diff --git a/plugins/Autocomplete/README.md b/plugins/Autocomplete/README.md
new file mode 100644
index 000000000..e8ac3ceb9
--- /dev/null
+++ b/plugins/Autocomplete/README.md
@@ -0,0 +1,9 @@
+Provides command completion for IRC clients that support it.
+
+This plugin implements an early draft of the IRCv3 autocompletion client tags.
+As this is not yet a released specification, it does nothing unless
+`supybot.protocols.irc.experimentalExtensions` is set to True (keep it set to
+False unless you know what you are doing).
+
+If you are interested in this feature, please contribute to
+[the discussion](https://github.com/ircv3/ircv3-specifications/pull/415>)
diff --git a/plugins/Autocomplete/__init__.py b/plugins/Autocomplete/__init__.py
new file mode 100644
index 000000000..1dbb0d94a
--- /dev/null
+++ b/plugins/Autocomplete/__init__.py
@@ -0,0 +1,72 @@
+###
+# Copyright (c) 2020, Valentin Lorentz
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are met:
+#
+#   * Redistributions of source code must retain the above copyright notice,
+#     this list of conditions, and the following disclaimer.
+#   * Redistributions in binary form must reproduce the above copyright notice,
+#     this list of conditions, and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#   * Neither the name of the author of this software nor the name of
+#     contributors to this software may be used to endorse or promote products
+#     derived from this software without specific prior written consent.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+# LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+# ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+# POSSIBILITY OF SUCH DAMAGE.
+
+###
+
+"""
+Autocomplete: Provides command autocompletion for IRC clients that support it.
+"""
+
+import sys
+import supybot
+from supybot import world
+
+# Use this for the version of this plugin.
+__version__ = ""
+
+# XXX Replace this with an appropriate author or supybot.Author instance.
+__author__ = supybot.authors.unknown
+
+# This is a dictionary mapping supybot.Author instances to lists of
+# contributions.
+__contributors__ = {}
+
+# This is a url where the most recent plugin package can be downloaded.
+__url__ = ""
+
+from . import config
+from . import plugin
+
+if sys.version_info >= (3, 4):
+    from importlib import reload
+else:
+    from imp import reload
+# In case we're being reloaded.
+reload(config)
+reload(plugin)
+# Add more reloads here if you add third-party modules and want them to be
+# reloaded when this plugin is reloaded.  Don't forget to import them as well!
+
+if world.testing:
+    from . import test
+
+Class = plugin.Class
+configure = config.configure
+
+
+# vim:set shiftwidth=4 tabstop=4 expandtab textwidth=79:
diff --git a/plugins/Autocomplete/config.py b/plugins/Autocomplete/config.py
new file mode 100644
index 000000000..078185c4c
--- /dev/null
+++ b/plugins/Autocomplete/config.py
@@ -0,0 +1,70 @@
+###
+# Copyright (c) 2020, Valentin Lorentz
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are met:
+#
+#   * Redistributions of source code must retain the above copyright notice,
+#     this list of conditions, and the following disclaimer.
+#   * Redistributions in binary form must reproduce the above copyright notice,
+#     this list of conditions, and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#   * Neither the name of the author of this software nor the name of
+#     contributors to this software may be used to endorse or promote products
+#     derived from this software without specific prior written consent.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+# LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+# ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+# POSSIBILITY OF SUCH DAMAGE.
+
+###
+
+from supybot import conf, registry
+
+try:
+    from supybot.i18n import PluginInternationalization
+
+    _ = PluginInternationalization("Autocomplete")
+except:
+    # Placeholder that allows to run the plugin on a bot
+    # without the i18n module
+    _ = lambda x: x
+
+
+def configure(advanced):
+    # This will be called by supybot to configure this module.  advanced is
+    # a bool that specifies whether the user identified themself as an advanced
+    # user or not.  You should effect your configuration by manipulating the
+    # registry as appropriate.
