diff --git a/lib/mcp/server.rb b/lib/mcp/server.rb
index 0baa9738..910c8d8f 100644
--- a/lib/mcp/server.rb
+++ b/lib/mcp/server.rb
@@ -65,13 +65,15 @@ def initialize(elicitations)
     #
     # https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2575
     class UnsupportedProtocolVersionError < RequestHandlerError
-      def initialize(requested, request = nil, supported: Configuration::SUPPORTED_MODERN_PROTOCOL_VERSIONS)
+      # No keyword parameters here: with one present, Ruby 2.7 would split a trailing symbol-keyed `request` Hash
+      # into keywords and fail with "unknown keywords".
+      def initialize(requested, request = nil)
         super(
           "Unsupported protocol version",
           request,
           error_type: :unsupported_protocol_version,
           error_code: ErrorCodes::UNSUPPORTED_PROTOCOL_VERSION,
-          error_data: { supported: supported, requested: requested || "unknown" },
+          error_data: { supported: Configuration::SUPPORTED_MODERN_PROTOCOL_VERSIONS, requested: requested || "unknown" },
         )
       end
     end
@@ -560,25 +562,26 @@ def handle_request(request, method, session: nil, related_request_id: nil)
           server_context: { request: request },
           exception_already_reported: ->(e) { reported_exception.equal?(e) },
         ) do
+          envelope = lift_request_envelope(params, method: method, session: session)
           result = case method
           when Methods::INITIALIZE
             init(params, session: session)
           when Methods::RESOURCES_READ
-            build_read_resource_result(read_resource_contents(params, session: session, related_request_id: related_request_id, cancellation: cancellation))
+            build_read_resource_result(read_resource_contents(params, session: session, related_request_id: related_request_id, cancellation: cancellation, envelope: envelope))
           when Methods::RESOURCES_SUBSCRIBE, Methods::RESOURCES_UNSUBSCRIBE
             validate_resource_subscription_params!(params)
-            dispatch_optional_context_handler(@handlers[method], params, session: session, related_request_id: related_request_id, cancellation: cancellation)
+            dispatch_optional_context_handler(@handlers[method], params, session: session, related_request_id: related_request_id, cancellation: cancellation, envelope: envelope)
             {}
           when Methods::TOOLS_CALL
-            call_tool(params, session: session, related_request_id: related_request_id, cancellation: cancellation)
+            call_tool(params, session: session, related_request_id: related_request_id, cancellation: cancellation, envelope: envelope)
           when Methods::PROMPTS_GET
-            get_prompt(params, session: session, related_request_id: related_request_id, cancellation: cancellation)
+            get_prompt(params, session: session, related_request_id: related_request_id, cancellation: cancellation, envelope: envelope)
           when Methods::COMPLETION_COMPLETE
-            complete(params, session: session, related_request_id: related_request_id, cancellation: cancellation)
+            complete(params, session: session, related_request_id: related_request_id, cancellation: cancellation, envelope: envelope)
           when Methods::LOGGING_SET_LEVEL
             configure_logging_level(params, session: session)
           else
-            dispatch_optional_context_handler(@handlers[method], params, session: session, related_request_id: related_request_id, cancellation: cancellation)
+            dispatch_optional_context_handler(@handlers[method], params, session: session, related_request_id: related_request_id, cancellation: cancellation, envelope: envelope)
           end
           client = session&.client || @client
           add_instrumentation_data(client: client) if client
@@ -609,6 +612,34 @@ def handle_request(request, method, session: nil, related_request_id: nil)
       }
     end
 
+    # 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.
+    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
+
+      if modern_session && method == Methods::INITIALIZE
+        requested = params.is_a?(Hash) ? params[:protocolVersion] || params["protocolVersion"] : nil
+        raise UnsupportedProtocolVersionError.new(requested, params)
+      end
+
+      if RequestEnvelope.modern?(params)
+        RequestEnvelope.parse!(params, request: params)
+      elsif modern_session
+        raise RequestHandlerError.new(
+          "Invalid Request: modern sessions require the SEP-2575 `_meta` envelope",
+          params,
+          error_type: :invalid_request,
+        )
+      end
+    end
+
     def handle_cancelled_notification(params, session: nil)
       return unless session
       return unless params.is_a?(Hash)
@@ -748,7 +779,7 @@ def list_tools(request)
       apply_cache_metadata({ tools: page[:items], nextCursor: page[:next_cursor] }.compact)
     end
 
