diff --git a/crates/collab-net/src/driver.rs b/crates/collab-net/src/driver.rs
index b8f6c154..4ebbec15 100644
--- a/crates/collab-net/src/driver.rs
+++ b/crates/collab-net/src/driver.rs
@@ -16,7 +16,7 @@ use figcraft_engine::live::{Event, Status};
 use web_time::Instant;
 
 use crate::inbox::Inbox;
-use crate::join::{GuestConfig, Next, Outcome, Resuming, Seen, Taken, after, closed, hello, resolved, take, unresolved};
+use crate::join::{GuestConfig, Next, Outcome, Resuming, Seen, Taken, after, closed, hello, resolved, take, too_large, unresolved};
 use crate::throttle::{PRESENCE_INTERVAL, Throttle, earliest};
 
 /// A WebSocket the driver opens, writes and closes; what it receives comes back through the
@@ -114,6 +114,9 @@ impl<S: Socket> Driver<S> {
     /// A binary message from the host.
     pub(crate) fn message(&mut self, bytes: &[u8], now: Instant) {
         let Some(c) = self.conn.as_mut() else { return };
+        if bytes.len() > self.cfg.limits.max_server_message {
+            return self.end(too_large(&self.cfg.limits), now);
+        }
         c.last_heard = now;
         let taken = match message::decode::<ServerMsg>(bytes, self.cfg.limits.max_server_message) {
             // The join code's answer: join the file it opens, on a new connection.
diff --git a/crates/collab-net/src/driver_tests.rs b/crates/collab-net/src/driver_tests.rs
index ca6563c5..5e395b98 100644
--- a/crates/collab-net/src/driver_tests.rs
+++ b/crates/collab-net/src/driver_tests.rs
@@ -10,7 +10,7 @@ use std::time::{Duration, Instant};
 use figcraft_collab::message::{self, ClientMsg, Presence, ServerMsg};
 use figcraft_collab::{Client, Limits, Outgoing, Server};
 use figcraft_doc::{Document, NodeId};
-use figcraft_engine::live::{Event, Status};
+use figcraft_engine::live::{Event, Status, TOO_LARGE_TO_JOIN};
 use tungstenite::protocol::WebSocket;
 use tungstenite::{Bytes, Message};
 
@@ -262,6 +262,15 @@ fn refusals_and_the_end_of_the_session_are_final() {
         assert!(d.done());
         assert!(matches!(statuses(&inbox.drain()).last(), Some(Status::Failed { reason }) if reason.starts_with("The host refused")));
     }
+    // A message over the size limit: the file is too large for this app, not refused by the host.
+    let inbox = Inbox::new(Arc::new(|| {}));
+    let mut cfg = GuestConfig::new(invite(), "Ada".into());
+    cfg.limits.max_server_message = 2 << 20;
+    let mut d = Driver::start(Script::default(), cfg, inbox.clone(), t0);
+    d.opened(t0);
+    d.message(&vec![0; (2 << 20) + 1], t0);
+    assert!(d.done());
+    assert!(matches!(statuses(&inbox.drain()).last(), Some(Status::Failed { reason }) if *reason == TOO_LARGE_TO_JOIN.replace("{mb}", "2")));
 }
 
 #[test]
diff --git a/crates/collab-net/src/guest.rs b/crates/collab-net/src/guest.rs
index eb899b8d..2e1d5046 100644
--- a/crates/collab-net/src/guest.rs
+++ b/crates/collab-net/src/guest.rs
@@ -26,7 +26,7 @@ use tungstenite::{Bytes, Message};
 use crate::inbox::Inbox;
 use crate::invite::{Endpoint, Invite};
 pub use crate::join::GuestConfig;
-use crate::join::{Next, Outcome, Resuming, Seen, Taken, after, closed, hello, resolved, take, unresolved};
+use crate::join::{Next, Outcome, Resuming, Seen, Taken, after, closed, hello, resolved, take, too_large, unresolved};
 use crate::manage;
 use crate::net::{Duplex, NetLimits, config, open, shutdown};
 use crate::throttle::{PRESENCE_INTERVAL, Throttle, earliest};
@@ -367,7 +367,7 @@ fn read_loop(mut ws: WebSocket<Duplex>, ctx: &ReaderCtx) {
             Ok(Message::Close(f)) => break f.map_or_else(|| closed(0, ""), |f| closed(f.code.into(), &f.reason)),
             Ok(Message::Text(_)) => break Outcome::Refused("the host sent a text message".into()),
             Ok(_) => {}
-            Err(tungstenite::Error::Capacity(_)) => break Outcome::Refused("the host sent a message over the size limit".into()),
+            Err(tungstenite::Error::Capacity(_)) => break too_large(&ctx.cfg.limits),
             Err(e) => break Outcome::Retry(e.to_string()),
         }
     };
diff --git a/crates/collab-net/src/join.rs b/crates/collab-net/src/join.rs
index 81cd2437..cc3be839 100644
--- a/crates/collab-net/src/join.rs
+++ b/crates/collab-net/src/join.rs
@@ -7,7 +7,7 @@ use std::time::Duration;
 
