From f24f8e7ba3a69da2cc7f6f1d994356f6d47fba34 Mon Sep 17 00:00:00 2001
From: Logan Johnson <loganj@squareup.com>
Date: Mon, 19 Oct 2026 15:54:19 -0400
Subject: [PATCH] feat(relay): tag every membership notice with its channel
 type

Every kind:44100/44101 notice now carries `["t", <channel_type>]`. A
filter sees a notice only if it knows the type: a filter with `#t` sees
the types it names, and a filter without `#t` sees only the frozen legacy
types (stream, forum, dm, workflow). A type added later is hidden from old
clients by default, so a read path that is not updated fails closed.

An untagged notice, stored before this change, counts as a legacy type, so
replay across the upgrade stays complete. One filter that names every type
reads all notices.

The rule lives in buzz-core (`notice_visible`) and in one SQL predicate
(`push_notice_visibility`), which defaults to legacy types. The relay reads
the channel type in one place, `emit_membership_notification`, and sends no
notice when the channel row is missing.

Co-authored-by: Larry <627498bd4bd1f281a16431e3c6cce3b5c25b6692798c78672298aefbf2f8f8b5@buzz.block.builderlab.xyz>
Signed-off-by: Logan Johnson <loganj@squareup.com>
---
 NOSTR.md                                      |  13 +-
 crates/buzz-core/src/filter.rs                | 175 ++++++++++++------
 crates/buzz-db/src/store/channel_members.rs   |  34 +++-
 crates/buzz-db/src/store/event.rs             |  74 +++++---
 crates/buzz-relay/src/handlers/req.rs         |  21 +--
 .../buzz-relay/src/handlers/side_effects.rs   |  33 ++--
 .../handlers/system_channel_postgres_tests.rs | 137 +++++++++++---
 docs/nips/NIP-CL.md                           |  41 ++--
 8 files changed, 372 insertions(+), 156 deletions(-)

diff --git a/NOSTR.md b/NOSTR.md
index bbde09d425..96ccb0edc0 100644
--- a/NOSTR.md
+++ b/NOSTR.md
@@ -152,9 +152,10 @@ out unless they ask for it:
   combine with OR, so `#t:["system", <label>]` does not mean "a system
   channel with this label".
 
-Push notifications skip system channels. Their kind:44100/44101
-membership notifications carry `["t","system"]`, and the relay returns those
-only to a filter with `#t:["system"]` (see NIP-CL).
+Push notifications skip system channels. Every kind:44100/44101 membership
+notification carries its channel type as `t`, and a filter without `#t` sees
+only the legacy types, so system channel notices reach only a filter with
+`#t:["system"]` (see NIP-CL).
 
 Events are stored **channel-scoped** so access control applies — private channel member lists are
 only visible to members. Discover groups via historical REQ:
@@ -180,8 +181,10 @@ The relay emits relay-signed notifications when members are added or removed:
 | **44100** | Member added | `p` = target pubkey, `h` = channel UUID | Community-global |
 | **44101** | Member removed | `p` = target pubkey, `h` = channel UUID | Community-global |
 
-A notice about a system channel also has `t` = `system`. Only a filter with
-`#t:["system"]` returns it (NIP-CL).
+Each notice also has `t` = the channel type (`stream`, `forum`, `dm`,
+`workflow`, `system`). A filter with `#t` gets only the types it names. A
+filter without `#t` gets only `stream`, `forum`, `dm` and `workflow`, and so
+does an untagged notice stored before the tag existed (NIP-CL).
 
 Stored community-globally (`channel_id = None` inside the connected community) so agents and clients can subscribe without knowing channel
 UUIDs in advance. Client-submitted kind:44100/44101 events are rejected — only the relay keypair
diff --git a/crates/buzz-core/src/filter.rs b/crates/buzz-core/src/filter.rs
index 2e1dee333f..43753df94d 100644
--- a/crates/buzz-core/src/filter.rs
+++ b/crates/buzz-core/src/filter.rs
@@ -32,44 +32,74 @@ pub fn reader_authorized_for_event(event: &nostr::Event, reader_pubkey_hex: &str
         .any(|t| t.content() == Some(reader_pubkey_hex))
 }
 
-/// The tag that marks a kind:44100/44101 membership notice about a system
-/// channel (NIP-CL): `["t", "system"]`. Only the relay signs these kinds, and
-/// it adds the tag only when the channel is a system channel.
-pub const SYSTEM_NOTICE_TAG: [&str; 2] = ["t", "system"];
-
-/// The kinds that [`SYSTEM_NOTICE_TAG`] can mark.
-pub const SYSTEM_NOTICE_KINDS: [u32; 2] = [
+/// The membership notice kinds (44100 added, 44101 removed). Only the relay
+/// signs them, and it tags each one with its channel's type:
+/// `["t", <channel_type>]` (NIP-CL).
+pub const MEMBERSHIP_NOTICE_KINDS: [u32; 2] = [
     crate::kind::KIND_MEMBER_ADDED_NOTIFICATION,
     crate::kind::KIND_MEMBER_REMOVED_NOTIFICATION,
 ];
 