-    def call_tool(request, session: nil, related_request_id: nil, cancellation: nil)
+    def call_tool(request, session: nil, related_request_id: nil, cancellation: nil, envelope: nil)
       tool_name = request[:name]
 
       tool = tools[tool_name]
@@ -781,7 +812,7 @@ def call_tool(request, session: nil, related_request_id: nil, cancellation: nil)
       progress_token = request.dig(:_meta, :progressToken)
 
       response = call_tool_with_args(
-        tool, arguments, server_context_with_meta(request), progress_token: progress_token, session: session, related_request_id: related_request_id, cancellation: cancellation
+        tool, arguments, server_context_with_meta(request), progress_token: progress_token, session: session, related_request_id: related_request_id, cancellation: cancellation, envelope: envelope
       )
       result = response.to_h
       validate_tool_call_result!(tool, result)
@@ -808,7 +839,7 @@ def list_prompts(request)
       apply_cache_metadata({ prompts: page[:items], nextCursor: page[:next_cursor] }.compact)
     end
 
-    def get_prompt(request, session: nil, related_request_id: nil, cancellation: nil)
+    def get_prompt(request, session: nil, related_request_id: nil, cancellation: nil, envelope: nil)
       prompt_name = request[:name]
       prompt = @prompts[prompt_name]
       unless prompt
@@ -826,6 +857,7 @@ def get_prompt(request, session: nil, related_request_id: nil, cancellation: nil
         session: session,
         related_request_id: related_request_id,
         cancellation: cancellation,
+        envelope: envelope,
       )
 
       call_prompt_template_with_args(prompt, prompt_args, server_context)
@@ -921,7 +953,7 @@ def apply_cache_metadata(result)
       { ttlMs: @ttl_ms || 0, cacheScope: @cache_scope || "public" }.merge(result)
     end
 
-    def complete(params, session: nil, related_request_id: nil, cancellation: nil)
+    def complete(params, session: nil, related_request_id: nil, cancellation: nil, envelope: nil)
       validate_completion_params!(params)
 
       result = dispatch_optional_context_handler(
@@ -930,6 +962,7 @@ def complete(params, session: nil, related_request_id: nil, cancellation: nil)
         session: session,
         related_request_id: related_request_id,
         cancellation: cancellation,
+        envelope: envelope,
       )
 
       normalize_completion_result(result)
@@ -938,13 +971,14 @@ def complete(params, session: nil, related_request_id: nil, cancellation: nil)
     # Invokes `resources/read` via the registered handler. If the handler block opts in to `server_context:`,
     # pass an `MCP::ServerContext` so the handler can observe cancellation via `server_context.cancelled?` or
     # `server_context.raise_if_cancelled!`.
-    def read_resource_contents(request, session: nil, related_request_id: nil, cancellation: nil)
+    def read_resource_contents(request, session: nil, related_request_id: nil, cancellation: nil, envelope: nil)
       dispatch_optional_context_handler(
         @handlers[Methods::RESOURCES_READ],
         request,
         session: session,
         related_request_id: related_request_id,
         cancellation: cancellation,
+        envelope: envelope,
       )
     end
 
@@ -952,7 +986,7 @@ def read_resource_contents(request, session: nil, related_request_id: nil, cance
     # `completion_handler`, `resources_subscribe_handler`, `resources_unsubscribe_handler`, or `define_custom_method`.
     # Existing handlers that only accept `params` are called unchanged; handlers that declare a `server_context:`
     # keyword receive an `MCP::ServerContext` wrapping the raw server context with cancellation plumbing.
-    def dispatch_optional_context_handler(handler, params, session: nil, related_request_id: nil, cancellation: nil)
+    def dispatch_optional_context_handler(handler, params, session: nil, related_request_id: nil, cancellation: nil, envelope: nil)
       return handler.call(params) unless handler_declares_server_context?(handler)
 
       server_context = build_server_context(
@@ -960,6 +994,7 @@ def dispatch_optional_context_handler(handler, params, session: nil, related_req
         session: session,
         related_request_id: related_request_id,
         cancellation: cancellation,
+        envelope: envelope,
       )
       handler.call(params, server_context: server_context)
     end
@@ -984,7 +1019,7 @@ def handler_declares_server_context?(handler)
 
