs/ruby/warp_engine/app/services/warp_engine/software_response_builder.rb
+++ b/libs/ruby/warp_engine/app/services/warp_engine/software_response_builder.rb
@@ -2,7 +2,7 @@ module WarpEngine
   module SoftwareResponseBuilder
     private
 
-    def build_response(software, releases, download_counts)
+    def build_response(software, releases, download_counts, subject: nil)
       sorted = releases.sort_by { |r| r.created_at || Time.at(0) }.reverse
       latest = sorted.reject { |r| r.version.to_s.start_with?("dev-") }.first
       # web-playable, ha az utolsó (stabil) release-nek van webes assetje
@@ -14,10 +14,33 @@ module WarpEngine
         latest:          latest,
         web_playable:    web_playable,
         total_downloads: total_downloads,
-        download_counts: download_counts
+        download_counts: download_counts,
+        access:          access_for(software, subject)
       )
     end
 
+    # Always present, even under the open policy: a client should never have to tell
+    # "this catalog says nothing about access" from "this title is not gated". One of
+    # those is a question and the other is an answer.
+    def access_for(software, subject)
+      WarpEngine.access_policy.access_for(software: software, subject: subject)
+    rescue StandardError => e
+      # A policy that raises must not take the catalog down with it. The open answer is
+      # wrong here, so the closed one is what a broken policy gets: a title nobody can
+      # download is recoverable, a paid title handed out for free is not.
+      Rails.logger.error("[WarpEngine::AccessPolicy] #{e.class}: #{e.message}")
+      WarpEngine::Access.new(gated: true, entitled: false)
+    end
+
+    # The titles this subject may see. A policy that raises empties the catalog rather
+    # than leaking it — the safe direction — but it is still a bug, so it is logged.
+    def visible_scope(subject)
+      WarpEngine.access_policy.visible_software_scope(subject: subject)
+    rescue StandardError => e
+      Rails.logger.error("[WarpEngine::AccessPolicy] #{e.class}: #{e.message}")
+      WarpEngine::Software.none
+    end
+
     # Egyetlen csoportosított lekérdezés release-enkénti letöltésszámokhoz (N+1 helyett).
     def download_counts_for(release_ids)
       return {} if release_ids.empty?
diff --git a/libs/ruby/warp_engine/app/services/warp_engine/software_service.rb b/libs/ruby/warp_engine/app/services/warp_engine/software_service.rb
index 06ad14e..e3791a8 100644
--- a/libs/ruby/warp_engine/app/services/warp_engine/software_service.rb
+++ b/libs/ruby/warp_engine/app/services/warp_engine/software_service.rb
@@ -2,11 +2,16 @@ module WarpEngine
   class SoftwareService
     include SoftwareResponseBuilder
 
-    def index(owner_id: nil)
-      softwares = WarpEngine::Software.includes(releases: [ :release_assets ]).includes(:external_links, :software_images).all
+    # `subject` is whoever the request authenticated as, or nil. It decides two things:
+    # which titles are listed at all (the policy's scope) and what each one's `access`
+    # block says about entitlement.
+    def index(owner_id: nil, subject: nil)
+      softwares = visible_scope(subject)
+                  .includes(releases: [ :release_assets ])
+                  .includes(:external_links, :software_images)
       softwares = softwares.where(owner_id: owner_id) if owner_id.present?
       counts = download_counts_for(softwares.flat_map { |sw| sw.releases.map(&:id) })
-      { softwares: softwares.map { |sw| build_response(sw, sw.releases.to_a, counts) } }
+      { softwares: softwares.map { |sw| build_response(sw, sw.releases.to_a, counts, subject: subject) } }
     end
   end
 end
diff --git a/libs/ruby/warp_engine/config/routes.rb b/libs/ruby/warp_engine/config/routes.rb
index 410efa4..517907c 100644
--- a/libs/ruby/warp_engine/config/routes.rb
+++ b/libs/ruby/warp_engine/config/routes.rb
@@ -1,5 +1,17 @@
 WarpEngine::Engine.routes.draw do
   namespace :api do
+    # What this deployment is and what it can do. A client reads it before it can have
+    # a credential, so it is public and cheap.
+    get "service", to: "service#show"
+
+    # Device sign-in, for clients that have no browser of their own (RFC 8628).
+    # Inactive — 404 on every action — unless the host configured a subject class.
+    namespace :auth do
+      post   "device",       to: "devices#create"
+      post   "device/token", to: "devices#token"
+      delete "token",        to: "tokens#destroy"
+    end
+
     get "software",               to: "software#index"
     get "software/highlighted",   to: "software_highlighted#index"
     get "image/:id",              to: "images#show"
diff --git a/libs/ruby/warp_engine/db/migrate/20260819000001_create_device_grants.rb b/libs/ruby/warp_engine/db/migrate/20260819000001_create_device_grants.rb
new file mode 100644
index 0000000..204d8d1
--- /dev/null
+++ b/libs/ruby/warp_engine/db/migrate/20260819000001_create_device_grants.rb
@@ -0,0 +1,35 @@
+class CreateDeviceGrants < ActiveRecord::Migration[8.1]
+  def change
+    create_table :device_grants, id: { type: :bigint, unsigned: true },
+                                 charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci" do |t|
+      # Both codes are secrets in the sense that guessing one grants a session, but they
+      # have different jobs: the device code is long and never shown to a person, the
+      # user code is short enough to read off a screen and type into a browser.
