Complete chat with the server's custom chat completions

ClientboundCustomChatCompletionsPacket was only an id constant, so the
names and tags proxies and plugins add with it (players on other backend
servers, chat tags) never showed when Tab completes chat. Decode it, keep
the connection's set like ClientSuggestionProvider.modifyCustomCompletions
(ADD, REMOVE, SET; an unknown action id is ADD), clear it on a
reconfiguration, and complete chat from getCustomTabSuggestions: the
players and the custom completions.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

crates/horizon-client-core/src/registries.rs

@@ -28,6 +28,10 @@ pub struct Registries {
     /// Game rule name -> integer-valued.
     pub game_rules: Vec<(String, bool)>,
     pub players: Vec<String>,
+    /// `ClientSuggestionProvider.customCompletionSuggestions`: the extra chat
+    /// completions the server sent (ClientboundCustomChatCompletionsPacket),
+    /// each once.
+    pub custom_completions: std::collections::BTreeSet<String>,
     /// Scoreboard objective names, team names and tracked score holders.
     pub objectives: Vec<String>,
     pub teams: Vec<String>,
@@ -87,7 +91,55 @@ pub struct RegistryIds {
     pub tags: Vec<String>,
 }
 const EMPTY: &[String] = &[];
+/// ClientboundCustomChatCompletionsPacket.Action.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum CustomCompletionAction {
+    Add,
+    Remove,
+    Set,
+}
+impl CustomCompletionAction {
+    /// `Action.BY_ID`: `ByIdMap.continuous` with `OutOfBoundsStrategy.ZERO`,
+    /// so an unknown id is `ADD`.
+    pub fn from_id(id: i32) -> Self {
+        match id {
+            1 => Self::Remove,
+            2 => Self::Set,
+            _ => Self::Add,
+        }
+    }
+}
+/// `ClientSuggestionProvider.modifyCustomCompletions`: the server adds,
+/// removes or replaces the extra chat completions.
+pub fn modify_custom_completions(
+    set: &mut std::collections::BTreeSet<String>,
+    action: CustomCompletionAction,
+    entries: &[String],
+) {
+    match action {
+        CustomCompletionAction::Add => set.extend(entries.iter().cloned()),
+        CustomCompletionAction::Remove => {
+            for e in entries {
+                set.remove(e);
+            }
+        }
+        CustomCompletionAction::Set => {
+            set.clear();
+            set.extend(entries.iter().cloned());
+        }
+    }
+}
 impl Registries {
+    /// `ClientSuggestionProvider.getCustomTabSuggestions`: what chat (not a
+    /// command) completes, the online players' names with the server's custom
+    /// completions, each once.
+    pub fn custom_tab_suggestions(&self) -> std::collections::BTreeSet<&str> {
+        self.players
+            .iter()
+            .chain(&self.custom_completions)
+            .map(String::as_str)
+            .collect()
+    }
     /// Fill what the game did not build itself from vanilla's exported
     /// registries (`command-registries.json`, written by
     /// `VanillaCommandParseOracle registries`): built-in registries such as

crates/horizon-client-render/src/chat.rs

@@ -929,7 +929,9 @@ impl ChatScreen {
         } else if !command.chars().all(char::is_whitespace) {
             let partial = &command[..cursor_b];
             let mut builder = SuggestionsBuilder::new(partial, last_word_index(partial));
-            builder.suggest_matching(c.registries.players.iter().map(String::as_str));
+            // ClientSuggestionProvider.getCustomTabSuggestions: the players
+            // and the server's custom chat completions.
+            builder.suggest_matching(c.registries.custom_tab_suggestions());
             let (range, list) = merge_suggestions(partial, builder.out);
             self.awaiting = None;
             self.pending = Some((range.0, list));

crates/horizon-client-render/src/chat_tests.rs

@@ -244,6 +244,58 @@ fn mod_commands_are_coloured_and_complete_like_vanilla_ones() {
     assert_eq!(screen.value(), "/coords");
 }
 