     # Builds an `MCP::ServerContext` used to give a handler access to session-scoped helpers
     # (progress, cancellation, nested server-to-client requests).
-    def build_server_context(request:, session:, related_request_id:, cancellation:)
+    def build_server_context(request:, session:, related_request_id:, cancellation:, envelope: nil)
       meta_source = request.is_a?(Hash) ? request : {}
       progress_token = meta_source.dig(:_meta, :progressToken)
       progress = Progress.new(notification_target: session, progress_token: progress_token, related_request_id: related_request_id)
@@ -994,6 +1029,7 @@ def build_server_context(request:, session:, related_request_id:, cancellation:)
         notification_target: session,
         related_request_id: related_request_id,
         cancellation: cancellation,
+        envelope: envelope,
       )
     end
 
@@ -1053,7 +1089,7 @@ def accepts_server_context?(method_object)
       end
     end
 
-    def call_tool_with_args(tool, arguments, context, progress_token: nil, session: nil, related_request_id: nil, cancellation: nil)
+    def call_tool_with_args(tool, arguments, context, progress_token: nil, session: nil, related_request_id: nil, cancellation: nil, envelope: nil)
       # Transports parse incoming JSON with `symbolize_names: true`, so `arguments` already arrives symbolized
       # at every nesting level. This top-level transform only guards callers that hand in string-keyed top-level arguments;
       # it does not recurse, and nested object keys remain symbols. Tools therefore receive symbol keys all the way down.
@@ -1068,6 +1104,7 @@ def call_tool_with_args(tool, arguments, context, progress_token: nil, session:
           notification_target: session,
           related_request_id: related_request_id,
           cancellation: cancellation,
+          envelope: envelope,
         )
         tool.call(**args, server_context: server_context)
       else
diff --git a/lib/mcp/server_context.rb b/lib/mcp/server_context.rb
index dc5b20a5..d2abb527 100644
--- a/lib/mcp/server_context.rb
+++ b/lib/mcp/server_context.rb
@@ -4,12 +4,17 @@ module MCP
   class ServerContext
     attr_reader :cancellation
 
-    def initialize(context, progress:, notification_target:, related_request_id: nil, cancellation: nil)
+    # The SEP-2575 per-request envelope (`MCP::RequestEnvelope`) when the request was classified as modern;
+    # `nil` on legacy requests.
+    attr_reader :envelope
+
+    def initialize(context, progress:, notification_target:, related_request_id: nil, cancellation: nil, envelope: nil)
       @context = context
       @progress = progress
       @notification_target = notification_target
       @related_request_id = related_request_id
       @cancellation = cancellation
+      @envelope = envelope
     end
 
     def cancelled?
@@ -20,6 +25,52 @@ def raise_if_cancelled!
       @cancellation&.raise_if_cancelled!
     end
 
+    # Whether the current request follows the stateless modern lifecycle (SEP-2575).
+    def modern?
+      !@envelope.nil?
+    end
+
+    # Client identity for the current request. Modern requests carry it in the `_meta` envelope;
+    # legacy sessions fall back to the state stored by `initialize`. The envelope always wins
+    # because servers MUST NOT infer identity from prior requests.
+    def client_info
+      return @envelope.client_info if @envelope
+
+      @notification_target.client if @notification_target.respond_to?(:client)
+    end
+
+    # Client capabilities for the current request, with the same envelope-first resolution as {#client_info}.
+    def client_capabilities
+      return @envelope.client_capabilities if @envelope
+
+      @notification_target.client_capabilities if @notification_target.respond_to?(:client_capabilities)
+    end
+
+    # The protocol version the current request was made with. `nil` on legacy requests,
+    # where the version is a session-level negotiation result rather than per-request data.
+    def protocol_version
+      @envelope&.protocol_version
+    end
+
+    # Guards the current request on a declared client capability (SEP-2575). `path` names nested capability keys,
+    # e.g. `require_client_capability!(:elicitation, :form)`. Raises `Server::MissingRequiredClientCapabilityError`
+    # (JSON-RPC error `-32021` with `data: { requiredCapabilities: ... }`) when the capability was not declared.
+    def require_client_capability!(*path)
+      raise ArgumentError, "at least one capability key is required" if path.empty?
+
+      declared = client_capabilities
+      value = path.reduce(declared) do |acc, key|
+        break unless acc.is_a?(Hash)
+
+        symbol_value = acc[key.to_sym]
+        symbol_value.nil? ? acc[key.to_s] : symbol_value
+      end
+      return unless value.nil?
+
+      required = path.reverse.inject({}) { |acc, key| { key.to_sym => acc } }
+      raise Server::MissingRequiredClientCapabilityError, required
+    end
+
     # Reports progress for the current tool operation.
     # The notification is automatically scoped to the originating session.
     #
@@ -41,6 +92,14 @@ def report_progress(progress, total: nil, message: nil)
     def notify_log_message(data:, level:, logger: nil)
       return unless @notification_target
 
