diff --git a/app/controllers/oidc_controller.rb b/app/controllers/oidc_controller.rb
index f7c07f9..8fb4a96 100644
--- a/app/controllers/oidc_controller.rb
+++ b/app/controllers/oidc_controller.rb
@@ -1,6 +1,18 @@
 class OidcController < ApplicationController
   SUPPORTED_SCOPES = %w[openid profile email groups offline_access].freeze
 
+  # Grant types this authorization server supports. Single source of truth:
+  # advertised in discovery (grant_types_supported), accepted at dynamic client
+  # registration, and dispatched by the token endpoint. clinch offers all of these
+  # to every OIDC client — they are all user-context grants gated by consent and
+  # Application#user_allowed?, so there is no per-client grant restriction to
+  # enforce. Keep this in sync with the `case grant_type` dispatch in #token.
+  SUPPORTED_GRANT_TYPES = [
+    "authorization_code",
+    "refresh_token",
+    "urn:ietf:params:oauth:grant-type:device_code"
+  ].freeze
+
   # Discovery and JWKS endpoints are public
   # authorize is also unauthenticated to handle prompt=none and prompt=login specially
   allow_unauthenticated_access only: [:discovery, :jwks, :token, :revoke, :introspect, :userinfo, :logout, :authorize, :device_authorization]
@@ -54,7 +66,7 @@ class OidcController < ApplicationController
       end_session_endpoint: "#{base_url}/logout",
       response_types_supported: ["code"],
       response_modes_supported: ["query"],
-      grant_types_supported: ["authorization_code", "refresh_token", "urn:ietf:params:oauth:grant-type:device_code"],
+      grant_types_supported: SUPPORTED_GRANT_TYPES,
       subject_types_supported: ["pairwise"],
       id_token_signing_alg_values_supported: ["RS256"],
       scopes_supported: SUPPORTED_SCOPES,
diff --git a/app/controllers/oidc_registration_controller.rb b/app/controllers/oidc_registration_controller.rb
index 2aaaa26..81b97a3 100644
--- a/app/controllers/oidc_registration_controller.rb
+++ b/app/controllers/oidc_registration_controller.rb
@@ -16,9 +16,15 @@ class OidcRegistrationController < ApplicationController
   }
 
   AUTH_METHODS = %w[none client_secret_basic client_secret_post].freeze
-  SUPPORTED_GRANT_TYPES = %w[authorization_code refresh_token].freeze
   SUPPORTED_RESPONSE_TYPES = %w[code].freeze
 
+  # Accept exactly the grant types the authorization server advertises in
+  # discovery (single source of truth), so a client cannot be rejected for
+  # requesting a grant the server actually supports (e.g. the device_code grant).
+  def supported_grant_types
+    OidcController::SUPPORTED_GRANT_TYPES
+  end
+
   # POST /oauth/register
   def create
     unless Application.dynamic_registration_enabled?
@@ -37,8 +43,8 @@ class OidcRegistrationController < ApplicationController
     end
 
     grant_types = Array(metadata["grant_types"].presence || ["authorization_code"])
-    if (grant_types - SUPPORTED_GRANT_TYPES).any?
-      return register_error("invalid_client_metadata", "Unsupported grant_types; only #{SUPPORTED_GRANT_TYPES.join(", ")} are allowed")
+    if (grant_types - supported_grant_types).any?
+      return register_error("invalid_client_metadata", "Unsupported grant_types; only #{supported_grant_types.join(", ")} are allowed")
     end
 
     response_types = Array(metadata["response_types"].presence || ["code"])
diff --git a/test/controllers/oidc_registration_controller_test.rb b/test/controllers/oidc_registration_controller_test.rb
index a793410..e7bda37 100644
--- a/test/controllers/oidc_registration_controller_test.rb
+++ b/test/controllers/oidc_registration_controller_test.rb
@@ -89,6 +89,25 @@ class OidcRegistrationControllerTest < ActionDispatch::IntegrationTest
     assert_equal "invalid_client_metadata", JSON.parse(@response.body)["error"]
   end
 
+  test "registers a client requesting the device_code grant advertised in discovery" do
+    enable_dcr
+    register(
+      redirect_uris: ["https://client.example.com/cb"],
+      token_endpoint_auth_method: "none",
+      grant_types: ["urn:ietf:params:oauth:grant-type:device_code", "refresh_token"]
+    )
+    assert_response :created
+    assert_includes JSON.parse(@response.body)["grant_types"], "urn:ietf:params:oauth:grant-type:device_code"
+  end
+
+  test "registration accepts exactly the grant types discovery advertises" do
+    get "/.well-known/openid-configuration"
+    advertised = JSON.parse(@response.body)["grant_types_supported"]
+    # Single source of truth: what we advertise is what registration accepts.
+    assert_equal OidcController::SUPPORTED_GRANT_TYPES, advertised
+    assert_includes advertised, "urn:ietf:params:oauth:grant-type:device_code"
+  end
+
   test "rejects a non-JSON body" do
     enable_dcr
     post "/oauth/register", params: "not json", headers: JSON_HEADERS