+#[test]
+fn chat_completes_the_servers_custom_completions_with_the_players() {
+    // CommandSuggestions.updateCommandInfo completes chat from
+    // ClientSuggestionProvider.getCustomTabSuggestions: the online players
+    // and what ClientboundCustomChatCompletionsPacket added.
+    use crate::command::{CustomCompletionAction as Action, modify_custom_completions};
+    let font = font();
+    let m = Measure(&font);
+    let mut registries = Registries {
+        players: vec!["Steve".into()],
+        ..Registries::default()
+    };
+    let entries = |e: &[&str]| e.iter().map(|s| s.to_string()).collect::<Vec<_>>();
+    let set = &mut registries.custom_completions;
+    modify_custom_completions(set, Action::from_id(0), &entries(&["Alex", "#trade"]));
+    modify_custom_completions(set, Action::Add, &entries(&["Notch", "Steve"]));
+    modify_custom_completions(set, Action::Remove, &entries(&["Alex"]));
+    // An id past the last action is ADD (OutOfBoundsStrategy.ZERO).
+    modify_custom_completions(set, Action::from_id(7), &entries(&["Zuri"]));
+    assert_eq!(
+        registries
+            .custom_tab_suggestions()
+            .into_iter()
+            .collect::<Vec<_>>(),
+        ["#trade", "Notch", "Steve", "Zuri"]
+    );
+    let completed = |registries: &Registries, line: &str| {
+        let dispatcher = crate::command::vanilla::game_dispatcher();
+        let c = Commands {
+            dispatcher,
+            registries,
+        };
+        let mut screen = ChatScreen::new(&m, &c, "", 0, 640, 360, full());
+        screen.type_text(&m, &c, line);
+        // Chat shows no list by itself: Tab shows it, Tab again uses it.
+        for _ in 0..2 {
+            screen.key(&m, &c, &[], winit::keyboard::KeyCode::Tab, false, false);
+        }
+        screen.value().to_owned()
+    };
+    assert_eq!(completed(&registries, "hi no"), "hi Notch");
+    assert_eq!(completed(&registries, "sell #tr"), "sell #trade");
+    assert_eq!(completed(&registries, "hi st"), "hi Steve");
+    assert_eq!(completed(&registries, "hi al"), "hi al");
+    // SET replaces them all; the players stay.
+    let set = &mut registries.custom_completions;
+    modify_custom_completions(set, Action::Set, &entries(&["Alex"]));
+    assert_eq!(completed(&registries, "hi no"), "hi no");
+    assert_eq!(completed(&registries, "hi al"), "hi Alex");
+    assert_eq!(completed(&registries, "hi st"), "hi Steve");
+}
+
 #[test]
 fn ask_server_nodes_wait_for_the_servers_completions() {
     // ClientSuggestionProvider.customSuggestion: an `ask_server` node at the

crates/horizon-client/src/game/chat.rs

@@ -474,6 +474,7 @@ impl Game<'_> {
         if let Some(names) = self.remote_online_names() {
             self.ui.registries.players = names;
         }
+        self.ui.registries.custom_completions = self.remote_custom_completions();
     }
     /// `ChatComponent.createScreen`: T or / opens the chat with the draft kept
     /// last time, when this method can restore it (a command draft only for

crates/horizon-client/src/game/remote.rs

@@ -183,6 +183,9 @@ pub(super) struct Remote {
     chat_previous: Option<Instant>,
     /// The server's command tree (ClientboundCommandsPacket).
     pub commands: Option<horizon_client::command::Dispatcher>,
+    /// `ClientSuggestionProvider.customCompletionSuggestions`: the extra chat
+    /// completions this server sent (ClientboundCustomChatCompletionsPacket).
+    custom_completions: std::collections::BTreeSet<String>,
     players: HashMap<u128, String>,
     /// `ClientPacketListener.playerInfoMap` as player-list entries, and
     /// which of them are listed.
@@ -518,6 +521,7 @@ impl Game<'_> {
             chat_queue: VecDeque::new(),
             chat_previous: None,
             commands: None,
+            custom_completions: Default::default(),
             players: HashMap::new(),
             tab_players: HashMap::new(),
             skin_textures: HashMap::new(),
@@ -908,6 +912,14 @@ impl Game<'_> {
         )
     }
 