-/// Whether `event` is a membership notice about a system channel.
+/// The tag name that carries a membership notice's channel type.
+pub const NOTICE_CHANNEL_TYPE_TAG: &str = "t";
+
+/// The channel types that clients knew before NIP-CL typed membership
+/// notices. A filter without `#t` sees notices only about these types, and a
+/// notice without a type tag (stored before NIP-CL) counts as all of them.
 ///
-/// Such a notice is hidden by default: it matches only a filter that asks
-/// for it ([`filter_asks_for_system_notices`]). Clients that do not know
-/// system channels never ask, so they never see one, on any read or live
-/// path that matches through [`filters_match`] or applies the same rule in
-/// SQL (`EventQuery::include_system_notices`, which defaults to off).
-pub fn is_system_notice(event: &nostr::Event) -> bool {
-    SYSTEM_NOTICE_KINDS.contains(&crate::kind::event_kind_u32(event))
-        && event.tags.iter().any(|tag| {
-            let tag = tag.as_slice();
-            tag.first().map(String::as_str) == Some(SYSTEM_NOTICE_TAG[0])
-                && tag.get(1).map(String::as_str) == Some(SYSTEM_NOTICE_TAG[1])
-        })
+/// Frozen. Do not add a type here: a type added later stays hidden from every
+/// client that does not name it in `#t`.
+pub const LEGACY_NOTICE_CHANNEL_TYPES: [&str; 4] = ["stream", "forum", "dm", "workflow"];
+
+/// Whether `event` is a membership notice (kind 44100/44101).
+pub fn is_membership_notice(event: &nostr::Event) -> bool {
+    MEMBERSHIP_NOTICE_KINDS.contains(&crate::kind::event_kind_u32(event))
 }
 
-/// Whether `filter` asks for system-channel membership notices: its `#t`
-/// values include `"system"`.
-pub fn filter_asks_for_system_notices(filter: &Filter) -> bool {
+/// The channel types whose membership notices `filter` may see: its `#t`
+/// values, or [`LEGACY_NOTICE_CHANNEL_TYPES`] when it has no `#t`.
+///
+/// This is the one rule for notice visibility (NIP-CL). [`filters_match`]
+/// applies it, and `EventQuery::notice_channel_types` applies it in SQL.
+pub fn notice_channel_types(filter: &Filter) -> Option<Vec<String>> {
     filter
         .generic_tags
         .get(&nostr::SingleLetterTag::lowercase(nostr::Alphabet::T))
-        .is_some_and(|values| values.iter().any(|value| value == SYSTEM_NOTICE_TAG[1]))
+        .map(|values| values.iter().map(ToString::to_string).collect())
+}
+
+/// Whether a filter whose visible types are `visible` (see
+/// [`notice_channel_types`]) may see membership notice `event`.
+///
+/// A tagged notice matches when one of its types is visible. An untagged
+/// notice is a legacy type, so it matches when a legacy type is visible.
+pub fn notice_visible(visible: Option<&[String]>, event: &nostr::Event) -> bool {
+    let is_visible = |channel_type: &str| match visible {
+        Some(types) => types.iter().any(|t| t == channel_type),
+        None => LEGACY_NOTICE_CHANNEL_TYPES.contains(&channel_type),
+    };
+    let mut tagged = false;
+    for tag in event.tags.iter() {
+        let tag = tag.as_slice();
+        if tag.first().map(String::as_str) != Some(NOTICE_CHANNEL_TYPE_TAG) {
+            continue;
+        }
+        tagged = true;
+        if tag
+            .get(1)
+            .is_some_and(|channel_type| is_visible(channel_type))
+        {
+            return true;
+        }
+    }
+    !tagged && LEGACY_NOTICE_CHANNEL_TYPES.iter().any(|t| is_visible(t))
 }
 
 fn filter_match_one(f: &Filter, ev: &StoredEvent) -> bool {
-    if is_system_notice(&ev.event) && !filter_asks_for_system_notices(f) {
+    // A membership notice's `#t` is its channel type, matched by the notice
+    // rule above instead of the plain tag match below.
+    let notice = is_membership_notice(&ev.event);
+    if notice && !notice_visible(notice_channel_types(f).as_deref(), &ev.event) {
         return false;
     }
 
@@ -107,6 +137,9 @@ fn filter_match_one(f: &Filter, ev: &StoredEvent) -> bool {
 
     for (tag_key, tag_values) in f.generic_tags.iter() {
         let tag_key_str = tag_key.to_string();
+        if notice && tag_key_str == NOTICE_CHANNEL_TYPE_TAG {
+            continue;
+        }
         let has_match = tag_values.iter().any(|filter_val| {
             ev.event
                 .tags
@@ -160,16 +193,16 @@ mod tests {
     }
 
