modelcontextprotocol · koic · Aug 1, 2026 · Jul 31, 2026
diff --git a/conformance/expected_failures.yml b/conformance/expected_failures.yml
@@ -1,2 +1,6 @@
 server: []
-client: []
+client:
+  # TODO: The conformance framework's client-initialization check pins the expected requested version to 2025-11-25,
+  # while this SDK's latest protocol version is 2026-07-28. Remove once the @modelcontextprotocol/conformance package
+  # accepts 2026-07-28.
+  - initialize
diff --git a/docs/building-clients.md b/docs/building-clients.md
@@ -22,7 +22,7 @@ Call `MCP::Client#connect` to perform the MCP [initialization handshake](https:/
 
 ```ruby
 client.connect
-# => { "protocolVersion" => "2025-11-25", "capabilities" => {...}, "serverInfo" => {...} }
+# => { "protocolVersion" => "2026-07-28", "capabilities" => {...}, "serverInfo" => {...} }
 
 client.connected?  # => true
 client.server_info # => cached InitializeResult
@@ -99,7 +99,7 @@ After `connect` succeeds, the HTTP transport captures the `Mcp-Session-Id` heade
 
 ```ruby
 http_transport.session_id       # => "abc123..."
-http_transport.protocol_version # => "2025-11-25"
+http_transport.protocol_version # => "2026-07-28"
 ```
 
 If the server terminates the session, subsequent requests return HTTP 404 and the transport raises `MCP::Client::SessionExpiredError` (a subclass of `RequestHandlerError`). Session state is cleared automatically; callers should start a new session by calling `connect` again.

diff --git a/lib/mcp/configuration.rb b/lib/mcp/configuration.rb
@@ -2,16 +2,17 @@
 
 module MCP
   class Configuration
-    LATEST_STABLE_PROTOCOL_VERSION = "2025-11-25"
+    LATEST_STABLE_PROTOCOL_VERSION = "2026-07-28"
     SUPPORTED_STABLE_PROTOCOL_VERSIONS = [
-      LATEST_STABLE_PROTOCOL_VERSION, "2025-06-18", "2025-03-26", "2024-11-05",
+      LATEST_STABLE_PROTOCOL_VERSION, "2025-11-25", "2025-06-18", "2025-03-26", "2024-11-05",
     ].freeze
     DEFAULT_NEGOTIATED_PROTOCOL_VERSION = "2025-03-26"
 
     # Protocol versions of the stateless "modern" lifecycle introduced by the MCP 2026-07-28 spec release (SEP-2575).
-    # Modern versions are deliberately kept out of `SUPPORTED_STABLE_PROTOCOL_VERSIONS`: the modern lifecycle has
-    # no `initialize` handshake, so these versions are never negotiated (each request carries its own version in `_meta`
-    # and is validated against this list independently), and `protocol_version=` keeps rejecting them for the same reason.
+    # 2026-07-28 serves both lifecycles of the dual-era model: it is negotiable through the legacy `initialize`
+    # handshake (so it also appears in `SUPPORTED_STABLE_PROTOCOL_VERSIONS`), and it is the version of the modern
+    # lifecycle, where each request carries its own version in `_meta` and is validated against this list
+    # independently, with no handshake.
