Support session expiry controls for StreamableHTTPTransport

## Motivation and Context

The MCP specification recommends expiring session IDs to reduce session hijacking risks:
https://modelcontextprotocol.io/specification/latest/basic/security_best_practices#session-hijacking

Several other SDKs (Go, C#, PHP, Python) already implement session expiry controls, but
the Ruby SDK did not. Sessions persisted indefinitely unless explicitly deleted or
a stream error occurred, leaving abandoned sessions to accumulate in memory.

This adds a `session_idle_timeout:` option to `StreamableHTTPTransport#initialize`. When
set, sessions that receive no HTTP requests for the specified duration (in seconds) are
automatically expired. Expired sessions return 404 on subsequent requests (GET and POST),
matching the MCP specification's behavior for terminated sessions. Each request resets
the idle timer, so actively used sessions are not interrupted.

A background reaper thread periodically cleans up expired sessions to handle orphaned
sessions that receive no further requests. The reaper only starts when
`session_idle_timeout` is configured.

The default is `nil` (no expiry) for backward compatibility, consistent with the Python
SDK's approach. The Python SDK recommends 1800 seconds (30 minutes) for most deployments:
modelcontextprotocol/python-sdk#2022

Resolves #265.

## How Has This Been Tested?

Added tests for session expiry, idle timeout reset, reaper thread lifecycle, input
validation, and default behavior in `streamable_http_transport_test.rb`. All existing
tests continue to pass.

## Breaking Change

None. The default value of `session_idle_timeout` is `nil`, which preserves the existing
behavior of sessions never expiring. The new `last_active_at` field in the internal
session hash is not part of the public API. Existing code that instantiates
`StreamableHTTPTransport.new(server)` or
`StreamableHTTPTransport.new(server, stateless: true)` continues to work without changes.