+      t.string :device_code, limit: 64, null: false
+      t.string :user_code, limit: 16, null: false
+      # What the client called itself. Shown on the approval page, so a person can tell
+      # which machine is asking, and kept on the issued token as its name.
+      t.string :client_name, limit: 128
+      # The approving subject comes from the host
+      # (WarpEngine.config.access_token_owner_class), so no FK — same reasoning as
+      # application_tokens.owner_type.
+      t.string :subject_type, limit: 128
+      t.bigint :subject_id, unsigned: true
+      t.bigint :application_token_id, unsigned: true
+      # The issued token, in the clear, for the seconds between "approved" and "the
+      # client's next poll". It is cleared on the poll that hands it over, so this
+      # column holds a live secret only while somebody is waiting for it. There is no
+      # way around storing it: the approval happens in a browser and the poll arrives on
+      # a different request, so the two cannot share memory. Everything else about a
+      # token is stored as a digest — this is the one exception, and it is temporary.
+      t.string :issued_token, limit: 64
+      t.datetime :approved_at, precision: 3
+      t.datetime :denied_at, precision: 3
+      t.datetime :expires_at, precision: 3, null: false
+      t.timestamps precision: 3, null: true
+      t.index :device_code, name: "idx_device_grants_device_code", unique: true
+      t.index :user_code, name: "idx_device_grants_user_code", unique: true
+      t.index :expires_at, name: "idx_device_grants_expires_at"
+    end
+  end
+end
diff --git a/libs/ruby/warp_engine/lib/generators/warp_engine/install/templates/create_warp_engine_tables.rb b/libs/ruby/warp_engine/lib/generators/warp_engine/install/templates/create_warp_engine_tables.rb
index 73b6901..6ed7bf7 100644
--- a/libs/ruby/warp_engine/lib/generators/warp_engine/install/templates/create_warp_engine_tables.rb
+++ b/libs/ruby/warp_engine/lib/generators/warp_engine/install/templates/create_warp_engine_tables.rb
@@ -98,6 +98,28 @@ class CreateWarpEngineTables < ActiveRecord::Migration[8.0]
       t.index :deleted_at
     end
 
+    # Device sign-in for clients that have no browser of their own (RFC 8628).
+    # Short-lived rows: one exists for the minute or two between "the client asked"
+    # and "the person answered". Only used where access_token_owner_class is set.
+    create_table :device_grants do |t|
+      t.string :device_code, limit: 64, null: false
+      t.string :user_code, limit: 16, null: false
+      t.string :client_name, limit: 128
+      t.string :subject_type, limit: 128
+      t.bigint :subject_id
+      t.bigint :application_token_id
+      # The issued token in the clear, cleared on the poll that hands it over — see
+      # the model. Everything else about a token is stored as a digest.
+      t.string :issued_token, limit: 64
+      t.datetime :approved_at, precision: 3
+      t.datetime :denied_at, precision: 3
+      t.datetime :expires_at, precision: 3, null: false
+      t.timestamps precision: 3, null: true
+      t.index :device_code, unique: true
+      t.index :user_code, unique: true
+      t.index :expires_at
+    end
+
     create_table :downloads do |t|
       t.string :file_path, null: false
       t.references :release, foreign_key: { on_delete: :nullify }, index: false
diff --git a/libs/ruby/warp_engine/lib/generators/warp_engine/install/templates/initializer.rb b/libs/ruby/warp_engine/lib/generators/warp_engine/install/templates/initializer.rb
index 89d69a7..e160600 100644
--- a/libs/ruby/warp_engine/lib/generators/warp_engine/install/templates/initializer.rb
+++ b/libs/ruby/warp_engine/lib/generators/warp_engine/install/templates/initializer.rb
@@ -43,6 +43,26 @@ Rails.application.config.to_prepare do
     # softwares got an owner (backfill)!
     # c.enforce_software_ownership = true
 
+    # Who may see a title and who may download it. :open (the default) lists every
+    # software and serves every artifact — the behaviour of a catalog nobody sells
+    # from. A host that does sell supplies a policy answering three methods; see
+    # WarpEngine::AccessPolicy. What it returns is what clients are told, so a paid
+    # title can announce itself as paid instead of failing at the download.
+    # c.access_policy = MyStore::AccessPolicy.new
+
+    # Client sign-in. nil (default) means there is none: /api/auth/* is inactive and
+    # GET /api/service reports auth: null, so a client offers no sign-in at all. Set
+    # the class a *client* token belongs to — usually your user model — to turn on
+    # the device authorization grant.
+    # c.access_token_owner_class = "User"
+    #
+    # Your own page where a signed-in person types the code their client displayed.