     #[test]
-    fn a_system_notice_matches_only_a_filter_that_asks() {
+    fn a_notice_matches_only_filters_that_know_its_channel_type() {
         let relay = Keys::generate();
         let member = Keys::generate().public_key();
-        let notice = |kind: u32, system: bool| {
+        let notice = |kind: u32, channel_type: Option<&str>| {
             let mut tags = vec![
                 Tag::parse(["p", &member.to_hex()]).unwrap(),
                 Tag::parse(["h", "c"]).unwrap(),
             ];
-            if system {
-                tags.push(Tag::parse(SYSTEM_NOTICE_TAG).unwrap());
+            if let Some(channel_type) = channel_type {
+                tags.push(Tag::parse([NOTICE_CHANNEL_TYPE_TAG, channel_type]).unwrap());
             }
             let event = EventBuilder::new(Kind::Custom(kind as u16), "")
                 .tags(tags)
@@ -179,36 +212,66 @@ mod tests {
         };
         let filter = |json: serde_json::Value| -> Filter { serde_json::from_value(json).unwrap() };
         let p = member.to_hex();
-        let old = [
-            filter(serde_json::json!({"kinds": [44100, 44101], "#p": [p]})),
-            filter(serde_json::json!({"#p": [p]})),
-            filter(serde_json::json!({})),
-            filter(serde_json::json!({"#t": ["other"]})),
+        let all = ["stream", "forum", "dm", "workflow", "system"];
+        // (filter, sees untagged, stream, system, a type added later)
+        let cases = [
+            (
+                serde_json::json!({"kinds": [44100, 44101], "#p": [p]}),
+                [true, true, false, false],
+            ),
+            (serde_json::json!({"#p": [p]}), [true, true, false, false]),
+            (serde_json::json!({}), [true, true, false, false]),
+            (
+                serde_json::json!({"#t": ["other"]}),
+                [false, false, false, false],
+            ),
+            (
+                serde_json::json!({"kinds": [44100, 44101], "#p": [p], "#t": all}),
+                [true, true, true, false],
+            ),
+            (
+                serde_json::json!({"#t": ["system"]}),
+                [false, false, true, false],
+            ),
+            (
+                serde_json::json!({"#t": ["dm"]}),
+                [true, false, false, false],
+            ),
+            (
+                serde_json::json!({"#t": ["future"]}),
+                [false, false, false, true],
+            ),
         ];
-        let asks = [
-            filter(serde_json::json!({"kinds": [44100, 44101], "#p": [p], "#t": ["system"]})),
-            filter(serde_json::json!({"#t": ["other", "system"]})),
-        ];
-        for kind in SYSTEM_NOTICE_KINDS {
-            let system = notice(kind, true);
-            assert!(is_system_notice(&system.event));
-            for f in &old {
-                assert!(!filters_match(std::slice::from_ref(f), &system), "{f:?}");
+        for kind in MEMBERSHIP_NOTICE_KINDS {
+            let notices = [
+                notice(kind, None),
+                notice(kind, Some("stream")),
+                notice(kind, Some("system")),
+                notice(kind, Some("future")),
+            ];
+            for (json, expected) in &cases {
+                let f = filter(json.clone());
+                for (ev, expected) in notices.iter().zip(expected) {
+                    assert_eq!(
+                        filters_match(std::slice::from_ref(&f), ev),
+                        *expected,
+                        "kind {kind}, filter {json}, notice {:?}",
+                        ev.event.tags
+                    );
+                }
             }
-            for f in &asks {
-                assert!(filters_match(std::slice::from_ref(f), &system), "{f:?}");
-            }
-            // An ordinary notice is unchanged: old filters see it, and the
-            // asking filter's `#t` does not match it.
-            let ordinary = notice(kind, false);
-            assert!(!is_system_notice(&ordinary.event));
-            assert!(filters_match(&old[..1], &ordinary));
-            assert!(!filters_match(&asks[..1], &ordinary));
         }
-        // The tag hides nothing on other kinds.
-        let other = notice(44200, true);
-        assert!(!is_system_notice(&other.event));
-        assert!(filters_match(&old[2..3], &other));
+        // Other kinds keep the plain `#t` match.
+        let other = notice(44200, Some("system"));
+        assert!(filters_match(&[filter(serde_json::json!({}))], &other));
+        assert!(filters_match(
+            &[filter(serde_json::json!({"#t": ["system"]}))],
+            &other
+        ));
+        assert!(!filters_match(
+            &[filter(serde_json::json!({"#t": ["stream"]}))],
+            &other
+        ));
     }
 
     #[test]
diff --git a/crates/buzz-db/src/store/channel_members.rs b/crates/buzz-db/src/store/channel_members.rs
index 4855d32ccc..9e7679e0c3 100644
--- a/crates/buzz-db/src/store/channel_members.rs
+++ b/crates/buzz-db/src/store/channel_members.rs
@@ -1288,8 +1288,29 @@ pub async fn get_readable_system_channel_ids(
         .collect()
 }
 
+/// The type of channel `channel_id`, deleted or not, or `None` when no such
+/// channel exists. Membership notices carry it as their `t` tag (NIP-CL).
+pub async fn channel_type_of(
+    pool: &PgPool,
+    community_id: CommunityId,
+    channel_id: Uuid,
+) -> Result<Option<String>> {
+    let mut connection = crate::observability::acquire_writer(
+        pool,
+        crate::observability::WriterOperation::Authorization,
+    )
+    .await?;
+    Ok(sqlx::query_scalar(
+        "SELECT channel_type::text FROM channels WHERE community_id = $1 AND id = $2",
+    )
+    .bind(community_id.as_uuid())
+    .bind(channel_id)
+    .fetch_optional(&mut *connection)
+    .await?)
+}
+
 /// The IDs in `channel_ids` that name system channels, deleted or not.
-/// Push matching and membership notifications use it to skip them.
+/// Push matching uses it to skip them.
 pub async fn system_channel_ids_among(
     pool: &PgPool,
     community_id: CommunityId,
@@ -1954,6 +1975,17 @@ impl Db {
         get_readable_system_channel_ids(&self.pool, community_id, pubkey).await
     }
 
