Make event_stream_parser an optional dependency like Faraday

event_stream_parser is only used in the client HTTP transport for
parsing SSE responses. Since many SDK users only use the server
functionality, make it an optional dependency that users include in
their Gemfile when needed, consistent with how Faraday is handled.

Co-Authored-By: Claude Opus 4.6 <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"

lib/mcp/client/http.rb

@@ -136,6 +136,14 @@ def require_faraday!
           "See https://rubygems.org/gems/faraday for more details."
       end
 
+      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"]
 
@@ -156,6 +164,7 @@ def parse_response_body(response, method, params)
       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|

mcp.gemspec

@@ -30,6 +30,5 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_dependency("event_stream_parser", ">= 1.0")
   spec.add_dependency("json-schema", ">= 4.1")
 end