Merge pull request #546 from koic/parse_json_response_bodies_in_the_client

Parse HTTP client response bodies without Faraday's JSON middleware

Author: koic

Gemfile

@@ -19,6 +19,9 @@ gem "activesupport"
 gem "debug" if RUBY_VERSION >= "3.1"
 # Avoid i18n 1.15.0, which breaks on Ruby 3.1 (ruby-i18n/i18n#735).
 gem "i18n", "!= 1.15.0"
+# FIXME: Drop this once a json release includes https://github.com/ruby/json/pull/1072. json 3.0.0 and 3.0.1
+# forward arguments after a leading parameter, syntax Ruby 2.7.3 was the first to parse, while allowing Ruby 2.7.0.
+gem "json", "< 3" if RUBY_VERSION < "2.7.3"
 gem "rake", "~> 13.0"
 gem "sorbet-static-and-runtime" if RUBY_VERSION >= "3.0"
 gem "yard", "~> 0.9"

lib/mcp/client/http.rb

@@ -723,7 +723,8 @@ def client
         require_faraday!
         @client ||= Faraday.new(url) do |faraday|
           faraday.request(:json)
-          faraday.response(:json)
+          # The client parses response bodies itself (`resolve_response_body`), so streamed and buffered responses
+          # take the same path; Faraday's json response middleware is deliberately left out.
           faraday.response(:raise_error)
 
           faraday.headers["Accept"] = ACCEPT_HEADER
@@ -1074,8 +1075,8 @@ def resolve_response_body(stream, response, method, params)
         elsif content_type&.include?("application/json")
           return parse_json_buffer(stream.buffer, method, params) unless stream.buffer.empty?
 
-          # Adapters without `on_data` support deliver the body via `response.body`,
-          # already parsed by the json response middleware.
+          # Adapters without `on_data` support deliver the body via `response.body`; a JSON middleware
+          # added by the Faraday customizer may have parsed it already.
           response.body.is_a?(String) ? parse_json_buffer(response.body, method, params) : response.body
         else
           raise RequestHandlerError.new(

test/mcp/client/http_test.rb

@@ -940,6 +940,26 @@ def test_send_request_parses_json_response_when_adapter_does_not_stream
         assert_equal({ "result" => { "tools" => [] } }, response)
       end
 
+      def test_send_request_parses_a_json_body_with_a_parser_taking_keyword_options_only
+        # json 3.0 accepts parser options as keywords only, and Faraday's json response middleware
+        # passes them as a positional Hash, which Ruby 3 no longer converts. The stand-in parser has
+        # the json 3.0 signature, so the body must reach `JSON.parse` through the client's own call.
+        stubs = Faraday::Adapter::Test::Stubs.new do |stub|
+          stub.post("/") do
+            [200, { "Content-Type" => "application/json" }, { result: { tools: [] } }.to_json]
+          end
+        end
+        client = HTTP.new(url: url) { |faraday| faraday.adapter(:test, stubs) }
+        parse = JSON.method(:parse)
+        keyword_options_only_parse = ->(source, **options) { parse.call(source, **options) }
+
+        response = JSON.stub(:parse, keyword_options_only_parse) do
+          client.send_request(request: { jsonrpc: "2.0", id: "test_id", method: "tools/list" })
+        end
+
+        assert_equal({ "result" => { "tools" => [] } }, response)
+      end
+
       def test_send_request_mirrors_x_mcp_header_params_into_mcp_param_headers
         # SEP-2243: on a modern connection, `tools/list` teaches the transport the `x-mcp-header`
         # declarations, and the following `tools/call` mirrors the annotated arguments into