+    /// The type of channel `channel_id`, deleted or not, or `None` when no
+    /// such channel exists.
+    #[datastore_span(name = "channel_type_of", system = "postgresql")]
+    pub async fn channel_type_of(
+        &self,
+        community_id: CommunityId,
+        channel_id: Uuid,
+    ) -> Result<Option<String>> {
+        channel_type_of(&self.pool, community_id, channel_id).await
+    }
+
     /// The IDs in `channel_ids` that name system channels, deleted or not.
     #[datastore_span(name = "system_channel_ids_among", system = "postgresql")]
     pub async fn system_channel_ids_among(
diff --git a/crates/buzz-db/src/store/event.rs b/crates/buzz-db/src/store/event.rs
index 51e6d833aa..da685f6fe6 100644
--- a/crates/buzz-db/src/store/event.rs
+++ b/crates/buzz-db/src/store/event.rs
@@ -125,12 +125,12 @@ pub struct EventQuery {
     /// containment alone would also admit decoys like `["x","shared","true"]`.
     /// `event_visible_to_reader` remains the post-filter.
     pub shared_gated_reader: Option<Vec<u8>>,
-    /// Whether results may include membership notices about system channels
-    /// ([`buzz_core::filter::is_system_notice`]). Off by default, so a read
-    /// path that does not ask never returns one (NIP-CL). Set it only for a
-    /// filter that asks for them
-    /// ([`buzz_core::filter::filter_asks_for_system_notices`]).
-    pub include_system_notices: bool,
+    /// The channel types whose membership notices (44100/44101) the query may
+    /// return: the filter's `#t` values
+    /// ([`buzz_core::filter::notice_channel_types`]). `None` means the legacy
+    /// types only, so a read path that does not set it never returns a notice
+    /// about a type added later, such as a system channel (NIP-CL).
+    pub notice_channel_types: Option<Vec<String>>,
 }
 
 impl EventQuery {
@@ -164,7 +164,7 @@ impl EventQuery {
             channel_ids_include_global: true,
             max_limit: None,
             shared_gated_reader: None,
-            include_system_notices: false,
+            notice_channel_types: None,
         }
     }
 }
@@ -662,7 +662,7 @@ fn build_query_events_sql(q: &EventQuery) -> QueryBuilder<sqlx::Postgres> {
         let table = if q.p_tag_hex.is_some() { "e" } else { "events" };
         qb.push(format!(" AND NOT ({table}.kind = 45010 AND NOT EXISTS (SELECT 1 FROM artifact_heads ah WHERE ah.community_id={table}.community_id AND ah.event_id={table}.id))"));
     }
-    push_system_notice_exclusion(&mut qb, col_prefix, q);
+    push_notice_visibility(&mut qb, col_prefix, q);
 
     if let Some(ch) = q.channel_id {
         qb.push(format!(" AND {col_prefix}channel_id = "))
@@ -920,34 +920,52 @@ fn push_tag_value_filters(
     }
 }
 
-/// Hide membership notices about system channels unless the query asks for
-/// them (NIP-CL): `AND NOT (kind IN (44100, 44101) AND <exact ["t","system"]>)`.
-/// The same rule as [`buzz_core::filter::is_system_notice`] in `filters_match`,
-/// pushed before `LIMIT` so hidden rows cannot take page slots. Skipped when
-/// the query's kinds cannot match a notice.
-fn push_system_notice_exclusion(
-    qb: &mut QueryBuilder<sqlx::Postgres>,
-    col_prefix: &str,
-    q: &EventQuery,
-) {
-    use buzz_core::filter::{SYSTEM_NOTICE_KINDS, SYSTEM_NOTICE_TAG};
-    if q.include_system_notices {
-        return;
-    }
+/// Return only membership notices whose channel type the query may see
+/// (NIP-CL), as [`buzz_core::filter::notice_visible`] does in `filters_match`:
+/// `AND (kind NOT IN (44100, 44101) OR <type tag in visible types>
+/// [OR <no type tag>])`. An untagged notice is a legacy type, so it is kept
+/// only when a legacy type is visible. Pushed before `LIMIT` so hidden rows
+/// cannot take page slots. Skipped when the query's kinds cannot match a
+/// notice.
+fn push_notice_visibility(qb: &mut QueryBuilder<sqlx::Postgres>, col_prefix: &str, q: &EventQuery) {
+    use buzz_core::filter::{
+        LEGACY_NOTICE_CHANNEL_TYPES, MEMBERSHIP_NOTICE_KINDS, NOTICE_CHANNEL_TYPE_TAG,
+    };
     if let Some(kinds) = q.kinds.as_deref() {
         if !kinds
             .iter()
-            .any(|kind| SYSTEM_NOTICE_KINDS.contains(&(*kind as u32)))
+            .any(|kind| MEMBERSHIP_NOTICE_KINDS.contains(&(*kind as u32)))
         {
             return;
         }
     }
-    let [added, removed] = SYSTEM_NOTICE_KINDS;
-    let [name, value] = SYSTEM_NOTICE_TAG;
+    let visible: Vec<String> = match &q.notice_channel_types {
+        Some(types) => types.clone(),
+        None => LEGACY_NOTICE_CHANNEL_TYPES
+            .iter()
+            .map(|t| (*t).to_owned())
+            .collect(),
+    };
+    let untagged_visible = visible
+        .iter()
+        .any(|t| LEGACY_NOTICE_CHANNEL_TYPES.contains(&t.as_str()));
+    let [added, removed] = MEMBERSHIP_NOTICE_KINDS;
     qb.push(format!(
-        " AND NOT ({col_prefix}kind IN ({added}, {removed}) AND "
+        " AND ({col_prefix}kind NOT IN ({added}, {removed}) OR "
     ));
-    push_exact_tag_match(qb, col_prefix, name, &[value.to_owned()]);
+    if visible.is_empty() {
+        qb.push("FALSE");
+    } else {
+        push_exact_tag_match(qb, col_prefix, NOTICE_CHANNEL_TYPE_TAG, &visible);
+    }
+    if untagged_visible {
+        qb.push(format!(
+            " OR NOT EXISTS (SELECT 1 FROM jsonb_array_elements({col_prefix}tags) AS tag \
+             WHERE tag->>0 = "
+        ))
+        .push_bind(NOTICE_CHANNEL_TYPE_TAG.to_owned())
+        .push(")");
+    }
     qb.push(")");
 }
 