     # https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2575
     LATEST_MODERN_PROTOCOL_VERSION = "2026-07-28"
     SUPPORTED_MODERN_PROTOCOL_VERSIONS = [LATEST_MODERN_PROTOCOL_VERSION].freeze

diff --git a/test/mcp/configuration_test.rb b/test/mcp/configuration_test.rb
@@ -54,7 +54,7 @@ class ConfigurationTest < ActiveSupport::TestCase
         Configuration.new(protocol_version: "DRAFT-2025-v3")
       end
 
-      assert_equal("protocol_version must be 2025-11-25, 2025-06-18, 2025-03-26, or 2024-11-05", exception.message)
+      assert_equal("protocol_version must be 2026-07-28, 2025-11-25, 2025-06-18, 2025-03-26, or 2024-11-05", exception.message)
     end
 
     test "raises ArgumentError when protocol_version is not a supported protocol version" do
@@ -63,7 +63,7 @@ class ConfigurationTest < ActiveSupport::TestCase
         custom_version = "2025-03-27"
         config.protocol_version = custom_version
       end
-      assert_equal("protocol_version must be 2025-11-25, 2025-06-18, 2025-03-26, or 2024-11-05", exception.message)
+      assert_equal("protocol_version must be 2026-07-28, 2025-11-25, 2025-06-18, 2025-03-26, or 2024-11-05", exception.message)
     end
 
     test "exposes the SEP-2575 modern protocol versions" do
@@ -73,12 +73,11 @@ class ConfigurationTest < ActiveSupport::TestCase
       refute Configuration.modern_protocol_version?("2025-11-25")
     end
 
-    test "raises ArgumentError when setting a modern protocol version" do
-      # Modern versions (SEP-2575) are never negotiated via `initialize`, so they are deliberately not settable as
-      # the legacy fallback version.
-      assert_raises(ArgumentError) do
-        Configuration.new(protocol_version: Configuration::LATEST_MODERN_PROTOCOL_VERSION)
-      end
+    test "accepts 2026-07-28 as the protocol version" do
+      # 2026-07-28 serves both lifecycles of the dual-era model (SEP-2575), so it is negotiable through
+      # the legacy `initialize` handshake and settable as the fallback version.
+      config = Configuration.new(protocol_version: Configuration::LATEST_MODERN_PROTOCOL_VERSION)
+      assert_equal "2026-07-28", config.protocol_version
     end
 
     test "raises ArgumentError when protocol_version is not a boolean value" do

diff --git a/test/mcp/server/transports/streamable_http_transport_test.rb b/test/mcp/server/transports/streamable_http_transport_test.rb
@@ -1993,6 +1993,32 @@ def string
           assert_equal 200, response[0]
         end
 
+        test "POST request with the 2026-07-28 MCP-Protocol-Version header succeeds" do
+          init_request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            { jsonrpc: "2.0", method: "initialize", id: "init", params: initialize_params(protocolVersion: "2026-07-28") }.to_json,
+          )
+          init_response = @transport.handle_request(init_request)
+          assert_equal "2026-07-28", JSON.parse(init_response[2][0])["result"]["protocolVersion"]
+          session_id = init_response[1]["mcp-session-id"]
+
+          request = create_rack_request(
+            "POST",
+            "/",
+            {
+              "CONTENT_TYPE" => "application/json",
+              "HTTP_MCP_SESSION_ID" => session_id,
+              "HTTP_MCP_PROTOCOL_VERSION" => "2026-07-28",
+            },
+            { jsonrpc: "2.0", method: "tools/list", id: "list" }.to_json,
+          )
+
+          response = @transport.handle_request(request)
+          assert_equal 200, response[0]
+        end
+
         test "POST request without MCP-Protocol-Version header succeeds" do
           init_request = create_rack_request(
             "POST",

diff --git a/test/mcp/server_test.rb b/test/mcp/server_test.rb
@@ -2036,6 +2036,20 @@ def read_resource_request(uri)
       assert_equal Configuration::LATEST_STABLE_PROTOCOL_VERSION, response[:result][:protocolVersion]
     end
 
+    test "server negotiates 2026-07-28 when the client requests it via initialize" do
+      server = Server.new(name: "test_server")
+
+      request = {
+        jsonrpc: "2.0",
+        method: "initialize",
+        id: 1,
+        params: initialize_params(protocolVersion: "2026-07-28"),
+      }
+
+      response = server.handle(request)
+      assert_equal "2026-07-28", response[:result][:protocolVersion]
+    end
+
     test "server removes description and icons from server_info when negotiating to 2025-06-18" do
       server = Server.new(
         name: "test_server",