+    from supybot.questions import expect, anything, something, yn
+
+    conf.registerPlugin("Autocomplete", True)
+
+
+Autocomplete = conf.registerPlugin("Autocomplete")
+# This is where your configuration variables (if any) should go.  For example:
+# conf.registerGlobalValue(Autocomplete, 'someConfigVariableName',
+#     registry.Boolean(False, _("""Help for someConfigVariableName.""")))
+conf.registerChannelValue(
+    Autocomplete,
+    "enabled",
+    registry.Boolean(
+        False,
+        _(
+            """Whether the bot should reply to autocomplete
+    requests from clients."""
+        ),
+    ),
+)
+
+
+# vim:set shiftwidth=4 tabstop=4 expandtab textwidth=79:
diff --git a/plugins/Autocomplete/local/__init__.py b/plugins/Autocomplete/local/__init__.py
new file mode 100644
index 000000000..e86e97b86
--- /dev/null
+++ b/plugins/Autocomplete/local/__init__.py
@@ -0,0 +1 @@
+# Stub so local is a module, used for third-party modules
diff --git a/plugins/Autocomplete/plugin.py b/plugins/Autocomplete/plugin.py
new file mode 100644
index 000000000..d2325c4b3
--- /dev/null
+++ b/plugins/Autocomplete/plugin.py
@@ -0,0 +1,144 @@
+###
+# Copyright (c) 2020, Valentin Lorentz
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are met:
+#
+#   * Redistributions of source code must retain the above copyright notice,
+#     this list of conditions, and the following disclaimer.
+#   * Redistributions in binary form must reproduce the above copyright notice,
+#     this list of conditions, and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#   * Neither the name of the author of this software nor the name of
+#     contributors to this software may be used to endorse or promote products
+#     derived from this software without specific prior written consent.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+# LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+# ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+# POSSIBILITY OF SUCH DAMAGE.
+
+###
+
+from supybot import conf, utils, plugins, ircutils, ircmsgs, callbacks
+from supybot.commands import *
+
+try:
+    from supybot.i18n import PluginInternationalization
+
+    _ = PluginInternationalization("Autocomplete")
+except ImportError:
+    # Placeholder that allows to run the plugin on a bot
+    # without the i18n module
+    _ = lambda x: x
+
+
+REQUEST_TAG = "+draft/autocomplete-request"
+RESPONSE_TAG = "+draft/autocomplete"
+
+
+def _getAutocompleteResponse(irc, msg, payload):
+    """Returns the value of the +draft/autocomplete tag for the given
+    +draft/autocomplete-request payload."""
+    tokens = callbacks.tokenize(payload, channel=msg.channel, network=irc.network)
+    normalized_payload = " ".join(tokens)
+
+    candidates = _getCandidates(irc, normalized_payload)
+
+    if not candidates:
+        return ""
+
+    # strip what the user already typed
+    assert all(candidate.startswith(normalized_payload) for candidate in candidates)
+    normalized_payload_length = len(normalized_payload)
+    response_items = [candidate[normalized_payload_length:] for candidate in candidates]
+    response_items.sort()
+    return "\t".join(response_items)
+
+
+def _getCandidates(irc, normalized_payload):
+    """Returns a list of commands starting with the normalized_payload."""
+    candidates = set()
+    for cb in irc.callbacks:
+        cb_commands = cb.listCommands()
+
+        # copy them with the plugin name (optional when calling a command)
+        # at the beginning
+        plugin_name = cb.canonicalName()
+        cb_commands += [plugin_name + " " + command for command in cb_commands]
+
+        candidates |= {
+            command for command in cb_commands if command.startswith(normalized_payload)
+        }
+
+    return candidates
+
+
+class Autocomplete(callbacks.Plugin):
+    """Provides command completion for IRC clients that support it."""
