feat(matrix): bot only responds when directly addressed

Story 191: Matrix bot should only respond when directly addressed.
The bot now checks if it was mentioned by name or replied to before
responding, preventing it from answering every message in a room.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/matrix/bot.rs

@@ -7,13 +7,14 @@ use matrix_sdk::{
     event_handler::Ctx,
     room::Room,
     ruma::{
-        OwnedRoomId, OwnedUserId,
+        OwnedEventId, OwnedRoomId, OwnedUserId,
         events::room::message::{
-            MessageType, OriginalSyncRoomMessageEvent, RoomMessageEventContent,
+            MessageType, OriginalSyncRoomMessageEvent, Relation,
+            RoomMessageEventContent, RoomMessageEventContentWithoutRelation,
         },
     },
 };
-use std::collections::HashMap;
+use std::collections::{HashMap, HashSet};
 use std::path::PathBuf;
 use std::sync::Arc;
 use std::sync::atomic::{AtomicBool, Ordering};
@@ -66,6 +67,10 @@ pub struct BotContext {
     pub history: ConversationHistory,
     /// Maximum number of entries to keep per room before trimming the oldest.
     pub history_size: usize,
+    /// Event IDs of messages the bot has sent.  Used to detect replies to the
+    /// bot so it can continue a conversation thread without requiring an
+    /// explicit `@mention` on every follow-up.
+    pub bot_sent_event_ids: Arc<TokioMutex<HashSet<OwnedEventId>>>,
 }
 
 // ---------------------------------------------------------------------------
@@ -154,6 +159,7 @@ pub async fn run_bot(config: BotConfig, project_root: PathBuf) -> Result<(), Str
         allowed_users: config.allowed_users,
         history: Arc::new(TokioMutex::new(HashMap::new())),
         history_size: config.history_size,
+        bot_sent_event_ids: Arc::new(TokioMutex::new(HashSet::new())),
     };
 
     // Register event handler and inject shared context
@@ -171,6 +177,64 @@ pub async fn run_bot(config: BotConfig, project_root: PathBuf) -> Result<(), Str
     Ok(())
 }
 
+// ---------------------------------------------------------------------------
+// Address-filtering helpers
+// ---------------------------------------------------------------------------
+
+/// Returns `true` if `body` contains a mention of the bot.
+///
+/// Two forms are recognised:
+/// - The bot's full Matrix user ID (e.g. `@timmy:homeserver.local`)
+/// - The bot's local part prefixed with `@` (e.g. `@timmy`)
+///
+/// A short mention (`@timmy`) is only counted when it is not immediately
+/// followed by an alphanumeric character, hyphen, or underscore — this avoids
+/// false-positives where a longer username (e.g. `@timmybot`) shares the same
+/// prefix.
+pub fn mentions_bot(body: &str, bot_user_id: &OwnedUserId) -> bool {
+    let full_id = bot_user_id.as_str();
+    if body.contains(full_id) {
+        return true;
+    }
+
+    let short = format!("@{}", bot_user_id.localpart());
+    let mut start = 0;
+    while let Some(rel) = body[start..].find(short.as_str()) {
+        let abs = start + rel;
+        let after = abs + short.len();
+        let next = body[after..].chars().next();
+        let is_word_end =
+            next.is_none_or(|c| !c.is_alphanumeric() && c != '-' && c != '_');
+        if is_word_end {
+            return true;
+        }
+        start = abs + 1;
+    }
+    false
+}
+
+/// Returns `true` if the message's `relates_to` field references an event that
+/// the bot previously sent (i.e. the message is a reply or thread-reply to a
+/// bot message).
+async fn is_reply_to_bot(
+    relates_to: Option<&Relation<RoomMessageEventContentWithoutRelation>>,
+    bot_sent_event_ids: &TokioMutex<HashSet<OwnedEventId>>,
+) -> bool {
+    let candidate_ids: Vec<&OwnedEventId> = match relates_to {
+        Some(Relation::Reply { in_reply_to }) => vec![&in_reply_to.event_id],
+        Some(Relation::Thread(thread)) => {
+            let mut ids = vec![&thread.event_id];
+            if let Some(irti) = &thread.in_reply_to {
+                ids.push(&irti.event_id);
+            }
+            ids
+        }
+        _ => return false,
+    };
+    let guard = bot_sent_event_ids.lock().await;
+    candidate_ids.iter().any(|id| guard.contains(*id))
+}
+
 // ---------------------------------------------------------------------------
 // Event handler
 // ---------------------------------------------------------------------------
@@ -215,12 +279,29 @@ async fn on_room_message(
     }
 
     // Only handle plain text messages.
-    let MessageType::Text(text_content) = ev.content.msgtype else {
-        return;
+    let body = match &ev.content.msgtype {
+        MessageType::Text(t) => t.body.clone(),
+        _ => return,
     };
 
+    // Only respond when the bot is directly addressed (mentioned by name/ID)
+    // or when the message is a reply to one of the bot's own messages.
+    if !mentions_bot(&body, &ctx.bot_user_id)
+        && !is_reply_to_bot(
+            ev.content.relates_to.as_ref(),
+            &ctx.bot_sent_event_ids,
+        )
+        .await
+    {
+        slog!(
+            "[matrix-bot] Ignoring unaddressed message from {}",
+            ev.sender
+        );
+        return;
+    }
+
     let sender = ev.sender.to_string();
-    let user_message = text_content.body.clone();
+    let user_message = body;
     slog!("[matrix-bot] Message from {sender}: {user_message}");
 
