diff --git a/lib/net/imap.rb b/lib/net/imap.rb
index 72fb8b0df..9a93c5424 100644
--- a/lib/net/imap.rb
+++ b/lib/net/imap.rb
@@ -103,9 +103,9 @@ module Net
   #
   # == Capabilities
   #
-  # Net::IMAP does not _currently_ modify its behaviour according to the
-  # server's advertised #capabilities.  Users of this class must check that the
-  # server is capable of extension commands or command arguments before
+  # Most Net::IMAP methods do not _currently_ modify their behaviour according
+  # to the server's advertised #capabilities.  Users of this class must check
+  # that the server is capable of extension commands or command arguments before
   # sending them.  Special care should be taken to follow the #capabilities
   # requirements for #starttls, #login, and #authenticate.
   #
@@ -404,14 +404,14 @@ module Net
   #
   # Although IMAP4rev2[https://tools.ietf.org/html/rfc9051] is not supported
   # yet, Net::IMAP supports several extensions that have been folded into it:
-  # +ENABLE+, +IDLE+, +MOVE+, +NAMESPACE+, +UIDPLUS+, and +UNSELECT+.  Commands
-  # for these extensions are listed with the
-  # {Core IMAP commands}[rdoc-ref:Net::IMAP@Core+IMAP+commands], above.
+  # +ENABLE+, +IDLE+, +MOVE+, +NAMESPACE+, +SASL-IR+, +UIDPLUS+, and +UNSELECT+.
+  # Commands for these extensions are listed with the {Core IMAP
+  # commands}[rdoc-ref:Net::IMAP@Core+IMAP+commands], above.
   #
   # >>>
   #   <em>The following are folded into +IMAP4rev2+ but are currently
   #   unsupported or incompletely supported by</em> Net::IMAP<em>: RFC4466
-  #   extensions, +ESEARCH+, +SEARCHRES+, +SASL-IR+, +LIST-EXTENDED+,
+  #   extensions, +ESEARCH+, +SEARCHRES+, +LIST-EXTENDED+,
   #   +LIST-STATUS+, +LITERAL-+, +BINARY+ fetch, and +SPECIAL-USE+.  The
   #   following extensions are implicitly supported, but will be updated with
   #   more direct support: RFC5530 response codes, <tt>STATUS=SIZE</tt>, and
@@ -457,6 +457,10 @@ module Net
   # - Updates #append with the +APPENDUID+ ResponseCode
   # - Updates #copy, #move with the +COPYUID+ ResponseCode
   #
+  # ==== RFC4959: +SASL-IR+
+  # Folded into IMAP4rev2[https://tools.ietf.org/html/rfc9051].
+  # - Updates #authenticate with the option to send an initial response.
+  #
   # ==== RFC5161: +ENABLE+
   # Folded into IMAP4rev2[https://tools.ietf.org/html/rfc9051] and also included
   # above with {Core IMAP commands}[rdoc-ref:Net::IMAP@Core+IMAP+commands].
@@ -983,19 +987,17 @@ def starttls(options = {}, verify = true)
     end
 