+    # It calls WarpEngine::DeviceGrantService#approve. A path is made absolute
+    # against the request, so you need not know your own hostname (default "/devices").
+    # c.identity_verification_url = "/devices"
+    # c.device_code_ttl      = 600  # seconds a pending code lives
+    # c.device_code_interval = 5    # seconds a client is told to wait between polls
+
     # If host models also reference catalog images, register them so the
     # admin Images page's orphan detection takes them into account:
     # c.image_owners = [
diff --git a/libs/ruby/warp_engine/lib/warp_engine.rb b/libs/ruby/warp_engine/lib/warp_engine.rb
index 7ba007b..12f415e 100644
--- a/libs/ruby/warp_engine/lib/warp_engine.rb
+++ b/libs/ruby/warp_engine/lib/warp_engine.rb
@@ -10,6 +10,7 @@ require "apipie-rails"
 require "warp_engine/version"
 require "warp_engine/configuration"
 require "warp_engine/storage"
+require "warp_engine/access"
 
 module WarpEngine
   # A tábláink prefix nélküliek (softwares, releases, ...) — az isolate_namespace
@@ -42,6 +43,17 @@ module WarpEngine
   def self.storage
     Storage.adapter
   end
+
+  # Who may see and download what. :open by default — see WarpEngine::AccessPolicy.
+  def self.access_policy
+    AccessPolicy.current
+  end
+
+  # The host has configured a subject class, so client sign-in is available. A client
+  # asks GET /api/service rather than this, but the engine's own controllers need it.
+  def self.identity_configured?
+    config.access_token_owner_class.present?
+  end
 end
 
 require "warp_engine/engine"
diff --git a/libs/ruby/warp_engine/lib/warp_engine/access.rb b/libs/ruby/warp_engine/lib/warp_engine/access.rb
new file mode 100644
index 0000000..ff7cb26
--- /dev/null
+++ b/libs/ruby/warp_engine/lib/warp_engine/access.rb
@@ -0,0 +1,119 @@
+module WarpEngine
+  # Who may see a title, and who may download it.
+  #
+  # Until now every catalog entry was public and every artifact was free: the API
+  # listed all software and /api/download handed over any file it could find. That is
+  # the right default for a catalog nobody sells from, and it stays the default — but a
+  # host that does sell needs the engine to *say so*, because the clients reading this
+  # API have no other way to learn it. A desktop client cannot know that a title is
+  # paid; it can only be told.
+  #
+  # This module is that seam. The default policy is byte for byte the previous
+  # behaviour, and a host swaps it on the configuration:
+  #
+  #   c.access_policy = MyStorePolicy.new   # or :open (default)
+  #
+  # A policy is any object answering to this contract:
+  #
+  #   visible_software_scope(subject:)                  -> ActiveRecord::Relation
+  #   access_for(software:, subject:)                   -> WarpEngine::Access
+  #   authorize_download(asset:, subject:, request:)     -> Access::Grant or nil
+  #
+  # `subject` is whoever the request authenticated as (see SubjectAuthentication), or
+  # nil for an anonymous caller. It is deliberately untyped here: the engine has no user
+  # model, and whose object this is belongs to the host.
+  module AccessPolicy
+    # Everything visible, everything open, no prices. The catalog as it always was.
+    class Open
+      def visible_software_scope(subject: nil)
+        WarpEngine::Software.all
+      end
+
+      def access_for(software:, subject: nil)
+        Access::OPEN
+      end
+
+      # An open catalog authorises every asset it can find. Returning a bare grant
+      # rather than `true` keeps one return type across policies.
+      def authorize_download(asset: nil, subject: nil, request: nil)
+        Access::Grant::OPEN
+      end
+    end
+
+    class << self
+      def current
+        configured = WarpEngine.config.access_policy
+
+        case configured
+        when nil, :open, "open" then open_policy
+        else configured
+        end
+      end
+
+      def open? = current.is_a?(Open)
+
+      def open_policy
+        @open_policy ||= Open.new
+      end
+
+      # Tests and hosts that swap the configuration at runtime.
+      def reset!
+        @open_policy = nil
+      end
+    end
+  end
+
+  # What a client is told about one title's availability.
+  #
+  # The vocabulary is deliberately generic — `gated`, `entitled`, `price` — because
+  # every client reading it serves more than one store. A word from any particular
+  # host's domain ("product", "purchase order", "library") would make the client
+  # that reads it specific to that host, which is exactly what this engine exists
+  # to prevent.
+  class Access
+    # Where a hosted (browser) build is played, when the host serves it somewhere other
+    # than the engine's own /file/ path. nil leaves the client with what it already
+    # builds, which is the pre-existing behaviour.
+    attr_reader :gated, :entitled, :price_cents, :currency, :purchase_url, :web_url
+
+    def initialize(gated: false, entitled: true, price_cents: nil, currency: nil,
+                   purchase_url: nil, web_url: nil)
+      @gated        = gated ? true : false
+      @entitled     = entitled
+      @price_cents  = price_cents
+      @currency     = currency
+      @purchase_url = purchase_url
+      @web_url      = web_url
+    end
+
+    # An open catalog's answer, and the shape every response carries even when no
+    # policy is configured: a client should never have to tell "no access block" from
+    # "not gated". One of those is a question, the other is an answer.