+    /// `ClientSuggestionProvider.customCompletionSuggestions` of the server
+    /// connection (none without one).
+    pub(super) fn remote_custom_completions(&self) -> std::collections::BTreeSet<String> {
+        self.remote
+            .as_ref()
+            .map(|r| r.custom_completions.clone())
+            .unwrap_or_default()
+    }
     /// The names in the server's player list, sorted.
     pub(super) fn remote_online_names(&self) -> Option<Vec<String>> {
         let r = self.remote.as_ref()?;
@@ -1049,14 +1061,18 @@ impl Game<'_> {
                 }
                 Event::Reconfigured(registries) => {
                     // handleConfigurationStart → clearClientLevel, and the next
-                    // ClientPacketListener starts with no players or scoreboard.
+                    // ClientPacketListener starts with no players or scoreboard,
+                    // and its new ClientSuggestionProvider with no custom
+                    // chat completions.
                     self.hud_disconnected();
                     self.score.board = Default::default();
                     if let Some(r) = self.remote.as_mut() {
                         r.players.clear();
                         r.tab_players.clear();
                         r.listed.clear();
+                        r.custom_completions.clear();
                     }
+                    self.ui.registries.custom_completions.clear();
                     self.set_registries(registries);
                 }
                 // handleTransfer: "Cannot transfer to server from
@@ -3052,6 +3068,17 @@ impl Game<'_> {
             {
                 self.handle_command_suggestions(&data);
             }
+            C::CustomChatCompletions(p) => {
+                // ClientPacketListener.handleCustomChatCompletions.
+                if let Some(r) = self.remote.as_mut() {
+                    horizon_client::command::modify_custom_completions(
+                        &mut r.custom_completions,
+                        horizon_client::command::CustomCompletionAction::from_id(p.action),
+                        &p.entries,
+                    );
+                    self.ui.registries.custom_completions = r.custom_completions.clone();
+                }
+            }
             C::Commands(p) => {
                 let tree = horizon_client::command::network::from_packet(
                     &p,

crates/horizon-command/src/args.rs

@@ -2,7 +2,9 @@
 //! the vanilla error messages and completion suggestions.
 use super::{CmdResult, CommandError, Context, Reader, SuggestionsBuilder};
 
-pub use horizon_client_core::registries::{Registries, RegistryIds};
+pub use horizon_client_core::registries::{
+    CustomCompletionAction, Registries, RegistryIds, modify_custom_completions,
+};
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq)]
 pub enum Resource {

crates/horizon-command/src/lib.rs

@@ -22,7 +22,7 @@ pub mod tree_world;
 pub mod vanilla;
 mod vanilla_order;
 
-pub use args::{Arg, ArgType, Registries};
+pub use args::{Arg, ArgType, CustomCompletionAction, Registries, modify_custom_completions};
 use horizon_client_core::text::Text;
 
 /// The command id of a mod's command (`Dispatcher::with_mod_commands`).

crates/horizon-protocol/src/packets/play.rs

@@ -211,6 +211,18 @@ packet!(ContainerSetData { container: i32 => VarInt, id: i16 => Short, value: i1
 // ClientboundCooldownPacket: the cooldown group (an identifier) and the ticks
 // its entry lasts (0 ends it).
 packet!(Cooldown { group: String => Ident, duration: i32 => VarInt });
+packet!(
+    /// ClientboundCustomChatCompletionsPacket: how the server changes the
+    /// extra chat completions (`custom_chat_action`) and the entries.
+    CustomChatCompletions { action: i32 => VarInt, entries: Vec<String> => List<Str> }
+);
+/// ClientboundCustomChatCompletionsPacket.Action, by id (`ByIdMap.continuous`
+/// with `OutOfBoundsStrategy.ZERO`: an unknown id is `ADD`).
+pub mod custom_chat_action {
+    pub const ADD: i32 = 0;
+    pub const REMOVE: i32 = 1;
+    pub const SET: i32 = 2;
+}
 packet!(ContainerSetSlot { container: i32 => VarInt, state: i32 => VarInt, slot: i16 => Short, item: Option<Stack> => Slot });
 /// A holder reference: registry id + 1, or 0 for an inline value (unsupported).
 pub struct HolderId;
@@ -1840,6 +1852,7 @@ packet_enum!(Clientbound, crate::ids::play::clientbound {
     ContainerSetSlot(ContainerSetSlot) = CONTAINER_SET_SLOT,
     CookieRequest(CookieRequest) = COOKIE_REQUEST,
     Cooldown(Cooldown) = COOLDOWN,
+    CustomChatCompletions(CustomChatCompletions) = CUSTOM_CHAT_COMPLETIONS,
     CustomPayload(CustomPayload) = CUSTOM_PAYLOAD,
     Disconnect(Disconnect) = DISCONNECT,
     DisguisedChat(DisguisedChat) = DISGUISED_CHAT,
@@ -2401,6 +2414,34 @@ mod tests {
         }
     }
 