 use figcraft_collab::Client;
 use figcraft_collab::message::{ClientMsg, Peer, Resume, ServerMsg};
-use figcraft_engine::live::{Event, Role, SessionInfo, Status};
+use figcraft_engine::live::{Event, Role, SessionInfo, Status, TOO_LARGE_TO_JOIN};
 
 use crate::inbox::Inbox;
 use crate::invite::Invite;
@@ -95,6 +95,12 @@ pub(crate) fn unresolved(cfg: &GuestConfig, outcome: Outcome) -> Outcome {
     })
 }
 
+/// The host sent a message over this app's size limit (the file's welcome, usually): final, and
+/// not the host's doing.
+pub(crate) fn too_large(limits: &NetLimits) -> Outcome {
+    Outcome::Failed(TOO_LARGE_TO_JOIN.replace("{mb}", &(limits.max_server_message >> 20).to_string()))
+}
+
 /// The server's answer to the join code: the invite is whole now.
 pub(crate) fn resolved(cfg: &mut GuestConfig, file: String, token: String) -> Result<(), Outcome> {
     if !crate::invite::valid_id(&file, 1, 64) || !crate::invite::valid_id(&token, 16, 128) {
diff --git a/crates/collab-net/src/loopback_tests.rs b/crates/collab-net/src/loopback_tests.rs
index 32c86bd0..a043d8c7 100644
--- a/crates/collab-net/src/loopback_tests.rs
+++ b/crates/collab-net/src/loopback_tests.rs
@@ -10,7 +10,7 @@ use std::time::{Duration, Instant};
 use figcraft_collab::message::{self, ClientMsg, Hello, Presence, ServerMsg, Signal};
 use figcraft_collab::{Client, Limits};
 use figcraft_doc::{Document, NodeId};
-use figcraft_engine::live::{Event, PEOPLE, Status};
+use figcraft_engine::live::{Event, PEOPLE, Status, TOO_LARGE_TO_JOIN};
 use tungstenite::protocol::WebSocket;
 use tungstenite::{Bytes, Message};
 
@@ -642,3 +642,31 @@ fn the_host_removes_someone_and_they_cant_come_back() {
     host.stop();
     assert_eq!(host.connections(), 0);
 }
+
+#[test]
+fn a_welcome_over_the_size_limit_says_the_file_is_too_large() {
+    let mut doc = Document::new("B".repeat(1 << 20));
+    doc.repair();
+    let probe = Probe::new();
+    let cfg = HostConfig {
+        addr: SocketAddr::from(([127, 0, 0, 1], 0)),
+        any_port_if_taken: false,
+        file: FILE.into(),
+        token: TOKEN.into(),
+        doc,
+        name: "Hana".into(),
+        limits: fast_limits(),
+        core: Limits::default(),
+    };
+    let mut host = Host::start(cfg, probe.inbox.clone()).unwrap();
+    let invite = Invite::new(host.addr().to_string(), FILE, TOKEN);
+
+    let mut guest_cfg = quick(GuestConfig::new(invite, "Ada".into()));
+    guest_cfg.limits = NetLimits { max_server_message: 1 << 20, ..fast_limits() };
+    let mut gp = Probe::new();
+    let _g = Guest::start(guest_cfg, gp.inbox.clone()).unwrap();
+    let too_large = TOO_LARGE_TO_JOIN.replace("{mb}", "1");
+    gp.wait("the file too large to join", |e| e.iter().any(|ev| matches!(ev, Event::Status(Status::Failed { reason }) if *reason == too_large)));
+    assert!(!welcomed(&gp.events));
+    host.stop();
+}
diff --git a/crates/engine/src/live.rs b/crates/engine/src/live.rs
index ed13de5f..cd8a0f03 100644
--- a/crates/engine/src/live.rs
+++ b/crates/engine/src/live.rs
@@ -86,6 +86,10 @@ pub fn parse_color(s: &str) -> Option<(u8, u8, u8)> {
     Some((byte(0)?, byte(2)?, byte(4)?))
 }
 