     # :call-seq:
-    #   authenticate(mechanism, ...)                               -> ok_resp
-    #   authenticate(mech, *creds, **props) {|prop, auth| val }    -> ok_resp
-    #   authenticate(mechanism, authnid, credentials, authzid=nil) -> ok_resp
-    #   authenticate(mechanism, **properties)                      -> ok_resp
-    #   authenticate(mechanism) {|propname, authctx| prop_value }  -> ok_resp
+    #   authenticate(mechanism, ...) -> ok_resp
+    #   authenticate(mech, *creds, sasl_ir: true, **attrs, &callback) -> ok_resp
     #
     # Sends an {AUTHENTICATE command [IMAP4rev1 §6.2.2]}[https://www.rfc-editor.org/rfc/rfc3501#section-6.2.2]
     # to authenticate the client.  If successful, the connection enters the
     # "_authenticated_" state.
     #
     # +mechanism+ is the name of the \SASL authentication mechanism to be used.
-    # All other arguments are forwarded to the authenticator for the requested
-    # mechanism.  The listed call signatures are suggestions.  <em>The
+    # +sasl_ir+ allows or disallows sending an "initial response" (see the
+    # +SASL-IR+ capability, below).  All other arguments are forwarded to the
+    # registered SASL authenticator for the requested mechanism.  <em>The
     # documentation for each individual mechanism must be consulted for its
     # specific parameters.</em>
     #
@@ -1048,19 +1050,40 @@ def starttls(options = {}, verify = true)
     #      raise "No acceptable authentication mechanism is available"
     #    end
     #
-    # Server capabilities may change after #starttls, #login, and #authenticate.
-    # Cached #capabilities will be cleared when this method completes.
-    # If the TaggedResponse to #authenticate includes updated capabilities, they
-    # will be cached.
+    # The SASL exchange provides a method for server challenges and client
+    # responses, but many mechanisms expect the client to "respond" first.  When
+    # the server's capabilities include +SASL-IR+
+    # [RFC4959[https://tools.ietf.org/html/rfc4959]], this "initial response"
+    # may be sent as an argument to the +AUTHENTICATE+ command, saving a
+    # round-trip.  The initial response will _only_ be sent when it is supported
+    # by both the mechanism and the server.  Set +sasl_ir+ to +false+ to prevent
+    # sending an initial response, even when it is supported.
     #
-    def authenticate(mechanism, ...)
-      authenticator = self.class.authenticator(mechanism, ...)
-      send_command("AUTHENTICATE", mechanism) do |resp|
+    # Although servers _should_ advertise all supported auth mechanisms, it is
+    # possible to attempt to authenticate with a +mechanism+ that isn't listed.
+    # However the initial response will not be sent unless the appropriate
+    # <tt>"AUTH=#{mechanism}"</tt> capability is also present.
+    #
+    # Server capabilities may change after #starttls, #login, and #authenticate.
+    # Previously cached #capabilities will be cleared when this method
+    # completes.  If the TaggedResponse to #authenticate includes updated
+    # capabilities, they will be cached.
+    def authenticate(mechanism, *creds, sasl_ir: true, **props, &callback)
+      authenticator = self.class.authenticator(mechanism,
+                                               *creds,
+                                               **props,
+                                               &callback)
+      cmdargs = ["AUTHENTICATE", mechanism]
+      if sasl_ir && capable?("SASL-IR") && auth_capable?(mechanism) &&
+          SASL.initial_response?(authenticator)
+        cmdargs << [authenticator.process(nil)].pack("m0")
+      end
+      send_command(*cmdargs) do |resp|
         if resp.instance_of?(ContinuationRequest)
-          data = authenticator.process(resp.data.text.unpack("m")[0])
-          s = [data].pack("m0")
-          send_string_data(s)
-          put_string(CRLF)
+          challenge = resp.data.text.unpack1("m")
+          response  = authenticator.process(challenge)
+          response  = [response].pack("m0")
+          put_string(response + CRLF)
         end
       end
         .tap { @capabilities = capabilities_from_resp_code _1 }
diff --git a/lib/net/imap/authenticators/plain.rb b/lib/net/imap/authenticators/plain.rb
index a9d46c920..e7fe07c4d 100644
--- a/lib/net/imap/authenticators/plain.rb
+++ b/lib/net/imap/authenticators/plain.rb
@@ -11,6 +11,8 @@
 # can be secured by TLS encryption.
 class Net::IMAP::PlainAuthenticator
 
+  def initial_response?; true end
+
   def process(data)
     return "#@authzid\0#@username\0#@password"
   end
diff --git a/lib/net/imap/authenticators/xoauth2.rb b/lib/net/imap/authenticators/xoauth2.rb
index 410570202..67b6e5aac 100644
--- a/lib/net/imap/authenticators/xoauth2.rb
+++ b/lib/net/imap/authenticators/xoauth2.rb
@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class Net::IMAP::XOauth2Authenticator
+
+  def initial_response?; true end
+
   def process(_data)
     build_oauth2_string(@user, @oauth2_token)
   end
diff --git a/lib/net/imap/sasl.rb b/lib/net/imap/sasl.rb
index 658580728..eca77bdd3 100644
--- a/lib/net/imap/sasl.rb
+++ b/lib/net/imap/sasl.rb
@@ -39,6 +39,10 @@ def saslprep(string, **opts)
         Net::IMAP::StringPrep::SASLprep.saslprep(string, **opts)
       end
 