+
+    def _enabled(self, irc, msg):
+        return conf.supybot.protocols.irc.experimentalExtensions() and self.registryValue(
+            "enabled", msg.channel, irc.network
+        )
+
+    def doTagmsg(self, irc, msg):
+        if REQUEST_TAG not in msg.server_tags:
+            return
+        if not "msgid" in msg.server_tags:
+            return
+        if not self._enabled(irc, msg):
+            return
+
+        msgid = msg.server_tags["msgid"]
+
+        text = msg.server_tags[REQUEST_TAG]
+
+        # using callbacks._addressed instead of callbacks.addressed, as
+        # callbacks.addressed would tag the m
+        payload = callbacks._addressed(irc, msg, payload=text)
+
+        if not payload:
+            # not addressed
+            return
+
+        # marks used by '_addressed' are usually prefixes (char, string,
+        # nick), but may also be suffixes (with
+        # supybot.reply.whenAddressedBy.nick.atEnd); but there is no way to
+        # have it in the middle of the message AFAIK.
+        assert payload in text
+
+        if not text.endswith(payload):
+            # If there is a suffix, it means the end of the text is used to
+            # address the bot, so it can't be a method to be completed.
+            return
+
+        autocomplete_response = _getAutocompleteResponse(irc, msg, payload)
+        if not autocomplete_response:
+            return
+
+        target = msg.channel or ircutils.nickFromHostmask(msg.prefix)
+        irc.queueMsg(
+            ircmsgs.IrcMsg(
+                server_tags={
+                    "+draft/reply": msgid,
+                    "+draft/autocomplete": autocomplete_response,
+                },
+                command="TAGMSG",
+                args=[target],
+            )
+        )
+
+
+Class = Autocomplete
+
+
+# vim:set shiftwidth=4 softtabstop=4 expandtab textwidth=79:
diff --git a/plugins/Autocomplete/test.py b/plugins/Autocomplete/test.py
new file mode 100644
index 000000000..0b3d29ca9
--- /dev/null
+++ b/plugins/Autocomplete/test.py
@@ -0,0 +1,127 @@
+###
+# Copyright (c) 2020, Valentin Lorentz
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# modification, are permitted provided that the following conditions are met:
+#
+#   * Redistributions of source code must retain the above copyright notice,
+#     this list of conditions, and the following disclaimer.
+#   * Redistributions in binary form must reproduce the above copyright notice,
+#     this list of conditions, and the following disclaimer in the
+#     documentation and/or other materials provided with the distribution.
+#   * Neither the name of the author of this software nor the name of
+#     contributors to this software may be used to endorse or promote products
+#     derived from this software without specific prior written consent.
+#
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+# AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+# IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+# ARE DISCLAIMED.  IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
+# LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+# CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+# SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+# CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+# ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+# POSSIBILITY OF SUCH DAMAGE.
+
+###
+
+from supybot import conf, ircmsgs
+from supybot.test import *
+
+
+class AutocompleteTestCase(PluginTestCase):
+    plugins = ("Autocomplete", "Later", "Misc")
+    # Later and Misc both have a 'tell' command, this allows checking
+    # deduplication.
