diff --git a/src/channels.rs b/src/channels.rs
index 5eaf800..e5562a4 100644
--- a/src/channels.rs
+++ b/src/channels.rs
@@ -148,6 +148,7 @@ pub struct ChanModes {
     pub nickflood: Option<Rate>,     // +F
     pub redirect: Option<String>,    // +L <#target> — when full, send there
     pub history: Option<(u32, u64)>, // +H <lines>:<secs> — replay recent messages to joiners
+    pub anticaps: Option<u8>,        // +B <percent> — block messages that are mostly CAPS
 }
 
 impl ChanModes {
diff --git a/src/coremods/core_message.rs b/src/coremods/core_message.rs
index f85519c..c392995 100644
--- a/src/coremods/core_message.rs
+++ b/src/coremods/core_message.rs
@@ -119,6 +119,18 @@ fn apply_censor(body: &str, censor: &[(String, String)]) -> Option<String> {
     Some(out)
 }
 
+/// Percentage of the ASCII letters in `t` that are uppercase, or `None` when there
+/// are too few letters to judge (so short shouts like "OK" aren't blocked). Used
+/// by the +B anticaps channel mode.
+fn caps_percent(t: &str) -> Option<u8> {
+    let letters = t.chars().filter(|c| c.is_ascii_alphabetic()).count();
+    if letters < 8 {
+        return None;
+    }
+    let upper = t.chars().filter(|c| c.is_ascii_uppercase()).count();
+    Some(((upper * 100) / letters) as u8)
+}
+
 pub fn commands() -> Vec<Box<dyn Command>> {
     vec![Box::new(PrivMsg), Box::new(Notice), Box::new(TagMsg)]
 }
@@ -305,6 +317,21 @@ pub(crate) fn deliver(s: &mut Server, uid: Uid, params: &[String], notice: bool)
                 }
             }
         }
+        // +B anticaps — reject a mostly-uppercase message (ops/opers exempt)
+        if !flood_exempt {
+            if let Some(pct) = s.channels.get(&key).and_then(|c| c.modes.anticaps) {
+                if caps_percent(&body).map(|p| p >= pct).unwrap_or(false) {
+                    if !notice {
+                        s.numeric(
+                            uid,
+                            ERR_CANNOTSENDTOCHAN,
+                            &format!("{target} :Cannot send to channel (+B: too many capitals)"),
+                        );
+                    }
+                    return CmdResult::Fail;
+                }
+            }
+        }
         // deliver to every member except the sender and +D (deaf) users, tagging
         // per-recipient (server-time + any client-only tags on the line)
         let line = format!(":{prefix} {cmd} {target} :{body}");
diff --git a/src/mode.rs b/src/mode.rs
index f5c2a86..0860527 100644
--- a/src/mode.rs
+++ b/src/mode.rs
@@ -84,6 +84,7 @@ static CHAN_MODES: &[&(dyn ChanMode + Sync)] = &[
     &NICKFLOOD,
     &REDIRECT,
     &CHANHISTORY,
+    &ANTICAPS,
 ];
 
 // --- prefix modes (+q/+a/+o/+h/+v): a per-member rank, needs a nick ----------
@@ -796,6 +797,44 @@ impl ChanMode for RedirectMode {
     }
 }
 
+/// +B `<percent>` — reject channel messages that are at least `<percent>` uppercase
+/// (InspIRCd `m_anticaps`). Enforced in the message path; ops are exempt.
+struct AntiCapsMode;
+static ANTICAPS: AntiCapsMode = AntiCapsMode;
+impl ChanMode for AntiCapsMode {
+    fn letter(&self) -> char {
+        'B'
+    }
+    fn wants_param(&self, adding: bool) -> bool {
+        adding
+    }
+    fn apply(
+        &self,
+        s: &mut Server,
+        _chan: &str,
+        key: &str,
+        _uid: Uid,
+        adding: bool,
+        param: Option<&str>,
+    ) -> Applied {
+        if adding {
+            let pct = param.and_then(|p| p.parse::<u8>().ok()).filter(|&p| p >= 1);
+            let Some(pct) = pct.map(|p| p.min(100)) else {
+                return Applied::No; // needs a 1..=100 percentage
+            };
+            if let Some(c) = s.channels.get_mut(key) {
+                c.modes.anticaps = Some(pct);
+            }
+            Applied::Yes(Some(pct.to_string()))
+        } else {
+            if let Some(c) = s.channels.get_mut(key) {
+                c.modes.anticaps = None;
+            }
+            Applied::Yes(None)
+        }
+    }
+}
+
 // === user modes ============================================================
 
 /// A user mode (+i/+w/+o) — same handler-object shape as [`ChanMode`], and the
@@ -1020,7 +1059,7 @@ mod tests {
 
     #[test]
     fn registry_covers_all_channel_modes() {
-        for c in "qaohvbeIklmntiszpONCTcSRMfjFLgGu".chars() {
+        for c in "qaohvbeIklmntiszpONCTcSRMfjFLgGuB".chars() {
             assert!(chan_mode(c).is_some(), "missing handler for +{c}");
         }
         assert!(chan_mode('y').is_none());
diff --git a/src/users.rs b/src/users.rs
index c969e0f..a595780 100644
--- a/src/users.rs
+++ b/src/users.rs
@@ -409,7 +409,7 @@ impl Server {
             uid,
             RPL_MYINFO,
             &format!(
-                "{} echoircd-{VERSION} iowxsgBDIHrRz qaohvbeIklimnpstzCTcSNORMfjFLgGu",
+                "{} echoircd-{VERSION} iowxsgBDIHrRz qaohvbeIklimnpstzCTcSNORMfjFLgGuB",
                 self.name
             ),
         );
@@ -417,7 +417,7 @@ impl Server {
             uid,
             RPL_ISUPPORT,
             &format!(
-                "CHANTYPES=# PREFIX=(qaohv)~&@%+ CHANMODES=beIg,k,lfjFLH,CGMNORSTcimnpstuz EXTBAN=,cmn WATCH=128 MONITOR=128 SILENCE=32 CALLERID=g WHOX CHATHISTORY=256 MSGREFTYPES=timestamp,msgid UTF8ONLY CASEMAPPING=ascii NICKLEN=30 CHANNELLEN=50 NETWORK={} :are supported by this server",
+                "CHANTYPES=# PREFIX=(qaohv)~&@%+ CHANMODES=beIg,k,lfjFLHB,CGMNORSTcimnpstuz EXTBAN=,cmn WATCH=128 MONITOR=128 SILENCE=32 CALLERID=g WHOX CHATHISTORY=256 MSGREFTYPES=timestamp,msgid UTF8ONLY CASEMAPPING=ascii NICKLEN=30 CHANNELLEN=50 NETWORK={} :are supported by this server",
                 self.network
             ),
         );