+      # Modern requests opt in to logging per request (SEP-2575): without `io.modelcontextprotocol/logLevel` in `_meta`,
+      # the server MUST NOT send any `notifications/message` for the request, and an insufficient level drops
+      # the message the same way. Session- or server-level gating still applies downstream on delegation.
+      if @envelope
+        threshold = @envelope.log_level && LoggingMessageNotification.new(level: @envelope.log_level)
+        return unless threshold&.valid_level? && threshold.should_notify?(level)
+      end
+
       @notification_target.notify_log_message(data: data, level: level, logger: logger, related_request_id: @related_request_id)
     end
 
diff --git a/lib/mcp/server_session.rb b/lib/mcp/server_session.rb
index 754c1d00..7cbf2fbc 100644
--- a/lib/mcp/server_session.rb
+++ b/lib/mcp/server_session.rb
@@ -7,12 +7,22 @@ module MCP
   # Holds per-connection state for a single client session.
   # Created by the transport layer; delegates request handling to the shared `Server`.
   class ServerSession
+    ERAS = [:legacy, :modern].freeze
+
     attr_reader :session_id, :client, :logging_message_notification
 
-    def initialize(server:, transport:, session_id: nil)
+    # Connection-era lock of the dual-era serving model (SEP-2575): `nil` until the first era-distinctive message succeeds,
+    # then `:legacy` or `:modern` for the connection's lifetime. Modern-era transports construct their per-request sessions
+    # with `era: :modern` up front.
+    attr_reader :era
+
+    def initialize(server:, transport:, session_id: nil, era: nil)
+      validate_era!(era) if era
+
       @server = server
       @transport = transport
       @session_id = session_id
+      @era = era
       @client = nil
       @client_capabilities = nil
       @logging_message_notification = nil
@@ -31,6 +41,19 @@ def initialized?
     # (the initialization phase MUST be the first interaction).
     def mark_initialized!
       @initialized = true
+      # A successful `initialize` is the legacy-distinctive message of the dual-era serving model (SEP-2575),
+      # so it also locks the connection era.
+      @era ||= :legacy
+    end
+
+    # One-shot era lock. Locking the already-locked era is a no-op; flipping an established era raises,
+    # because a connection can never change eras.
+    def lock_era!(era)
+      validate_era!(era)
+      return if @era == era
+      raise "Session era already locked to #{@era}" if @era
+
+      @era = era
     end
 
     # Registers a `Cancellation` token for an in-flight request.
@@ -236,6 +259,10 @@ def notify_log_message(data:, level:, logger: nil, related_request_id: nil)
 
     private
 
+    def validate_era!(era)
+      raise ArgumentError, "era must be one of #{ERAS.inspect}" unless ERAS.include?(era)
+    end
+
     # Forwards `send_notification` to the transport with only the kwargs the transport's method signature
     # actually accepts. Custom transports that implement the abstract `send_notification(method, params = nil)`
     # contract continue to work unchanged; bundled transports that declare `session_id:` / `related_request_id:`
diff --git a/test/mcp/server_context_test.rb b/test/mcp/server_context_test.rb
index 91258052..07d60727 100644
--- a/test/mcp/server_context_test.rb
+++ b/test/mcp/server_context_test.rb
@@ -311,6 +311,71 @@ def context.combine(prefix, suffix:, &block)
       assert_nothing_raised { server_context.notify_log_message(data: "test", level: "info") }
     end
 