@@ -1069,7 +1087,7 @@ pub(crate) async fn count_events_on(conn: &mut sqlx::PgConnection, q: &EventQuer
         let table = if q.p_tag_hex.is_some() { "e" } else { "events" };
         qb.push(format!(" AND NOT ({table}.kind = 45010 AND NOT EXISTS (SELECT 1 FROM artifact_heads ah WHERE ah.community_id={table}.community_id AND ah.event_id={table}.id))"));
     }
-    push_system_notice_exclusion(&mut qb, col_prefix, q);
+    push_notice_visibility(&mut qb, col_prefix, q);
 
     if let Some(ch) = q.channel_id {
         qb.push(format!(" AND {col_prefix}channel_id = "))
diff --git a/crates/buzz-relay/src/handlers/req.rs b/crates/buzz-relay/src/handlers/req.rs
index cf29bdff08..9c6b5d70a6 100644
--- a/crates/buzz-relay/src/handlers/req.rs
+++ b/crates/buzz-relay/src/handlers/req.rs
@@ -1252,24 +1252,13 @@ fn filter_to_query_params(
         .filter(|values| !values.is_empty())
         .map(|values| values.iter().map(|v| v.to_string()).collect());
 
-    let mut tag_values = group_state_tag_values(filter);
-    // A filter that asks for system-channel membership notices (`#t` has
-    // "system") may read them; match its `#t` in SQL so ordinary notices do
-    // not take its page slots. Every other filter keeps the default, which
-    // hides them (NIP-CL).
-    let include_system_notices = buzz_core::filter::filter_asks_for_system_notices(filter);
-    if include_system_notices && !tag_values.iter().any(|(name, _)| name == "t") {
-        let t = nostr::SingleLetterTag::lowercase(nostr::Alphabet::T);
-        if let Some(values) = filter.generic_tags.get(&t) {
-            tag_values.push((
-                t.to_string(),
-                values.iter().map(|value| value.to_string()).collect(),
-            ));
-        }
-    }
+    let tag_values = group_state_tag_values(filter);
+    // NIP-CL: a membership notice's `#t` is its channel type. SQL keeps only
+    // the types this filter may see; a filter without `#t` sees legacy types.
+    let notice_channel_types = buzz_core::filter::notice_channel_types(filter);
 
     EventQuery {
-        include_system_notices,
+        notice_channel_types,
         channel_id,
         kinds,
         pubkey,
diff --git a/crates/buzz-relay/src/handlers/side_effects.rs b/crates/buzz-relay/src/handlers/side_effects.rs
index c0e6f08eb0..f5b55410d3 100644
--- a/crates/buzz-relay/src/handlers/side_effects.rs
+++ b/crates/buzz-relay/src/handlers/side_effects.rs
@@ -1059,11 +1059,12 @@ pub fn emit_live_thread_summary(
 
