Merge pull request #478 from koic/stdio_era_lock

Serve both lifecycle eras over stdio with an era lock per SEP-2575

lib/mcp/server.rb

@@ -628,23 +628,32 @@ module MCP
     # Lifts the SEP-2575 per-request `_meta` envelope for modern requests. Only a request whose `_meta` carries
     # the full required triple is classified as modern; a partial triple keeps flowing through the legacy path untouched.
     # Notifications carry no envelope (their `_meta` is a `NotificationMetaObject`), and `server/discover` is
-    # pre-version discovery, so both are exempt. On a session already era-locked to modern, `initialize` is
-    # rejected with `-32022` (the modern lifecycle has no handshake) and the triple becomes required for
-    # every other request.
+    # pre-version discovery, so both are exempt. Era-locked sessions additionally enforce the dual-era rules:
+    # on a modern session, `initialize` is rejected with `-32022` (the modern lifecycle has no handshake)
+    # and the triple becomes required for every other request; on a legacy session, a modern envelope is rejected as
+    # an invalid request because a connection can never change eras.
     def lift_request_envelope(params, method:, session:)
       return if Methods.notification?(method)
       return if method == Methods::SERVER_DISCOVER
 
-      modern_session = session.respond_to?(:era) && session.era == :modern
+      era = session.respond_to?(:era) ? session.era : nil
 
-      if modern_session && method == Methods::INITIALIZE
+      if era == :modern && method == Methods::INITIALIZE
         requested = params.is_a?(Hash) ? params[:protocolVersion] || params["protocolVersion"] : nil
         raise UnsupportedProtocolVersionError.new(requested, params)
       end
 
       if RequestEnvelope.modern?(params)
+        if era == :legacy
+          raise RequestHandlerError.new(
+            "Invalid Request: the session already negotiated the legacy lifecycle via `initialize`",
+            params,
+            error_type: :invalid_request,
+          )
+        end
+
         RequestEnvelope.parse!(params, request: params)
-      elsif modern_session
+      elsif era == :modern
         raise RequestHandlerError.new(
           "Invalid Request: modern sessions require the SEP-2575 `_meta` envelope",
           params,

lib/mcp/server/transports/stdio_transport.rb

@@ -49,7 +49,9 @@ module MCP
             end
             break if line.nil?
 
-            response = @session.handle_json(line.strip)
+            line = line.strip
+            parsed = parse_line(line)
+            response = parsed ? dispatch_with_era(parsed) : @session.handle_json(line)
             send_response(response) if response
           end
         rescue Interrupt
@@ -90,6 +92,12 @@ module MCP
         # cancellation has very limited value here regardless; servers that need cancellation propagation for nested
         # server-to-client requests should use `StreamableHTTPTransport`.
         def send_request(method, params = nil)
+          # The modern lifecycle (SEP-2575) forbids server-initiated JSON-RPC requests;
+          # multi round-trip `input_required` results (SEP-2322) replace them.
+          if @session && @session.era == :modern
+            raise "Server-initiated requests are not available in the modern lifecycle (SEP-2575)."
+          end
+
           request_id = generate_request_id
           request = { jsonrpc: "2.0", id: request_id, method: method }
           request[:params] = params if params
@@ -116,7 +124,7 @@ module MCP
 
               return parsed[:result]
             else
-              response = @session ? @session.handle(parsed) : @server.handle(parsed)
+              response = @session ? dispatch_with_era(parsed) : @server.handle(parsed)
               send_response(response) if response
             end
           end
@@ -143,6 +151,35 @@ module MCP
 
           line
         end
+
+        # Parses a frame once so era classification can inspect its method and `_meta`.
+        # Returns `nil` for frames that are not JSON objects; those fall back to
+        # `ServerSession#handle_json` so protocol-level error responses stay identical.
+        def parse_line(line)
+          parsed = JSON.parse(line, symbolize_names: true)
+          parsed.is_a?(Hash) ? parsed : nil
+        rescue JSON::ParserError
+          nil
+        end
+
+        # Serves one frame under the dual-era model (SEP-2575): the first era-distinctive message to succeed locks
+        # the connection era. A successful `initialize` locks `:legacy` inside `Server#init`; a successful `server/discover`
+        # or a successful request carrying the full modern `_meta` triple locks `:modern`. Era-violating frames
+        # (an `initialize` after a modern lock, a modern envelope after a legacy lock, or a missing envelope after a modern lock)
+        # are rejected in-band by `Server#lift_request_envelope`.
+        def dispatch_with_era(parsed)
+          response = @session.handle(parsed)
+          lock_modern_era_on_success(parsed, response)
+          response
+        end
+
+        def lock_modern_era_on_success(parsed, response)
+          return if @session.era
+          return if !response.is_a?(Hash) || response.key?(:error)
+          return if parsed[:method] != Methods::SERVER_DISCOVER && !RequestEnvelope.modern?(parsed[:params])
+
+          @session.lock_era!(:modern)
+        end
       end
     end
   end