+    test "ServerContext#notify_log_message drops messages on modern requests without a logLevel" do
+      # Per SEP-2575, without `io.modelcontextprotocol/logLevel` in `_meta`, the server MUST NOT send
+      # any `notifications/message` for the request.
+      notification_target = mock
+      notification_target.expects(:notify_log_message).never
+      server_context = build_modern_server_context(notification_target, log_level: nil)
+
+      server_context.notify_log_message(data: "test", level: "error")
+    end
+
+    test "ServerContext#notify_log_message drops messages below the modern request logLevel" do
+      notification_target = mock
+      notification_target.expects(:notify_log_message).never
+      server_context = build_modern_server_context(notification_target, log_level: "error")
+
+      server_context.notify_log_message(data: "test", level: "info")
+    end
+
+    test "ServerContext#notify_log_message delegates messages at or above the modern request logLevel" do
+      notification_target = mock
+      notification_target.expects(:notify_log_message).with(
+        data: "test", level: "error", logger: nil, related_request_id: nil,
+      ).once
+      server_context = build_modern_server_context(notification_target, log_level: "warning")
+
+      server_context.notify_log_message(data: "test", level: "error")
+    end
+
+    test "ServerContext exposes envelope data and falls back to the session on legacy requests" do
+      notification_target = mock
+      notification_target.stubs(:client).returns({ name: "legacy_client", version: "1.0" })
+      notification_target.stubs(:client_capabilities).returns({ roots: {} })
+      progress = Progress.new(notification_target: notification_target, progress_token: nil)
+
+      legacy_context = ServerContext.new(nil, progress: progress, notification_target: notification_target)
+      modern_context = build_modern_server_context(notification_target)
+
+      refute_predicate legacy_context, :modern?
+      assert_equal({ name: "legacy_client", version: "1.0" }, legacy_context.client_info)
+      assert_equal({ roots: {} }, legacy_context.client_capabilities)
+      assert_nil legacy_context.protocol_version
+
+      assert_predicate modern_context, :modern?
+      assert_equal({ name: "modern_client", version: "2.0" }, modern_context.client_info)
+      assert_equal({ elicitation: { form: {} } }, modern_context.client_capabilities)
+      assert_equal "2026-07-28", modern_context.protocol_version
+    end
+
+    test "ServerContext#require_client_capability! matches string-keyed capabilities and requires a path" do
+      envelope = RequestEnvelope.new(
+        protocol_version: "2026-07-28",
+        client_info: { name: "c", version: "1" },
+        client_capabilities: { "elicitation" => { "form" => {} } },
+      )
+      progress = Progress.new(notification_target: nil, progress_token: nil)
+      server_context = ServerContext.new(nil, progress: progress, notification_target: nil, envelope: envelope)
+
+      assert_nothing_raised { server_context.require_client_capability!(:elicitation, :form) }
+      assert_raises(ArgumentError) { server_context.require_client_capability! }
+      error = assert_raises(Server::MissingRequiredClientCapabilityError) do
+        server_context.require_client_capability!(:sampling)
+      end
+      assert_equal({ requiredCapabilities: { sampling: {} } }, error.error_data)
+    end
+
     test "ServerContext#notify_resources_updated delegates to notification_target" do
       notification_target = mock
       notification_target.expects(:notify_resources_updated).with(uri: "test://resource-1").once
@@ -1000,5 +1065,18 @@ def template(args, server_context:)
       assert_equal "User: prompt_user, Trace: trace_xyz789, Message: World",
         response[:result][:messages][0][:content][:text]
     end
+
+    private
+
+    def build_modern_server_context(notification_target, log_level: nil)
+      envelope = RequestEnvelope.new(
+        protocol_version: "2026-07-28",
+        client_info: { name: "modern_client", version: "2.0" },
+        client_capabilities: { elicitation: { form: {} } },
+        log_level: log_level,
+      )
+      progress = Progress.new(notification_target: notification_target, progress_token: nil)
+      ServerContext.new(nil, progress: progress, notification_target: notification_target, envelope: envelope)
+    end
   end
 end
diff --git a/test/mcp/server_test.rb b/test/mcp/server_test.rb
index c51c2109..d8cce38d 100644
--- a/test/mcp/server_test.rb
+++ b/test/mcp/server_test.rb
@@ -197,6 +197,17 @@ class ServerTest < ActiveSupport::TestCase
       assert_equal "unknown", error.error_data[:requested]
     end
 