+    OPEN = new.freeze
+
+    def as_json(*)
+      {
+        gated:       gated,
+        entitled:    entitled.nil? ? nil : (entitled ? true : false),
+        price:       price_cents.nil? ? nil : { amountCents: price_cents, currency: currency },
+        purchaseUrl: purchase_url,
+        webUrl:      web_url
+      }
+    end
+
+    # A download the policy allowed.
+    #
+    # `filename` and `expires_in` let a host override what the engine would otherwise
+    # decide on its own; both nil means "you choose", which is what the open policy says.
+    class Grant
+      attr_reader :filename, :expires_in
+
+      def initialize(filename: nil, expires_in: nil)
+        @filename   = filename
+        @expires_in = expires_in
+      end
+
+      OPEN = new.freeze
+    end
+  end
+end
diff --git a/libs/ruby/warp_engine/lib/warp_engine/configuration.rb b/libs/ruby/warp_engine/lib/warp_engine/configuration.rb
index 502782c..1e8790c 100644
--- a/libs/ruby/warp_engine/lib/warp_engine/configuration.rb
+++ b/libs/ruby/warp_engine/lib/warp_engine/configuration.rb
@@ -26,6 +26,22 @@ module WarpEngine
     #   PEM, or a URL to fetch it from (e.g. https://ci.../api/signature/public-key).
     #   With neither set, POST /build/config rejects every request.
     # ci_update_server: server URL written into the upload/publish steps; nil → the request's base_url.
+    # access_policy: who may see a title and who may download it.
+    #   :open (default) — every software listed, every artifact served, no prices:
+    #                     byte for byte the previous behaviour;
+    #   any object      — must answer visible_software_scope/access_for/
+    #                     authorize_download, see WarpEngine::AccessPolicy.
+    # access_token_owner_class: class name of the subject a *client* token belongs to
+    #   (e.g. "Accounts::User"). nil (default) means no client sign-in: /api/auth/* is
+    #   inactive and GET /api/service reports auth: null. Deliberately separate from
+    #   application_token_owner_class, which owns *publishing* tokens — a publisher and
+    #   a customer are rarely the same kind of thing.
+    # identity_verification_url: the host's own page where a person approves a device
+    #   code. Path or absolute URL; nil falls back to "/devices". The page is the host's
+    #   because approving needs a session, a login and HTML — none of which is the
+    #   engine's business.
+    # device_code_ttl / device_code_interval: how long a pending device code lives, and
+    #   how often a client is told to poll for it.
     # woodpecker_url / woodpecker_api_token / woodpecker_repo_owner:
     #   Woodpecker CI management (repo sync, secret provisioning, pipeline control).
     #   All nil → the management features are inactive.
@@ -44,7 +60,12 @@ module WarpEngine
                   :woodpecker_api_token,
                   :woodpecker_repo_owner,
                   :image_owners,
-                  :storage_adapter
+                  :storage_adapter,
+                  :access_policy,
+                  :access_token_owner_class,
+                  :identity_verification_url,
+                  :device_code_ttl,
+                  :device_code_interval
 
     def initialize
       @file_container_path           = ENV.fetch("FILE_CONTAINER_PATH", "/softwares")
@@ -63,6 +84,11 @@ module WarpEngine
       @woodpecker_repo_owner         = ENV["WOODPECKER_REPO_OWNER"]
       @image_owners                  = []
       @storage_adapter               = :local
+      @access_policy                 = :open
+      @access_token_owner_class      = nil
+      @identity_verification_url     = nil
+      @device_code_ttl               = 600
+      @device_code_interval          = 5
     end
   end
 end
diff --git a/libs/ruby/warp_engine/lib/warp_engine/version.rb b/libs/ruby/warp_engine/lib/warp_engine/version.rb
index 02a8188..991e73b 100644
--- a/libs/ruby/warp_engine/lib/warp_engine/version.rb
+++ b/libs/ruby/warp_engine/lib/warp_engine/version.rb
@@ -1,5 +1,5 @@
 module WarpEngine
-  VERSION = "0.4.0"
+  VERSION = "0.5.0"
 
   # The header every API response carries. Named here rather than written out at the one
   # place that sets it: clients read it, the README documents it, and a string in three
diff --git a/libs/ruby/warp_engine/spec/dummy/db/schema.rb b/libs/ruby/warp_engine/spec/dummy/db/schema.rb
index cfb98ef..6dbcfdf 100644
--- a/libs/ruby/warp_engine/spec/dummy/db/schema.rb
+++ b/libs/ruby/warp_engine/spec/dummy/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_08_06_000002) do
+ActiveRecord::Schema[8.1].define(version: 2026_08_19_000001) do