+      def initial_response?(mechanism)
+        mechanism.respond_to?(:initial_response?) && mechanism.initial_response?
+      end
+
     end
   end
 
diff --git a/test/net/imap/fake_server/command_reader.rb b/test/net/imap/fake_server/command_reader.rb
index 2abe2f8a4..2fd15ef2f 100644
--- a/test/net/imap/fake_server/command_reader.rb
+++ b/test/net/imap/fake_server/command_reader.rb
@@ -32,7 +32,7 @@ def get_command
     def parse(buf)
       /\A([^ ]+) ((?:UID )?\w+)(?: (.+))?\r\n\z/min =~ buf or raise "bad request"
       case $2.upcase
-      when "LOGIN", "SELECT", "ENABLE"
+      when "LOGIN", "SELECT", "ENABLE", "AUTHENTICATE"
         Command.new $1, $2, scan_astrings($3), buf
       else
         Command.new $1, $2, $3, buf # TODO...
diff --git a/test/net/imap/fake_server/command_router.rb b/test/net/imap/fake_server/command_router.rb
index c99a0a989..3c46daa17 100644
--- a/test/net/imap/fake_server/command_router.rb
+++ b/test/net/imap/fake_server/command_router.rb
@@ -55,7 +55,17 @@ def handler_for(command)
       resp.args.nil? or return resp.fail_bad_args
       resp.bye
       state.logout
-      resp.done_ok
+      begin
+        resp.done_ok
+      rescue IOError
+        # TODO: fix whatever is causing this!
+        warn "connection issue after bye but before LOGOUT could complete"
+        if $!.respond_to :detailed_message
+          warn $!.detailed_message highlight: true, order: :bottom
+        else
+          warn $!.full_message     highlight: true, order: :bottom
+        end
+      end
     end
 
     on "STARTTLS" do |resp|
@@ -79,8 +89,14 @@ def handler_for(command)
     on "AUTHENTICATE" do |resp|
       state.not_authenticated?           or return resp.fail_bad_state(state)
       args = resp.command.args
-      args == "PLAIN"                    or return resp.fail_no "unsupported"
-      response_b64 = resp.request_continuation("") || ""
+      (1..2) === args.length             or return resp.fail_bad_args
+      args.first == "PLAIN"              or return resp.fail_no "unsupported"
+      if args.length == 2
+        response_b64 = args.last
+      else
+        response_b64 = resp.request_continuation("") || ""
+        state.commands << {continuation: response_b64}
+      end
       response = Base64.decode64(response_b64)
       response.empty?                   and return resp.fail_bad "canceled"
       # TODO: support mechanisms other than PLAIN.
diff --git a/test/net/imap/fake_server/configuration.rb b/test/net/imap/fake_server/configuration.rb
index b215f019f..91fe72a42 100644
--- a/test/net/imap/fake_server/configuration.rb
+++ b/test/net/imap/fake_server/configuration.rb
@@ -22,6 +22,7 @@ class Configuration
       encrypted_login: true,
       cleartext_auth:  false,
       sasl_mechanisms: %i[PLAIN].freeze,
+      sasl_ir: false,
 
       rev1: true,
       rev2: false,
@@ -66,6 +67,7 @@ def initialize(with_extensions: [], without_extensions: [], **opts, &block)
     alias cleartext_auth?        cleartext_auth
     alias greeting_bye?          greeting_bye
     alias greeting_capabilities? greeting_capabilities
+    alias sasl_ir?               sasl_ir
 
     def on(event, &handler)
       handler or raise ArgumentError
@@ -104,6 +106,7 @@ def capabilities_pre_tls
       capa << "STARTTLS"            if starttls?
       capa << "LOGINDISABLED"   unless cleartext_login?
       capa.concat auth_capabilities if cleartext_auth?
+      capa << "SASL-IR" if sasl_ir? && cleartext_auth?
       capa
     end
 
@@ -111,6 +114,7 @@ def capabilities_pre_auth
       capa = basic_capabilities
       capa << "LOGINDISABLED" unless encrypted_login?
       capa.concat auth_capabilities
+      capa << "SASL-IR" if sasl_ir?
       capa
     end
 
diff --git a/test/net/imap/test_imap.rb b/test/net/imap/test_imap.rb
index 949710227..f794fde2c 100644
--- a/test/net/imap/test_imap.rb
+++ b/test/net/imap/test_imap.rb
@@ -776,6 +776,96 @@ def test_id
     end
   end
 