+    test "UnsupportedProtocolVersionError accepts a symbol-keyed request Hash on every Ruby version" do
+      # On Ruby 2.7, a keyword parameter in `initialize` would make the trailing symbol-keyed Hash split
+      # into keywords and raise an ArgumentError.
+      request = { name: "echo", arguments: {}, _meta: {} }
+
+      error = Server::UnsupportedProtocolVersionError.new("1900-01-01", request)
+
+      assert_equal "1900-01-01", error.error_data[:requested]
+      assert_equal Configuration::SUPPORTED_MODERN_PROTOCOL_VERSIONS, error.error_data[:supported]
+    end
+
     test "MissingRequiredClientCapabilityError surfaces as -32021 with the SEP-2575 data shape" do
       server = Server.new(name: "error_test", tools: [TestTool])
       server.define_tool(name: "missing_capability_tool") do
@@ -215,6 +226,152 @@ class ServerTest < ActiveSupport::TestCase
       assert_equal({ elicitation: {} }, response.dig(:error, :data, :requiredCapabilities))
     end
 
+    test "#handle tools/call with a modern envelope exposes per-request client data without touching the session" do
+      server = Server.new(name: "modern_test", tools: [])
+      received = nil
+      server.define_tool(name: "modern_tool") do |server_context:|
+        received = {
+          modern: server_context.modern?,
+          client_info: server_context.client_info,
+          client_capabilities: server_context.client_capabilities,
+          protocol_version: server_context.protocol_version,
+        }
+        Tool::Response.new([{ type: "text", text: "ok" }])
+      end
+      session = ServerSession.new(server: server, transport: mock)
+
+      response = server.handle(
+        modern_request("tools/call", { name: "modern_tool", arguments: {} }, capabilities: { elicitation: {} }),
+        session: session,
+      )
+
+      refute_nil response[:result]
+      assert received[:modern]
+      assert_equal({ name: "modern_client", version: "2.0" }, received[:client_info])
+      assert_equal({ elicitation: {} }, received[:client_capabilities])
+      assert_equal "2026-07-28", received[:protocol_version]
+      # Per SEP-2575, servers MUST NOT infer client state from prior requests, so nothing is stored.
+      assert_nil session.client
+      refute_predicate session, :initialized?
+    end
+
+    test "#handle tools/call with an unsupported envelope version returns -32022" do
+      server = Server.new(name: "modern_test", tools: [])
+      called = false
+      server.define_tool(name: "modern_tool") do
+        called = true
+        Tool::Response.new([{ type: "text", text: "ok" }])
+      end
+
+      response = server.handle(modern_request("tools/call", { name: "modern_tool" }, version: "2027-01-01"))
+
+      refute called
+      assert_equal ErrorCodes::UNSUPPORTED_PROTOCOL_VERSION, response.dig(:error, :code)
+      assert_equal Configuration::SUPPORTED_MODERN_PROTOCOL_VERSIONS, response.dig(:error, :data, :supported)
+      assert_equal "2027-01-01", response.dig(:error, :data, :requested)
+    end
+
+    test "#handle treats a partial modern triple as a legacy request" do
+      server = Server.new(name: "modern_test", tools: [])
+      received_context = nil
+      server.define_tool(name: "modern_tool") do |server_context:|
+        received_context = server_context
+        Tool::Response.new([{ type: "text", text: "ok" }])
+      end
+
+      response = server.handle({
+        jsonrpc: "2.0",
+        method: "tools/call",
+        id: 1,
+        params: {
+          name: "modern_tool",
+          arguments: {},
+          _meta: { "io.modelcontextprotocol/protocolVersion": "2026-07-28" },
+        },
+      })
+
+      refute_nil response[:result]
+      refute_predicate received_context, :modern?
+    end
+
+    test "#handle requires the envelope for requests on a modern-locked session" do
+      server = Server.new(name: "modern_test", tools: [])