   create_table "application_tokens", id: { type: :bigint, unsigned: true }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.datetime "created_at", precision: 3
     t.datetime "deleted_at", precision: 3
@@ -29,6 +29,24 @@ ActiveRecord::Schema[8.1].define(version: 2026_08_06_000002) do
     t.index ["token_digest"], name: "idx_application_tokens_token_digest", unique: true
   end
 
+  create_table "device_grants", id: { type: :bigint, unsigned: true }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
+    t.bigint "application_token_id", unsigned: true
+    t.datetime "approved_at", precision: 3
+    t.string "client_name", limit: 128
+    t.datetime "created_at", precision: 3
+    t.datetime "denied_at", precision: 3
+    t.string "device_code", limit: 64, null: false
+    t.datetime "expires_at", precision: 3, null: false
+    t.string "issued_token", limit: 64
+    t.bigint "subject_id", unsigned: true
+    t.string "subject_type", limit: 128
+    t.datetime "updated_at", precision: 3
+    t.string "user_code", limit: 16, null: false
+    t.index ["device_code"], name: "idx_device_grants_device_code", unique: true
+    t.index ["expires_at"], name: "idx_device_grants_expires_at"
+    t.index ["user_code"], name: "idx_device_grants_user_code", unique: true
+  end
+
   create_table "downloads", id: { type: :bigint, unsigned: true }, charset: "utf8mb4", collation: "utf8mb4_0900_ai_ci", force: :cascade do |t|
     t.datetime "created_at", precision: 3
     t.datetime "deleted_at", precision: 3
diff --git a/libs/ruby/warp_engine/spec/requests/device_auth_spec.rb b/libs/ruby/warp_engine/spec/requests/device_auth_spec.rb
new file mode 100644
index 0000000..4583810
--- /dev/null
+++ b/libs/ruby/warp_engine/spec/requests/device_auth_spec.rb
@@ -0,0 +1,199 @@
+require "rails_helper"
+
+# Signing a client in, end to end: the client asks for a code, a person approves it on
+# the host's page, the client's next poll carries the token away, and the token then
+# works as a bearer credential on the read-only API.
+RSpec.describe "Device sign-in", type: :request do
+  let(:owner) { create(:test_owner) }
+
+  # The identity seam is off by default. Configuring the subject class is what turns
+  # the whole flow on — including whether it exists at all.
+  def configure_identity!(verification: "/devices")
+    allow(WarpEngine.config).to receive(:access_token_owner_class).and_return("TestOwner")
+    allow(WarpEngine.config).to receive(:identity_verification_url).and_return(verification)
+  end
+
+  describe "when the host configured no client identity" do
+    it "has no device endpoint at all" do
+      post "/api/auth/device", params: { client_name: "laptop" }
+
+      expect(response).to have_http_status(:not_found)
+    end
+
+    it "says so in the service descriptor rather than by erroring" do
+      get "/api/service"
+
+      expect(response).to have_http_status(:ok)
+      expect(JSON.parse(response.body)["auth"]).to be_nil
+    end
+  end
+
+  describe "the full flow" do
+    before { configure_identity! }
+
+    it "issues a code pair a person can read off a screen" do
+      post "/api/auth/device", params: { client_name: "Zsolt's laptop" }
+
+      expect(response).to have_http_status(:ok)
+      json = JSON.parse(response.body)
+      expect(json["deviceCode"]).to be_present
+      # Grouped and free of I/O/0/1, because it is typed by hand into a browser.
+      expect(json["userCode"]).to match(/\A[A-HJ-NP-Z2-9]{4}-[A-HJ-NP-Z2-9]{4}\z/)
+      expect(json["verificationUrl"]).to eq("http://www.example.com/devices")
+      expect(json["interval"]).to eq(5)
+    end
+
+    it "keeps the client waiting until somebody approves" do
+      post "/api/auth/device", params: { client_name: "laptop" }
+      device_code = JSON.parse(response.body)["deviceCode"]
+
+      post "/api/auth/device/token", params: { device_code: device_code }
+
+      expect(JSON.parse(response.body)).to eq("state" => "pending")
+    end
+
+    it "hands over the token on the first poll after approval" do
+      post "/api/auth/device", params: { client_name: "laptop" }
+      json = JSON.parse(response.body)
+
+      WarpEngine::DeviceGrantService.new.approve(user_code: json["userCode"], subject: owner)
+      post "/api/auth/device/token", params: { device_code: json["deviceCode"] }
+
+      body = JSON.parse(response.body)
+      expect(body["state"]).to eq("approved")
+      expect(body["token"]).to be_present
+    end
+
+    # The plain token is never stored, so it cannot be handed out twice. A client that
+    # loses it starts again — which is cheaper than a database full of live secrets.