+  test("#authenticate sends an initial response " \
+       "when supported by both the mechanism and the server") do
+    with_fake_server(
+      preauth: false, cleartext_auth: true, sasl_ir: true
+    ) do |server, imap|
+      imap.authenticate("PLAIN", "test_user", "test-password")
+      cmd = server.commands.pop
+      assert_equal "AUTHENTICATE", cmd.name
+      assert_equal(["PLAIN", ["\x00test_user\x00test-password"].pack("m0")],
+                   cmd.args)
+      assert_empty server.commands
+    end
+  end
+
+  test("#authenticate never sends an initial response " \
+       "when the server doesn't explicitly support the mechanism") do
+    with_fake_server(
+      preauth: false, cleartext_auth: true,
+      sasl_ir: true, sasl_mechanisms: %i[SCRAM-SHA-1 SCRAM-SHA-256],
+    ) do |server, imap|
+      imap.authenticate("PLAIN", "test_user", "test-password")
+      cmd, cont = 2.times.map { server.commands.pop }
+      assert_equal %w[AUTHENTICATE PLAIN], [cmd.name, *cmd.args]
+      assert_equal(["\x00test_user\x00test-password"].pack("m0"),
+                   cont[:continuation].strip)
+      assert_empty server.commands
+    end
+  end
+
+  test("#authenticate never sends an initial response " \
+       "when the server isn't capable") do
+    with_fake_server(
+      preauth: false, cleartext_auth: true, sasl_ir: false
+    ) do |server, imap|
+      imap.authenticate("PLAIN", "test_user", "test-password")
+      cmd, cont = 2.times.map { server.commands.pop }
+      assert_equal %w[AUTHENTICATE PLAIN], [cmd.name, *cmd.args]
+      assert_equal(["\x00test_user\x00test-password"].pack("m0"),
+                   cont[:continuation].strip)
+      assert_empty server.commands
+    end
+  end
+
+  test("#authenticate never sends an initial response " \
+       "when sasl_ir: false") do
+    [true, false].each do |server_support|
+      with_fake_server(
+        preauth: false, cleartext_auth: true, sasl_ir: server_support
+      ) do |server, imap|
+        imap.authenticate("PLAIN", "test_user", "test-password", sasl_ir: false)
+        cmd, cont = 2.times.map { server.commands.pop }
+        assert_equal %w[AUTHENTICATE PLAIN], [cmd.name, *cmd.args]
+        assert_equal(["\x00test_user\x00test-password"].pack("m0"),
+                     cont[:continuation].strip)
+        assert_empty server.commands
+      end
+    end
+  end
+
+  test("#authenticate never sends an initial response " \
+       "when the mechanism does not support client-first") do
+    with_fake_server(
+      preauth: false, cleartext_auth: true,
+      sasl_ir: true, sasl_mechanisms: %i[DIGEST-MD5]
+    ) do |server, imap|
+      server.on "AUTHENTICATE" do |cmd|
+        response_b64 = cmd.request_continuation(
+          [
+            %w[
+              realm="somerealm"
+              nonce="OA6MG9tEQGm2hh"
+              qop="auth"
+              charset=utf-8
+              algorithm=md5-sess
+            ].join(",")
+          ].pack("m0")
+        )
+        state.commands << {continuation: response_b64}
+        server.state.authenticate(server.config.user)
+        cmd.done_ok
+      end
+      imap.authenticate("DIGEST-MD5", "test_user", "test-password",
+                        warn_deprecation: false)
+      cmd, cont = 2.times.map { server.commands.pop }
+      assert_equal %w[AUTHENTICATE DIGEST-MD5], [cmd.name, *cmd.args]
+      assert_match(%r{\A[a-z0-9+/]+=*\z}i, cont[:continuation].strip)
+      assert_empty server.commands
+    end
+  end
+
   def test_uidplus_uid_expunge
     with_fake_server(select: "INBOX",
                      extensions: %i[UIDPLUS]) do |server, imap|
diff --git a/test/net/imap/test_imap_authenticators.rb b/test/net/imap/test_imap_authenticators.rb
index ceffd04af..cdf32e0a3 100644
--- a/test/net/imap/test_imap_authenticators.rb
+++ b/test/net/imap/test_imap_authenticators.rb
@@ -9,14 +9,17 @@ class IMAPAuthenticatorsTest < Test::Unit::TestCase
   # PLAIN
   # ----------------------
 