 /// Emit a relay-signed membership notification event stored globally (channel_id = None).
 ///
-/// kind:44100 = member added, kind:44101 = member removed. A notice about a
-/// system channel carries `["t","system"]` (NIP-CL). This is the only place
-/// that checks the channel type for notices: every read and live path hides a
-/// tagged notice unless the filter asks for it (`buzz_core::filter`), so
-/// clients that do not know system channels never see one.
+/// kind:44100 = member added, kind:44101 = member removed. Every notice
+/// carries its channel's type as `["t", <channel_type>]` (NIP-CL). This is
+/// the only place that reads the channel type for notices. Every read and live
+/// path shows a notice only to a filter that knows its type
+/// (`buzz_core::filter::notice_visible`), so a client never sees a notice
+/// about a type it does not know, such as a system channel.
 /// The p tag addresses the target pubkey; the h tag carries the channel UUID as metadata.
 /// Stored with channel_id = None so global subscribers receive it via slow-path fan-out.
 pub async fn emit_membership_notification(
@@ -1074,11 +1075,16 @@ pub async fn emit_membership_notification(
     actor_pubkey: &[u8],
     notification_kind: u32,
 ) -> anyhow::Result<()> {
-    let system = !state
+    // No channel row means no type to tag. Send nothing rather than an
+    // untagged notice, which every client would treat as a legacy type.
+    let Some(channel_type) = state
         .db
-        .system_channel_ids_among(tenant.community(), &[channel_id])
+        .channel_type_of(tenant.community(), channel_id)
         .await?
-        .is_empty();
+    else {
+        warn!(%channel_id, "membership notice skipped: channel not found");
+        return Ok(());
+    };
     let target_hex = hex::encode(target_pubkey);
     let actor_hex = hex::encode(actor_pubkey);
     let channel_id_str = channel_id.to_string();
@@ -1105,15 +1111,10 @@ pub async fn emit_membership_notification(
     })
     .to_string();
 
-    let mut tags = vec![p_tag, h_tag];
-    if system {
-        tags.push(
-            Tag::parse(buzz_core::filter::SYSTEM_NOTICE_TAG)
-                .map_err(|e| anyhow::anyhow!("failed to build system tag: {e}"))?,
-        );
-    }
+    let type_tag = Tag::parse([buzz_core::filter::NOTICE_CHANNEL_TYPE_TAG, &channel_type])
+        .map_err(|e| anyhow::anyhow!("failed to build channel type tag: {e}"))?;
     let event = EventBuilder::new(Kind::Custom(notification_kind as u16), content)
-        .tags(tags)
+        .tags([p_tag, h_tag, type_tag])
         .sign_with_keys(&state.relay_keypair)
         .map_err(|e| anyhow::anyhow!("failed to sign membership notification: {e}"))?;
 
diff --git a/crates/buzz-relay/src/handlers/system_channel_postgres_tests.rs b/crates/buzz-relay/src/handlers/system_channel_postgres_tests.rs
index 6b3153713e..a74644e21e 100644
--- a/crates/buzz-relay/src/handlers/system_channel_postgres_tests.rs
+++ b/crates/buzz-relay/src/handlers/system_channel_postgres_tests.rs
@@ -428,19 +428,20 @@ async fn named_channels_missing_from_the_cache_are_confirmed_together() {
     assert!(readable_system.is_empty());
 }
 
-/// A system channel's membership notices are stored kind:44100/44101 with
-/// `["t","system"]`, and only a filter that asks with `#t:["system"]` sees
-/// them. Every other filter, on every path, sees only the ordinary channel's
-/// notice, as it does today: live fan-out, REQ history, COUNT, and the HTTP
-/// bridge.
-/// Mutation: drop the tag in `emit_membership_notification` → RED (the
-/// opt-in filter finds nothing, and old filters see the system channel);
-/// drop the `is_system_notice` check in `filters_match` → RED (live and
-/// post-filtered reads show it); drop `push_system_notice_exclusion` → RED
-/// (COUNT counts it).
+/// Every membership notice is a stored kind:44100/44101 tagged with its
+/// channel type (`["t","stream"]`, `["t","system"]`). A filter sees a notice
+/// only when it knows the type: a filter without `#t` (every client today)
+/// sees the legacy types, and a filter with `#t` sees the types it names. One
+/// filter can name all types. This holds on every path: live fan-out, REQ
+/// history, COUNT, and the HTTP bridge. An untagged notice stored before the
+/// tag existed counts as a legacy type.
+/// Mutation: drop the type tag in `emit_membership_notification` → RED;
+/// drop the `notice_visible` check in `filters_match` → RED (live and
+/// post-filtered reads show the system notice); drop the untagged clause in
+/// `push_notice_visibility` → RED (the legacy notice disappears).
 #[tokio::test]
 #[ignore = "requires Postgres and Redis"]
-async fn system_channel_notices_reach_only_filters_that_ask() {
+async fn membership_notices_reach_only_filters_that_know_their_type() {
     use buzz_core::kind::{KIND_MEMBER_ADDED_NOTIFICATION, KIND_MEMBER_REMOVED_NOTIFICATION};
     let f = fixture().await;
     let owner = Keys::generate();
@@ -452,9 +453,14 @@ async fn system_channel_notices_reach_only_filters_that_ask() {
     ];
     let old = json!({"kinds": kinds, "#p": [member_hex]});
     let asks = json!({"kinds": kinds, "#p": [member_hex], "#t": ["system"]});
+    let all = json!({
+        "kinds": kinds,
+        "#p": [member_hex],
+        "#t": ["stream", "forum", "dm", "workflow", "system"],
+    });
 
