Isolate Stateless Requests in Ephemeral Sessions per SEP-2567

## Motivation and Context

SEP-2567 (modelcontextprotocol/modelcontextprotocol#2567, merged for the 2026-07-28 spec release)
makes MCP sessionless: every Streamable HTTP POST must be fully self-contained,
with no protocol-level session state shared between requests.

`StreamableHTTPTransport` already implements the SEP's transport surface via
`stateless: true` (no `Mcp-Session-Id` issued or required, GET returns 405,
DELETE is a no-op, server-to-client requests raise), but its dispatch had a state leak:
stateless POSTs were handled with `session: nil`, so `Server#init` wrote `@client` and
`@client_capabilities` onto the shared `Server` instance.
Concurrent stateless requests could therefore observe another client's identity,
and the data persisted across requests, which is exactly what SEP-2567 forbids.
The TypeScript SDK's stateless prototypes (the closed typescript-sdk#2058/#2131/#2251 stack)
solve this with per-request dispatch; this change applies the equivalent fix within the existing architecture:

- Stateless `handle_initialization`, `handle_regular_request`, and `dispatch_notification` now
  run handlers against an ephemeral per-request `ServerSession` (with `session_id: nil`),
  so client info, logging level, and initialized state live only for the duration of that POST.
  Repeated `initialize` requests are naturally permitted because each POST gets a fresh,
  never-initialized session.
- `send_notification` in stateless mode now returns `false` (non-delivery) instead of raising.
  With ephemeral sessions in place, a tool calling `server_context.report_progress` or `notify_log_message`
  would otherwise route every call into the exception reporter; non-delivery matches how
  these helpers already degrade when no session exists. `send_request` (server-to-client requests) still raises,
  as those are genuinely unsupported without a stream.

Resolves modelcontextprotocol#388.

## How Has This Been Tested?

New tests in `test/mcp/server/transports/streamable_http_transport_test.rb`:

- a stateless `initialize` POST leaves `Server#client_capabilities` and the server's `@client` untouched
  (the leak regression)
- repeated `initialize` POSTs both succeed with 200 and no `Mcp-Session-Id` header
- a tool calling `server_context.report_progress` under stateless mode returns its result normally and
  the exception reporter is never invoked
- the existing "stateless mode does not support server-sent events" test is updated to assert
  the new `false` return instead of the removed raise

All other existing stateless-mode tests pass unchanged.

## Breaking Changes

`StreamableHTTPTransport#send_notification` in stateless mode now returns `false` instead of raising `RuntimeError`.
The raise message was not a documented contract, broadcasting in stateless mode was always a non-deliverable operation,
and the boolean return matches the method's documented semantics in every other non-delivery case.
Default (session-oriented) mode is unchanged.