+/// Why a join failed when a message from the host is over this app's size limit (`{mb}`: the
+/// limit in MB). The UI translates it.
+pub const TOO_LARGE_TO_JOIN: &str = "This file is too large to join here (up to {mb} MB)";
+
 /// The connection to a live session.
 #[derive(Clone, Debug, Default, PartialEq, Serialize)]
 #[serde(rename_all = "camelCase", tag = "state")]
diff --git a/crates/ui-egui/locales/en.toml b/crates/ui-egui/locales/en.toml
index ccf833be..ae566fc8 100644
--- a/crates/ui-egui/locales/en.toml
+++ b/crates/ui-egui/locales/en.toml
@@ -1393,6 +1393,7 @@ name = "English"
 "This condition is edited as an expression" = "This condition is edited as an expression"
 "This file" = "This file"
 "This file has unsaved changes" = "This file has unsaved changes"
+"This file is too large to join here (up to {mb} MB)" = "This file is too large to join here (up to {mb} MB)"
 "This file uses fonts that aren't installed on this computer. Their text is drawn in a fallback font until you install them." = "This file uses fonts that aren't installed on this computer. Their text is drawn in a fallback font until you install them."
 "This file is on {server}, open only to you. Share it to let others in with a new code." = "This file is on {server}, open only to you. Share it to let others in with a new code."
 "This frame is new on the branch." = "This frame is new on the branch."
diff --git a/crates/ui-egui/locales/es.toml b/crates/ui-egui/locales/es.toml
index 1187c658..596feda6 100644
--- a/crates/ui-egui/locales/es.toml
+++ b/crates/ui-egui/locales/es.toml
@@ -1394,6 +1394,7 @@ status = "draft — needs native review"
 "This condition is edited as an expression" = "Esta condición se edita como expresión"
 "This file" = "Este archivo"
 "This file has unsaved changes" = "Este archivo tiene cambios sin guardar"
+"This file is too large to join here (up to {mb} MB)" = "Este archivo es demasiado grande para unirte aquí (hasta {mb} MB)"
 "This file uses fonts that aren't installed on this computer. Their text is drawn in a fallback font until you install them." = "Este archivo usa fuentes que no están instaladas en este equipo. Su texto se dibuja con una fuente de reserva hasta que las instales."
 "This file is on {server}, open only to you. Share it to let others in with a new code." = "Este archivo está en {server}, abierto solo para ti. Compártelo para dejar entrar a otros con un código nuevo."
 "This frame is new on the branch." = "Este marco es nuevo en la rama."
diff --git a/crates/ui-egui/src/i18n_tests.rs b/crates/ui-egui/src/i18n_tests.rs
index fcf21fde..046212c1 100644
--- a/crates/ui-egui/src/i18n_tests.rs
+++ b/crates/ui-egui/src/i18n_tests.rs
@@ -135,6 +135,7 @@ fn dynamic_labels() -> Vec<&'static str> {
     v.extend(Group::ALL.iter().map(|g| g.label()));
     v.extend(GESTURES.iter().flat_map(|(_, label, mac, other)| [*label, *mac, *other]));
     v.extend(DATA_LABELS);
+    v.push(figcraft_engine::live::TOO_LARGE_TO_JOIN);
     // The starter templates' names, descriptions and kinds.
     v.extend(figcraft_engine::templates::TEMPLATES.iter().flat_map(|t| [t.name, t.description, t.kind.label()]));
     v
@@ -327,6 +328,14 @@ fn lookups_fall_back_to_english_and_fill_placeholders() {
     assert!(matches!(lookup(es, k), Some(x) if std::ptr::eq(x, t)));
 }
 