     // Spawn a separate task so the Matrix sync loop is not blocked while we
@@ -292,12 +373,16 @@ async fn handle_message(
     // Spawn a task to post messages to Matrix as they arrive so we don't
     // block the LLM stream while waiting for Matrix send round-trips.
     let post_room = room.clone();
+    let sent_ids = Arc::clone(&ctx.bot_sent_event_ids);
     let post_task = tokio::spawn(async move {
         while let Some(chunk) = msg_rx.recv().await {
             let html = markdown_to_html(&chunk);
-            let _ = post_room
+            if let Ok(response) = post_room
                 .send(RoomMessageEventContent::text_html(chunk, html))
-                .await;
+                .await
+            {
+                sent_ids.lock().await.insert(response.event_id);
+            }
         }
     });
 
@@ -439,6 +524,112 @@ pub fn drain_complete_paragraphs(buffer: &mut String) -> Vec<String> {
 mod tests {
     use super::*;
 
+    // -- mentions_bot -------------------------------------------------------
+
+    fn make_user_id(s: &str) -> OwnedUserId {
+        s.parse().unwrap()
+    }
+
+    #[test]
+    fn mentions_bot_by_full_id() {
+        let uid = make_user_id("@timmy:homeserver.local");
+        assert!(mentions_bot("hello @timmy:homeserver.local can you help?", &uid));
+    }
+
+    #[test]
+    fn mentions_bot_by_localpart_at_start() {
+        let uid = make_user_id("@timmy:homeserver.local");
+        assert!(mentions_bot("@timmy please list open stories", &uid));
+    }
+
+    #[test]
+    fn mentions_bot_by_localpart_mid_sentence() {
+        let uid = make_user_id("@timmy:homeserver.local");
+        assert!(mentions_bot("hey @timmy what's the status?", &uid));
+    }
+
+    #[test]
+    fn mentions_bot_not_mentioned() {
+        let uid = make_user_id("@timmy:homeserver.local");
+        assert!(!mentions_bot("can someone help me with this PR?", &uid));
+    }
+
+    #[test]
+    fn mentions_bot_no_false_positive_longer_username() {
+        // "@timmybot" must NOT match "@timmy"
+        let uid = make_user_id("@timmy:homeserver.local");
+        assert!(!mentions_bot("hey @timmybot can you help?", &uid));
+    }
+
+    #[test]
+    fn mentions_bot_at_end_of_string() {
+        let uid = make_user_id("@timmy:homeserver.local");
+        assert!(mentions_bot("shoutout to @timmy", &uid));
+    }
+
+    #[test]
+    fn mentions_bot_followed_by_comma() {
+        let uid = make_user_id("@timmy:homeserver.local");
+        assert!(mentions_bot("@timmy, can you help?", &uid));
+    }
+
+    // -- is_reply_to_bot ----------------------------------------------------
+
+    #[tokio::test]
+    async fn is_reply_to_bot_direct_reply_match() {
+        let sent: Arc<TokioMutex<HashSet<OwnedEventId>>> =
+            Arc::new(TokioMutex::new(HashSet::new()));
+        let event_id: OwnedEventId = "$abc123:example.com".parse().unwrap();
+        sent.lock().await.insert(event_id.clone());
+
+        let in_reply_to = matrix_sdk::ruma::events::relation::InReplyTo::new(event_id);
+        let relates_to: Option<Relation<RoomMessageEventContentWithoutRelation>> =
+            Some(Relation::Reply { in_reply_to });
+
+        assert!(is_reply_to_bot(relates_to.as_ref(), &sent).await);
+    }
+
+    #[tokio::test]
+    async fn is_reply_to_bot_direct_reply_no_match() {
+        let sent: Arc<TokioMutex<HashSet<OwnedEventId>>> =
+            Arc::new(TokioMutex::new(HashSet::new()));
+        // sent is empty — this event was not sent by the bot
+
+        let in_reply_to = matrix_sdk::ruma::events::relation::InReplyTo::new(
+            "$other:example.com".parse::<OwnedEventId>().unwrap(),
+        );
+        let relates_to: Option<Relation<RoomMessageEventContentWithoutRelation>> =
+            Some(Relation::Reply { in_reply_to });
+
+        assert!(!is_reply_to_bot(relates_to.as_ref(), &sent).await);
+    }
+
+    #[tokio::test]
+    async fn is_reply_to_bot_no_relation() {
+        let sent: Arc<TokioMutex<HashSet<OwnedEventId>>> =
+            Arc::new(TokioMutex::new(HashSet::new()));
+        let relates_to: Option<Relation<RoomMessageEventContentWithoutRelation>> = None;
+        assert!(!is_reply_to_bot(relates_to.as_ref(), &sent).await);
+    }
+
+    #[tokio::test]
+    async fn is_reply_to_bot_thread_root_match() {
+        let sent: Arc<TokioMutex<HashSet<OwnedEventId>>> =
+            Arc::new(TokioMutex::new(HashSet::new()));
+        let root_id: OwnedEventId = "$root123:example.com".parse().unwrap();
+        sent.lock().await.insert(root_id.clone());
+
+        // Thread reply where the thread root is the bot's message
+        let thread = matrix_sdk::ruma::events::relation::Thread::plain(
+            root_id,
+            "$latest:example.com".parse::<OwnedEventId>().unwrap(),
+        );
+        let relates_to: Option<Relation<RoomMessageEventContentWithoutRelation>> =
+            Some(Relation::Thread(thread));
+
+        assert!(is_reply_to_bot(relates_to.as_ref(), &sent).await);
+    }
+
     // -- markdown_to_html ---------------------------------------------------
 
     #[test]