     // The member's live global subscriptions: one as clients send today, one
-    // that asks for system notices. An outsider also holds a subscription
+    // that asks for system notices, and one that names every type. An outsider also holds a subscription
     // that asks. It is registered directly, as if some filter (for example a
     // kindless `ids` lookup) had been admitted, so only the fan-out gate can
     // keep the notices from it.
@@ -490,7 +496,7 @@ async fn system_channel_notices_reach_only_filters_that_ask() {
         }
         rx
     };
-    let mut rx = register(&member, &[("old", &old), ("asks", &asks)]);
+    let mut rx = register(&member, &[("old", &old), ("asks", &asks), ("all", &all)]);
     let outsider = Keys::generate();
     let mut outsider_rx = register(&outsider, &[("outsider", &json!({"#t": ["system"]}))]);
 
@@ -515,10 +521,10 @@ async fn system_channel_notices_reach_only_filters_that_ask() {
     )
     .await;
 
-    // Notifications are sent after the reply; collect the three frames.
+    // Notifications are sent after the reply; collect the six frames.
     let mut live = Vec::new();
     let mut ids = std::collections::HashMap::new();
-    while live.len() < 3 {
+    while live.len() < 6 {
         let frame = tokio::time::timeout(std::time::Duration::from_secs(5), rx.recv())
             .await
             .unwrap_or_else(|_| panic!("notices so far: {live:?}"))
@@ -542,6 +548,17 @@ async fn system_channel_notices_reach_only_filters_that_ask() {
             .find(|tag| tag.as_slice().first().map(String::as_str) == Some("h"))
             .and_then(|tag| tag.as_slice().get(1).cloned())
             .expect("h tag");
+        let channel_type = event
+            .tags
+            .iter()
+            .find(|tag| tag.as_slice().first().map(String::as_str) == Some("t"))
+            .and_then(|tag| tag.as_slice().get(1).cloned());
+        let expected_type = if h == system.to_string() {
+            "system"
+        } else {
+            "stream"
+        };
+        assert_eq!(channel_type.as_deref(), Some(expected_type), "type tag");
         let kind = u32::from(event.kind.as_u16());
         ids.insert((kind, h.clone()), event.id.to_hex());
         live.push((message[1].as_str().expect("sub id").to_owned(), kind, h));
@@ -550,7 +567,7 @@ async fn system_channel_notices_reach_only_filters_that_ask() {
         tokio::time::timeout(std::time::Duration::from_millis(200), rx.recv())
             .await
             .is_err(),
-        "no fourth notice"
+        "no seventh notice"
     );
     assert_eq!(
         sorted(live),
@@ -570,6 +587,21 @@ async fn system_channel_notices_reach_only_filters_that_ask() {
                 KIND_MEMBER_ADDED_NOTIFICATION,
                 ordinary.to_string()
             ),
+            (
+                "all".into(),
+                KIND_MEMBER_ADDED_NOTIFICATION,
+                system.to_string()
+            ),
+            (
+                "all".into(),
+                KIND_MEMBER_REMOVED_NOTIFICATION,
+                system.to_string()
+            ),
+            (
+                "all".into(),
+                KIND_MEMBER_ADDED_NOTIFICATION,
+                ordinary.to_string()
+            ),
         ])
     );
     let ordinary_added = ids[&(KIND_MEMBER_ADDED_NOTIFICATION, ordinary.to_string())].clone();
@@ -605,6 +637,56 @@ async fn system_channel_notices_reach_only_filters_that_ask() {
     let mut since = asks.clone();
     since["since"] = json!(0);
     assert_eq!(sorted(req(&f, &member, since).await), system_notices);
+    // One filter that names every type reads all three notices.
+    let every_notice = {
+        let mut ids = system_notices.clone();
+        ids.push(ordinary_added.clone());
+        sorted(ids)
+    };
+    assert_eq!(sorted(req(&f, &member, all.clone()).await), every_notice);
+    assert_eq!(count(&f, &member, all.clone()).await, 3);
+    assert_eq!(
+        sorted(bridge_ids(
+            bridge(&f, &member, "/query", json!([all])).await
+        )),
+        every_notice
+    );
+
+    // A notice stored before notices had a type tag counts as a legacy type:
+    // filters without `#t`, and filters that name a legacy type, read it.
+    let legacy_channel = Uuid::new_v4().to_string();
+    let legacy = nostr::EventBuilder::new(
+        nostr::Kind::Custom(KIND_MEMBER_ADDED_NOTIFICATION as u16),
+        "",
+    )
+    .tags([
+        nostr::Tag::parse(["p", &member_hex]).unwrap(),
+        nostr::Tag::parse(["h", &legacy_channel]).unwrap(),
+    ])
+    .sign_with_keys(&f.state.relay_keypair)
+    .expect("sign");
+    f.state
+        .db
+        .insert_event(f.tenant.community(), &legacy, None)
+        .await
+        .expect("store legacy notice");
+    let legacy_id = legacy.id.to_hex();
+    let with_legacy = |ids: &[String]| {
+        let mut ids = ids.to_vec();
+        ids.push(legacy_id.clone());
+        sorted(ids)
+    };
+    assert_eq!(
+        sorted(req(&f, &member, old.clone()).await),
+        with_legacy(std::slice::from_ref(&ordinary_added))
+    );
+    assert_eq!(count(&f, &member, old.clone()).await, 2);
+    assert_eq!(
+        sorted(req(&f, &member, all.clone()).await),
+        with_legacy(&every_notice)
+    );
+    assert_eq!(count(&f, &member, all.clone()).await, 4);
+    assert_eq!(sorted(req(&f, &member, asks.clone()).await), system_notices);
 