+#[test]
+fn a_live_failure_the_ui_knows_is_read_back_for_translation() {
+    use figcraft_engine::live::TOO_LARGE_TO_JOIN;
+    let reason = TOO_LARGE_TO_JOIN.replace("{mb}", "1024");
+    assert!(matches!(crate::live::failure_text(&reason), std::borrow::Cow::Owned(t) if t == reason));
+    assert!(matches!(crate::live::failure_text("Couldn't reach the server"), std::borrow::Cow::Borrowed(_)));
+}
+
 #[test]
 fn system_languages_pick_the_first_one_with_a_catalog() {
     let code = |pref: &str, system: Option<&str>| LANGS[resolve(pref, system)].code;
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index 84aedd6a..8495c30d 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -1510,7 +1510,7 @@ impl FigApp {
         // What happened in a live session (the network woke us for it).
         if self.session.live_poll() {
             if let Some(m) = self.session.message.take() {
-                self.toast(m);
+                self.toast(live::failure_text(&m));
             }
             self.background_done(&ctx);
         }
diff --git a/crates/ui-egui/src/live.rs b/crates/ui-egui/src/live.rs
index 513060ab..a4baa303 100644
--- a/crates/ui-egui/src/live.rs
+++ b/crates/ui-egui/src/live.rs
@@ -13,10 +13,11 @@
 //! and it fades a few seconds after the last change (frames are asked for only until then).
 //! Spotlight: your avatar's menu asks everyone to follow you; a banner says so and stops it.
 
+use std::borrow::Cow;
 use std::collections::BTreeMap;
 
 use egui::{Align2, Color32, CornerRadius, Pos2, Rect, Sense, Shape, Stroke, Ui, pos2, vec2};
-use figcraft_engine::live::{Access, PEOPLE, Role, SessionId, Share, Signal, Status, Via, initials, parse_color, presence_page};
+use figcraft_engine::live::{Access, PEOPLE, Role, SessionId, Share, Signal, Status, TOO_LARGE_TO_JOIN, Via, initials, parse_color, presence_page};
 use serde_json::{Value, json};
 
 use crate::FigApp;
@@ -595,7 +596,7 @@ pub fn share_popover(app: &mut FigApp, anchor: &egui::Response) {
                 (Status::Reconnecting { attempt }, _) => {
                     (trf("Connection lost, reconnecting (attempt {attempt})…", &[("attempt", attempt)]), t.text_secondary)
                 }
-                (Status::Failed { .. }, _) => (status.label(), t.danger),
+                (Status::Failed { reason }, _) => (failure_text(reason).into_owned(), t.danger),
                 _ => (tr(&status.label()).to_string(), t.text_secondary),
             };
             if !line.is_empty() {
@@ -695,11 +696,18 @@ pub fn join_field(app: &mut FigApp, ui: &mut Ui, typed: &mut String, id: egui::I
 pub fn join_status(app: &FigApp, ui: &mut Ui, t: &Tokens, session: SessionId) {
     match app.session.live.get(session).map(|l| &l.status) {
         Some(Status::Connecting) => widgets::label(ui, tr("Joining…"), t.text_secondary),
-        Some(Status::Failed { reason }) => widgets::label(ui, reason, t.danger),
+        Some(Status::Failed { reason }) => widgets::label(ui, &failure_text(reason), t.danger),
         _ => {}
     }
 }
 
+/// Why a live session failed, in the UI language when it is a reason the UI knows (the transport's
+/// English otherwise).
+pub(crate) fn failure_text(reason: &str) -> Cow<'_, str> {
+    let mb = TOO_LARGE_TO_JOIN.split_once("{mb}").and_then(|(before, after)| reason.strip_prefix(before)?.strip_suffix(after));
+    mb.map_or(Cow::Borrowed(reason), |mb| Cow::Owned(trf(TOO_LARGE_TO_JOIN, &[("mb", &mb)])))
+}
+
 /// Not in a session: your name, how to share (online or on the local network), start; or join
 /// one with a code. Everything else is under Advanced.
 fn start_ui(app: &mut FigApp, ui: &mut Ui, t: &Tokens) {