Author: koic

README.md

@@ -1664,6 +1664,11 @@ Set `stateless: true` in `MCP::Server::Transports::StreamableHTTPTransport.new`
 transport = MCP::Server::Transports::StreamableHTTPTransport.new(server, stateless: true)
 ```
 
+In stateless mode, each POST is fully self-contained per SEP-2567: no `Mcp-Session-Id` is issued or required,
+handlers run against an ephemeral per-request session (so client identity never leaks across requests or onto the shared server),
+and repeated `initialize` requests are permitted. Request-scoped notifications such as progress and log messages are skipped
+(there is no stream to deliver them), while server-to-client requests (`sampling/createMessage`, `roots/list`, `elicitation/create`) raise an error.
+
 You can enable JSON response mode, where the server returns `application/json` instead of `text/event-stream`.
 Set `enable_json_response: true` in `MCP::Server::Transports::StreamableHTTPTransport.new`:
 

lib/mcp/server/transports/streamable_http_transport.rb

@@ -85,8 +85,10 @@ def close
         end
 
         def send_notification(method, params = nil, session_id: nil, related_request_id: nil)
-          # Stateless mode doesn't support notifications
-          raise "Stateless mode does not support notifications" if @stateless
+          # Stateless mode has no streams to deliver notifications on. Report non-delivery instead of raising
+          # so the ephemeral per-request session's notify_* helpers (e.g. progress or log notifications from
+          # a tool handler) degrade gracefully rather than spamming the exception reporter on every call.
+          return false if @stateless
 
           notification = {
             jsonrpc: "2.0",
@@ -575,7 +577,9 @@ def notification?(body)
         # `notifications/initialized`) through the server so it can update session state.
         def dispatch_notification(body_string, session_id)
           server_session = nil
-          if session_id && !@stateless
+          if @stateless
+            server_session = ephemeral_session
+          elsif session_id
             @mutex.synchronize do
               session = @sessions[session_id]
               server_session = session[:server_session] if session
@@ -611,9 +615,10 @@ def handle_response(body, session_id:)
 
         def handle_initialization(body_string, body)
           session_id = nil
-          server_session = nil
 
-          unless @stateless
+          if @stateless
+            server_session = ephemeral_session
+          else
             session_id = SecureRandom.uuid
             server_session = ServerSession.new(server: @server, transport: self, session_id: session_id)
 
@@ -626,17 +631,13 @@ def handle_initialization(body_string, body)
             end
           end
 
-          response = if server_session
-            server_session.handle_json(body_string)
-          else
-            @server.handle_json(body_string)
-          end
+          response = server_session.handle_json(body_string)
 
           # If `Server#init` produced an error response (e.g., malformed JSON-RPC envelope),
           # `mark_initialized!` was never called. Discard the orphaned session and omit
           # the `Mcp-Session-Id` header so the client retries from a clean state instead of
           # reusing a never-initialized ID that would later look like a duplicate `initialize`.
-          if server_session && !server_session.initialized?
+          if session_id && !server_session.initialized?
             cleanup_session(session_id)
             session_id = nil
           end
@@ -657,15 +658,15 @@ def handle_accepted
         def handle_regular_request(body_string, session_id, related_request_id: nil)
           server_session = nil
 
-          unless @stateless
-            if session_id
-              error_response = validate_and_touch_session(session_id)
-              return error_response if error_response
+          if @stateless
+            server_session = ephemeral_session
+          elsif session_id
+            error_response = validate_and_touch_session(session_id)
+            return error_response if error_response
 
-              @mutex.synchronize do
-                session = @sessions[session_id]
-                server_session = session[:server_session] if session
-              end
+            @mutex.synchronize do
+              session = @sessions[session_id]
+              server_session = session[:server_session] if session
             end
           end
 
@@ -775,6 +776,13 @@ def session_exists?(session_id)
           @mutex.synchronize { @sessions.key?(session_id) }
         end
 
+        # Each stateless POST is self-contained (SEP-2567): handlers run against an ephemeral per-request `ServerSession`
+        # so client info, logging level, and initialized state never leak onto the shared `Server` instance or across concurrent requests.
+        # https://github.com/modelcontextprotocol/modelcontextprotocol/pull/2567
+        def ephemeral_session
+          ServerSession.new(server: @server, transport: self, session_id: nil)
+        end
+
         # Returns true iff a session exists and is not past its idle timeout. Expired sessions
         # are evicted as a side effect so a live request never observes a zombie session that
         # the reaper hasn't yet pruned. Does NOT update `last_active_at`; callers that are

test/mcp/server/transports/streamable_http_transport_test.rb

@@ -2061,17 +2061,102 @@ def string
         end
 
         test "stateless mode does not support server-sent events" do
+          # Notifications have no stream to ride in stateless mode; the transport reports non-delivery
+          # instead of raising so per-request session notify_* helpers degrade gracefully (SEP-2567).
           stateless_transport = StreamableHTTPTransport.new(@server, stateless: true)
 
-          e = assert_raises(RuntimeError) do
-            stateless_transport.send_notification(
-              "test_notification",
-              { message: "Hello" },
-              session_id: "some_session_id",
+          result = stateless_transport.send_notification(
+            "test_notification",
+            { message: "Hello" },
+            session_id: "some_session_id",
+          )
+
+          refute result
+        end
+
+        test "stateless mode does not leak client info onto the shared server" do
+          # Each stateless POST runs against an ephemeral per-request session (SEP-2567); concurrent requests
+          # must never observe another client's identity through the shared Server instance.
+          stateless_transport = StreamableHTTPTransport.new(@server, stateless: true)
+
+          request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            {
+              jsonrpc: "2.0",
+              method: "initialize",
+              id: 1,
+              params: {
+                protocolVersion: "2025-11-25",
+                capabilities: { roots: {} },
+                clientInfo: { name: "client-a", version: "1.0" },
+              },
+            }.to_json,
+          )
+          response = stateless_transport.handle_request(request)
+
+          assert_equal 200, response[0]
+          assert_nil @server.client_capabilities
+          assert_nil @server.instance_variable_get(:@client)
+        end
+
+        test "stateless mode allows repeated initialize requests" do
+          stateless_transport = StreamableHTTPTransport.new(@server, stateless: true)
+
+          2.times do |i|
+            request = create_rack_request(
+              "POST",
+              "/",
+              { "CONTENT_TYPE" => "application/json" },
+              {
+                jsonrpc: "2.0",
+                method: "initialize",
+                id: i + 1,
+                params: {
+                  protocolVersion: "2025-11-25",
+                  clientInfo: { name: "client-#{i}", version: "1.0" },
+                },
+              }.to_json,
             )
+            response = stateless_transport.handle_request(request)
+
+            assert_equal 200, response[0]
+            body = JSON.parse(response[2][0])
+            assert body.key?("result"), "initialize ##{i + 1} should succeed, got #{body.inspect}"
+            refute response[1].key?("Mcp-Session-Id")
+          end
+        end
+
+        test "stateless mode skips progress notifications without raising" do
+          reported = []
+          configuration = MCP::Configuration.new
+          configuration.exception_reporter = ->(exception, _context) { reported << exception }
+
+          server = Server.new(name: "stateless_progress_test", configuration: configuration)
+          server.define_tool(name: "progress_tool") do |server_context:|
+            server_context.report_progress(50, total: 100)
+            Tool::Response.new([{ type: "text", text: "ok" }])
           end
+          stateless_transport = StreamableHTTPTransport.new(server, stateless: true)
 
-          assert_equal("Stateless mode does not support notifications", e.message)
+          request = create_rack_request(
+            "POST",
+            "/",
+            { "CONTENT_TYPE" => "application/json" },
+            {
+              jsonrpc: "2.0",
+              method: "tools/call",
+              id: 1,
+              params: { name: "progress_tool", arguments: {}, _meta: { progressToken: "tok" } },
+            }.to_json,
+          )
+          response = stateless_transport.handle_request(request)
+
+          assert_equal 200, response[0]
+          body = JSON.parse(response[2][0])
+          assert_equal "ok", body.dig("result", "content", 0, "text")
+          assert_empty reported
         end
 
         test "stateless mode responds with 202 when client sends a notification/initialized request" do