diff --git a/extensions/extended-isupport.md b/extensions/extended-isupport.md
new file mode 100644
index 00000000..3857eda5
--- /dev/null
+++ b/extensions/extended-isupport.md
@@ -0,0 +1,94 @@
+---
+title: "`extended-isupport` Extension"
+layout: spec
+copyrights:
+  -
+    name: "Simon Ser"
+    period: "2024"
+    email: "contact@emersion.fr"
+---
+
+## Notes for implementing work-in-progress version
+
+This is a work-in-progress specification.
+
+Software implementing this work-in-progress specification MUST NOT use the
+unprefixed `extended-isupport` capability name. Instead, implementations SHOULD
+use the `draft/extended-isupport` capability name to be interoperable with
+other software implementing a compatible work-in-progress version. The final
+version of the specification will use unprefixed capability names.
+
+## Introduction
+
+`RPL_ISUPPORT` is used to advertise metadata about the server as key-value
+entries. However, `RPL_ISUPPORT` is only sent by servers after connection
+registration. This undermines the usefulness of `RPL_ISUPPORT`: some of the
+metadata would be useful to clients prior to connection registration. This
+extension adds a mechanism to send `RPL_ISUPPORT` messages before connection
+registration, as well as a dedicated command to request the `ISUPPORT` list and
+a batch for `RPL_ISUPPORT` messages.
+
+## Implementation
+
+### `ISUPPORT` command
+
+A new `ISUPPORT` command is introduced to request the full `RPL_ISUPPORT` list.
+When receiving this command, the server MUST reply with one or more
+`RPL_ISUPPORT` messages (grouped inside a `draft/isupport` batch if the `batch`
+and `draft/extended-isupport` capabilities are enabled, see below).
+
+### `draft/extended-isupport` capability
+
+When the `draft/extended-isupport` capability is enabled by the client, the
+server MUST accept `ISUPPORT` commands before connection registration
+completes (ie, before the client sends `CAP END`, and before the server sends
+`RPL_WELCOME`).
+
+Before connection registration completes, while `draft/extended-isupport` is
+enabled, the server MAY send updates to the key-value entries via subsequent
+`RPL_ISUPPORT` messages (the same way it would after connection registration
+completes without this extension).
+
+The server MAY skip the `RPL_ISUPPORT` replies usually sent when connection
+registration completes, if it already sent all information.
+
+Before connection registration, the server MAY send only a subset of the full
+`RPL_ISUPPORT` list. In that case, the server MUST send a `RPL_ISUPPORT` list
+when connection registration completes with entries previously omitted.
+
+### `draft/isupport` batch
+
+The server MUST group all `RPL_ISUPPORT` messages inside a `draft/isupport`
+batch when the [`batch`][] and `draft/extended-isupport` capabilities are
+enabled. The server MUST NOT send any unbatched `RPL_ISUPPORT` message while
+both of these capabilities are enabled. The order in which the capabilities are
+enabled is not significant.
+
+The batch MUST only contain one or more `RPL_ISUPPORT` messages, it MUST NOT
+contain any other message.
+
+As usual, servers can update or delete existing values by sending additional
+`RPL_ISUPPORT` messages in a `draft/isupport` batch after the initial batch.
+
+## Examples
+
+Enabling the capability:
+
+    C: CAP LS 302
+    S: :irc.example.org CAP * LS :multi-prefix sasl batch draft/extended-isupport
+    C: CAP REQ batch draft/extended-isupport
+    S: :irc.example.org CAP * ACK :batch draft/extended-isupport
+    C: ISUPPORT
+    S: :irc.example.org BATCH +asdf draft/isupport
+    S: @batch=asdf :irc.example.org 005 * NETWORK=Example NICKLEN=30 FOO=bar
+    S: :irc.example.org BATCH -asdf
+    C: NICK emersion
+    C: USER emersion 0 * :Simon Ser
+    C: CAP END
+    S: 001 emersion :어서 오세요
+
+Sending a change:
+
+    S: :irc.example.org BATCH +asdf draft/isupport
+    S: @batch=asdf :irc.example.org 005 * CHANNELLEN=64 NICKLEN=42 -FOO
+    S: :irc.example.org BATCH -asdf