-  def plain(*args, **kwargs, &block)
-    Net::IMAP.authenticator("PLAIN", *args, **kwargs, &block)
-  end
+  def plain(...) Net::IMAP.authenticator("PLAIN", ...) end
 
   def test_plain_authenticator_matches_mechanism
     assert_kind_of(Net::IMAP::PlainAuthenticator, plain("user", "pass"))
   end
 
+  def test_plain_supports_initial_response
+    assert plain("foo", "bar").initial_response?
+    assert Net::IMAP::SASL.initial_response?(plain("foo", "bar"))
+  end
+
   def test_plain_response
     assert_equal("\0authc\0passwd", plain("authc", "passwd").process(nil))
     assert_equal("authz\0user\0pass",
@@ -33,13 +36,24 @@ def test_plain_no_null_chars
   # XOAUTH2
   # ----------------------
 
+  def xoauth2(...) Net::IMAP.authenticator("XOAUTH2", ...) end
+
+  def test_xoauth2_authenticator_matches_mechanism
+    assert_kind_of(Net::IMAP::XOauth2Authenticator, xoauth2("user", "pass"))
+  end
+
   def test_xoauth2
     assert_equal(
       "user=username\1auth=Bearer token\1\1",
-      Net::IMAP::XOauth2Authenticator.new("username", "token").process(nil)
+      xoauth2("username", "token").process(nil)
     )
   end
 
+  def test_xoauth2_supports_initial_response
+    assert xoauth2("foo", "bar").initial_response?
+    assert Net::IMAP::SASL.initial_response?(xoauth2("foo", "bar"))
+  end
+
   # ----------------------
   # LOGIN (obsolete)
   # ----------------------
@@ -54,6 +68,10 @@ def test_login_authenticator_matches_mechanism
     assert_kind_of(Net::IMAP::LoginAuthenticator, login("n", "p"))
   end
 
+  def test_login_does_not_support_initial_response
+    refute Net::IMAP::SASL.initial_response?(login("foo", "bar"))
+  end
+
   def test_login_authenticator_deprecated
     assert_warn(/LOGIN.+deprecated.+PLAIN/) do
       Net::IMAP.authenticator("LOGIN", "user", "pass")
@@ -80,6 +98,10 @@ def test_cram_md5_authenticator_matches_mechanism
     assert_kind_of(Net::IMAP::CramMD5Authenticator, cram_md5("n", "p"))
   end
 
+  def test_cram_md5_does_not_support_initial_response
+    refute Net::IMAP::SASL.initial_response?(cram_md5("foo", "bar"))
+  end
+
   def test_cram_md5_authenticator_deprecated
     assert_warn(/CRAM-MD5.+deprecated./) do
       Net::IMAP.authenticator("CRAM-MD5", "user", "pass")
@@ -112,6 +134,10 @@ def test_digest_md5_authenticator_deprecated
     end
   end
 
+  def test_digest_md5_does_not_support_initial_response
+    refute Net::IMAP::SASL.initial_response?(digest_md5("foo", "bar"))
+  end
+
   def test_digest_md5_authenticator
     auth = digest_md5("cid", "password", "zid")
     assert_match(
diff --git a/test/net/imap/test_imap_capabilities.rb b/test/net/imap/test_imap_capabilities.rb
index 31f4d0ff8..14e1f79ba 100644
--- a/test/net/imap/test_imap_capabilities.rb
+++ b/test/net/imap/test_imap_capabilities.rb
@@ -190,6 +190,7 @@ def teardown
 
       imap.authenticate("PLAIN", "test_user", "test-password")
       assert_equal "AUTHENTICATE", server.commands.pop.name
+      assert server.commands.pop[:continuation]
       refute imap.capabilities_cached?
 
       assert imap.capable? :IMAP4rev1
@@ -277,6 +278,7 @@ def teardown
       rescue Net::IMAP::NoResponseError
       end
       assert_equal "AUTHENTICATE", server.commands.pop.name
+      assert server.commands.pop[:continuation]
       assert_equal original_capabilities, imap.capabilities
       assert_empty server.commands
     end