+    #[test]
+    fn custom_chat_completions_are_an_action_and_a_string_list() {
+        // ClientboundCustomChatCompletionsPacket.STREAM_CODEC: Action.STREAM_CODEC
+        // (ByteBufCodecs.idMapper, a VarInt), then STRING_UTF8 in a list.
+        let payload = b"Steve#tag";
+        let packet = Clientbound::CustomChatCompletions(CustomChatCompletions {
+            action: custom_chat_action::SET,
+            entries: vec!["Steve".into(), "#tag".into()],
+        });
+        assert_eq!(
+            Clientbound::decode(
+                crate::ids::play::clientbound::CUSTOM_CHAT_COMPLETIONS,
+                payload
+            )
+            .unwrap(),
+            packet
+        );
+        assert_eq!(&packet.encode()[1..], payload);
+        round_trip(packet, Clientbound::encode, Clientbound::decode);
+        assert!(
+            Clientbound::decode(
+                crate::ids::play::clientbound::CUSTOM_CHAT_COMPLETIONS,
+                &payload[..payload.len() - 1]
+            )
+            .is_err()
+        );
+    }
+
     #[test]
     fn serverbound_abilities_carry_only_flying() {
         // ServerboundPlayerAbilitiesPacket.write: one byte, 2 when flying.

tools/port_map/client-002.tsv

@@ -65,7 +65,7 @@ ClientPacketListener.handleSetBorderSize	port	crates/horizon-client/src/game/rem
 ClientPacketListener.handleSetBorderWarningDistance	port	crates/horizon-client/src/game/remote.rs::Game::handle_packet	SetBorderWarningDistance arm
 ClientPacketListener.handleSetBorderWarningDelay	port	crates/horizon-client/src/game/remote.rs::Game::handle_packet	SetBorderWarningDelay arm
 ClientPacketListener.handleTitlesClear	port	crates/horizon-client/src/game/remote.rs::Game::handle_packet	ClearTitles arm: clear, optionally reset the times
-ClientPacketListener.handleCustomChatCompletions	missing		minor: server-sent custom chat completions are not used
+ClientPacketListener.handleCustomChatCompletions	port	crates/horizon-client/src/game/remote.rs::Game::handle_packet	CustomChatCompletions arm: modify_custom_completions on the connection's set
 ClientPacketListener.setTitleText	port	crates/horizon-client/src/game/remote.rs::Game::handle_packet	SetTitleText arm
 ClientPacketListener.setSubtitleText	port	crates/horizon-client/src/game/remote.rs::Game::handle_packet	SetSubtitleText arm
 ClientPacketListener.setTitlesAnimation	port	crates/horizon-client/src/game/remote.rs::Game::handle_packet	SetTitlesAnimation arm
@@ -116,12 +116,12 @@ ClientPacketListener.notifyPlayerLoaded	port	crates/horizon-client/src/game/remo
 ClientPacketListener.setKeyPair	missing		player: no chat key pair is ever set, so chat and commands stay unsigned
 ClientRegistryLayer.createRegistryAccess	skip:data		registry layers come from Horizon's own tables
 ClientSuggestionProvider.getOnlinePlayerNames	port	crates/horizon-client/src/game/remote.rs::Game::remote_online_names	sorted names of the server's player list
-ClientSuggestionProvider.getCustomTabSuggestions	missing		minor: server-sent custom completions are not used
+ClientSuggestionProvider.getCustomTabSuggestions	port	crates/horizon-client-core/src/registries.rs::Registries::custom_tab_suggestions	the player names and the server's custom completions, each once
 ClientSuggestionProvider.getSelectedEntities	missing		minor: no suggestion for the entity under the crosshair
 ClientSuggestionProvider.suggestRegistryElements	port	crates/horizon-command/src/args.rs::suggest_resources	registry ids come from Horizon's registry tables
 ClientSuggestionProvider.getRelevantCoordinates	port	crates/horizon-command/src/args.rs::suggest_coordinates	the block under the crosshair for block positions
 ClientSuggestionProvider.getAbsoluteCoordinates	port	crates/horizon-command/src/args.rs::suggest_coordinates	gap: minor: the exact hit point is not offered; Vec3 arguments only get ~
-ClientSuggestionProvider.modifyCustomCompletions	missing		minor: server-sent custom completions are not stored
+ClientSuggestionProvider.modifyCustomCompletions	port	crates/horizon-client-core/src/registries.rs::modify_custom_completions	ADD, REMOVE or SET on the set the connection keeps
 KnownPacksManager.trySelectingPacks	skip:data		Horizon answers SelectKnownPacks with no packs, so the server sends full registries (net.rs configure)
 LegacyServerPinger.channelActive	skip:other		legacy pre-1.7 server-list ping, not used by this client
 LegacyServerPinger.channelRead0	skip:other		legacy pre-1.7 server-list ping, not used by this client