+    it "does not repeat the token on a second poll" do
+      post "/api/auth/device", params: { client_name: "laptop" }
+      json = JSON.parse(response.body)
+      WarpEngine::DeviceGrantService.new.approve(user_code: json["userCode"], subject: owner)
+      post "/api/auth/device/token", params: { device_code: json["deviceCode"] }
+
+      post "/api/auth/device/token", params: { device_code: json["deviceCode"] }
+
+      body = JSON.parse(response.body)
+      expect(body["state"]).to eq("approved")
+      expect(body).not_to have_key("token")
+    end
+
+    it "reports a denied grant as denied" do
+      post "/api/auth/device", params: { client_name: "laptop" }
+      json = JSON.parse(response.body)
+      WarpEngine::DeviceGrantService.new.deny(user_code: json["userCode"])
+
+      post "/api/auth/device/token", params: { device_code: json["deviceCode"] }
+
+      expect(JSON.parse(response.body)["state"]).to eq("denied")
+    end
+
+    it "reports an expired grant as expired" do
+      post "/api/auth/device", params: { client_name: "laptop" }
+      json = JSON.parse(response.body)
+      WarpEngine::DeviceGrant.last.update!(expires_at: 1.minute.ago)
+
+      post "/api/auth/device/token", params: { device_code: json["deviceCode"] }
+
+      expect(JSON.parse(response.body)["state"]).to eq("expired")
+    end
+
+    it "404s an unknown device code" do
+      post "/api/auth/device/token", params: { device_code: "nope" }
+
+      expect(response).to have_http_status(:not_found)
+    end
+
+    it "will not approve the same code twice" do
+      post "/api/auth/device", params: { client_name: "laptop" }
+      code = JSON.parse(response.body)["userCode"]
+      WarpEngine::DeviceGrantService.new.approve(user_code: code, subject: owner)
+
+      expect { WarpEngine::DeviceGrantService.new.approve(user_code: code, subject: owner) }
+        .to raise_error(WarpEngine::DeviceGrantService::UnknownCode)
+    end
+
+    it "accepts the user code however a person typed it" do
+      post "/api/auth/device", params: { client_name: "laptop" }
+      code = JSON.parse(response.body)["userCode"]
+
+      grant = WarpEngine::DeviceGrantService.new.approve(
+        user_code: code.downcase.delete("-"), subject: owner
+      )
+
+      expect(grant).to be_approved
+    end
+  end
+
+  describe "the issued token" do
+    before { configure_identity! }
+
+    let(:token) do
+      post "/api/auth/device", params: { client_name: "laptop" }
+      json = JSON.parse(response.body)
+      WarpEngine::DeviceGrantService.new.approve(user_code: json["userCode"], subject: owner)
+      post "/api/auth/device/token", params: { device_code: json["deviceCode"] }
+      JSON.parse(response.body)["token"]
+    end
+
+    it "belongs to the subject who approved it, and may only read the catalog" do
+      token
+      record = WarpEngine::ApplicationToken.last
+
+      expect(record.owner).to eq(owner)
+      expect(record.scopes).to eq([ "catalog" ])
+    end
+
+    # A publishing token must not become a client token by accident, and vice versa:
+    # the scope is what separates them, and the catalog endpoint requires its own.
+    it "is not accepted as a publishing credential" do
+      token
+
+      expect(WarpEngine::ApplicationToken.authenticate(token, required_scope: "update")).to be_nil
+    end
+
+    it "identifies the subject on a catalog request" do
+      create(:software)
+      seen = nil
+      policy = Class.new do
+        def initialize(sink) = @sink = sink
+        def visible_software_scope(subject: nil) = WarpEngine::Software.all
+        def access_for(software:, subject: nil)
+          @sink.call(subject)
+          WarpEngine::Access::OPEN
+        end
+        def authorize_download(asset: nil, subject: nil, request: nil) = WarpEngine::Access::Grant::OPEN
+      end.new(->(s) { seen = s })
+      allow(WarpEngine.config).to receive(:access_policy).and_return(policy)
+
+      get "/api/software", headers: { "Authorization" => "Bearer #{token}" }
+
+      expect(seen).to eq(owner)
+    end
+
+    it "is ignored when it is not a bearer credential" do
+      value = token
+
+      get "/api/software", headers: { "Authorization" => value }
+
+      expect(response).to have_http_status(:ok)
+      expect(WarpEngine::ApplicationToken.last.last_used_at).to be_nil
+    end
+
+    it "stops working once revoked" do
+      value = token
+
+      delete "/api/auth/token", headers: { "Authorization" => "Bearer #{value}" }
+
+      expect(response).to have_http_status(:no_content)
+      expect(WarpEngine::ApplicationToken.authenticate(value, required_scope: "catalog")).to be_nil
+    end
+
+    it "refuses to revoke without a token" do
+      delete "/api/auth/token"
+
+      expect(response).to have_http_status(:unauthorized)
+    end
+  end
+end
diff --git a/libs/ruby/warp_engine/spec/requests/service_controller_spec.rb b/libs/ruby/warp_engine/spec/requests/service_controller_spec.rb
new file mode 100644
index 0000000..4c9bf04
--- /dev/null
+++ b/libs/ruby/warp_engine/spec/requests/service_controller_spec.rb
@@ -0,0 +1,75 @@
+require "rails_helper"
+
+# The descriptor is how a client stops being built for one particular store: everything
+# it used to have compiled in — is there a sign-in, where does it live, can titles be
+# gated — is answered here instead.