+
+    def _sendRequest(self, request):
+        self.irc.feedMsg(
+            ircmsgs.IrcMsg(
+                prefix="foo!bar@baz",
+                server_tags={"msgid": "1234", "+draft/autocomplete-request": request},
+                command="TAGMSG",
+                args=[self.nick],
+            )
+        )
+
+    def _assertAutocompleteResponse(self, request, expectedResponse):
+        self._sendRequest(request)
+        m = self.irc.takeMsg()
+        self.assertEqual(
+            m,
+            ircmsgs.IrcMsg(
+                server_tags={
+                    "+draft/reply": "1234",
+                    "+draft/autocomplete": expectedResponse,
+                },
+                command="TAGMSG",
+                args=["foo"],
+            ),
+        )
+
+    def testResponse(self):
+        with conf.supybot.protocols.irc.experimentalExtensions.context(True):
+            with conf.supybot.plugins.Autocomplete.enabled.context(True):
+                self._assertAutocompleteResponse("apro", "pos")
+                self._assertAutocompleteResponse("apr", "opos")
+                self._assertAutocompleteResponse("te", "ll\tstplugin eval")
+                self._assertAutocompleteResponse("tel", "l")
+                self._assertAutocompleteResponse("misc t", "ell")
+                self._assertAutocompleteResponse("misc c", "learmores\tompletenick")
+
+    def testNoResponse(self):
+        with conf.supybot.protocols.irc.experimentalExtensions.context(True):
+            self._sendRequest("apro")
+            self.assertIsNone(self.irc.takeMsg())
+
+        with conf.supybot.plugins.Autocomplete.enabled.context(True):
+            self._sendRequest("apro")
+            self.assertIsNone(self.irc.takeMsg())
+
+
+class AutocompleteChannelTestCase(ChannelPluginTestCase):
+    plugins = ("Autocomplete", "Later", "Misc")
+
+    def _sendRequest(self, request):
+        self.irc.feedMsg(
+            ircmsgs.IrcMsg(
+                prefix="foo!bar@baz",
+                server_tags={"msgid": "1234", "+draft/autocomplete-request": request},
+                command="TAGMSG",
+                args=[self.channel],
+            )
+        )
+
+    def _assertAutocompleteResponse(self, request, expectedResponse):
+        self._sendRequest(request)
+        m = self.irc.takeMsg()
+        self.assertEqual(
+            m,
+            ircmsgs.IrcMsg(
+                server_tags={
+                    "+draft/reply": "1234",
+                    "+draft/autocomplete": expectedResponse,
+                },
+                command="TAGMSG",
+                args=[self.channel],
+            ),
+        )
+    def testResponse(self):
+        with conf.supybot.protocols.irc.experimentalExtensions.context(True):
+            with conf.supybot.plugins.Autocomplete.enabled.context(True):
+                self._assertAutocompleteResponse("@apro", "pos")
+
+    def testNoResponse(self):
+        with conf.supybot.protocols.irc.experimentalExtensions.context(True):
+            self._sendRequest("@apro")
+            self.assertIsNone(self.irc.takeMsg())
+
+        with conf.supybot.plugins.Autocomplete.enabled.context(True):
+            self._sendRequest("@apro")
+            self.assertIsNone(self.irc.takeMsg())
+
+
+# vim:set shiftwidth=4 tabstop=4 expandtab textwidth=79:
diff --git a/src/callbacks.py b/src/callbacks.py
index ddacf9a7f..a9458f1bb 100644
--- a/src/callbacks.py
+++ b/src/callbacks.py
@@ -51,7 +51,15 @@ _ = PluginInternationalization()
 
 def _addressed(irc, msg, prefixChars=None, nicks=None,
               prefixStrings=None, whenAddressedByNick=None,
-              whenAddressedByNickAtEnd=None):
+              whenAddressedByNickAtEnd=None, payload=None):
+    """Determines whether this message is a command to the bot (because of a
+    prefix char/string, or because the bot's nick is used as prefix, or because
+    it's a private message, etc.).
+    Returns the actual content of the command (ie. the content of the message,
+    stripped of the prefix that was used to determine if it's addressed).
+
+    If 'payload' is not None, its value is used instead of msg.args[1] as the
+    content of the message."""
     if isinstance(irc, str):
         warnings.warn(
             "callbacks.addressed's first argument should now be be the Irc "
@@ -71,9 +79,10 @@ def _addressed(irc, msg, prefixChars=None, nicks=None,
                 payload = payload[len(prefixString):].lstrip()
         return payload
 
-    assert msg.command == 'PRIVMSG'
+    assert msg.command in ('PRIVMSG', 'TAGMSG')
     target = msg.channel or msg.args[0]
-    payload = msg.args[1]
+    if not payload:
+        payload = msg.args[1]
     if not payload:
         return ''
     if prefixChars is None: