diff --git a/Gemfile b/Gemfile
index 7aef0bb..103e22e 100644
--- a/Gemfile
+++ b/Gemfile
@@ -5,3 +5,5 @@ gemspec
 gem "base64" if RUBY_VERSION >= "3.4.0"
 # No longer a default gem on Ruby 4.0 and TruffleRuby 40, and logtail 0.1.17 requires it undeclared
 gem "logger"
+# Rack 3 moved the session middlewares the tests use into this gem; its 1.x releases for older Racks are empty
+gem "rack-session"
diff --git a/lib/logtail-rack/session_context.rb b/lib/logtail-rack/session_context.rb
index 4618d78..e4d808b 100755
--- a/lib/logtail-rack/session_context.rb
+++ b/lib/logtail-rack/session_context.rb
@@ -25,7 +25,10 @@ def get_session_id(env)
             if session = env['rack.session']
               if session.respond_to?(:id)
                 Logtail::Config.instance.debug { "Rack env session detected, using id attribute" }
-                session.id
+                id = session.id
+                # Since Rack 1.6.12 and 2.0.8 a Rack::Session::SessionId, which MessagePack can't encode. Its public id
+                # is the session cookie of server-side stores, the private id a hash of it that can't be used as one.
+                id.respond_to?(:private_id) ? id.private_id : id
               elsif session.respond_to?(:[])
                 Logtail::Config.instance.debug { "Rack env session detected, using the session_id key" }
                 session["session_id"]
diff --git a/spec/logtail-rack/session_context_spec.rb b/spec/logtail-rack/session_context_spec.rb
new file mode 100644
index 0000000..252bae0
--- /dev/null
+++ b/spec/logtail-rack/session_context_spec.rb
@@ -0,0 +1,67 @@
+require "spec_helper"
+require "rack/session/cookie"
+require "rack/session/pool"
+
+RSpec.describe Logtail::Integrations::Rack::SessionContext do
+  [Rack::Session::Cookie, Rack::Session::Pool].each do |session_store|
+    it "log the private id of a #{session_store.name} session as a String", :aggregate_failures do
+      skip "Rack::Session::SessionId is new in Rack 1.6.12 and 2.0.8" unless defined?(Rack::Session::SessionId)
+      # Makes the session store generate a known session id
+      secure_random = Object.new
+      def secure_random.hex(_length)
+        "5e55102d0f1c4a6b9e2d7c8a1b3f6e90"
+      end
+      app = Rack::Builder.new do
+        use session_store, secret: "x" * 64, secure_random: secure_random
+        use Logtail::Integrations::Rack::SessionContext
+        run lambda { |env|
+          env["rack.session"]["visits"] = 1
+          Logtail::Config.instance.logger.info("inside the app")
+          [200, {}, ["ok"]]
+        }
+      end.to_app
+      request = Rack::MockRequest.new(app)
+
+      entries = capture_log_entries do
+        session_cookie = Array(request.get("/").headers["Set-Cookie"]).first.split(";").first
+        request.get("/", "HTTP_COOKIE" => session_cookie)
+      end
+
+      # The first request has no session cookie yet. The private id is "2::" and the SHA-256 digest of the
+      # session id, the key Rack's server-side stores use. The session id is the cookie value of those stores.
+      expect(entries.map { |entry| entry.context_snapshot[:session] }).to eq([nil, { id: "2::5ea0adef81b3967d06c0bfc9021a8d4657afbe5a960a41977b99a3e0739e9087" }])
+      expect { entries.map(&:to_hash).to_msgpack }.not_to raise_error
+    end
+  end
+
+  it "log a String session id as it is" do
+    app = lambda { |env|
+      Logtail::Config.instance.logger.info("inside the app")
+      [200, {}, ["ok"]]
+    }
+    env = Rack::MockRequest.env_for("/", "rack.session" => Struct.new(:id).new("plain-session-id"))
+
+    entries = capture_log_entries { described_class.new(app).call(env) }
+
+    expect(entries.map { |entry| entry.context_snapshot[:session] }).to eq([{ id: "plain-session-id" }])
+  end
+
+  # Collects the LogEntry objects the way the HTTP log device receives them, before MessagePack encodes them
+  def capture_log_entries(&blk)
+    old_logger = Logtail::Config.instance.logger
+
+    entries = []
+    device = Object.new
+    device.define_singleton_method(:write) { |entry| entries << entry }
+    device.define_singleton_method(:close) {}
+    logger = Logtail::Logger.new(device)
+    logger.formatter = Logtail::Logger::PassThroughFormatter.new
+    Logtail::Config.instance.logger = logger
+
+    blk.call
+
+    entries
+  ensure
+    Logtail::Config.instance.logger = old_logger
+  end
+end