Author: koic

README.md

@@ -293,6 +293,14 @@ Set `stateless: true` in `MCP::Server::Transports::StreamableHTTPTransport.new`
 transport = MCP::Server::Transports::StreamableHTTPTransport.new(server, stateless: true)
 ```
 
+By default, sessions do not expire. To mitigate session hijacking risks, you can set a `session_idle_timeout` (in seconds).
+When configured, sessions that receive no HTTP requests for this duration are automatically expired and cleaned up:
+
+```ruby
+# Session timeout of 30 minutes
+transport = MCP::Server::Transports::StreamableHTTPTransport.new(server, session_idle_timeout: 1800)
+```
+
 ### Unsupported Features (to be implemented in future versions)
 
 - Resource subscriptions

lib/mcp/server/transports/streamable_http_transport.rb

@@ -8,18 +8,30 @@ module MCP
   class Server
     module Transports
       class StreamableHTTPTransport < Transport
-        def initialize(server, stateless: false)
+        def initialize(server, stateless: false, session_idle_timeout: nil)
           super(server)
-          # { session_id => { stream: stream_object }
+          # Session data structure: `{ session_id => { stream: stream_object, last_active_at: float_from_monotonic_clock } }`.
           @sessions = {}
           @mutex = Mutex.new
 
           @stateless = stateless
+          @session_idle_timeout = session_idle_timeout
+
+          if @session_idle_timeout
+            if @stateless
+              raise ArgumentError, "session_idle_timeout is not supported in stateless mode."
+            elsif @session_idle_timeout <= 0
+              raise ArgumentError, "session_idle_timeout must be a positive number."
+            end
+          end
+
+          start_reaper_thread if @session_idle_timeout
         end
 
         REQUIRED_POST_ACCEPT_TYPES = ["application/json", "text/event-stream"].freeze
         REQUIRED_GET_ACCEPT_TYPES = ["text/event-stream"].freeze
         STREAM_WRITE_ERRORS = [IOError, Errno::EPIPE, Errno::ECONNRESET].freeze
+        SESSION_REAP_INTERVAL = 60
 
         def handle_request(request)
           case request.env["REQUEST_METHOD"]
@@ -35,6 +47,9 @@ def handle_request(request)
         end
 
         def close
+          @reaper_thread&.kill
+          @reaper_thread = nil
+
           @mutex.synchronize do
             @sessions.each_key { |session_id| cleanup_session_unsafe(session_id) }
           end
@@ -56,6 +71,11 @@ def send_notification(method, params = nil, session_id: nil)
               session = @sessions[session_id]
               return false unless session && session[:stream]
 
+              if session_expired?(session)
+                cleanup_session_unsafe(session_id)
+                return false
+              end
+
               begin
                 send_to_stream(session[:stream], notification)
                 true
@@ -75,6 +95,11 @@ def send_notification(method, params = nil, session_id: nil)
               @sessions.each do |sid, session|
                 next unless session[:stream]
 
+                if session_expired?(session)
+                  failed_sessions << sid
+                  next
+                end
+
                 begin
                   send_to_stream(session[:stream], notification)
                   sent_count += 1
@@ -97,6 +122,36 @@ def send_notification(method, params = nil, session_id: nil)
 
         private
 
+        def start_reaper_thread
+          @reaper_thread = Thread.new do
+            loop do
+              sleep(SESSION_REAP_INTERVAL)
+              reap_expired_sessions
+            rescue StandardError => e
+              MCP.configuration.exception_reporter.call(e, error: "Session reaper error")
+            end
+          end
+        end
+
+        def reap_expired_sessions
+          return unless @session_idle_timeout
+
+          expired_streams = @mutex.synchronize do
+            @sessions.each_with_object([]) do |(session_id, session), streams|
+              next unless session_expired?(session)
+
+              streams << session[:stream] if session[:stream]
+              @sessions.delete(session_id)
+            end
+          end
+
+          expired_streams.each do |stream|
+            stream.close
+          rescue
+            nil
+          end
+        end
+
         def send_to_stream(stream, data)
           message = data.is_a?(String) ? data : data.to_json
           stream.write("data: #{message}\n\n")
@@ -141,7 +196,9 @@ def handle_get(request)
           session_id = extract_session_id(request)
 
           return missing_session_id_response unless session_id
-          return session_not_found_response unless session_exists?(session_id)
+
+          error_response = validate_and_touch_session(session_id)
+          return error_response if error_response
 
           setup_sse_stream(session_id)
         end
@@ -234,6 +291,7 @@ def handle_initialization(body_string, body)
             @mutex.synchronize do
               @sessions[session_id] = {
                 stream: nil,
+                last_active_at: Process.clock_gettime(Process::CLOCK_MONOTONIC),
               }
             end
           end
@@ -255,8 +313,9 @@ def handle_accepted
 
         def handle_regular_request(body_string, session_id)
           unless @stateless
-            if session_id && !session_exists?(session_id)
-              return session_not_found_response
+            if session_id
+              error_response = validate_and_touch_session(session_id)
+              return error_response if error_response
             end
           end
 
@@ -272,6 +331,22 @@ def handle_regular_request(body_string, session_id)
           end
         end
 
+        def validate_and_touch_session(session_id)
+          @mutex.synchronize do
+            return session_not_found_response unless (session = @sessions[session_id])
+            return unless @session_idle_timeout
+
+            if session_expired?(session)
+              cleanup_session_unsafe(session_id)
+              return session_not_found_response
+            end
+
+            session[:last_active_at] = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+          end
+
+          nil
+        end
+
         def get_session_stream(session_id)
           @mutex.synchronize { @sessions[session_id]&.fetch(:stream, nil) }
         end
@@ -364,6 +439,12 @@ def send_keepalive_ping(session_id)
           )
           raise # Re-raise to exit the keepalive loop
         end
+
+        def session_expired?(session)
+          return false unless @session_idle_timeout
+
+          Process.clock_gettime(Process::CLOCK_MONOTONIC) - session[:last_active_at] > @session_idle_timeout
+        end
       end
     end
   end