     // Knowing a notice's id is not authorization: only its target reads,
     // counts or receives it, with or without `kinds`.
@@ -641,12 +723,13 @@ async fn system_channel_notices_reach_only_filters_that_ask() {
     assert_eq!(sorted(req(&f, &member, by_id(false)).await), system_notices);
 }
 
-/// The database hides system notices unless the query asks, so a read path
-/// that builds its own `EventQuery` and forgets the rule still hides them.
-/// Mutation: drop `push_system_notice_exclusion` → RED.
+/// The database returns only legacy-type notices unless the query names
+/// other types, so a read path that builds its own `EventQuery` and forgets
+/// the rule still hides system notices.
+/// Mutation: drop `push_notice_visibility` → RED.
 #[tokio::test]
 #[ignore = "requires Postgres and Redis"]
-async fn the_database_hides_system_notices_by_default() {
+async fn the_database_returns_only_legacy_type_notices_by_default() {
     use buzz_core::kind::KIND_MEMBER_ADDED_NOTIFICATION;
     let f = fixture().await;
     let owner = Keys::generate();
@@ -694,7 +777,7 @@ async fn the_database_hides_system_notices_by_default() {
             f.state
                 .db
                 .query_events(&EventQuery {
-                    include_system_notices: true,
+                    notice_channel_types: Some(vec!["stream".into(), "system".into()]),
                     ..base.clone()
                 })
                 .await
@@ -720,6 +803,18 @@ async fn the_database_hides_system_notices_by_default() {
         );
         assert_eq!(f.state.db.count_events(&query).await.expect("count"), 1);
     }
+    let only_system = EventQuery {
+        notice_channel_types: Some(vec!["system".into()]),
+        ..base.clone()
+    };
+    assert_eq!(
+        channels_of(f.state.db.query_events(&only_system).await.expect("query")),
+        [system.to_string()]
+    );
+    assert_eq!(
+        f.state.db.count_events(&only_system).await.expect("count"),
+        1
+    );
 }
 
 /// A group-state filter with `#t:["system"]` or any `#P` finds the system
diff --git a/docs/nips/NIP-CL.md b/docs/nips/NIP-CL.md
index 864e006060..b7c415a73d 100644
--- a/docs/nips/NIP-CL.md
+++ b/docs/nips/NIP-CL.md
@@ -188,29 +188,44 @@ does not add it to another filter in the same request.
 
 The relay MUST NOT send push notifications for system channels.
 
-### Membership notices for system channels
+### Membership notices carry the channel type
 
-When a member is added to or removed from a system channel, the relay sends
-the usual kind:44100 or kind:44101 notice with one more tag,
-`["t", "system"]`. It stores the notice like any other 44100/44101.
+Every kind:44100 (added) and kind:44101 (removed) notice has one more tag,
+`["t", <channel_type>]`, for example `["t", "stream"]` or `["t", "system"]`.
+The relay sets it from the channel type when it sends the notice. A channel's
+type does not change. The relay stores the notice like any other 44100/44101.
 
-A client that does not know system channels would show the channel from such
-a notice. So the relay MUST return a notice with `["t", "system"]` only to a
-filter whose `#t` has the value `system`. This applies on every path: REQ
-history, live delivery, COUNT, and any relay-specific query API. Existing
-clients do not send `#t` on these kinds, so they see no change.
+A client sees a notice only if it knows the notice's channel type. Old
+clients do not know types added later, such as `system`, and would show the
+channel from such a notice. So the relay MUST apply this rule:
 
-A client that knows system channels sends one more filter:
+- **A filter with `#t`** matches a notice only when the notice's type is one
+  of the `#t` values.
+- **A filter without `#t`** matches a notice only when its type is one of the
+  **legacy types**: `stream`, `forum`, `dm`, `workflow`. This list is fixed.
+  A type added later is never on it, so old clients never see its notices.
+- **A notice without a `t` tag** was stored before this rule. Every such
+  notice is about a legacy type, so the relay treats it as all of them: it
+  matches a filter without `#t`, and a filter whose `#t` has a legacy type.
+  On these two kinds, this replaces the plain NIP-01 `#t` match.
+
+The rule applies on every path: REQ history, live delivery, COUNT, and any
+relay-specific query API. Existing clients do not send `#t` on these kinds,
+so they see no change.
+
+A client that knows every current type sends one filter:
 
 ```json
-{"kinds": [44100, 44101], "#p": ["<me>"], "#t": ["system"]}
+{"kinds": [44100, 44101], "#p": ["<me>"], "#t": ["stream", "forum", "dm", "workflow", "system"]}
 ```
 
+When Buzz adds a type, a client adds its value to `#t` when it supports that
+type.
+
 The `#p` rule for 44100/44101 still applies: it must be the reader's own
 pubkey. An `ids` filter does not lift it: the relay returns, counts or
 delivers a 44100/44101 only to the pubkey in its `p` tag, even to a reader
-who knows the event id. The tag comes from the channel type when the relay
-sends the notice.
+who knows the event id.
 
 Because the notices are stored, a client that reconnects can set `since` to
 the newest notice it has seen. One subscription then returns the notices it