Merge pull request #408 from koic/dcr_application_type

Set OIDC `application_type` on Dynamic Client Registration per SEP-837

lib/mcp/client/oauth/discovery.rb

@@ -237,6 +237,23 @@ module MCP
             false
           end
 
+          # Infers the OIDC Dynamic Client Registration `application_type` for a client from its `redirect_uris`.
+          # Per SEP-837, MCP clients MUST specify an appropriate application type during Dynamic Client Registration
+          # so the authorization server can apply the matching redirect URI policy.
+          #
+          # Returns `"native"` when every redirect URI is a native-app URI: a custom non-http(s) scheme (RFC 8252 Section 7.1)
+          # or an http(s) URI whose host is a loopback address (`localhost`, `127.0.0.0/8`, or `::1`, RFC 8252 Section 7.3).
+          # Returns `"web"` otherwise, including when `redirect_uris` is nil, empty, or contains an unparseable URI.
+          #
+          # - https://github.com/modelcontextprotocol/modelcontextprotocol/pull/837
+          # - https://openid.net/specs/openid-connect-registration-1_0.html#ClientMetadata
+          def infer_application_type(redirect_uris)
+            uris = Array(redirect_uris)
+            return "web" if uris.empty?
+
+            uris.all? { |uri| native_redirect_uri?(uri) } ? "native" : "web"
+          end
+
           # Like `canonicalize_url` but also strips query string, fragment, and
           # userinfo. This variant is used for identity comparison against
           # the request URL Faraday actually sends, which differs from the value
@@ -345,6 +362,20 @@ module MCP
             nil
           end
 
+          # A redirect URI counts as native when it uses a custom non-http(s) scheme
+          # (e.g. `com.example.app:/callback`) or when it is an http(s) URI whose host is
+          # a loopback address. A URI without a scheme or one that fails to parse is not native.
+          def native_redirect_uri?(url)
+            uri = URI.parse(url.to_s)
+            scheme = uri.scheme&.downcase
+            return false if scheme.nil?
+            return loopback_host?(uri.host) if ["http", "https"].include?(scheme)
+
+            true
+          rescue URI::InvalidURIError
+            false
+          end
+
           def base_url(uri)
             port_part = uri.port && uri.port != uri.default_port ? ":#{uri.port}" : ""
             "#{uri.scheme}://#{uri.host}#{port_part}"

lib/mcp/client/oauth/flow.rb

@@ -440,7 +440,7 @@ module MCP
           end
 
           response = begin
-            http_post_json(registration_endpoint, @provider.client_metadata)
+            http_post_json(registration_endpoint, registration_client_metadata)
           rescue Faraday::Error => e
             raise AuthorizationError,
               "Dynamic client registration failed: #{e.class}: #{e.message}."
@@ -466,6 +466,20 @@ module MCP
           info
         end
 
+        # Returns the client metadata to submit on Dynamic Client Registration.
+        # Per SEP-837, MCP clients MUST specify an appropriate OIDC `application_type`
+        # so the authorization server can apply the matching redirect URI policy.
+        # When the user did not set one explicitly, infer `"native"` vs `"web"` from
+        # the registered `redirect_uris`; an explicit value always wins.
+        # https://github.com/modelcontextprotocol/modelcontextprotocol/pull/837
+        def registration_client_metadata
+          metadata = @provider.client_metadata
+          return metadata if metadata[:application_type] || metadata["application_type"]
+
+          redirect_uris = metadata[:redirect_uris] || metadata["redirect_uris"]
+          metadata.merge("application_type" => Discovery.infer_application_type(redirect_uris))
+        end
+
         # Reads `key` from a `client_information` hash that may use either string or
         # symbol keys, so users can persist the result of `JSON.parse` *or* a hand-built
         # `{ client_id:, client_secret: }` and have both work.

lib/mcp/client/oauth/provider.rb

@@ -13,6 +13,9 @@ module MCP
       # - `client_metadata`  - Hash sent to the authorization server's Dynamic Client
       #   Registration endpoint. Must include at minimum `redirect_uris`,
       #   `grant_types`, `response_types`, and `token_endpoint_auth_method`.
+      #   When `application_type` is omitted, the SDK infers `"native"` or `"web"`
+      #   from `redirect_uris` per SEP-837 before registering; an explicit value
+      #   always wins.
       # - `redirect_uri`     - String: the redirect URI used for the authorization
       #   request. Must be one of `redirect_uris` in `client_metadata`.
       # - `redirect_handler` - Callable invoked with the fully-built authorization