Parse SSE responses in HTTP client via event_stream_parser

The Streamable HTTP spec allows servers to respond with either
`application/json` or `text/event-stream`. The client previously
rejected the latter. Add SSE parsing via the optional
`event_stream_parser` gem (users add it to their Gemfile when their
server uses SSE), dispatch on the response Content-Type, and scan the
event stream for the first JSON-RPC response message.

No other Streamable HTTP features (202 handling, session IDs,
`connect`, DELETE termination) are included here; those will land in
follow-up PRs.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: atesgoral

Gemfile

@@ -23,6 +23,7 @@ gem "yard", "~> 0.9"
 gem "yard-sorbet", "~> 0.9" if RUBY_VERSION >= "3.1"
 
 group :test do
+  gem "event_stream_parser", ">= 1.0"
   gem "faraday", ">= 2.0"
   gem "minitest", "~> 5.1", require: false
   gem "mocha"

docs/building-clients.md

@@ -51,11 +51,12 @@ stdio_transport.close
 
 ## HTTP Transport
 
-Use `MCP::Client::HTTP` to interact with MCP servers over HTTP. Requires the `faraday` gem:
+Use `MCP::Client::HTTP` to interact with MCP servers over HTTP. Requires the `faraday` gem, plus `event_stream_parser` if the server uses SSE (`text/event-stream`) responses:
 
 ```ruby
 gem 'mcp'
 gem 'faraday', '>= 2.0'
+gem 'event_stream_parser', '>= 1.0' # optional, required only for SSE responses
 ```
 
 ```ruby

lib/mcp/client/http.rb

@@ -18,8 +18,7 @@ def send_request(request:)
         params = request[:params] || request["params"]
 
         response = client.post("", request)
-        validate_response_content_type!(response, method, params)
-        response.body
+        parse_response_body(response, method, params)
       rescue Faraday::BadRequestError => e
         raise RequestHandlerError.new(
           "The #{method} request is invalid",
@@ -92,14 +91,52 @@ def require_faraday!
           "See https://rubygems.org/gems/faraday for more details."
       end
 
-      def validate_response_content_type!(response, method, params)
+      def require_event_stream_parser!
+        require "event_stream_parser"
+      rescue LoadError
+        raise LoadError, "The 'event_stream_parser' gem is required to parse SSE responses. " \
+          "Add it to your Gemfile: gem 'event_stream_parser', '>= 1.0'. " \
+          "See https://rubygems.org/gems/event_stream_parser for more details."
+      end
+
+      def parse_response_body(response, method, params)
         content_type = response.headers["Content-Type"]
-        return if content_type&.include?("application/json")
+
+        if content_type&.include?("text/event-stream")
+          parse_sse_response(response.body, method, params)
+        elsif content_type&.include?("application/json")
+          response.body
+        else
+          raise RequestHandlerError.new(
+            "Unsupported Content-Type: #{content_type.inspect}. Expected application/json or text/event-stream.",
+            { method: method, params: params },
+            error_type: :unsupported_media_type,
+          )
+        end
+      end
+
+      def parse_sse_response(body, method, params)
+        require_event_stream_parser!
+
+        json_rpc_response = nil
+        parser = EventStreamParser::Parser.new
+        parser.feed(body.to_s) do |_type, data, _id|
+          next if data.empty?
+
+          begin
+            parsed = JSON.parse(data)
+            json_rpc_response = parsed if parsed.is_a?(Hash) && (parsed.key?("result") || parsed.key?("error"))
+          rescue JSON::ParserError
+            next
+          end
+        end
+
+        return json_rpc_response if json_rpc_response
 
         raise RequestHandlerError.new(
-          "Unsupported Content-Type: #{content_type.inspect}. This client only supports JSON responses.",
+          "No valid JSON-RPC response found in SSE stream",
           { method: method, params: params },
-          error_type: :unsupported_media_type,
+          error_type: :parse_error,
         )
       end
     end

test/mcp/client/http_test.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 require "test_helper"
+require "event_stream_parser"
 require "faraday"
 require "webmock/minitest"
 require "mcp/client/http"
@@ -25,6 +26,26 @@ def test_raises_load_error_when_faraday_not_available
         assert_includes(error.message, "Add it to your Gemfile: gem 'faraday', '>= 2.0'")
       end
 
+      def test_raises_load_error_when_event_stream_parser_not_available
+        stub_request(:post, url)
+          .to_return(
+            status: 200,