+RSpec.describe "GET /api/service", type: :request do
+  after { WarpEngine::AccessPolicy.reset! }
+
+  it "names the engine and its version" do
+    get "/api/service"
+
+    json = JSON.parse(response.body)
+    expect(json["engine"]).to eq("warp_engine")
+    expect(json["version"]).to eq(WarpEngine::VERSION)
+    expect(response.headers["WarpEngine-Version"]).to eq(WarpEngine::VERSION)
+  end
+
+  it "reports an open catalog as ungated and offering no sign-in" do
+    get "/api/service"
+
+    json = JSON.parse(response.body)
+    expect(json["catalog"]).to eq("gated" => false)
+    expect(json["auth"]).to be_nil
+  end
+
+  it "reports a configured policy as a catalog that can gate" do
+    policy = Class.new do
+      def visible_software_scope(subject: nil) = WarpEngine::Software.all
+      def access_for(software:, subject: nil) = WarpEngine::Access::OPEN
+      def authorize_download(asset: nil, subject: nil, request: nil) = WarpEngine::Access::Grant::OPEN
+    end.new
+    allow(WarpEngine.config).to receive(:access_policy).and_return(policy)
+
+    get "/api/service"
+
+    expect(JSON.parse(response.body)["catalog"]).to eq("gated" => true)
+  end
+
+  describe "with a client identity configured" do
+    before do
+      allow(WarpEngine.config).to receive(:access_token_owner_class).and_return("TestOwner")
+      allow(WarpEngine.config).to receive(:identity_verification_url).and_return("/devices")
+    end
+
+    it "describes the device flow, so a client needs no addresses of its own" do
+      get "/api/service"
+
+      auth = JSON.parse(response.body)["auth"]
+      expect(auth["schemes"]).to eq([ "bearer" ])
+      expect(auth["device"]["authorizeUrl"]).to eq("http://www.example.com/api/auth/device")
+      expect(auth["device"]["tokenUrl"]).to eq("http://www.example.com/api/auth/device/token")
+      expect(auth["device"]["revokeUrl"]).to eq("http://www.example.com/api/auth/token")
+      expect(auth["device"]["interval"]).to eq(5)
+    end
+
+    # A host that configured a bare path should not have to know its own hostname; one
+    # that put the approval page on another domain should keep it.
+    it "makes a configured path absolute against the request" do
+      get "/api/service"
+
+      expect(JSON.parse(response.body)["auth"]["device"]["verificationUrl"])
+        .to eq("http://www.example.com/devices")
+    end
+
+    it "leaves an absolute verification URL alone" do
+      allow(WarpEngine.config).to receive(:identity_verification_url)
+        .and_return("https://accounts.example.org/devices")
+
+      get "/api/service"
+
+      expect(JSON.parse(response.body)["auth"]["device"]["verificationUrl"])
+        .to eq("https://accounts.example.org/devices")
+    end
+  end
+end
diff --git a/libs/ruby/warp_engine/spec/services/access_policy_spec.rb b/libs/ruby/warp_engine/spec/services/access_policy_spec.rb
new file mode 100644
index 0000000..14f0054
--- /dev/null
+++ b/libs/ruby/warp_engine/spec/services/access_policy_spec.rb
@@ -0,0 +1,174 @@
+require "rails_helper"
+require "tmpdir"
+
+# The access seam, from both sides: what the catalog says about a title, and whether an
+# artifact is handed over. The load-bearing case is the *default* one — a catalog with
+# no policy configured has to behave exactly as it did before this existed.
+RSpec.describe "The access policy" do
+  let(:tmpdir) { Dir.mktmpdir }
+
+  # A policy that gates everything except what the subject is named after. Small enough
+  # to read, and it exercises every method of the contract.
+  let(:gating_policy) do
+    Class.new do
+      def initialize(open_name) = @open_name = open_name
+
+      def visible_software_scope(subject: nil)
+        WarpEngine::Software.where.not(status: "development")
+      end
+
+      def access_for(software:, subject: nil)
+        return WarpEngine::Access.new if software.name == @open_name
+
+        WarpEngine::Access.new(
+          gated: true, entitled: subject.present?, price_cents: 1490, currency: "EUR",
+          purchase_url: "https://shop.example/#{software.name}",
+          web_url: "https://shop.example/play/#{software.name}"
+        )
+      end
+
+      def authorize_download(asset: nil, subject: nil, request: nil)
+        return WarpEngine::Access::Grant.new if asset&.release&.software&.name == @open_name
+
+        subject.nil? ? nil : WarpEngine::Access::Grant.new
+      end
+    end
+  end
+
+  before do
+    allow(WarpEngine.config).to receive(:file_container_path).and_return(tmpdir)
+    WarpEngine::Storage.reset!
+    WarpEngine::AccessPolicy.reset!
+  end
+
+  after do
+    FileUtils.rm_rf(tmpdir)
+    WarpEngine::AccessPolicy.reset!
