Add protocol version negotiation to server initialization

[duboff]

Motivation and Context

Currently server launched enforces the latest protocol regardless of the protocols supported by the client. This leads to issues when a client (eg. Claude Desktop) supports only protocol versions (2025-06-18) behind the current version supported by the server (2025-11-25).

This PR adds full protocol version negotiation logic (as per the MCP Spec), including dropping parameters not supported by each individual protocol version, to maximize compatibility.

How Has This Been Tested?

Tested with the Chattermill MCP (proprietary).

Breaking Changes

No breaking changes expected.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context

A potential consideration could be to add a config option to enumerate supported protocols if the server developer wants to explicitly avoid supporting all possible protocols. Not implemented here as doesn't seem necessary.

[atesgoral]

Thanks for doing this!

[koic]

Thanks! Can you rebase on the latest main branch and resolve the conflicts?

[koic]

The filtering loop logic seems a bit redundant, and the terminology could be clearer. Can you update it?

diff --git a/lib/mcp/server.rb b/lib/mcp/server.rb
index af3ea15..03628bb 100644
--- a/lib/mcp/server.rb
+++ b/lib/mcp/server.rb
@@ -18,6 +18,9 @@ module MCP
   class Server
     DEFAULT_VERSION = "0.1.0"
 
+    UNSUPPORTED_PROPERTIES_UNTIL_2025_06_18 = [:description, :icons].freeze
+    UNSUPPORTED_PROPERTIES_UNTIL_2025_03_26 = [:title, :websiteUrl].freeze
+
     class RequestHandlerError < StandardError
       attr_reader :error_type
       attr_reader :original_error
@@ -321,9 +324,10 @@ module MCP
         configuration.protocol_version
       end
 
-      info = server_info
-      info = info.reject { |key| [:description, :icons].include?(key) } if negotiated_version <= "2025-06-18"
-      info = info.reject { |key| [:title, :websiteUrl].include?(key) } if negotiated_version <= "2025-03-26"
+      info = server_info.reject do |property|
+        negotiated_version <= "2025-06-18" && UNSUPPORTED_PROPERTIES_UNTIL_2025_06_18.include?(property) ||
+          negotiated_version <= "2025-03-26" && UNSUPPORTED_PROPERTIES_UNTIL_2025_03_26.include?(property)
+      end
 
       response_instructions = instructions