+            headers: { "Content-Type" => "text/event-stream" },
+            body: "data: {}\n\n",
+          )
+
+        HTTP.any_instance.stubs(:require).with("faraday").returns(true)
+        HTTP.any_instance.stubs(:require).with("event_stream_parser")
+          .raises(LoadError, "cannot load such file -- event_stream_parser")
+
+        error = assert_raises(LoadError) do
+          client.send_request(request: { method: "tools/list" })
+        end
+
+        assert_includes(error.message, "The 'event_stream_parser' gem is required to parse SSE responses")
+        assert_includes(error.message, "Add it to your Gemfile: gem 'event_stream_parser', '>= 1.0'")
+      end
+
       def test_headers_are_added_to_the_request
         headers = { "Authorization" => "Bearer token" }
         client = HTTP.new(url: url, headers: headers)
@@ -267,7 +288,7 @@ def test_block_customizes_faraday_connection
         custom_client.send_request(request: request)
       end
 
-      def test_send_request_raises_error_for_non_json_response
+      def test_send_request_raises_error_for_unsupported_content_type
         request = {
           jsonrpc: "2.0",
           id: "test_id",
@@ -278,22 +299,105 @@ def test_send_request_raises_error_for_non_json_response
           .with(body: request.to_json)
           .to_return(
             status: 200,
-            headers: { "Content-Type" => "text/event-stream" },
-            body: "data: {}\n\n",
+            headers: { "Content-Type" => "text/html" },
+            body: "<html></html>",
           )
 
         error = assert_raises(RequestHandlerError) do
           client.send_request(request: request)
         end
 
         assert_equal(
-          'Unsupported Content-Type: "text/event-stream". This client only supports JSON responses.',
+          'Unsupported Content-Type: "text/html". Expected application/json or text/event-stream.',
           error.message,
         )
         assert_equal(:unsupported_media_type, error.error_type)
         assert_equal({ method: "tools/list", params: nil }, error.request)
       end
 
+      def test_send_request_parses_sse_response
+        request = {
+          jsonrpc: "2.0",
+          id: "test_id",
+          method: "tools/list",
+        }
+
+        sse_body = <<~SSE
+          : comment
+          data: {"jsonrpc":"2.0","method":"notifications/progress","params":{}}
+
+          data: {"jsonrpc":"2.0","id":"test_id","result":{"tools":[{"name":"echo"}]}}
+
+        SSE
+
+        stub_request(:post, url)
+          .with(body: request.to_json)
+          .to_return(
+            status: 200,
+            headers: { "Content-Type" => "text/event-stream" },
+            body: sse_body,
+          )
+
+        response = client.send_request(request: request)
+
+        assert_equal({ "tools" => [{ "name" => "echo" }] }, response["result"])
+      end
+
+      def test_send_request_parses_sse_error_response
+        request = {
+          jsonrpc: "2.0",
+          id: "test_id",
+          method: "tools/list",
+        }
+
+        sse_body = <<~SSE
+          data: {"jsonrpc":"2.0","id":"test_id","error":{"code":-32600,"message":"Invalid request"}}
+
+        SSE
+
+        stub_request(:post, url)
+          .with(body: request.to_json)
+          .to_return(
+            status: 200,
+            headers: { "Content-Type" => "text/event-stream" },
+            body: sse_body,
+          )
+
+        response = client.send_request(request: request)
+
+        assert_equal(-32600, response.dig("error", "code"))
+        assert_equal("Invalid request", response.dig("error", "message"))
+      end
+
+      def test_send_request_raises_error_for_sse_without_response
+        request = {
+          jsonrpc: "2.0",
+          id: "test_id",
+          method: "tools/list",
+        }
+
+        sse_body = <<~SSE
+          : just a comment
+          data: {"jsonrpc":"2.0","method":"notifications/progress","params":{}}
+
+        SSE
+
+        stub_request(:post, url)
+          .with(body: request.to_json)
+          .to_return(
+            status: 200,
+            headers: { "Content-Type" => "text/event-stream" },
+            body: sse_body,
+          )
+
+        error = assert_raises(RequestHandlerError) do
+          client.send_request(request: request)
+        end
+
+        assert_includes(error.message, "No valid JSON-RPC response found in SSE stream")
+        assert_equal(:parse_error, error.error_type)
+      end
+
       private
 
       def stub_request(method, url)