+      server.define_tool(name: "modern_tool") { Tool::Response.new([{ type: "text", text: "ok" }]) }
+      session = ServerSession.new(server: server, transport: mock, era: :modern)
+
+      response = server.handle(
+        { jsonrpc: "2.0", method: "tools/call", id: 1, params: { name: "modern_tool" } },
+        session: session,
+      )
+
+      assert_equal JsonRpcHandler::ErrorCode::INVALID_REQUEST, response.dig(:error, :code)
+    end
+
+    test "#handle rejects initialize on a modern-locked session with -32022" do
+      session = ServerSession.new(server: @server, transport: mock, era: :modern)
+
+      response = @server.handle(
+        {
+          jsonrpc: "2.0",
+          method: "initialize",
+          id: 1,
+          params: { protocolVersion: "2025-11-25", clientInfo: { name: "c", version: "1" } },
+        },
+        session: session,
+      )
+
+      assert_equal ErrorCodes::UNSUPPORTED_PROTOCOL_VERSION, response.dig(:error, :code)
+      assert_equal "2025-11-25", response.dig(:error, :data, :requested)
+    end
+
+    test "#handle server/discover succeeds without an envelope on a modern-locked session" do
+      session = ServerSession.new(server: @server, transport: mock, era: :modern)
+
+      response = @server.handle({ jsonrpc: "2.0", method: "server/discover", id: 1 }, session: session)
+
+      refute_nil response[:result]
+    end
+
+    test "#handle tools/call enforces require_client_capability! from the envelope" do
+      server = Server.new(name: "modern_test", tools: [])
+      server.define_tool(name: "guarded_tool") do |server_context:|
+        server_context.require_client_capability!(:elicitation, :form)
+        Tool::Response.new([{ type: "text", text: "ok" }])
+      end
+
+      declared = server.handle(
+        modern_request("tools/call", { name: "guarded_tool" }, capabilities: { elicitation: { form: {} } }),
+      )
+      undeclared = server.handle(modern_request("tools/call", { name: "guarded_tool" }))
+
+      refute_nil declared[:result]
+      assert_equal ErrorCodes::MISSING_REQUIRED_CLIENT_CAPABILITY, undeclared.dig(:error, :code)
+      assert_equal(
+        { elicitation: { form: {} } },
+        undeclared.dig(:error, :data, :requiredCapabilities),
+      )
+    end
+
+    test "ServerSession locks the legacy era on mark_initialized! and refuses to flip eras" do
+      session = ServerSession.new(server: @server, transport: mock)
+
+      assert_nil session.era
+      session.mark_initialized!
+      assert_equal :legacy, session.era
+
+      session.lock_era!(:legacy)
+      assert_equal :legacy, session.era
+      assert_raises(RuntimeError) { session.lock_era!(:modern) }
+    end
+
+    test "ServerSession accepts a preset era and validates era values" do
+      session = ServerSession.new(server: @server, transport: mock, era: :modern)
+
+      assert_equal :modern, session.era
+      assert_raises(ArgumentError) { ServerSession.new(server: @server, transport: mock, era: :future) }
+      assert_raises(ArgumentError) { session.lock_era!(:future) }
+    end
+
     test "#handle initialize request returns protocol info, server info, and capabilities" do
       request = {
         jsonrpc: "2.0",
@@ -3726,5 +3883,23 @@ def server_context
 
       refute Apps.client_supports?(server.client_capabilities)
     end
+
+    private
+
+    # Builds a request carrying the SEP-2575 modern `_meta` envelope.
+    def modern_request(method, params, version: "2026-07-28", capabilities: {})
+      {
+        jsonrpc: "2.0",
+        method: method,
+        id: 1,
+        params: params.merge(
+          _meta: {
+            "io.modelcontextprotocol/protocolVersion": version,
+            "io.modelcontextprotocol/clientInfo": { name: "modern_client", version: "2.0" },
+            "io.modelcontextprotocol/clientCapabilities": capabilities,
+          },
+        ),
+      }
+    end
   end
 end