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 #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.
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Koichi ITO
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1 changed files with 123 additions and 25 deletions
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@ -2061,17 +2061,102 @@ module MCP
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end
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test "stateless mode does not support server-sent events" do
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# Notifications have no stream to ride in stateless mode; the transport reports non-delivery
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# instead of raising so per-request session notify_* helpers degrade gracefully (SEP-2567).
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stateless_transport = StreamableHTTPTransport.new(@server, stateless: true)
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e = assert_raises(RuntimeError) do
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stateless_transport.send_notification(
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"test_notification",
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{ message: "Hello" },
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session_id: "some_session_id",
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)
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end
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result = stateless_transport.send_notification(
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"test_notification",
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{ message: "Hello" },
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session_id: "some_session_id",
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)
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assert_equal("Stateless mode does not support notifications", e.message)
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refute result
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end
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test "stateless mode does not leak client info onto the shared server" do
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# Each stateless POST runs against an ephemeral per-request session (SEP-2567); concurrent requests
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# must never observe another client's identity through the shared Server instance.
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stateless_transport = StreamableHTTPTransport.new(@server, stateless: true)
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request = create_rack_request(
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"POST",
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"/",
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{ "CONTENT_TYPE" => "application/json" },
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{
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jsonrpc: "2.0",
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method: "initialize",
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id: 1,
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params: {
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protocolVersion: "2025-11-25",
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capabilities: { roots: {} },
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clientInfo: { name: "client-a", version: "1.0" },
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},
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}.to_json,
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)
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response = stateless_transport.handle_request(request)
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assert_equal 200, response[0]
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assert_nil @server.client_capabilities
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assert_nil @server.instance_variable_get(:@client)
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end
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test "stateless mode allows repeated initialize requests" do
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stateless_transport = StreamableHTTPTransport.new(@server, stateless: true)
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2.times do |i|
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request = create_rack_request(
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"POST",
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"/",
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{ "CONTENT_TYPE" => "application/json" },
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{
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jsonrpc: "2.0",
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method: "initialize",
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id: i + 1,
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params: {
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protocolVersion: "2025-11-25",
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clientInfo: { name: "client-#{i}", version: "1.0" },
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},
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}.to_json,
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)
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response = stateless_transport.handle_request(request)
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assert_equal 200, response[0]
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body = JSON.parse(response[2][0])
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assert body.key?("result"), "initialize ##{i + 1} should succeed, got #{body.inspect}"
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refute response[1].key?("Mcp-Session-Id")
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end
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end
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test "stateless mode skips progress notifications without raising" do
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reported = []
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configuration = MCP::Configuration.new
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configuration.exception_reporter = ->(exception, _context) { reported << exception }
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server = Server.new(name: "stateless_progress_test", configuration: configuration)
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server.define_tool(name: "progress_tool") do |server_context:|
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server_context.report_progress(50, total: 100)
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Tool::Response.new([{ type: "text", text: "ok" }])
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end
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stateless_transport = StreamableHTTPTransport.new(server, stateless: true)
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request = create_rack_request(
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"POST",
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"/",
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{ "CONTENT_TYPE" => "application/json" },
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{
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jsonrpc: "2.0",
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method: "tools/call",
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id: 1,
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params: { name: "progress_tool", arguments: {}, _meta: { progressToken: "tok" } },
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}.to_json,
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)
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response = stateless_transport.handle_request(request)
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assert_equal 200, response[0]
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body = JSON.parse(response[2][0])
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assert_equal "ok", body.dig("result", "content", 0, "text")
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assert_empty reported
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end
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test "stateless mode responds with 202 when client sends a notification/initialized request" do
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