+  end
+
+  describe "the default (:open) policy" do
+    it "lists every software, whatever its status" do
+      create(:software, status: "development")
+      create(:software, status: "released")
+
+      result = WarpEngine::SoftwareService.new.index
+
+      expect(result[:softwares].size).to eq(2)
+    end
+
+    it "reports every title as open, so a client never has to guess" do
+      create(:software)
+
+      entry = WarpEngine::SoftwareService.new.index[:softwares].first
+
+      expect(entry[:access]).to eq(
+        gated: false, entitled: true, price: nil, purchaseUrl: nil, webUrl: nil
+      )
+    end
+
+    it "hands over an artifact with no subject at all" do
+      File.write(File.join(tmpdir, "game-1.0.zip"), "zip")
+
+      path = WarpEngine::DownloadService.new.create(
+        path: "game-1.0.zip", ip: "127.0.0.1", user_agent: "rspec", referer: nil
+      )
+
+      expect(path).to eq(File.join(tmpdir, "game-1.0.zip"))
+    end
+  end
+
+  describe "a configured policy" do
+    let(:open_software)   { create(:software, name: "free-game", status: "released") }
+    let(:gated_software)  { create(:software, name: "paid-game", status: "released") }
+    let(:subject_record)  { create(:test_owner) }
+
+    before do
+      open_software
+      gated_software
+      create(:software, name: "draft-game", status: "development")
+      allow(WarpEngine.config).to receive(:access_policy).and_return(gating_policy.new("free-game"))
+    end
+
+    it "narrows the catalog to what the policy scope allows" do
+      names = WarpEngine::SoftwareService.new.index[:softwares].map { |e| e[:software][:name] }
+
+      expect(names).to contain_exactly("free-game", "paid-game")
+    end
+
+    it "describes a gated title with its price and where to buy it" do
+      entry = WarpEngine::SoftwareService.new.index[:softwares]
+                                         .find { |e| e[:software][:name] == "paid-game" }
+
+      expect(entry[:access]).to eq(
+        gated: true, entitled: false,
+        price: { amountCents: 1490, currency: "EUR" },
+        purchaseUrl: "https://shop.example/paid-game",
+        webUrl: "https://shop.example/play/paid-game"
+      )
+    end
+
+    it "reports entitlement against the authenticated subject" do
+      entry = WarpEngine::SoftwareService.new.index(subject: subject_record)[:softwares]
+                                         .find { |e| e[:software][:name] == "paid-game" }
+
+      expect(entry[:access][:entitled]).to be(true)
+    end
+
+    it "refuses an artifact the policy will not authorise" do
+      File.write(File.join(tmpdir, "paid-game-1.0.zip"), "zip")
+      release = create(:release, software: gated_software)
+      WarpEngine::ReleaseAsset.create!(release: release, kind: "win_x64",
+                                       path: File.join(tmpdir, "paid-game-1.0.zip"))
+
+      expect {
+        WarpEngine::DownloadService.new.create(
+          path: "paid-game-1.0.zip", ip: "127.0.0.1", user_agent: "rspec", referer: nil
+        )
+      }.to raise_error(WarpEngine::DownloadService::Denied)
+    end
+
+    it "hands the same artifact over to a subject the policy accepts" do
+      File.write(File.join(tmpdir, "paid-game-1.0.zip"), "zip")
+      release = create(:release, software: gated_software)
+      WarpEngine::ReleaseAsset.create!(release: release, kind: "win_x64",
+                                       path: File.join(tmpdir, "paid-game-1.0.zip"))
+
+      path = WarpEngine::DownloadService.new.create(
+        path: "paid-game-1.0.zip", ip: "127.0.0.1", user_agent: "rspec", referer: nil,
+        subject: subject_record
+      )
+
+      expect(path).to eq(File.join(tmpdir, "paid-game-1.0.zip"))
+    end
+  end
+
+  describe "a policy that raises" do
+    let(:broken_policy) do
+      Class.new do
+        def visible_software_scope(subject: nil) = raise("boom")
+        def access_for(software:, subject: nil) = raise("boom")
+        def authorize_download(asset: nil, subject: nil, request: nil) = raise("boom")
+      end.new
+    end
+
+    before { allow(WarpEngine.config).to receive(:access_policy).and_return(broken_policy) }
+
+    # The direction of the failure is the point. A broken gatekeeper must not become an
+    # open one: an empty catalog is recoverable, a paid title given away is not.
+    it "empties the catalog rather than leaking it" do
+      create(:software, status: "released")
+
+      expect(WarpEngine::SoftwareService.new.index[:softwares]).to be_empty
+    end
+
+    it "refuses the download rather than serving it" do
+      File.write(File.join(tmpdir, "game-1.0.zip"), "zip")
+
+      expect {
+        WarpEngine::DownloadService.new.create(
+          path: "game-1.0.zip", ip: "127.0.0.1", user_agent: "rspec", referer: nil
+        )
+      }.to raise_error(WarpEngine::DownloadService::Denied)
+    end
+  end
+end